User:Abyssal/Ceratopsian paleobiogeography

Ceratopsian paleobiogeography refers to the geographical distributions of the extinct ceratopsian dinosaurs. Research into paleobiogepgraphy has implications for reconstructing ancient climates, extinction events, and the geography of the ancient world. It can also help provide information about a group's evolutionary history that couldn't be inferred directly from their fossils. Dinosaur paleobiogeography studies where evolutionary lineages originated, as well as where and how they arrrived at their final locations before going extinct. Scientists have traditionally thought that ceratopsian paleobiogeography was a simple matter because ceratopsians were known only from Asia and North America. Since the most primitive known ceratopsians are Asian and the most derived ceratopsians are were found in North America, paleontologists have generally believed that ceratopsians originated in Asia and spread from there to North America. However possible fragmentary ceratopsian fossils from other locations have strained earlier consensus and suggest that the ceratopsians had a more complicated geographic history than formerly recognized. Individual ceratopsid species tended to have small ranges. Most species have only been documented from a single geologic formation. In Late Cretaceous North America many ceratopsid species were unique to their own environment in a north–south series of distinct ecosystems. If this interpretation is correct it has profound implications for ceratopsian paleobiology and behavior.

Research techniques

In a 2010 review of ceratopsian paleobiogeography, Chinnery-Allgeier and Kirkland noted that while there is an abundance of information available for geologists to use for the reconstruction of ancient earth's geography, there are certain geological process that inevitably cause gaps within this record. Erosion of sediment is one such process. Gaps like these tend to influence reconstructions of ancient continent shapes, coastline positions, and the existence or shape of islands. These gaps tend to be minor and subject to small-scale controversies but can still have major influence on scientific reconstruction of dispersal events. Chinnery-Allgeier and Kirkland argue that ceratopsians first arrived in North America from Europe during the Late Jurassic and Early Cretaceous because the presence of ceratopsians in North America seems to predate the formation of the land bridge connecting Asia to western North America.

Dinosaur paleobiogeography has been studied in broad strokes by including most of the different kinds of dinosaur group in the analyses. This approach uses complicated analyses that produce informative results. Nevertheless, fragmentary remains are often excluded from these studies because they can lower its "resolution". This absence prevents such studies from revealing "the finer patterns with clades".

In 2004 Holtz and others used a phenetic method to build a distance matrix with localities on one axis and taxa on the other. They used the Unweighted Pair-Group Method with Arithmetic Averages technique to analyze their data. Chinnery-Allgeier and Kirkland criticized their choice of method on the grounds that it included all taxa as equal entities even though some may contribute more information about dinosaur paleobiogeography than others. They also excluded some taxa because when all were included in the analysis the results were low resolution. They only used taxa known from at least two localities and only used localities with at least two identified taxa. The study was also biased for more recent dinosaurs because more fossils are known from more recent strata. This research provided the foundation for a hypothetical connection between North America and Europe because their Early Cretaceous faunas clustered together. There was something of a discrepancy betwee these clusters and the cluster that included the faunas of western North America and Asia. Holtz and the other researchers concluded that there may have been a connection between Europe and North America during the Early Cretaceous and a connection between Asia and North American after the Albian-Cenomanian boundary when the Bering Land Bridge became accessible.

With the exception of the 2004 study by Holtz, most paleobiogeographic dinosaur research, including Sereno and Upchurch and others', has seen their researcher's views of relationships within the dinosaurs as its primary influence. The researchers compared their ideas about how dinosaurs were related to one another through evolution to contemporary understandings of ancient earth's geography. All previous research only used scientifically described and formally named taxa. Ceratopsian paleobiogeography has often been neglected or given only cursory examination "except at the lower clade level". The basal neoceratopsians of Asia have not had their biogeography studied thoroughly.

Cladistics-based approaches

One way to help reconstruct ceratopsian paleobiogeography is to infer what dispersal events would be necessary to account for ceratopsian phylogeny as reconstructed by a cladogram. This method has some issue, though. Cladograms vary from one another based on the researchers' choices of characters to examine and interpretations of their states. Cladistic analyses also tend to exclude very fragmentary remains.

Cladograms have a major implication for paleobiogeography due to differing positions of protoceratopsids relative to leptoceratopsids and ceratopsids. Coronosauria is the clade descended from the most recent common ancestor of Protoceratops andrewsi and Triceratops horridus. Some analyses have included the leptoceratopsids, or some taxa generally regarded as leptoceratopsids within this clade. Chinnery-Allgeier and Kirkland found this interesting because if any leptoceratopsid is more closely related to ceratopsids that means that ceratopsids evolved from their shared common ancestor. This scenario would require only one dispersal event from Asia.

Other analyses have found that protoceratopsids are most closely related to ceratopsids and therefore the coronosauria excludes the ceratopsidae. This implies that ceratopsids are descended from an ancestor shared with protoceratopsids. This means that there were at least two dispersal events from Asia into North America, one for the line leading up to ceratopsids and one leading up to the leptoceratopsids. Chinnery and Horner's 2007 cladistic analysis was the most recently performed one as of 2010, and it more strongly supports the second idea.

Cladograms can leave out biogeographically important fossil solely on the grounds of how fragmentary they are. Neoceratopsian teeth have been identified from Maryland's Arundel Formation and Utah's Cedar Mountain Formation based on the oval shape of crown when viewed from the side, pronounced cingulum, indendations on the side of the primary ridge and especially the convex outward-facing surface. The wear patterns of the Arundel neoceratopsian teeth resemble the vertical notch pattern from all known basal neoceratopsian of North America as well as the Asian Udanoceratops. The wear patterns on the Cedar Mountain Formation's basal neoceratopsian teeth indicate oblique shear, however. This may mean that the Cedar Mountain neoceratopsian had different evolutionary affinities than its east coast relative or the teeth were maxillary teeth.

Chinnery-Allgeier and Kirkland observed that even fossils too fragmentary to be useful for reconstructing phylogenies can be useful for understanding paleobiogeography. There are many ghost lineages that suggests that science has many early ceratopsians to discover.

The construction of Area cladograms is an approach that was employed by Paul Sereno in 1999 to graphically arrange regions like continents based on their fossils to uncover the vicariance and dispersal patterns of different dinosaur groups. The nodes on these cladograms represent more recently shared centers of origin for the areas' fossil content. This approach was advantageous because previous research which relied more on the subjective opinion of the researcher. Chinnery-Allgeier and Kirkland criticized Sereno's approach as flawed by its omission of areas with few preserved dinosaur fossils and possible errors in its cladistic analyses. Because time wasn't factored in when producing the cladogram ghost lineages had to be inferred.

Tree Reconciliation Analysis finds the most probable geologic connections based on the area relationships in a cladogram. In 2002 Upchurch and others used Tree Reconciliaion Analysis to find the most probable geologic connections based on the area relationships in a cladogram. Upchurch and others took this approach in 2002, basing on their work on earlier cladogram-centered approaches, but their efforts factored in the time and geologic position of their subject taxa. In 2010, Chinnery-Allgeier and Kirkland criticized their work as afflicted by some of the same problems as earlier research based on area cladograms Issues they noted included inaccurate cladograms or missing data from different geologic sites. However, it also had the unique flaw that the method would fail if the main factor determining the presence of a taxon in the area of study was extinction or dispersal. Extinction or dispersal caused problems because Upchurch his colleagues' approach focused on finding vicariance.

Continental distribution

In a 2010 review of ceratopsian paleobiogeography, Chinnery-Allgeier and Kirkland concluded that ceratopsians underwent multiple geographic dispersals during their evolutionary history. However, the large number of possible courses that could explain ceratopsian paleobiogeography makes pinpointing the specific routes difficult. One or two of these dispersals involved the lineages that became leptoceratopsids and ceratopsids and originated within Asia. The presence of Turanoceratops and Udanoceratops in Asia may require one or two more dispersion events to be explained. Ceratopsians could have traveled from Asia to North America through Europe to eastern North America or from eastern Asia to western North America. Chinnery-Allgeier and Kirkland argued that the first dispersal of ceratopsians from Asia came through Europe to eastern North America, 15 million years before an eastern Asia to western North America dispersal.

Scientists have traditionally thought that ceratopsian paleobiogeography has been simple because they are known only from Asia and North America. Ceratopsians are believed to have originated in Asia because both the chronologically oldest and most primitive forms are found there while more derived ceratopsians are found in North America. Consequently, most experts have interpreted ceratopsian paleobiogeography in terms of dispersal from eastern Asia to western North America. Chinnery-Allgeier and Kirkland allege that scientists with this "simple, bimodal" interpretation of ceratopsian paleobiogeography have tended to ignore possible fragmentary ceratopsian fossils from other locations.

Chinnery-Allgeier and Kirkland summarized their interpretation of ceratopsian paleobiogeography.

They thought that ceratopsians first originated and diversified in Asia due to the most primitive forms being located there as well as the3 presence of its sister groups.

Then the ancestor of leptoceratopsids spread to Europe and from there to North America. some populations descended from this group may have stayed in Europe and evolved into endemic groups here that would leave fossils of unusual teeth, but the absence of ceratopsian fossils in other European strata of the age suggests otherwise.

Then basal neoceratopsians began divsersifying in Asia. This radiation may have included the evolution of ceratopsids from Turanoceratops or a relative and the dispersal of one or more taxa into North America when the Bering Land Bridge was exposed around the Turonian.

Ceratopsids in North America diversified greatly. If the ceratopsids didn't originate in Asia then one or more dispersal events must have gone back over the Bering Land Bridge into Asia to account for Turanoceratops. Another dispersal event back into Asia may have taken leptoceratopsids there, which would account for Udanoceratops. Chinnery-Allgeier and Kirkland regarded this as unconfirmed, however.

One or more dispersal events from North America into Europe during the middle part of the Cretaceous Period.

Climate, vegetation distributions, landmass size, and physical barriers are all factors that could affect which environments ceratopsians lived in. Evolution is also affected by perturbations in the environment like habitat tracking, extinction, and the formation of isolated populations. This makes the environmental contexts of ceratopsian remains important for understanding their biology and evolution.

