Theretairus

Theretairus is a Late Jurassic genus of sphenodont reptile from the Morrison Formation of western North America,^[1] present in stratigraphic zones 5 and 6.^[2]

Theretairus Temporal range: Late Jurassic PreꞒ Ꞓ O S D C P T J K Pg N
Illustration of the Theretairus antiquus holotype mandible.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	Lepidosauria
Order:	Sphenodontia
Genus:	Theretairus
Type species
†Theretairus antiquus Simpson, 1926

History and naming

The holotype consists of a right mandible and several in-socket teeth from Quarry 9 at Como Bluff, Wyoming where it was excavated by paleontologist William Harlow Reed, who then was working for Othniel Charles Marsh,^[1][3] and it was deposited at the Yale Peabody Museum under YPM VP 13764.^[3] The locality comes from strata of the Brushy Basin Member of the Morrison Formation, which also bears other sphenodontians Opisthias and Eilenodon.^[4][3] It was not named until George Gaylord Simpson described it as a new genus and species in a 1926 paper in the American Journal of Science. The generic name means "mammal companion", due to its association with the jaws of mammals in the Como Bluff quarry, and the specific name means "ancient", due to its old age as a sphenodontian.^[3]

Description and classification

The type specimen preserves a very strange morphology, with masseteric fossae in the posterior end similar to those in mammals. The teeth progressively get larger as they get closer to the anterior end, a trait also similar to mammals.^[3] Recently, the symphysis and two (perhaps three) enlarged acrodont fangs at the anterior end of the jaw, but separated one from the other to a lesser degree of the type have linked the taxon to the Mexican taxon Sphenovipera. The anterior border of the symphysis is steeply inclined in the holotype of Theretarius, suggesting it is an adult or older individual. Theretarius preserves five fossae along the posterior end of the mandible, making the mandible lighter, a trait only shared with Sphenovipera.^[5]

The following is a cladogram of Rhynchocephalia after DeMar et al. 2022.^[6]

Younginia capensis Prolacerta broomi Lepidosauria Pan-Squamata Sophineta cracoviensis Pristidactylus Eichstaettisaurus schroederi Megachirella wachtleri Marmoretta oxoniensis Rhynchocephalia 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 Brachyrhinodon taylori Colobops noviportensis Sphenodontidae Sphenodon punctatus (tuatara) Cynosphenodon huizachalensis Sphenovipera jimmysjoyi Kawasphenodon expectatus Kawasphenodon peligrensis Pelecymala robustus Eilenodontinae Fraserosphenodon latidens Opisthias rarus Eilenodon robustus Sphenotitan leyesi Toxolophosaurus cloudi Priosphenodon avelasi Leptorhynchia Homoeosaurus maximiliani Kallimodon pulchellus Sigmala sigmala Vadasaurus herzogi Pleurosauridae Palaeopleurosaurus posidonae Pleurosaurus goldfussi Pleurosaurus ginsburgi Kallimodon cerinensis Sapheosauridae Sapheosaurus thiollierei Ankylosphenodon pachyostosus Oenosaurus muehlheimensis

See also

• Paleontology portal

• Prehistoric reptile
• Paleobiota of the Morrison Formation

References

1. Foster, J. (2007). "Table 2.1: Fossil Vertebrates of the Morrison Formation." Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Indiana University Press. pp. 58-59.
2. Foster, J. (2007). "Appendix." Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Indiana University Press. pp. 327-329.
3. Simpson, G. G. (1926-07-01). "American terrestrial Rhynchocephalia". American Journal of Science. s5-12 (67): 12–16. Bibcode:1926AmJS...12...12S. doi:10.2475/ajs.s5-12.67.12. ISSN 0002-9599.^{[permanent dead link‍]}
4. Rasmussen, T. E.; Callison, George (1981). "A New Herbivorous Sphenodontid (Rhynchocephalia: Reptilia) from the Jurassic of Colorado". Journal of Paleontology. 55 (5): 1109–1116. JSTOR 1304536.
5. Reynoso, Víctor-Hugo (2005-09-30). "Possible evidence of a venom apparatus in a Middle Jurassic Sphenodontian from the Huizachal red beds of Tamaulipas, México". Journal of Vertebrate Paleontology. 25 (3): 646–654. doi:10.1671/0272-4634(2005)025[0646:PEOAVA]2.0.CO;2. ISSN 0272-4634. S2CID 131602253.
6. 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.