Planocephalosaurus Explained

Planocephalosaurus is an extinct genus of basal rhynchocephalian. Fossils of the genus are primarily known from fissure fill deposits from the Late Triassic of southwest Britain, with fragmentary remains possibly belonging to the genus also known from the Late Triassic of Texas.

Taxonomy

The genus and the type species P. robinsonae was described in 1982 by Fraser from remains found in fissure fill deposits in Gloucestershire in southwest Britain.^[1] In 2004, a second species P. lucasi was described from remains found in the Tecovas Formation in Texas.^[2] P. robinsonae is known from disarticulated remains covering most of the skeleton,^[3] , while P. lucasi is known from fragments of the jaws.^[4] While originally classified in Sphenodontidae, later studies have placed it as a basal sphenodontian, more derived than Diphydontosaurus but more basal than Clevosaurus.^[5]

Cladogram after:^[6]

Description

Planocephalosaurus robinsonae was a small sphenodontian, with a skull length of around 2cm (01inches), with a total length of around NaNcm (-2,147,483,648inches). Unlike the modern tuatara, the skull is thought to lack a complete lower temporal bar connecting the jugal and the quadrate, though a complete temporal bar may have been variably present in some adult individuals. While initially suggested to have had fully acrodont dentition, later X-ray analysis suggested that the teeth at the front of the jaws of Planocephalosaurus were attached to a shelf on the inward side of the jaw, technically making them pleurodont, though they had some characters typically associated with acrodonty, such as having no clear boundary between the teeth and jaw bone. The back of the jaws had enlarged fully acrodont teeth fused to the apex of the jaw bone, similar to Diphydontosaurus.^{[7] [8]}

Ecology

Planocephalosaurus is thought to have been insectivorous, with it possibly also feeding on small vertebrates.

Notes and References

1. Fraser . N.C. . November 1982 . A New Rhynchocephalian from the British Upper Trias . Palaeontology . 25 . 4 . 709–725.
2. Heckert, A. B. 2004. Late Triassic microvertebrates from the lower Chinle Group (Otischalkian-Adamanian: Carnian), southwestern U.S.A. New Mexico Museum of Natural History & Science Bulletin 27:1–170.
3. Fraser . N.C. . Walkden . G.M. . August 1984 . The postcranial skeleton of the Upper Triassic sphenodontid Planocephalosaurus robinsonae . Palaeontology . 27 . 3 . 575–595.
4. Heckert . Andrew B. . Lucas . Spencer G. . Rinehart . Larry F. . Hunt . Adrian P. . July 2008 . A New Genus and Species of Sphenodontian From the Ghost Ranch Coelphysis Quarryu (Upper Triassic: Apachean), Rock Point Formation, New Mexico, USA . Palaeontology . en . 51 . 4 . 827–845 . 10.1111/j.1475-4983.2008.00786.x . 0031-0239. free .
5. Chambi-Trowell . Sofia A. V. . Martinelli . Agustín G. . Whiteside . David I. . Vivar . Paulo R. Romo de . Soares . Marina Bento . Schultz . Cesar L. . Gill . Pamela G. . Benton . Michael J. . Rayfield . Emily J. . 2021-06-03 . The diversity of Triassic South American sphenodontians: a new basal form, clevosaurs, and a revision of rhynchocephalian phylogeny . Journal of Systematic Palaeontology . en . 19 . 11 . 787–820 . 10.1080/14772019.2021.1976292 . 1477-2019. free . 1983/af14affc-a26e-426b-83ca-e1833e355882 . free .
6. 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 . en . 10.1002/ar.25339 . 1932-8486 . 37937325 . 265050255.
7. Fraser . N.C. . Shelton . C.G. . 1988 . Studies of tooth impantation in fossil tetrapods using high-resolution X-radiography . Geological Magazine . 125 . 2 . 117–122 . 1988GeoM..125..117F . 10.1017/s0016756800009523.
8. Jenkins KM, Jones ME, Zikmund T, Boyde A, Daza JD . September 2017 . A Review of Tooth Implantation Among Rhynchocephalians (Lepidosauria) . . en . 51 . 3 . 300–306 . 10.1670/16-146 . 0022-1511 . 90519352.