Typothorax

''Typothorax'' is an extinct genus of typothoracine aetosaur that lived in the Late Triassic. Its remains have been found in North America. Two species are known: ''T. coccinarum'', the type species, and ''T. antiquum''.

Description

''Typothorax'' was an aetosaur, a pseudosuchian distantly related to modern crocodilians. Unlike modern crocodilians, aetosaurs were herbivorous. ''Typothorax'' and other aetosaurs possess small, leaf-shaped teeth that were unsuited for a diet consisting of meat.Martz, J.W. 2002. The morphology and ontogeny of Typothorax coccinarum (Archosauria, Stagonolepididae) from the Upper Triassic of the American southwest. M.S. thesis, Geosciences, Texas Tech University, Lubbock, 279 pp.
Unlike some aetosaurs such as ''Desmatosuchus'', ''Typothorax'' does not have large shoulder spikes. It does, however, have a pair of enlarged spikes on the neck projecting from the third row of scutes. It has lateral scutes that bear horns that are posteriorly hooked along its back, while its sides and underbelly are covered with ornamented scutes. Although fossils of aetosaurs are not as common as other Triassic archosaurs, with their armor plates being the most common, ''Typothorax'' has been represented by fewer skeletal elements than other aetosaurs. The largest known specimen of ''T. coccinarum'', described in 2023, would have been similar in size to ''Desmatosuchus'', measuring over long, though it has been identified as a skeletally immature individual (16 to 19 years old) based on the open neurocentral suture and the lines of arrested growth. However, the authors warn that, since smaller individuals have co-ossified neurocentral sutures, they may not be a reliable factor in determining maturity in aetosaurs. So this individual still may be an adult.

The vertebral column of ''Typothorax'' is shortened, with individual vertebrae being reduced in length. However, the osteoderms that overly the vertebrae are not shortened. Instead, they are reduced in number so that each dorsal paramedian osteoderm (osteoderm that covers the back) overlies several dorsal vertebrae. In nearly all other crurotarsans, there is one row of osteoderms per vertebra. ''T. coccinarum'' has around 20 rows of presacral osteoderms and about 26 presacral vertebrae. If the cervical spikes of ''Typothorax'' are homologous to those of ''Desmatosuchus'', it is likely that rows of osteoderms were removed from the front. This is because in ''Desmatosuchus'' the spikes are present in the fifth row, while in ''Typothorax'' they are present in the third.

In ''T. coccinarum'', there are 10 thoracic columns and four caudal columns of ventral osteoderms on the underside. Unlike all other aetosaurs, ''Typothorax'' possesses spiked osteoderms on the underside of the tail, near the cloaca. This region is not covered by osteoderms to the extent that it is in other aetosaurs such as '' Aetosaurus'' and '' Coahomasuchus''.

History

''Typothorax'' was one of the earliest vertebrates named from the Triassic of western North AmericaCope, E.D. 1875. The geology of New Mexico. Philadelphia Academy of Natural Sciences Proceedings, pp. 263-267. but has been poorly understood since that naming. Paleontologists have found the characteristic armor plates for decades,Case, E.C. 1922. New reptiles and stegocephalians from the Upper Triassic of western Texas. Carnegie Institution of Washington Publication, no. 321 (October, 1922). Carnegie Institution of Washington, Washington D.C., 84 pp.Long, R.A., and Murry P.A. 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science Bulletin 4, 254 pp. but only recently has a comprehensive study of ''Typothorax'' appeared. It has been found in Arizona, in the Chinle Formation, as well as in New Mexico and Texas in the Bull Canyon Formation of the Dockum Group.

Paleobiology

Like all aetosaurs and many other early crurotarsans, ''Typothorax'' had erect hindlimbs held beneath the body. This is evident by a straight femur, an anteriorly directed pes (foot), and the projection of the lateral surface of the ilium over the femur. Because the length of the femur is almost equal to that of the tibia and fibula (lower leg) and astragalus and calcaneum (ankle), ''Typothorax'' was probably slow-moving. The forelimbs are reduced in size and were directed outward in a sprawling position. This posture is also seen in ankylosaurs, ceratopsians, and the early cynodont '' Procynosuchus''.

Several aspects of the forelimbs have been interpreted as adaptations to digging. Like many digging tetrapods, the radius is significantly shorter than the humerus. Like other aetosaurs, there is a prominent deltopectoral crest on the humerus. The manus is short and wide, a characteristic of digging animals. There is also an entepicondyle on the humerus, which is the origin of forearm pronator and manual flexor muscles. ''Typothorax'' also possesses a relatively long olecranon process for the insertion of the '' M. triceps'' muscle, but not as long as those of digging animals. Heckert ''et al.'' (2010) concluded that ''Typothorax'' was not specifically adapted for a fossorial lifestyle but had an ability to dig that other aetosaurs didn't have. The upturned snout of ''Typothorax'' and other aetosaurs suggests that they may have searched for food by rooting around in soil.

Classification

''Typothorax'' belongs to a clade of aetosaurs called the Typothoracinae, within the subfamily Aetosaurinae. It is closely related to '' Redondasuchus''. ''Redondasuchus reseri'' was first named in 1991, but was later synonymized with ''T. coccinarum'' in 1995 on the basis of the similarity between the osteoderms used to describe the species and the anterior osteoderms of ''T. coccinarum''. Martz (2002) suggested that ''R. reseri'' belongs to ''Typothorax'', but is still its own species, called ''T. reseri''. More recently, the distinctiveness of ''Redondasuchus'' from ''Typothorax'' has been supported with the description of a new species, ''R. rineharti'', in 2006. With new skeletal material from ''T. coccinarum'', Heckert ''et al.'' (2010) claim that ''Redondasuchus'' is distinct from ''Typothorax'' because it has strongly flexed paramedian osteoderms, while ''Typothorax'' has more gently arching paramedian osteoderms.

Parker (2013) redescribed the holotype of the type species of ''Typothorax'', ''T. coccinarum''. It was found to be too fragmentary to be diagnosable and therefore represents a ''nomen dubium''. '' Episcoposaurus horridus'', the type species of its genus usually considered to be a junior synonym of ''T. coccinarum'', is the next available name for material referred to ''T. coccinarum''. Therefore, other species of ''Typothorax'' as well as material referred to ''T. coccinarum'', can be reassigned to ''Episcoposaurus''. Nevertheless, to preserve taxonomic stability Parker (2013) suggested to attempt to petition for a neotype for ''T. coccinarum'' from well preserved specimens which have been previously referred to it.

References

External links

''Typothorax''
in the Paleobiology Database
*Jeff Martz'
Master's Thesis
from Texas Tech University.

{{Taxonbar, from=Q1012908

Aetosaurs of North America
Chinle fauna
Prehistoric pseudosuchian genera
Taxa named by Edward Drinker Cope