''Theretairus'' is a

Late Jurassic The Late Jurassic is the third Epoch (geology), epoch of the Jurassic Period, and it spans the geologic time scale, geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratum, strata.Owen ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of sphenodont reptile from the

Morrison Formation The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltston ...

of western

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

,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. present in stratigraphic zones 5 and 6.Foster, J. (2007). "Appendix." ''Jurassic West: The Dinosaurs of the Morrison Formation and Their World''. Indiana University Press. pp. 327-329.

History and naming

The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

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 Othniel Charles Marsh (October 29, 1831 – March 18, 1899) was an American professor of paleontology. A prolific fossil collector, Marsh was one of the preeminent paleontologists of the nineteenth century. Among his legacies are the discovery or ...

, and it was deposited at the

Yale Peabody Museum The Peabody Museum of Natural History at Yale University (also known as the Yale Peabody Museum of Natural History or the Yale Peabody Museum) is one of the oldest, largest, and most prolific university natural history museums in the world. It ...

under YPM VP 13764. The locality comes from strata of the Brushy Basin Member of the

, which also bears other sphenodontians '' Opisthias'' and '' Eilenodon.''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 JSTOR ( ; short for ''Journal Storage'') is a digital library of academic journals, books, and primary sources founded in 1994. Originally containing digitized back issues of academic journals, it now encompasses books and other primary source ...

1304536. It was not named until

George Gaylord Simpson George Gaylord Simpson (June 16, 1902 – October 6, 1984) was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern synthesis, contributing '' Tempo ...

described it as a new genus and species in a 1926 paper in the ''

American Journal of Science The ''American Journal of Science'' (''AJS'') is the United States of America's longest-running scientific journal, having been published continuously since its conception in 1818 by Professor Benjamin Silliman, who edited and financed it himself ...

.'' 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.

Description and classification

The

type specimen In biology, a type is a particular wikt:en:specimen, specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally associated. In other words, a type is an example that serves to ancho ...

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. Recently, the symphysis and two (perhaps three) enlarged

acrodont Acrodonty (from Greek ''akros'' 'highest' + ''odont-'' 'tooth') is an anatomical placement of the teeth at the summit of the alveolar ridge of the jaw, without sockets, characteristic of bony fish. Functionally, acrodont tooth implantation may be r ...

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 ''Sphenovipera jimmysjoyi'' is an extinct species of sphenodontian dated from the Jurassic. If was discovered in the lower part of the La Boca Formation located in Tamaulipas, Mexico. Only the lower jaw of this organism has been discovered and s ...

.'' 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.'' The following is a

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

of Rhynchocephalia after DeMar et al. 2022.

References

Morrison fauna Jurassic lepidosaurs Rhynchocephalia Prehistoric reptile genera {{jurassic-reptile-stub

History and naming

Description and classification

See also

References