Sereno found that ceratopsians were limited to Asia and western North America. They were thought to be a clear case of dispersal across the Bering Land Bridge. At the time this was consistent with geological consensus. Sereno's study also predated the discovery of early ceratopsians Yinlong and Chaoyangsaurus.

Chinnery-Allgeier and Kirkland expressed sympathy for the traditional view because it closeley fit the fossil data until relatively recently. Ten to fifteen years before their 2010 study of ceratopsian paleobiogeography fossil finds in North America and Asia as well as cladistic analyses have complicated the traditional view of ceratopsian distribution.

Chinnery-Allgeier and Kirkland argue that ceratopsians first arrived in North America from Europe during the Late Jurassic and Early Cretaceous because the presence of ceratopsians in North America seems to predate the formation of the land bridge connecting Asia to western North America.

The presence of very primitive Asian ceratopsians like Yinlong and Chaoyangsaurus during the Late Jurassic to earliest Cretaceous opens the possibility that ceratopsians dispersed to North America prior to the appearance of the Bering Land Bridge in the middle Cretaceous. Chaoyangsaurus has either been found one step before or after the Psittacosauridae in cladistic analyses.

Chinnery-Allgeier and Kirkland wanted to know the answers to four questions:

When did the Bering Land Bridge form?

When was it possible to travel to North America from Asia and Europe?

When did the Western Interior Seaway divide North America in two, preventing travel between the different side of the continent.

How do the answers to these questions relate to other evidence regarding ceratopsian paleobiogeography.

Multiple dispersal events must have occurred in order to account for the distributions and relationships of ceratopsian dinosaurs. Up to two dispersal events brought ceratopsians from Asia where they evolved in to North America. The first was the ancestor of the leptoceratopsid clade. The second was the ancestor of ceratopsids. Although most ceratopsids are known from North America the possibility that Turanoceratops might be the sister taxon of ceratopsidae could imply that they first evolved in Asia. If this isn't true then there must have been at least one dispersal event from Asia back into North America. European ceratopsians are so primitive that they probably immigrated there from Asia.

Asia

LJ

Asia is probably where ceratopsians first evolved since both the oldest and most primitive taxa are found there. These primitive forms include middle-to-late Jurassic genera like Yinlong and Chaoyangsaurus, with Yinlong being the older of the two. Preliminary paleobiogeographic research suggests that the wildlife of Asia was unique at the time. Upon the arrival of the Late Jurassic there's still no evidence for a connection between Asia and North America. The biogeography of cryptobranchoid salamanders also contradicts the idea of a Late Jurassic connection between Asia and North America. In 1993 Russel thought that Asia was also separated from Europe by a body of water called the Turgai Sea. This likely geographic isolation that makes it difficult to understand how ceratopsians dispersed from Asia to Europe and North America. If Asia really was isolated from the rest of the world during the mid-to-late Jurassic it puts some constraints on when and how ceratopsians could have traveled from Asia to North America. Ceratopsians could have dispersed from Asia prior to its isolation in the middle Jurassic or not dispersed until the Bering Land Bridge had come into existence. It's also possible that ceratopsians originated in Europe or North America instead of Asia, but again, this isn't consistent with the presence of old, primitive ceratopsians in Asia like Yinlong.

EC

Asia may have been the source of two waves of ceratopsian dispersal into North America. The first dispersal event include the ancestor of the leptoceratopsids and the second was the ancestor of ceratopsids. However, since Turanoceratops might be the sister taxon of Ceratopsidae, the family may have evolved in Asia instead. If this isn't true then there must have been at least one dispersal event from Asia back into North America. Asia is likely the original source for European ceratopsians as well. It's difficult to reconstruct the paths these dispersal events may have taken because the ceratopsians could have traveled across the Bering Land Bridge between Asia and western North America or between eastern North America and Europe. Further, these options aren't mutually exclusive and both of these routes were probably taken at some points. The geology of Europe hasn't been studied well enough to conclude whether it could form a link connecting Asia to western North America during the Cretaceous. However, there is no evidence against the idea either. The first dispersal of ceratopsians out of Asia probably occurred during or before the Aptian because basal neoceratopsian teeth are already known in the eastern US by this time. This could possibly be accounted for if the ceratopsians crossed the Bering Land Bridge into western North America and crossed the entire continent. However, geological evidence suggests that the Bering Land Bridge didn't appear until 15 million years after this dispersal event. This suggests that the best explanation for the early presence of ceratopsians in eastern North America was an indirect route out of Asia, through Europe, and into North America during the Late Jurassic or Early Cretaceous.

The more advanced psittacosaurs and neoceratopsians appear further to Asia's north than the more primitive forms during the Early Cretaceous, from Barremian to Albian time. Chinnery-Allgeier and Kirkland point out Auroraceratops, Archaeoceratops, and Liaoceratops, as representative Asian basal neoceratopsians from that interval of time.

From the early Late Cretaceous (94-85 million years ago) onward all evidence of dispersal events of ancient dinosaurs and mammals occurs from Asia into North America across the Bering Land Bridge. This is likely due to the well-supported presence of the Bering Land Bridge and the increasing distance between Europe and North America.

The basal neoceratopsians of Asia survived all the way into the Late Cretaceous. Their biogeography, however, hasn't been studied thoroughly.

Udanoceratops tischovi is the only known leptoceratopsid from Asia. Characteristics it shares with North America leptoceratopsids include the shape of its dentary, curved tooth row, and tooth occlusion pattern. Despite the preliminary similarities there are major differences in body size, jaw anatomy and nostril size that distinguish Udanoceratops from the leptoceratopsids.

Turanoceratops is known from very fragmentary remains. Features identifying Turanoceratops as a ceratopsid include double rooted teeth and brow horns. The presence of a ceratopsid in Asia during the Turonian calls into question the idea that they originated in North America. Turanoceratops is important not only because at the time Chinnery-Allgeier and Kirkland were writing it was the only known ceratopsid from Asia, but also because its Turonian age placed it early in the Late Cretaceous.

Australia

Possible ceratopsian remains from around the Barremian to Albian time range have been reported from Australia. The find consisted of two ulnae possibly left behind by basal neoceratopsians, although their ceratopsian identity is "questionable" and "suspect".

Europe

Between the Late Jurassic and Early Cretaceous its possible there was a temporary connection between Europe and North America.

Some reconstructions, however, prohibit the idea of a connection of Europe and North America through Asia.

Ceratopsian fossils have been found in Asia, North America, Europe, and possibly Australia.

Europe is thought to have resembled an island chain in a shallow sea during the Late Jurassic. Different reconstructions of ancient Europe made by different scientists tend to be similar to one another varying only in the shape and placement of the European islands.

In 1993 Russel argued for the possibility of intermittant connections between Europe and North America during the Middle and Late Jurassic. However, he also thought that Europe was isolated form Asia by the Turgai Sea. Tentative paleobiogeographic evidence suggests that the wildlife of Asia was endemic at the time. If all this is true it puts some constraints on when and how ceratopsians could have gotten to North America. Ceratopsians could have dispersed from Asia prior to the middle Jurassic, originated in Europe or North America instead of Asia, or not dispersed until the Bering Land Bridge had come into existence. The existence of Yinlong in the Middle Jurassic of Asia is support for the former.

Blakey's reconstructions portray the Western Interior Seaway as being absent 140 million years ago, but has mostly divided North America in half by 120 million years ago. Other reconstructions show North America as only somewhat split by this time. The early appearance of ceratopsians in northeastern North America adds significant paleobiogeographic significance to the exact timing of the division of North American by the Western Interior Seaway. Basal neoceratopsian teeth are found in 120 million years strata in Maryland. The Bering Land Bridge probably did not exist prior to 125 million years ago, so if ceratopsians really came in from Asia they only had 5 million years to reach the east coast.

By 115 million years ago the Western Interior Seaway became longer and wider. By 100 million years ago it began shrinking in both dimensions again.CITETHISEITHER394OR395 At this time Europe was an archipelago separated from North America and Asia.

In Russel's 1993 reconstructions Europe is connected to both Asia and North America by land. Scotese didn't have a reconstruction for this time period, but shows the existence of the Bering Land Bridge by 140 ma at most. He depicts it as breaking up around 120 million years ago and then reforming in the Late Cretaceous. There is strong evidence for a connection between Europe and North America based on paleobiogeographic data.

At the time of writing pachycephalosaurids were only known in Asia and North America. However the problematic possible pachycephalosaurids Yaverlandia bitholu from the Barremian aged Wealden Formation of England and Stenopelix valdensis from Barremian Germany may indicate they were more widely spread than generally acknowledged.

Non-dinosaurs also provide evidence for dispersal events during or prior to the Early Cretaceous. Gobiconodontid mammals are known from Europe and England although before they were only known from Asia and North America. The oldest members of this group are from the Hauterivian of Asia. Asian forms probably moved into Europe during the Barremian. Cuenca-Bescos and Canudo theorized in 2003 that gobiconodontids dispersed into North America across the Bering Land Bridge during the Aptian or Albian. However, Chinnery-Allgeier and Kirkland argue that this biogeographic data could also support the idea that the gobiconodontids traveled from Europe to North America.

None of the reconstructions of Early Cretaceous Earth that Chinnery-Allgeier and Kirkland consulted showed the existence of the Bering Land Bridge before 120 million years ago. In all reconstructions the Fenno-Scandinavian Shield is included in an island chain within a shallow sea.

Smith and others portray the Western Interior Seaway as completely dividing North America by 105 million years ago. The Smith and others and Rusell reconstructions portray Europe as connected with Asia by 105 million years ago. Smith and others and Russel portray the Bering Land Bridge as existing by the Aptian in 120 million years ago. Blakey however, doesn't show the land bridge until 90 million years ago.

