Reesodus is an extinct genus of hybodontiform. It lived from the Tournaisian age of the Early Carboniferous to the Wordian age of the Permian, and remains have been found in England, Russia and Oman. The generic name honors Jan Rees, who first realized that the fossils belong to a distinct genus.[1]
| Reesodus Temporal range:
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Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Order: | †Hybodontiformes |
| Genus: | †Reesodus Koot, Cuny, Tintori & Twitchett, 2013 |
| Type species | |
| Reesodus underwoodi Koot, Cuny, Tintori & Twitchett, 2013
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| Species | |
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Taxonomy
editRees & Underwood (2002) found that Paleozoic material referred to the genus Lissodus should be assigned to two separate genera which they left unnamed. They assigned 'Lissodus' wirksworthensis from southern England, 'Lissodus' pectinatus from western Russia and 'Lissodus' sp. from central Russia to 'Palaeozoic Genus 2'.[2] The genus Reesodus was erected in 2013 as a replacement name for 'Palaeozoic Genus 2', with the newly described Reesodus underwoodi from the Khuff Formation of Oman as the type species. Though the known material is identified as belonging to a hybodontiform, Reesodus cannot be confidently assigned to any family (though it is assumed to be close to Lonchidiidae) as the heterodonty pattern and the possible presence of enlarged lateral teeth cannot be assessed, thus it is listed as Hybodontiformes incertae sedis.[1]
Species
editCurrently, the following species have been assigned to the genus Reesodus:
The type species, Reesodus underwoodi, is known from 6 complete teeth collected from the Guadalupian-aged Khuff Formation of Oman. The specific name honors Charlie Underwood, who worked on the taxonomy of Lissodus and recognized the Paleozoic specimens to represent separate genera. Teeth of this species always have a triangular labial peg and a lingual peg, and sometimes two accessory nodes. The crown shoulder has a horizontal rim and acute longitudinal crest. The teeth are small and symmetrical, measuring 0.4–0.5 mm high.[1]
Originally described in 1985 as Lissodus wirksworthensis, Reesodus wirksworthensis is known from fossilized teeth found in the Early Carboniferous-aged Eyam Limestone of Derbyshire, England. Its teeth measure 0.75 mm in height and 2 mm mesiodistally, with a longitudinal ridge on the crown shoulder and a moderate labial peg. The basal root has many foramina and is well-developed, reaching a height of 0.35 mm.[3][4]
Reesodus pectinatus was originally described in 1996 as Lissodus pectinatus, but has since been recognized to belong in this separate genus. It is known from teeth collected in Tournaisian-Viséan deposits in western Russia.[5]
Description
editReesodus is known from fossil teeth characterized in having a mesio-distally expanded crown with a low profile, as many as 4 lateral cusplets and a very strong crown shoulder. These teeth may be ornamented with varying amounts and density of coarse folds. The labial peg is moderately or well-developed, often accompanied by protuberances near the base of the lateral cusplets and/or a labial root buttress. Compared to those of other hybodonts, the tooth roots of Reesodus are less porous.[1][2]
References
edit- ^ a b c d Koot, Martha B.; Cuny, Gilles; Tintori, Andrea; Twitchett, Richard J. (2013-03-13). "A new diverse shark fauna from the Wordian (Middle Permian) Khuff Formation in the interior Haushi-Huqf area, Sultanate of Oman: CHONDRICHTHYANS FROM THE WORDIAN KHUFF FORMATION OF OMAN". Palaeontology. 56 (2): 303–343. doi:10.1111/j.1475-4983.2012.01199.x. S2CID 86428264.
- ^ a b Rees, Jan; Underwood, Charlie J. (2002-09-19). "The status of the shark genus Lissodus Brough, 1935, and the position of nominal Lissodus species within the Hybodontoidea (Selachii)". Journal of Vertebrate Paleontology. 22 (3): 471–479. doi:10.1671/0272-4634(2002)022[0471:TSOTSG]2.0.CO;2. ISSN 0272-4634. S2CID 13836075.
- ^ Smith, Roy; Martill, David M.; Duffin, Christopher (2017-06-01). "The shark-beds of the Eyam Limestone Formation (Lower Carboniferous, Viséan) of Steeplehouse Quarry, Wirksworth, Derbyshire, UK". Proceedings of the Geologists' Association. 128 (3): 374–400. doi:10.1016/j.pgeola.2017.04.004. ISSN 0016-7878.
- ^ Duffin, C. (1985). "Revision of the hybodont selachian genus Lissodus BROUGH (1935)". Palaeontographica Abteilung A. S2CID 233061921.
- ^ Lebedev, O. A. (January 1996). "Fish assemblages in the Tournaisian-Viséan environments of the East European Platform". Geological Society, London, Special Publications. 107 (1): 387–415. doi:10.1144/GSL.SP.1996.107.01.28. ISSN 0305-8719. S2CID 129309437.