{{Short description|Extinct genus of reptiles}} {{Italic title}} {{Automatic_taxobox | fossil_range = {{fossilrange|Late Jurassic}} | image = Theretairus holotype.png | image_caption = Illustration of the ''Theretairus antiquus'' holotype mandible. | taxon = Theretairus | authority = | synonyms = | type_species = {{extinct}}''Theretairus antiquus'' | type_species_authority = Simpson, 1926 }}
'''''Theretairus''''' is a Late Jurassic genus of sphenodont reptile from the Morrison Formation of western North America,<ref name="vert-table">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.</ref> present in stratigraphic zones 5 and 6.<ref name="foster-appendix">Foster, J. (2007). "Appendix." ''Jurassic West: The Dinosaurs of the Morrison Formation and Their World''. Indiana University Press. pp. 327-329.</ref>
== 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,<ref name="vert-table" /><ref name=":0" /> and it was deposited at the Yale Peabody Museum under YPM VP 13764.<ref name=":0">{{Cite journal |last=Simpson |first=G. G. |date=1926-07-01 |title=American terrestrial Rhynchocephalia |url=http://www.ajsonline.org/cgi/doi/10.2475/ajs.s5-12.67.12 |journal=American Journal of Science |language=en |volume=s5-12 |issue=67 |pages=12–16 |doi=10.2475/ajs.s5-12.67.12 |bibcode=1926AmJS...12...12S |issn=0002-9599 |url-access=subscription }}{{Dead link|date=February 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> The locality comes from strata of the Brushy Basin Member of the Morrison Formation, which also bears other sphenodontians ''Opisthias'' and ''Eilenodon.''<ref>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.</ref><ref name=":0" /> 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.<ref name=":0" />
== 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.<ref name=":0" /> 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.''<ref name=":1">{{Cite journal |last=Reynoso |first=Víctor-Hugo |date=2005-09-30 |title=Possible evidence of a venom apparatus in a Middle Jurassic Sphenodontian from the Huizachal red beds of Tamaulipas, México |url=https://doi.org/10.1671/0272-4634(2005)025[0646:PEOAVA]2.0.CO;2 |journal=Journal of Vertebrate Paleontology |volume=25 |issue=3 |pages=646–654 |doi=10.1671/0272-4634(2005)025[0646:PEOAVA]2.0.CO;2 |s2cid=131602253 |issn=0272-4634|url-access=subscription }}</ref>
The following is a cladogram of Rhynchocephalia after DeMar et al. 2022.<ref name=":14">{{Cite journal |last1=DeMar |first1=David G. |last2=Jones |first2=Marc E. H. |last3=Carrano |first3=Matthew T. |date=2022-12-31 |title=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) |url=https://www.tandfonline.com/doi/full/10.1080/14772019.2022.2093139 |journal=Journal of Systematic Palaeontology |language=en |volume=20 |issue=1 |pages=1–64 |doi=10.1080/14772019.2022.2093139 |issn=1477-2019 |s2cid=252325953 |hdl-access=free |hdl=2440/136608}}</ref>
{{clade|{{clade |1=''Younginia capensis'' |2=''Prolacerta broomi'' |label3=Lepidosauria |3={{clade |label1=Pan-Squamata |1={{clade |1=''Sophineta cracoviensis'' |2=''Pristidactylus'' |3=''Eichstaettisaurus schroederi'' |4={{clade |1=''Megachirella wachtleri'' |2=''Marmoretta oxoniensis''}} }} |label2=Rhynchocephalia |2={{clade |1=''Gephyrosaurus bridensis'' |label2=Sphenodontia |2={{clade |1=''Diphydontosaurus avonis'' |label2=Acrosphenodontia |2={{clade |1=''Planocephalosaurus robinsonae'' |2={{clade |1=''Rebbanasaurus jaini'' |2={{clade |1={{clade |1=''Godavarisaurus lateefi'' |2='''''Theretairus antiquus'''''}} |label2=Eusphenodontia |2={{clade |1=''Polysphenodon mulleri'' |2={{clade |1=''Opisthiamimus gregori'' |2={{clade |label1=Clevosauridae |1={{clade |1={{clade |1=''Clevosaurus convallis'' |2={{clade |1=''Clevosaurus brasiliensis'' |2=''Clevosaurus hadroprodon''}}}} |2={{clade |1=''Clevosaurus bairdi'' |2={{clade |1=''Clevosaurus hudsoni'' |2=''Clevosaurus cambrica''}}}}}} |label2=Neosphenodontia |2={{clade |1=''Brachyrhinodon taylori'' |2={{clade |1=''Colobops noviportensis'' |2={{clade |label1=Sphenodontidae |1={{clade |1=''Sphenodon punctatus'' (tuatara) |2=''Cynosphenodon huizachalensis'' |3=''Sphenovipera jimmysjoyi'' |4={{clade |1=''Kawasphenodon expectatus'' |2=''Kawasphenodon peligrensis''}} |5=''Pelecymala robustus'' |label6=Eilenodontinae |6={{clade |1={{clade |1=''Fraserosphenodon latidens'' |2=''Opisthias rarus''}} |2={{clade |1=''Eilenodon robustus'' |2={{clade |1=''Sphenotitan leyesi'' |2={{clade |1=''Toxolophosaurus cloudi'' |2=''Priosphenodon avelasi''}}}}}}}}}} |label2=Leptorhynchia |2={{clade |1={{clade |1=''Homoeosaurus maximiliani'' |2=''Kallimodon pulchellus''}} |2={{clade |1={{clade |1=''Sigmala sigmala'' |2=''Vadasaurus herzogi''}} |label2=Pleurosauridae |2={{clade |1=''Palaeopleurosaurus posidonae'' |2={{clade |1=''Pleurosaurus goldfussi'' |2=''Pleurosaurus ginsburgi''}}}}}} |3=''Kallimodon cerinensis'' |label4=Sapheosauridae |4={{clade |1=''Sapheosaurus thiollierei'' |2={{clade |1=''Ankylosphenodon pachyostosus'' |2=''Oenosaurus muehlheimensis''}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}|style=font-size:85%; line-height:85%}}
==See also== {{Portal|Paleontology}} * Prehistoric reptile * Paleobiota of the Morrison Formation
==References== {{Reflist}}
{{Sphenodontia}} {{Taxonbar|from=Q7782944}}
Category:Morrison fauna Category:Jurassic lepidosaurs Category:Rhynchocephalia Category:Prehistoric reptile genera
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