It's difficult to reconstruct the paths these dispersal events took because the ceratopsians could have traveled across the Bering Land Bridge between Asia and western North America or between eastern North America and Europe. Both of these routes were probably taken at some points. The first dispersal of ceratopsians out of Asia probably occurred during or before the Aptian because basal neoceratopsian teeth are already known in the Arundel Formation of Maryland by this time. This could possibly be accounted for if the ceratopsians crossed the Bering Land Bridge into western North America and crossed the entire continent. However, geological evidence suggests that the Bering Land Bridge didn't appear until 15 million years after this dispersal event. This suggests that the best explanation for the early presence of ceratopsians in eastern North America is dispersal from Europe during the Late Jurassic or Early Cretaceous. Terrestrial environments in Europe at this time are poorly understood but it looks like the Turgai Sea that separated Asia from Europe was spanned by land bridges at some time as, at times, was the newly forming Atlantic Ocean.

Life forms with Asian affinities other than neoceratopsians also appeared in North America prior to the formation of the Bering Land Bridge, like therizinosaurs. Commonalities between North American and European wildlife from the same time period suggest travel between the continent in one or both directions at the time.

Some of the ceratopsians of North America are older than those known from Europe. In 2007 both Lindgren and others and Godefroit and Lambert argued that the European ceratopsians were the descendants of ceratopsians from Asia. Chinnery-Allgeier and Kirkland, however, suggested the possibility that these forms were descended from North American forms due to the existence of older teeth from the Aptian of Maryland.

Most reconstructions of Cretaceous North America have the Bering Land Bridge appear near the end of the Albian. The geology of Europe hasn't been studied well enough to conclude whether it could form a link connecting Asia to western North America. However, there is no evidence against the idea either. Also North America was divided by the KWIS by the late Early Cretaceous. This is significant because the earliest known North American ceratopsian fossils are known from the Aptian of northeastern North America.

All reconstructions show the Bering Land Bridge by the early Late Cretaceous (94-85 million years ago). All likewise show Europe as more distant to North America. From this point onward all evidence of dispersal events of ancient dinosaurs and mammals occurs from Asia into North America across the Bering Land Bridge.

Ceratopsid fossils were known only from western North America in rocks of middle Cretaceous age and later.

The known geographic range of basal neoceratopsians has expanded to included Sweden, which has produced teeth and vertebrae, and Belgium, which has produced teeth referred to cf. Craspedodon lonzeensis.

Basal neoceratopsian teeth have been reported from the earliest and latest Early Campanian of southern Sweden. Teeth resembling basal neoceratopsians ahve been found in Belgium's middle Late Cretaceous rocks. Their late date may cause more confusion than utility in trying to reconstruct paleobiogeography.

The recently discovered teeth in Sweden show similarities and differences to those of North America. These teeth have enamel on both sides of the crown. This is a characteristic of ornithopods but also very primitive ceratopsians like Chaoyangsaurus and psittacosaurids. all other neoceratopsians have teeth with enamel only on the non-occlusive surface. The Swedish teeth have thinner enamel on their outer surface, which may be an intermediate trait. Like other basal neoceratopsian teeth, the teeth are wide with respect to the inside and outside of the mouth, however their outer surface is concave like ornithopod teeth. All have prominent primary ridges and a cingulum, although the cingulum is only pronounced on the speicmen RM PZ R1833. In all other specimens the primary ridge is confluent with the cingulum, a feature found in all ornithischians except leptoceratopsids. The wear surface on four of the five teeth show vertical notch or oblique notch patterns, but is very different from the wear seen on leptoceratopsid teeth. RM PZ R1833, according to Chinnery-Allgeier and Kirkland, is the only neoceratopsian tooth in the set. The vertebrae described by Lindgren and others in 2007 is not of sufficient quality for a confident identification. The possession of enamel on both sides of the tooth suggets the specimen warrants inclusion at the ver base of the neoceratopsia. RM PZ R1833 has a pronounced cingulum set back from the primary ridge otherwise found only in leptoceratopsids.

In 2007 Godefroit and Lambert referred teeth found in Belgium that had once been regarded as belong to the iguanodont Craspedodon lonzeensis. They were recognized as such based on their distribution of enamel, shape and prominent secondary ridges. The Belgian teeth have wear patterns suggestive of oblique shear like in Asian neoceratopsians and the maxillary teeth of leptoceratopsids. These teeth also have a prominent cingulum which is separarte from their primary ridge which are further evidence for affinities with leptoceratopsids.

Although not formally subjected to cladistic analysis, the Belgian Craspedodon teeth seem to belong at the base of the leptoceratopsidae and the Swedish tooth between Psittacosaurids and Liaoceratops. The teeth from Aptian strata in Maryland and Albian strata in Utah were unresolved and Chinnery-Allgeier and Kirkland thought them to belong one step below leptoceratopsidae on a ceratopsian cladogram.

The neoceratopsian teeth of Utah's Cedar Mountain Formation form "one of the more compelling lines of evidence" that ceratopsians dispersed into North America from Europe. A large carnosaur is also known from the formation. Many of the dinosaurs from the Cedar Mountain Formation have relatives in Europe. Pelorosaurus is a sauropod the same age as the Cedar Mountain Formation related to Cedarosaurus and Venenosaurus. The advanced iguanodonts of the Cedar Mountain Formation have teeth that are similar to those of their relatives in the Albian to Cenomanian of England like "Iguanodon" hilli and "Trachodon" cantabrigiensis. Gastonia is very similar to Polacanthus from western Europe. Nedcolbertia resembles European coelurosaurs from the same time period. These and other similarities between the dinosaurs of the Cedar Mountain Formation and contemporary Europe support the idea that a landbridge connected it with North America 120-125 million years ago, before the formation of the Bering Land Bridge.

North America

In the Late Jurassic there were no land connections between Asia and North America.

Between the Late Jurassic and Early Cretaceous its possible there was a temporary connection between Europe and North America.

Some reconstructions, however, prohibit the idea of a connection of Europe and North America through Asia.

Biogeographic evidence from cryptobranchoid salamanders also contradicts the idea of a Late Jurassic connection between Asia and North America.

In 1993 Russel argued for the possibility of intermittant connections between Europe and North America during the Middle and Late Jurassic. However, he also thought that Europe was isolated form Asia by the Turgai Sea. Tentative paleobiogeographic evidence suggests that the wildlife of Asia was endemic at the time. If all this is true it puts some constraints on when and how ceratopsians could have gotten to North America. Ceratopsians could have dispersed from Asia prior to the middle Jurassic, originated in Europe or North America instead of Asia, or not dispersed until the Bering Land Bridge had come into existence. The existence of Yinlong in the Middle Jurassic of Asia is support for the former.

Blakey's reconstructions portray the Western Interior Seaway as being absent 140 million years ago, but has mostly divided North America in half by 120 million years ago. Other reconstructions show North America as only somewhat split by this time. The early appearance of ceratopsians in northeastern North America adds significant paleobiogeographic significance to the exact timing of the division of North American by the Western Interior Seaway. Basal neoceratopsian teeth are found in 120 million years strata in Maryland. The Bering Land Bridge probably did not exist prior to 125 million years ago, so if ceratopsians really came in from Asia they only had 5 million years to reach the east coast.

By 115 million years ago the Western Interior Seaway became longer and wider. By 100 million years ago it began shrinking in both dimensions again.CITETHISEITHER394OR395 At this time Europe was an archipelago separated from North America and Asia.

In Russel's 1993 reconstructions Europe is connected to both Asia and North America by land. Scotese didn't have a reconstruction for this time period, but shows the existence of the Bering Land Bridge by 140 ma at most. He depicts it as breaking up around 120 million years ago and then reforming in the Late Cretaceous. There is strong evidence for a connection between Europe and North America based on paleobiogeographic data.

The neoceratopsian teeth of Utah's Cedar Mountain Formation form "one of the more compelling lines of evidence" that ceratopsians dispersed into North America from Europe. A large carnosaur is also known from the formation. Many of the dinosaurs from the Cedar Mountain Formation have relatives in Europe. Pelorosaurus is a sauropod the same age as the Cedar Mountain Formation related to Cedarosaurus and Venenosaurus. The advanced iguanodonts of the Cedar Mountain Formation have teeth that are similar to those of their relatives in the Albian to Cenomanian of England like "Iguanodon" hilli and "Trachodon" cantabrigiensis. Gastonia is very similar to Polacanthus from western Europe. Nedcolbertia resembles European coelurosaurs from the same time period. These and other similarities between the dinosaurs of the Cedar Mountain Formation and contemporary Europe support the idea that a landbridge connected it with North America 120-125 million years ago, before the formation of the Bering Land Bridge.

At the time of writing pachycephalosaurids were only known in Asia and North America. However the problematic possible pachycephalosaurids Yaverlandia bitholu from the Barremian aged Wealden Formation of England and Stenopelix valdensis from Barremian Germany may indicate they were more widely spread than generally acknowledged.

Non-dinosaurs also provide evidence for dispersal events during or prior to the Early Cretaceous. Gobiconodontid mammals are known from Europe and England although before they were only known from Asia and North America. The oldest members of this group are from the Hauterivian of Asia. Asian forms probably moved into Europe during the Barremian. Cuenca-Bescos and Canudo theorized in 2003 that gobiconodontids dispersed into North America across the Bering Land Bridge during the Aptian or Albian. However, Chinnery-Allgeier and Kirkland argue that this biogeographic data could also support the idea that the gobiconodontids traveled from Europe to North America.

None of the reconstructions of Early Cretaceous Earth that Chinnery-Allgeier and Kirkland consulted showed the existence of the Bering Land Bridge before 120 million years ago. In all reconstructions the Fenno-Scandinavian Shield is included in an island chain within a shallow sea.

Smith and others portray the Western Interior Seaway as completely dividing North America by 105 million years ago. The Smith and others and Rusell reconstructions portray Europe as connected with Asia by 105 million years ago. Smith and others and Russel portray the Bering Land Bridge as existing by the Aptian in 120 million years ago. Blakey however, doesn't show the land bridge until 90 million years ago.

It's difficult to reconstruct the paths these dispersal events took because the ceratopsians could have traveled across the Bering Land Bridge between Asia and western North America or between eastern North America and Europe. Both of these routes were probably taken at some points. The first dispersal of ceratopsians out of Asia probably occurred during or before the Aptian because basal neoceratopsian teeth are already known in the Arundel Formation of Maryland by this time. This could possibly be accounted for if the ceratopsians crossed the Bering Land Bridge into western North America and crossed the entire continent. However, geological evidence suggests that the Bering Land Bridge didn't appear until 15 million years after this dispersal event. This suggests that the best explanation for the early presence of ceratopsians in eastern North America is dispersal from Europe during the Late Jurassic or Early Cretaceous. Terrestrial environments in Europe at this time are poorly understood but it looks like the Turgai Sea that separated Asia from Europe was spanned by land bridges at some time as, at times, was the newly forming Atlantic Ocean.

