''Libognathus'' (meaning "southwest jaw" in

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

) is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

procolophonid Procolophonidae is an extinct family of small, lizard-like parareptiles known from the Late Permian to Late Triassic that were distributed across Pangaea, having been reported from Europe, North America, China, South Africa, South America, Antar ...

parareptile Parareptilia ("at the side of reptiles") is a subclass or clade of basal sauropsids (reptiles), typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near the ...

from the

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...

Texas Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...

. The type and only species, ''Libognathus sheddi'', was named in 1997 from the

Cooper Canyon Formation The Cooper Canyon Formation is a geological formation of Norian age in Texas and New Mexico.Weishampel, David B; et al. (2004).Dinosaur distribution (Late Triassic, North America)" In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (ed ...

in the fossil-rich Post Quarry, which is found in

Garza County Garza County is a county located in the U.S. state of Texas. As of the 2020 census, its population was 5,816, of which most of the population were residing in Its county seat, and only incorporated municipality, Post. The county was created i ...

. ''Libognathus'' was the first definite procolophonid discovered in the southwestern United States, although another possible procolophonid called '' Chinleogomphius'' was reported from the southwest before ''Libognathus'' was named.

Description

''Libognathus'' is known from a single

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...

specimen including the left

dentary bone In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable ...

and coronoid process of the lower jaw, as well as six teeth implanted in the jaw. ''Libognathus'' can be identified as a procolophonid by its wide molar-like teeth, each of which has a ridge running along its width and several projections or cusps around it. It is considered a member of the procolophonid subfamily Leptopleuroninae based on the presence of more conically shaped teeth at the front of the jaw. ''Libognathus'' is distinguished from other procolophonids by the steep angle of the lower margin of the dentary (indicating a deeper lower jaw than in related procolophonids such as ''

Hypsognathus ''Hypsognathus'' (from el, ῠ̔́ψος , 'height' and el, γνάθος , 'jaw') is an extinct genus of procolophonid parareptile from the Late Triassic of New Jersey, Connecticut, and Nova Scotia.Sues, H.-D.,Olsen, P.E., Scott,D.M, and Spen ...

'' and ''

Leptopleuron ''Leptopleuron'' is an extinct genus of procolophonid that lived in the dry lands during the late Triassic in Elgin of northern Scotland and was the first to be included in the clade of Procolophonidae. First described by English paleontologist ...

'') and a small hole or

foramen In anatomy and osteology, a foramen (;Entry "foramen"
in

on the jaw's outer surface below the fifth tooth. Several other jaw fragments have been identified as possible North American procolophonids, and have been given names such as ''Chinleogomphius'', '' Xenodiphyodon'', and '' Gomphiosauridion''. The holotype ''Libognathus'' can be distinguished from these other remains by features in its teeth; while ''Chinleogomphius'' and ''Xenodiphyodon'' have three cusps per tooth, ''Libognathus'' has only two, and while ''Gomphiodon'' has a ridge positioned centrally on the tooth surface, the ridge on each tooth of ''Libognathus'' is positioned farther forward. Tooth replacement is evident on the inner surface of the jaw of ''Libognathus'', where the exposed roots of teeth show smaller teeth pushing up from underneath them. This form of tooth replacement is similar to that in most other reptiles, but different from the condition in most other procolophonids, which have teeth that are fused to the bone of the jaw.

References

Leptopleuronines Triassic parareptiles Fossils of the United States Prehistoric reptile genera {{paleo-reptile-stub