Gephyrosaurus

Gephyrosaurus Temporal range: Late Triassic-Early Jurassic, Rhaetian–195 PreꞒ Ꞓ O S D C P T J K Pg N
Restoration of the skull and lower jaw
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Rhynchocephalia
Family:	†Gephyrosauridae
Genus:	†Gephyrosaurus Evans, 1980
Type species
†Gephyrosaurus bridensis Evans, 1980
Other species
†G. evansae Whiteside & Guffin, 2017

Gephyrosaurus is an extinct genus of lepidosaurian reptile known from the Late Triassic to Early Jurassic of the United Kingdom. It is generally considered to be one of the most primitive members of the clade Rhynchocephalia.

Description

Gephyrosaurus bridensis was relatively small in size,[1] with a skull around 3 centimetres (1.2 in) long, 2 centimetres (0.79 in) wide and 1 centimetre (0.39 in) deep. The skull of Gephyrosaurus bridensis lacks a complete temporal bar, with a gap between the jugal and quadrate bones.[2] Unlike more advanced rhynchocephalians belonging to Sphenodontia,[3] Gephyrosaurus bridensis retains a lacrimal bone in the skull, though it is considerably reduced in size compared to more primitive reptiles. The frontal and parietal bones are unpaired. Each half of the upper and lower jaws have around 35-40 teeth.[2] Unlike other known rhynchocephalians, all of these teeth are pleurodont, being attached to a shelf on the inner side of the jaw, with this being particularly apparent in the front of the jaw. However, the teeth at the back of the jaws have relatively shallow roots and appear to have undergone slower replacement than the front teeth.[4] The postcranial skeleton of G. bridensis was lizard-like, with long legs.[1]

Taxonomy

The type species G. bridensis was described by Susan E. Evans in 1980 based on fossils found in Early Jurassic fissure fill deposits in South Wales.[2] In 2017 a second species G. evansae was described from a maxilla found in fissure fill deposits from the Late Triassic (Rhaetian) of nearby Somerset. This species differs from the type by having distinctly smaller, more pointed and more densely packed teeth.[5]

When originally described, Gephyrosaurus was placed in "Eosuchia",[1][2] which is now considered to be a non-monophyletic group that included a wide variety of unrelated small diapsid reptiles.[6]

Gephyrosaurus is now generally considered to be among the most primitive rhynchocephalians, being outside the clade Sphenodontia, which contains the vast majority of rhynchocephalians. Cladogram after Sues and Schoch (2023):[7]

Rhynchocephalia Wirtembergia hauboldae Gephyrosaurus bridensis Sphenodontia Diphydontosaurus avonis Acrosphenodontia Planocephalosaurus robinsonae Rebbanasaurus jaini Godavarisaurus lateefi Theretairus antiquus Eusphenodontia Polysphenodon mulleri Opisthiamimus gregori Clevosauridae Clevosaurus convallis Clevosaurus brasiliensis Clevosaurus hadroprodon Clevosaurus bairdi Clevosaurus hudsoni Clevosaurus cambrica Neosphenodontia

Some other authors have found that instead that Gephyrosaurus is more closely related to Squamata (which contains lizards and snakes).[8]

During its original description Evans placed Gephyrosaurus into its own family, Gephyrosauridae, which was originally monotypic,[2] though later authors have also included other genera within the family.[5]

Ecology

Gephyrosaurus is suggested to have been an insectivore that was probably diurnal (active during the day).[2] The postcranial skeleton suggests that it was an agile animal that was capable of climbing. It is suggested to have used a "sit and wait" ambush strategy for catching prey. Findings of numerous jaw bones with healed fractures suggests that Gephyrosaurus may have engaged in fights with other conspecifics over territory, as occurs in some modern lizards.[9]

References

1. Evans, Susan E. (1981). "The postcranial skeleton of the Lower Jurassic eosuchian Gephyrosaurus bridensis". Zoological Journal of the Linnean Society. 73 (1): 81–116. doi:10.1111/j.1096-3642.1981.tb01580.x.
2. Evans, S.E. 1980. The skull of a new eosuchian reptile from the Lower Jurassic of South Wales. Zoological Journal of the Linnean Society 70: 203–264.
3. DeMar, David G.; Jones, Marc E. H.; Carrano, Matthew T. (2022-12-31). "A nearly complete skeleton of a new eusphenodontian from the Upper Jurassic Morrison Formation, Wyoming, USA, provides insight into the evolution and diversity of Rhynchocephalia (Reptilia: Lepidosauria)". Journal of Systematic Palaeontology. 20 (1): 1–64. doi:10.1080/14772019.2022.2093139. hdl:2440/136608. ISSN 1477-2019. S2CID 252325953.
4. Jenkins KM, Jones ME, Zikmund T, Boyde A, Daza JD (September 2017). "A Review of Tooth Implantation Among Rhynchocephalians (Lepidosauria)". Journal of Herpetology. 51 (3): 300–306. doi:10.1670/16-146. ISSN 0022-1511. S2CID 90519352.
5. David I. Whiteside, FLS; Christopher J. Duffin, FLS (2017). "Late Triassic terrestrial microvertebrates from Charles Moore's "Microlestes" quarry, Holwell, Somerset, UK". Zoological Journal of the Linnean Society. 179 (3): 677–705. doi:10.1111/zoj.12458.
6. Ezcurra, Martín D. (2016-04-28). "The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms". PeerJ. 4: e1778. doi:10.7717/peerj.1778. hdl:11336/46557. ISSN 2167-8359.
7. Sues, Hans-Dieter; Schoch, Rainer R. (2023-11-07). "The oldest known rhynchocephalian reptile from the Middle Triassic (Ladinian) of Germany and its phylogenetic position among Lepidosauromorpha". The Anatomical Record. doi:10.1002/ar.25339. ISSN 1932-8486. PMID 37937325. S2CID 265050255.
8. Simões, Tiago R.; Caldwell, Michael W.; Pierce, Stephanie E. (December 2020). "Sphenodontian phylogeny and the impact of model choice in Bayesian morphological clock estimates of divergence times and evolutionary rates". BMC Biology. 18 (1): 191. doi:10.1186/s12915-020-00901-5. ISSN 1741-7007. PMC 7720557. PMID 33287835.
9. Evans, Susan E. (August 16, 1983). "Mandibular Fracture and Inferred Behavior in a Fossil Reptile". Copeia. 1983 (3): 845–847. doi:10.2307/1444363. JSTOR 1444363.