Life forms with Asian affinities other than neoceratopsians also appeared in North America prior to the formation of the Bering Land Bridge, like therizinosaurs. Commonalities between North American and European wildlife from the same time period suggest travel between the continent in one or both directions at the time.

Some of the ceratopsians of North America are older than those known from Europe. In 2007 both Lindgren and others and Godefroit and Lambert argued that the European ceratopsians were the descendants of ceratopsians from Asia. Chinnery-Allgeier and Kirkland, however, suggested the possibility that these forms were descended from North American forms due to the existence of older teeth from the Aptian of Maryland.

Most reconstructions of Cretaceous North America have the Bering Land Bridge appear near the end of the Albian. The geology of Europe hasn't been studied well enough to conclude whether it could form a link connecting Asia to western North America. However, there is no evidence against the idea either. Also North America was divided by the KWIS by the late Early Cretaceous. This is significant because the earliest known North American ceratopsian fossils are known from the Aptian of northeastern North America.

All reconstructions show the Bering Land Bridge by the early Late Cretaceous (94-85 million years ago). All likewise show Europe as more distant to North America. From this point onward all evidence of dispersal events of ancient dinosaurs and mammals occurs from Asia into North America across the Bering Land Bridge.

Ceratopsid fossils were known only from western North America in rocks of middle Cretaceous age and later.

Early Cretaceous

Ceratopsian remains from around the Barremian to Albian time range have been reported from North American geologic units like the Arundel Formation, Cloverly Formation, and Wayan Formation. An unnamed Albian-aged basal neoceratopsian is the oldest example of such from North America. North American basal neoceratopsians persisted until the end of the Cretaceous. Fragmentary remains of North American basal neoceratopsians including teeth and partial skeletons are known from both eastern and western North America. Basal neoceratopsian remains from Maryland date back to the Aptian and remains from Idaho and Utah are known from the Albian. Basal neoceratopsian teeth have been known from the Arundel Formation in Maryland for many years although they have been excluded from cladisitic analyses on the ground of their fragmentary preservation. According to Lipka, the Arundel Formation is unlikely to produce any more substantial fossils of basal neoceratopsians. Fragmentary remains are likweise known from western North America and are just as fragmentary as their eastern counterparts.

Basal neoceratopsian remains from Maryland date back to the Aptian and remains from Idaho and Utah are known from the Albian. One undescribed partial skull and two partial basal neoceratopsian skeletons are known from the Albian of western North America. Basal neoceratopsian teeth are known from the Cenomanian. Udanoceratops tischovi is the only known leptoceratopsid from Asia. Known fossils of Udanoceratops includes a partial skull that's missing its roof, braincase, and frill. Characteristics it shares with North America leptoceratopsids include the shape of its dentary, curved tooth row, and tooth occlusion pattern. Despite the preliminary similarities there are major differences in body size, jaw anatomy and nostril size that distinguish Udanoceratops from the leptoceratopsids. This combined with its poorly understood anatomy implies that science's opinion on its place in evolution may change.

The Cedar Mountain Formation has been dated using Argon isotopes. This confirms the presence of certain kinds of animal life by the Albian. The close relationship between the wildlife in the Cedar Mountain Formation and contemporary European and Asian wildlife suggests that by this point some means of travel existed connecting North America with Asia, possibly the Bering Land Bridge. The Cedar Mountain Formation has been dated using Argon isotopes. This confirms the presence of certain kinds of animal life by the Albian. The close relationship between the wildlife in the Cedar Mountain Formation and contemporary European and Asian wildlife suggests that by this point some means of travel existed connecting North America with Asia, possibly the Bering Land Bridge.

Most reconstructions of Cretaceous North America have the Bering Land Bridge appear near the end of the Albian. The geology of Europe hasn't been studied well enough to conclude whether it could form a link connecting Asia to western North America. However, there is no evidence against the idea either. Also North America was divided by the KWIS by the late Early Cretaceous. This is significant because the earliest known North American ceratopsian fossils are known from the Aptian of northeastern North America.

The recently discovered teeth in Sweden show similarities and differences to those of North America. These teeth have enamel on both sides of the crown. This is a characteristic of ornithopods but also very primitive ceratopsians like Chaoyangsaurus and psittacosaurids. all other neoceratopsians have teeth with enamel only on the non-occlusive surface. The Swedish teeth have thinner enamel on their outer surface, which may be an intermediate trait. Like other basal neoceratopsian teeth, the teeth are wide with respect to the inside and outside of the mouth, however their outer surface is concave like ornithopod teeth. All have prominent primary ridges and a cingulum, although the cingulum is only pronounced on the speicmen RM PZ R1833. In all other specimens the primary ridge is confluent with the cingulum, a feature found in all ornithischians except leptoceratopsids. The wear surface on four of the five teeth show vertical notch or oblique notch patterns, but is very different from the wear seen on leptoceratopsid teeth. RM PZ R1833, according to Chinnery-Allgeier and Kirkland, is the only neoceratopsian tooth in the set. The vertebrae described by Lindgren and others in 2007 is not of sufficient quality for a confident identification. The possession of enamel on both sides of the tooth suggets the specimen warrants inclusion at the ver base of the neoceratopsia. RM PZ R1833 has a pronounced cingulum set back from the primary ridge otherwise found only in leptoceratopsids.

Early Late Cretaceous

Most Mesozoic biogeography has focused on comparing the lifeforms between and among continents. Because there are many fossils and the area is well studied, Cretaceous western North America can be examined for regional differences. The total combined area of Cretaceous dinosaur-rich habitats in western North America adds up to 4,000,000 kilometers or 16% of the continent, a "remarkabl[e]" figure. Although North America was sometimes linked to Asia during the Late Cretaceous, the lack of species shared between the two continents suggest that there was only limited faunal exchange over this land bridge. The meant that western North America was effectively isolated. Sampson and Loewen called it a "sweepstakes filter". Late Cretaceous wwestern North America is probably the best studied "slice" of space-time of any major Mesozoic land mass.

Changes in the level of the Western Interior Seaway had profound impacts on ceratopsian. About 100 million years ago the epieric sea flooded the interior of western North America. This formed two partially isolated land masses, known as Laramidia in the west and Appalachia in the east. The Sevier Fault Belt bordered the Western Interior Seaway to the west. Over the next 35 million years the Wester Interior Seaway rose and fell, changing the size of the Western Interior Basin and the kinds of environments found there. Several fossil-bearing rock formations found between the Sevier and the KWIS preserve evidence of environments like a near shore coastal plain, more distal alluvial plain and upland intermontane basins during the Maastrichtian. The last major expansion into norther Laramidia of the Western Interior Seaway was the Bearpaw Transgression. For 3.5 million years of the late Campanian the KWIS migrated 200 miles west, with occasional short regressive fluctuations. During the Maastrichtian, the KWIS retreated to the Northeast, eventually exposing connections between western and eastern North America.

The discovery of centrosaurine fossil in the Parras Basin of Mexico and reports of Pachyrhinosaurus from Alaska means that centrosaurs spanned all of North America from Mexico to the Arctic. This is a "much larger" range than has been documented in chasmosaurs. The discovery of new centrosaurs in Utah is evidence of previously unrealized diversity in the south of the Western Interior Basin. Zuniceratops is evidence that long eye horns were present in the common ancestor of centrosaurs and chasmosaurs. The presence of centrosaurs with long eye horns like Albertaceratops and the new taxa from the Kaiparowits and Wahweap Formations indicates they were still present in primitive centrosaurs and that these primitive centrosaurs already achieved a wide latitudinal range. Pachyrhinosaurs with facial bosses seem restricted to the northern part of western North America.

Zuniceratops is the sister taxon of the ceratopsids and already existed 90 million years ago, significantly after the Western Interior Seaway came into existence. The earliest known ceratopsid occurs at about 79 million years. The lack of ceratopsids in Appalachia suggests they evolved after the KWIS isolated the two landmasses from each other. Triceratops is the last known ceratopsid and coincides with the retreat of the canonball sea at the end of the Cretaceous. Triceratops, Torosaurus, and Diceratops are the only known ceratopsids who weren't restricted to the narrow band of alluvial coastal plain sediments lying between the Sevier orogenic belt and the Western Interior Seaway. They lived in the area once occupied by the sea before it began retreating and the two halves of North America were reunited.

Zuniceratops has been regarded as one of the most significant ceratopsian finds in North America. Zuniceratops is a transitional form linking primitive neoceratopsians with ceratopsids. Zuniceratops brow horns link it with the ceratopsids, but its single rooted teeth, lack of a nose horn and others are primitive. The Turonian age of this taxon is important to the study of biogeography because it illustrates that evolution within the Neoceratopsia occurred earlier than once thought.

Multiple dispersal events must have occurred in order to account for the distributions and relationships of ceratopsian dinosaurs. Up to two dispersal events brought ceratopsians from Asia where they evolved in to North America. The first was the ancestor of the leptoceratopsid clade. The second was the ancestor of ceratopsids. Although most ceratopsids are known from North America the possibility that Turanoceratops might be the sister taxon of ceratopsidae could imply that they first evolved in Asia. If this isn't true then there must have been at least one dispersal event from Asia back into North America. European ceratopsians are so primitive that they probably immigrated there from Asia.

Thomas M. Lehman, in a study of Late Cretaceous dinosaur distribution, concluded that they were "remarkably provincial," with herbivorous dinosaurs exhibiting "persistent latitudinal and altitudinal zonation" in "[d]istinctive endemic associations."

In some points in time few or no kinds of ceratopsids are known. Very few ceratopsids older than 76 million years are known. In the north only the 77 million year old Centrosaurus brinkmani, the 78 million year old Albertaceratops nesmoi, and Medusaceratops lokii. In the south is the 79 million years old Wahweap new taxon. Only the new taxon from the Wahweap Formation has significant implications for understanding early ceratopsid evolution. Before 79 million years ago is a 10 million year gap before encountering the ceratopsids' sister taxon, Zuniceratops christopheri.

At any single point in time there were likely only a single species of chasmosaur and centrosaur in the northern and southern regions of western North America.

Campanian

Ceratopsids in North America are only known from the Late Cretaceous.

The best studied span of time in ceratopsid research is 76-74 million years ago during th late Campanian. Of the 32 known ceratopsids, almost half are from this time span. Taking into account the late Campanian generally, 76–70.5 million years ago, more than twenty of the thirty two known ceratopsids are accounted for. This means that two thirds of known ceratopsid diversity is known from one third of their lifespan. Late Campanian strata are also more widely studied across a wide geographic range than those of other ages. They are known from Mexico to Alaska. Only for the 76-74 million years ago timespan can distinct northern and southern faunas be perceived.

Multiple factors probably contributed to the short lifespans of ceratopsid species. These include cyclical envrinomental changes like transgression and regrassion of the Cretaceous Western Interior Seaway splitting larger populations into smaller ones. Succinctly, many new species were evolving in an environment that lacked the capacity to hold all of them led to the development of a pattern of continuous replacement.

Later studies on earlier ceratopsids from the Campanian supported the coastal lowland ideas. In the northern region of the Western Interior Basin, ceratopsian fossils are most abundant when the sea expanded. The clastic wedge that preserves dinosaur fossils in the region includes geologic formations like the Judith River Formation, Dinosaur Park Formation, and Two Medicine. All of these formations are overlain by the marine shales of the Bearpaw Formation. This implies that fossil sites located stratigraphically higher within these formations represent environments closer to the coast. In 1998 Brinkman and others published a study documenting evidence of this same stratigraphic pattern. They also studied sites of the same age geographically varying in placement from east to west within southern Alberta. If ceratopsians really preferred coastal habitats they should be seen in increasing numbers as the sites progressed farther east because those sites would be closest to the coast. The researchers found that this was indeed the case in both macrovertebrate and microvertebrate sites.

The Two Medicine Formation is particularly significant because it's one of the few Campanian deposits in the Western Interior Basin to preserve more arid upland environments. In 2007 Ryan and others speculated that Styracosaurus albertensis of the Dinosaur Park Formation was adapted to more coastal mesic settings while S. ovatus of the Two Medicine Formation was adapted for more inland xeric settings. Most dinosaur remains in the Two Medicine Formation are known from the upper portion 76-74 million years ago. Ceratopsians, however, are known only from the 75-74 million year old uppermost part of the formation. Below this level dinosaur remains are relatively abundant and include animals like the Egg Mountain fauna including Maiasaura, Orodromeus, and Troodon. Sampson and Loewen feel that the absence of ceratopsian fossils below the uppermost Two Medicine reflects and absence in life. They hypothesize that ceratopsians didn't appear in the Two Medicine depositional environment until the Bear Paw transgression had changed the environment to a coastal setting. Since S. ovatus is slightly younger than S. albertensis and lived during a period of rapid transgression it's possible that it too could have been a coastal animal.

In 1998 Rogers concluded that the uppermost Two Medicine strata were deposited in a semi-arid environment, which may be evidence against Sampson and Loewen's hypothesis. They said the coastal hypothesis can be tested with increased sampling of strata lower in the Two Medicine and further study into how its and other formations' environments changed over time. Dinosaurs of the Campanian WIB are only known from west coastal plains and cooler alluvial plains, a third paleoenvironment appears during the Maastrichtian.

Dinosaurs of the Campanian WIB are only known from west coastal plains and cooler alluvial plains, a third paleoenvironment appears during the Maastrichtian. After the Laramide orogeny began in the early Maastrichtian, semi-arid intermontane basins appeared. These were characterized by a unique fauna of herbivorous dinosaurs that includes the titanosaur Alamosaurus and the chasmosaur Torosaurus. These environments are generally only found in the wouth of the WIB from Utah to Texas. The presence of Torosaurus in upland intermontane basins a significant distance away from the coasts suggests it may have been an ecological outlier that lived in a paleoenvironment significantly different from those preferred by other ceratopsians. Sampson and Loewen encouraged future research into understanding ceratopsian habitat preferences in order to better understand their evolutionary history.

In 1998 Brinkman and others proposed a hypothesis Sampson and Loewen called "intriguing" regarding ceratopsid migration. The observed that the many monodominant ceratopsid bonbeds tended to occur in the middle of the Dinosaur Park Formation. The rock here was deposited in an inland deposit. This is in contradiction to most ceratopsid fossil finds which tend to be closer to the shore of the KWIS. The authors hypothesized that ceratopsids may have gathered into herds seasonally and engage in east–west migrations perhaps in accordance with food availability or mating season. Sampson and Loewen encouraged more research into this idea as more evidence comes to light.

Some paleontologists have argued that during the Late Cretaceous western North America had distinct northern and southern biomes. Others have challenged thhis hypothesis on the grounds that some geologic formations being compared were of different ages. Recent radiometric dates for different strata have supported the idea of faunas varying by latitude. This is because the Dinosaur Park, Two Medicine, and Judith River Formations all overlap with eacother in times. This is also true of the southern Kaiparowits and Fruitland Formations. The discovery of a new macrovertebrate fauna in the Campanian Kaiparowits Formation of Utah in particular has supported this idea. Sampson and Loewen consider the most remarkable fact about purported latitudinal variations in the fauna of Late Cretaceous Western North America in that so conclusively identified fossils from a single species of dinosaur from the time period is known from both the north and south of the continent. In fact most dinosaurs are known only from a single geologic formation, although Triceratops and Torosaurus were exceptions to that rule.

After the Dinosaur Renaissance, ceratopsians were viewed as rhino or antelope-like animals living in mixed-sex herds in upland habitats. In 1987, Lehman challenged this view by noting that Triceratops lived in humid lowlands near the coast. In fact most dinosaurs are known only from a single geologic formation, although Triceratops and Torosaurus were exceptions to that rule.

The low ceratopsid species diversity at any given point in time may have important implications for understanding Late Cretaceous dinosaur diversity in general. Many scientists have argued that in North America dinosaur diversity peaked in the late Campanian and then dwindled to very low diversity near the end of the Cretaceous. Much of the support for this dwindling diversity hypothesis has been based on the large variety of dinosaur found in the Dinosaur Park Formation. However, the Dinosaur Park Formation preserves a series of dinosaur faunas, so most of its preserved diversity would not have been alive at one time. In 2002, Russell and Manabe published evidence that the Hell Creek Formation is more diverse than previously credited. Some dinosaur groups present in the late Campanian are absent in the late Maastrichtian. These include lambeosaurs, centrosaurs, and albertosaurs. The reason the Dinosaur Park Formation preserved so many different species is because they existed for brief periods before evolving and going extinct. This led to a high number of species being preserved over the period of deposition.

Dinosaur diversity during the Campanian was still high despite the above comments. At any given time during the late Campanian just among large herbivores it would be typical to find a centrosaur, chasmosaur, lambeosaur, hadrosaur, nodosaur and ankylosaur. The newly discovered Kaiparowits Formation fauna in Utah suggests the south was just as biodiverse as the north at the time. This implies that during the late Campanian a land area a fraction the size of modern western North America supported a dozen or more herbivores ranging in sizes comparable to rhinos or elephants.

Many paleontologists have thought that the Judithian, or late Campanian was the peak of dinosaur evolution, at least in North America, with large numbers of species both within and between geologic formations. In 2001, Lehman proposed that there was an association between Corythosaurus and Centrosaurus in the Dinosaur Park Formation and a Maiasaura-Einiosaurus fauna in more inland habitats of the same time period in the Two Medicine Formation. Sampson and Loewen felt that stratigraphic research since then had discredited this hypothesis. Centrosaurus and Einiosaurus don't seem to have lived at the same time period and were separated in age by about a million years. There also isn't any evidence for multiple chasmosaurs inhabiting the same latitude. In fact the scientific evidence suggests that at any given time in Late Cretaceous western North America there were one each of centrosaurs and chasmosaurs living in the northern and southern regions of the Western Interior Basin.

This high diversity of large herbivores crowded into such a small land area suggests a greater abundance of plant material, lower metabolisms, or both, compared to those enjoyed by modern mammals. Science still has much to learn about ceratopsid dinosaurs. This can be said despite the fact that ceratopsids are probably the best documented clade of dinosaurs. North America probably still has many new kinds of ceratopsid dinosaurs waiting to be discovered. There is a complete lack of overlap in time between genera and species from the north and south of the Western Interior Basin.

In 2005, Ryan and Evans reported a possible overlap between Centrosaurus apertus and Styracosaurus albertensis. However, the "Styracosaurus" specimen this was based on was found by Ryan and others in 2007 to be the first process on the caudal parietal of a Centrosaurus apertus. The only other possible co-occurrence is Avaceratops lammersi with Centrosaurus apertus. However, since both the age and classification of Avaceratops. As it now stands there is no evidence for multiple kinds of ceratopsid living in the same time and place within either the northern or southern regions of the Western Interior Basin.

Many dinosaur species in North America during the Late Cretaceous had "remarkably small georgraphic ranges" despite their large body size and high mobility. Large herbivores like ceratopsians and hadrosaurs exhibited the most obvious endemism, which strongly contrasts with modern mammalian faunas whose large herbivores' ranges "typical[ly] ... span much of a continent." Lehman observes that "it is often the most conspicuous and abundant species with the most restricted distributions." He notes that Corythosaurus and Centrosaurus haven't been discovered outside of southern Alberta even though they are the most abundant Judithian dinosaurs in the region. Another example is Pentaceratops, the only known Judithian ceratopsian from New Mexico. Restrictions in herbivorous dinosaur distribution may be due to foliage preferences, narrow tolerance for variation in climate or other environmental factors. The restrictions on herbivorous dinosaur distribution must have been due to ecological factors rather than physical barriers because carnivorous dinosaurs tended to have wider distributions, especially smaller ones.

Pentaceratops.

As of his 2001 paper, restrictions in dinosaur occurrences based on distance from the paleo-shoreline had already been well documented. Terrestrial sedimentary strata from the Judithian to the Lancian are generally regressive through-out the entire sequence the preserved changes in fossil communities represent not only phylogenetic changes but ecological zones from the submontane habitats to near-sea level coastal habitats. Modern life at high elevations in lower altitudes resembles life at low elevation in higher latitudes. There may be parallels to this phenomenon in Cretaceous ecosystems, for instance, Pachyrhinosaurus is found in both Alaska and upland environments in southern Alberta.

The Judithian has been regarded as the interval of time with the greatest diversity of large herbivorous dinosaurs in North America, with ten genera of ceratopsians in Montana and Southern Alberta alone. This An association between Centrosaurus and Corythosaurus is characteristic of southern Alberta. Earlier research had found that lambeosaurines are less common in contemporary Montanan strata and with different centrosaurs as Monoclonius takes the place of Centrosaurus. Inland environments also differed, with the contemporary two medicine formation preserving an inland fauna characterized by Maiasaura and the early pachyrhinosaur Einiosaurus. Farther south was characterized by lower taxonomic diversity in communities where lambeosaurine were less common and centrosaurs were completely lacking. There Kritosaurus, Parasaurolophus and Pentaceratops are the dominant fauna. Farther south, in Texas, Kritosaurus predominates. The biomes of the Eastern US may have resembled those of Texas except completely lacking in ceratopsians. Parasaurolophus and Kritosaurus are also present in northern latitudes, so evidently exchange between them occurred, but both are uncommon outside of the southern biome.

Alamosaurus.

A dispersal event near the Cenomanian preceded the development of endemic northern and souther biomes in Western North America during the Late Cretaceous. Environments with highly favorable conditions led to diverse ecosystems populated by ornate herbivores with complex social interactions. These ecosystems were able to support such diversity because the herbivores had specialized diets that minimized their needs to compete with one another for resources. Migration between ecosystems was probably limited due to the efficient occupation of every niche in these environments. There doesn't seem to have been any physical barrier inhibiting physical travel between the different ecological zones. The widespread prominence of hadrosaurs in these ecosystems may imply that the dominant ecosystem of the time and place were coastal wetlands. Latitudinal zonation was pervasive across these ecosystems and likely arising from similar causes as modern provinciality, which exhibits similar characteristics. Northern biomes were dominated by pachyrhinosaurs and protoceratopsians. Ecological zonation based on altitude seems to be present as well. Lehman speculates that the Judithian dinosaur faunas may represent the "climax" of "individuality" in dinosaur communities.

Maastrichtian

In the south, little changes in the transition to the Edmontonian. However, in the northern biome a general trend in reduction of centrosaurines, with only pachrhinosaurus surviving. Likewise among lambeosaurs, only the single genus Hypacrosaurus remains. Inland faunas are distinguished by a Saurolophus-Anchiceratops association while more coastal areas were characterized by Pachyrhinosaurus and Edmontosaurus. Pachyrhinosaurus occurred as far north as Alaska. "Archaic" elements such as hypilophodonts like Parksosaurus and the "(re)appearance" of basal neoceratopsians like Montanoceratops begin characterizing inland faunas. Lehman described Arrhinoceratops is a likely ancestor for Triceratops.

Maiasaura.

By the Lancian hadrosaurs are no longer the dominant inhabitant of any province of western North America. Lehman records two surviving chasmosaurs, Triceratops and Torosaurus.

Saurolophus.

The extreme changes occurring in the make-up of herbivore communities during the faunal turnover suggests that a change in the ecosystems' flora was "the most immediate cause...though perhaps not the ultimate one." The rapid expansion of land and drying of inland climate accompanying a drop in sea level could explain some of the environmental changes occurring Late Cretaceous western North America. The wetland habitat enjoyed by many dinosaurs would have shrunk and fragmented. Since many species had very limited geographic ranges its plausible that some of the fragments would be smaller than the area needed to support the species. However, there's no direct evidence for the shrinking of wetland environments. Lehman contends that the actual area of coastal lowlands within 150m of the shoreline must have actually increased significantly. Further, dinosaurs that inhabited inland or arid environments were among the most prevalent in the Lancian. In previous research Jack Horner speculated that a rise in sea level during the Bearpaw Transgression created selective pressure as coastal lowlands were swallowed up the sea, resulting in anagenesis. If the geographic range of some dinosaur species were truly as limited as the fossil record suggests, then a rapid rise and in sea level could cause intense pressure even the event was local. Additionally, a rapid drop in sea level could allow for "rapid colonization by a few dinosaur generalists." The appearance or reappearance of basal neoceratopsians could be explained by immigration from Asia. Dinosaurs like Nodocephalosaurus resembled Asian forms, and some like Saurolophus co-occurred in Asia as well as North America. Potential Asian immigrants were especially common in upland environments. Early Cretaceous deposits in North America reveal that basal neoceratopsian were already present on the continent before their apparent reemergence in the Lancian, so an immigration event from Asia is unnecessary to explain their appearance.

The faunal turnover may be explained by the descent of more primitive forms existing in upland refugia characterized by conifer-dominated flora into areas that were formerly coastal lowlands as the seas retreated and conditions became more arid.

The decline of mammal diversity in Western North America from the Miocene to present primarily effected large herbivores and occurred over roughly the same length of time as the Late Cretaceous changes, and so may be parallel. They have many commonalities, including the replacement of diversity with single species environments caribou in the north, bison to the south. The most spectacular and specialized forms went extinct. The turnover was preceded by an episode of immigration. Associated with the rapid expanse of terrestrial habitat due to melting glaciars. By contrast, with the mammalian turnovers the newly emerging dominant fauna were clearly old world immigrants, the cervids and bovids.

In fact most dinosaurs are known only from a single geologic formation, although Triceratops and Torosaurus were exceptions to that rule.

It is currently unclear whether the ceratopsids of the Maastrichtian had the same levels of endemism as those of the late Campanian. Torosaurus latus has been reported from both the north in formations like the Hell Creek of Montana, North Dakota, and South Dakota as well as the Frenchman Formation of Saskatchewan. Torosaurus latus is also known from the south in formations like the North Horn Formation of Utah, McCrae Formation of New Mexico, and Javelina Formation of Texas. Triceratops horridus also has a broad range, although not as broad as Torosaurus latus. Some paleontologists have regarded southern specimens of Torosaurus as belonging to a second species, Torosaurus utahensis. The number of species in Triceratops has also been controversial. Most specimens have been referred to T. horridus. There is some tentative evidence for geographically isolated species. Andrew Farke thinks that northern Triceratops specimens and southern Triceratops specimens may be distinct form each other.

If these dinosaurs really were divided into different species by area, this implies that whatever factors may have led to enedmism in the Campanian was still around in the Maastrichtian. If they weren't divided into different species and instead had larger ranges than the Campanian ceratopsids then that may mean those factors were gone. The disappearance of these factors could have been related to changing climate and the retreat of the KWIS. Also, if they were predominantly coastal animals ceratopsids like Triceratops may have followed after the retreating KWIS.

The best studied region of ceratopsid habitation is the northern region of the Western Interior Basin, with southern Alberta and Northern Montana being the best studied area of that northern region. Most kinds of Maastrichtian or Campanian ceratopsids are known only from this area. Almost half of known ceratopsids are known from southern Alberta and most of those are unknown any place else. Most of these taxa's stratigraphic distribution are well understood. It's also important to note that the fossil-bearing geologic units in the area represent multiple different kinds of environments from nearshore coastal plain to more distal alluvial plain settings.

Another major stratigraphic gap in the fossil record of ceratopsids spans the Maastrichtian, the final five million years of the Cretaceous Period, from 70.5 to 65.5 million years ago. Dramatic environmental changes occurred at this time including dropping temperatures and withdrawal of the Cretaceous Interior Seaway. The latter resulted in renewed connection between Laramidia and Appalachia. Known ceratopsians from this time period include relatively well-known dinosaurs like Triceratops horridus and Torosaurus latus from the latest Maastrichtian. Arrhinoceratops brachyops and Eotriceratops xerinsularis date to the latest Campanian or early Maastrichtian.

It's not known whether a major extinction exterminated most ceratopsids at the end of the Campanian leaving mostly chasmosaurine descendants. It's also possible that ceratopsids continued to produce many short-lived species into the Maastrichtian the way they had in the Campanian.

Related to the Maastrichtian gap is the relative ignorance about southern ceratopsids. There have been enough recent finds of new ceratopsids in the south of the Western Interior Basin to suggest the prospect that southern ceratopsid diversity was at a similar level as the north. It's not known if southern ceratopsians experienced the same rapid evolution and replacement of species as the northern ecosystems did. It's also not known if the regional division of faunas continued to exist in the Maastrichtian or if the environments of western North America became more uniform. If these places with unique faunas existed, how long did they last, and what stopped animals from one fauna from moving into another faunal region.

Sampson and Loewen noted possible exceptions to this generalization include the co-occurrence of Diceratops hatcheri, Triceratops horridus and Tatankaceratops in the Late Maastrichtian Hell Creek Formation in the northern region. And in the south the co-occurrence of Kaiparowits new species B and C in the Kaiparowits Formation.

Sampson and Loewen noted that it was possible that the latest Cretaceous only supported a single lineage each of centrosaurs and chasmosaurs, although earlier estimates anticipated higher levels of diversity.

The low ceratopsid species diversity at any given point in time may have important implications for understanding Late Cretaceous dinosaur diversity in general. Many scientists have argued that in North America dinosaur diversity peaked in the late Campanian and then dwindled to very low diversity near the end of the Cretaceous. Much of the support for this dwindling diversity hypothesis has been based on the large variety of dinosaur found in the Dinosaur Park Formation. However, the Dinosaur Park Formation preserves a series of dinosaur faunas, so most of its preserved diversity would not have been alive at one time. In 2002, Russell and Manabe published evidence that the Hell Creek Formation is more diverse than previously credited. Some dinosaur groups present in the late Campanian are absent in the late Maastrichtian. These include lambeosaurs, centrosaurs, and albertosaurs. The reason the Dinosaur Park Formation preserved so many different species is because they existed for brief periods before evolving and going extinct. This led to a high number of species being preserved over the period of deposition.

Provincialism was even present during the turbulent faunal turnover of the Maastrichtian stage, when the Laramide Orogeny triggered "the most dramatic event that affected Late Cretaceous dinosaur communities in North America prior to their extinction." This turnover event saw specialized and highly ornamented centrosaurine and lambeosaurines be replaced by more primitive upland dinosaurs in the south while northern biomes became dominated by Triceratops with a greatly reduced hadrosaur community.

Ecological disturbance brought them to an end during the Edmontonian. Relative sea levels fell very rapidly due to the Laramide Orogeny. Opportunistic generalist herbivores filled the vacated niches that were once filled by a diverse number of specialist forms. The newly formed ecosystems tended to be dominated by a single herbivorous species each. The new dominant herbivores were usually less ornamented and probably represent "survivors from indigenous lineages" rather than immigrants from other areas. Gradually however "relict" dinosaurs such as protoceratopsids and sauropods began expanding into lower altitude areas as sea-levels fell. In the southern biome by Lancian time sauropods had replaced both hadrosaurs and ceratopsians in the southern biome. In the north both were still present although hadrosaurs were demoted to a "subordinate" role in dinosaur ecosystems. Edmontosaurus was the dominant northern hadrosaurid. At the end of the Cretaceous most ecosystems were dominated by a single herbivore. The northern biome was dominated by Triceratops and the southern biome by Alamosaurus. This faunal turnover coincides with the Laramide orogeny and the uplift of the central Rockies. Strata exhibit changes in lithology and the direction of paleocurrents, and a severe drop in relative sea level. At the very least, Lehman argues, the altitudinal life zones would shift, and a change in the distribution of vegetation utilized by herbivorous dinosaurs would have probably resulted. By the end of the Judithian, North America had 7.7 million km² of land area, but by the end of the Lancian it had reached 17.9 million km², nearly the modern value of 22.5 million km².

Implications

Research into paleobiogepgraphy has implications for reconstructing ancient climates, extinction events, and the geography of the ancient world. It can help provide information about evolutionary history that couldn't be inferred directly from fossils. Dinosaur paleobiogeography studies where evolutionary lineages originated, as well as where and how they arrrived at their final locations before going extinct. Dinosaur paleobiogeography can be studied both at local and global scales.

Behavior

Centrosaurus apertus.

Despite their large body sizes both hadrosaurs and ceratopsids seem to be divided into a northern faunal group including Montana and Alberta, as well as a southern faunal group that included Utah, Colorado, New Mexico and Texas. These separate geographic ranges occur within well-constrained overlapping time intervals, which is strong support for the idea of dinosaur provincialism in western Late Cretaceous North America as advocated by Lehman. The narrow geographic ranges of dinosaurs living in Cretaceous western North America contradicts the idea that some of them engaged in long distance migrations the way caribous and wildebeest do.

In 1998 Brinkman and others proposed a hypothesis Sampson and Loewen called "intriguing" regarding ceratopsid migration. The observed that the many monodominant ceratopsid bonbeds tended to occur in the middle of the Dinosaur Park Formation. The rock here was deposited in an inland deposit. This is in contradiction to most ceratopsid fossil finds which tend to be closer to the shore of the KWIS. The authors hypothesized that ceratopsids may have gathered into herds seasonally and engage in east–west migrations perhaps in accordance with food availability or mating season. Sampson and Loewen encouraged more research into this idea as more evidence comes to light.

Biodiversity

Chinnery-Allgeier and Kirkland observed that even fossils too fragmentary to be useful for reconstructing phylogenies can be useful for understanding paleobiogeography. There are many ghost lineages that suggests that science has many early ceratopsians to discover.

Many paleontologists have thought that the Judithian, or late Campanian was the peak of dinosaur evolution, at least in North America, with large numbers of species both within and between geologic formations. In 2001, Lehman proposed that there was an association between Corythosaurus and Centrosaurus in the Dinosaur Park Formation and a Maiasaura-Einiosaurus fauna in more inland habitats of the same time period in the Two Medicine Formation. Sampson and Loewen felt that stratigraphic research since then had discredited this hypothesis. Centrosaurus and Einiosaurus don't seem to have lived at the same time period and were separated in age by about a million years. There also isn't any evidence for multiple chasmosaurs inhabiting the same latitude. In fact the scientific evidence suggests that at any given time in Late Cretaceous western North America there were one each of centrosaurs and chasmosaurs living in the northern and southern regions of the Western Interior Basin.

Sampson and Loewen noted possible exceptions to this generalization include the co-occurrence of Diceratops hatcheri, Triceratops horridus and Tatankaceratops in the Late Maastrichtian Hell Creek Formation in the northern region. And in the south the co-occurrence of Kaiparowits new species B and C in the Kaiparowits Formation.

They also observed that Diceratops hatcheri's status as a legitimate species has been questioned and it is only known from one specimen. The stratigraphy of the Kaiparowits new taxa is currently under investigation. The current pattern in ceratopsian stratigraphy might change with the acquisition of more data. There might truly be divisions between coastal and inland faunas. However, Sampson and Loewen, called the current level of understanding "striking" and the evidence currently indicates that ceratopsids mostly lived near the coast. There is little evidence supporting the idea that multiple chasmosaurs or multiple centrosaurs inhabited the same place at the same time in either the south or the north of the Western Interior Basin.

The relatively low diversity of ceratopsids living at the same time coupled with their high diversity over time suggests that ceratopsid species turnover rates were very high. The average ceratopsid species probably survived significantly less than a million years. The scarcity of data and wholes in the fossil record prevent scientists from making detailed analysis, however, the existence of documented series of distinct ceratopsid species that don't overlap chronologically offers further support to the idea of short species lifetimes and rapid turnover.

Sampson and Loewen noted that it was possible that the latest Cretaceous only supported a single lineage each of centrosaurs and chasmosaurs, although earlier estimates anticipated higher levels of diversity.

Ceratopsid traits suited for survival underwent very little evolution compared to traits related to reproduction. This phylogenetic conservatism might have resulted in a lack of niche partitioning necessary to allow multiple related species to coexist without overcompetition. Some of the features that distinguish chasmosaurs from centrosaurs may have been adaptations that occurred early in the clade's evolution to prevent them from competing for the same food resources.

Multiple factors probably contributed to the short lifespans of ceratopsid species. These include cyclical envrinomental changes like transgression and regrassion of the Cretaceous Western Interior Seaway splitting larger populations into smaller ones. Succinctly, many new species were evolving in an environment that lacked the capacity to hold all of them led to the development of a pattern of continuous replacement.

Sampson and Loewen admitted that this scenario was highly speculative. It predicts that there would be a rapid pattern of replacement where two taxa from the same lineage overlap only for a brief period of time. This brief period of overlap implies that there would be little evidence for the co-occurrence in the fossil record. The fossil record implied by this scenario would closely resemble one left by an anagenetic series where one taxon directly gave rise to another, as proposed by Horner and others in 1992. Sampson and Loewen's scenario, however, is cladogenetic with the formation of each new branch in the lineage followed by an extinction event. One of the predictive differences between the above scenarioes is that Sampson and Loewen's would predict relative morphological stasis within species instead of gradual drift in the direction of their descendant taxon's as would be expected of the anagenetic scenario. Instead, while there may be variation about a mean, the species should remain roughly the same until they go extinct.

Sampson and Loewen noted that morphometric analysis could turn up evidence for gradual evolution within species that would weigh against their own scenario and in favor of the anagenetic scenario. The reaffirmed that the current evidence, however, did support their scenario.

If it turns out that pulses of speciation did occur simultaneously across groups it must then be ascertained whether or not this pattern holds across multiple geologic formations. It must also be ascertained how similar or different patterns of species turnover were between the northern and souther regions of western North America. If the expansion and retraction of the western interior seaway were responsible for the species turnover events then they wouldn't have occurred at the same time in the north and south since expansion and retraction events of the Cretaceous Western Interior Seaway didn't occur at the same time across all regions of western North America.

At any single point in time there were likely only a single species of chasmosaur and centrosaur in the northern and southern regions of western North America.

The low ceratopsid species diversity at any given point in time may have important implications for understanding Late Cretaceous dinosaur diversity in general. Many scientists have argued that in North America dinosaur diversity peaked in the late Campanian and then dwindled to very low diversity near the end of the Cretaceous. Much of the support for this dwindling diversity hypothesis has been based on the large variety of dinosaur found in the Dinosaur Park Formation. However, the Dinosaur Park Formation preserves a series of dinosaur faunas, so most of its preserved diversity would not have been alive at one time. In 2002, Russell and Manabe published evidence that the Hell Creek Formation is more diverse than previously credited. Some dinosaur groups present in the late Campanian are absent in the late Maastrichtian. These include lambeosaurs, centrosaurs, and albertosaurs. The reason the Dinosaur Park Formation preserved so many different species is because they existed for brief periods before evolving and going extinct. This led to a high number of species being preserved over the period of deposition.

Dinosaur diversity during the Campanian was still high despite the above comments. At any given time during the late Campanian just among large herbivores it would be typical to find a centrosaur, chasmosaur, lambeosaur, hadrosaur, nodosaur and ankylosaur. The newly discovered Kaiparowits Formation fauna in Utah suggests the south was just as biodiverse as the north at the time. This implies that during the late Campanian a land area a fraction the size of modern western North America supported a dozen or more herbivores ranging in sizes comparable to rhinos or elephants.

In 2005, Ryan and Evans reported a possible overlap between Centrosaurus apertus and Styracosaurus albertensis. However, the "Styracosaurus" specimen this was based on was found by Ryan and others in 2007 to be the first process on the caudal parietal of a Centrosaurus apertus. The only other possible co-occurrence is Avaceratops lammersi with Centrosaurus apertus. However, since both the age and classification of Avaceratops. As it now stands there is no evidence for multiple kinds of ceratopsid living in the same time and place within either the northern or southern regions of the Western Interior Basin.

Since ceratopsids are known primarily from "a restricted region of the northern region of the Western Inerior Basin" there may be new discoveries to be made outside it.

Environmental change

If these dinosaurs really were divided into different species by area, this implies that whatever factors may have led to enedmism in the Campanian was still around in the Maastrichtian. If they weren't divided into different species and instead had larger ranges than the Campanian ceratopsids then that may mean those factors were gone. The disappearance of these factors could have been related to changing climate and the retreat of the KWIS. Also, if they were predominantly coastal animals ceratopsids like Triceratops may have followed after the retreating KWIS.

Evolution

There are many ghost lineages that suggests that science has many early ceratopsians to discover.

The discovery of centrosaurine fossil in the Parras Basin of Mexico and reports of Pachyrhinosaurus from Alaska means that centrosaurs spanned all of North America from Mexico to the Arctic. This is a "much larger" range than has been documented in chasmosaurs. Zuniceratops is evidence that long eye horns were present in the common ancestor of centrosaurs and chasmosaurs. The presence of centrosaurs with long eye horns like Albertaceratops and the new taxa from the Kaiparowits and Wahweap Formations indicates they were still present in primitive centrosaurs and that these primitive centrosaurs already achieved a wide latitudinal range.

The relatively low diversity of ceratopsids living at the same time coupled with their high diversity over time suggests that ceratopsid species turnover rates were very high. The average ceratopsid species probably survived significantly less than a million years. The scarcity of data and wholes in the fossil record prevent scientists from making detailed analysis, however, the existence of documented series of distinct ceratopsid species that don't overlap chronologically offers further support to the idea of short species lifetimes and rapid turnover.

Ceratopsid traits suited for survival underwent very little evolution compared to traits related to reproduction. This phylogenetic conservatism might have resulted in a lack of niche partitioning necessary to allow multiple related species to coexist without overcompetition. Some of the features that distinguish chasmosaurs from centrosaurs may have been adaptations that occurred early in the clade's evolution to prevent them from competing for the same food resources.

Multiple factors probably contributed to the short lifespans of ceratopsid species. These include cyclical envrinomental changes like transgression and regrassion of the Cretaceous Western Interior Seaway splitting larger populations into smaller ones. Succinctly, many new species were evolving in an environment that lacked the capacity to hold all of them led to the development of a pattern of continuous replacement.

Sampson and Loewen admitted that this scenario was highly speculative. It predicts that there would be a rapid pattern of replacement where two taxa from the same lineage overlap only for a brief period of time. This brief period of overlap implies that there would be little evidence for the co-occurrence in the fossil record. The fossil record implied by this scenario would closely resemble one left by an anagenetic series where one taxon directly gave rise to another, as proposed by Horner and others in 1992. Sampson and Loewen's scenario, however, is cladogenetic with the formation of each new branch in the lineage followed by an extinction event. One of the predictive differences between the above scenarioes is that Sampson and Loewen's would predict relative morphological stasis within species instead of gradual drift in the direction of their descendant taxon's as would be expected of the anagenetic scenario. Instead, while there may be variation about a mean, the species should remain roughly the same until they go extinct.

Sampson and Loewen noted that morphometric analysis could turn up evidence for gradual evolution within species that would weigh against their own scenario and in favor of the anagenetic scenario. The reaffirmed that the current evidence, however, did support their scenario.

If it turns out that pulses of speciation did occur simultaneously across groups it must then be ascertained whether or not this pattern holds across multiple geologic formations. It must also be ascertained how similar or different patterns of species turnover were between the northern and souther regions of western North America. If the expansion and retraction of the western interior seaway were responsible for the species turnover events then they wouldn't have occurred at the same time in the north and south since expansion and retraction events of the Cretaceous Western Interior Seaway didn't occur at the same time across all regions of western North America.

At any single point in time there were likely only a single species of chasmosaur and centrosaur in the northern and southern regions of western North America.

Habitat preferences

After the Dinosaur Renaissance, prevailing opinion on ceratopsian habitats interpreted them as upland animals. In 1987, Lehman challenged this view by noting that Triceratops lived in humid lowlands near the coast. Later studies on earlier ceratopsids from the Campanian supported the coastal lowland ideas. In the northern region of the Western Interior Basin, ceratopsian fossils are most abundant when the sea expanded. The clastic wedge that preserves dinosaur fossils in the region includes geologic formations like the Judith River Formation, Dinosaur Park Formation, and Two Medicine. All of these formations are overlain by the marine shales of the Bearpaw Formation. This implies that fossil sites located stratigraphically higher within these formations represent environments closer to the coast. In 1998 Brinkman and others published a study documenting evidence of this same stratigraphic pattern. They also studied sites of the same age geographically varying in placement from east to west within southern Alberta. If ceratopsians really preferred coastal habitats they should be seen in increasing numbers as the sites progressed farther east because those sites would be closest to the coast. The researchers found that this was indeed the case in both macrovertebrate and microvertebrate sites.

In 2007 Eberth concluded that most ceratopsians lived in alluvial to coastal plain habitats with poorly drained soil but a small number lived in better drained alluvial plain settings. In 2007 Butler performed a "relatively coarse analysis" that concluded that ceratopsians preferred drier inland habitats.

Sampson and Loewen surveyed the literature for every known ceratopsian species and found "strong support" for the coastal hypothesis. They found that most ceratopsians are found in strata near coastal environments, just like Brinkman and others did. Deposits preserving ceratopsian fossils tend to be located in close stratigraphic proximity to marine deposits associated with transgression and regression cycles. This is true of Agujaceratops mariscalensis, Cerro del Pueblo new taxon, and Eotriceratops xerinsularis.

The Two Medicine Formation is particularly significant because it's one of the few Campanian deposits in the Western Interior Basin to preserve more arid upland environments. In 2007 Ryan and others speculated that Styracosaurus albertensis of the Dinosaur Park Formation was adapted to more coastal mesic settings while S. ovatus of the Two Medicine Formation was adapted for more inland xeric settings. Most dinosaur remains in the Two Medicine Formation are known from the upper portion 76-74 million years ago. Ceratopsians, however, are known only from the 75-74 million year old uppermost part of the formation. Below this level dinosaur remains are relatively abundant and include animals like the Egg Mountain fauna including Maiasaura, Orodromeus, and Troodon. Sampson and Loewen feel that the absence of ceratopsian fossils below the uppermost Two Medicine reflects and absence in life. They hypothesize that ceratopsians didn't appear in the Two Medicine depositional environment until the Bear Paw transgression had changed the environment to a coastal setting. Since S. ovatus is slightly younger than S. albertensis and lived during a period of rapid transgression it's possible that it too could have been a coastal animal.

In 1998 Rogers concluded that the uppermost Two Medicine strata were deposited in a semi-arid environment, which may be evidence against Sampson and Loewen's hypothesis. They said the coastal hypothesis can be tested with increased sampling of strata lower in the Two Medicine and further study into how its and other formations' environments changed over time. Dinosaurs of the Campanian WIB are only known from west coastal plains and cooler alluvial plains, a third paleoenvironment appears during the Maastrichtian. After the Laramide orogeny began in the early Maastrichtian, semi-arid intermontane basins appeared. These were characterized by a unique fauna of herbivorous dinosaurs that includes the titanosaur Alamosaurus and the chasmosaur Torosaurus. These environments are generally only found in the wouth of the WIB from Utah to Texas. The presence of Torosaurus in upland intermontane basins a significant distance away from the coasts suggests it may have been an ecological outlier that lived in a paleoenvironment significantly different from those preferred by other ceratopsians. Sampson and Loewen encouraged future research into understanding ceratopsian habitat preferences in order to better understand their evolutionary history.

Many paleontologists have thought that the Judithian, or late Campanian was the peak of dinosaur evolution, at least in North America, with large numbers of species both within and between geologic formations. In 2001, Lehman proposed that there was an association between Corythosaurus and Centrosaurus in the Dinosaur Park Formation and a Maiasaura-Einiosaurus fauna in more inland habitats of the same time period in the Two Medicine Formation. Sampson and Loewen felt that stratigraphic research since then had discredited this hypothesis. Centrosaurus and Einiosaurus don't seem to have lived at the same time period and were separated in age by about a million years. There also isn't any evidence for multiple chasmosaurs inhabiting the same latitude. In fact the scientific evidence suggests that at any given time in Late Cretaceous western North America there were one each of centrosaurs and chasmosaurs living in the northern and southern regions of the Western Interior Basin.

Physiology

Most ceratopsids were confined to small geographic ranges, which is unusual because such large animals would be expected to need large ranges just to get enough food. Further, the ceratopsids of Campanian North America would have coexisted at any given time with a lambeosaur, hadrosaur, nodosaur and ankylosaur simultaneously. This implies that during the late Campanian a land area a fraction the size of modern western North America supported a dozen or more herbivores of body sizes comparable to rhinos or elephants. This would be especially unusual if ceratopsids were endotherms. According to Sampson and Loewen, there are only a few possible explanations for how herbivores as large as ceratopsids could survive at high population densities in the small geographic ranges indicated by their fossil record in North America. The first is that their metabolic rates could be much lower than those of modern mammals. Secondly, they could have had access to larger amounts of high quality food than is available to modern mammals. The third possibility is that both of the preceding are true and play a role in explaining ceratopsids' small ranges.

References

• Chinnery-Allgeier, B.J., Kirkland, J.I. (2010): An Update on the Paleobiogeography of Ceratopsian Dinosaurs. In: M.J. Ryan, B.J. Chinnery-Allgeier, D.A. Eberth (eds.). New Perspectives on Horned Dinosaurs. Indiana University Press, Bloomington: Pp. 387 - 404. ISBN 0253353580.
• Lehman, T. M., 2001, Late Cretaceous dinosaur provinciality: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 310–328.
• Sampson, S. D. and Loewen, M. A. 2010. Unraveling a radiation: a review of the diversity, stratigraphic distribution, biogeography, and evolution of horned dinosaurs. (Ornithischia:Ceratopsidae). Pp. 405–427, In: Michael J. Ryan, Brenda J. Chinnery-Allgeier, and David A. Eberth (eds), New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium, Indiana University Press, 656 pp. ISBN 0253353580.