Thecachampsa Sericodon Tooth Clark

''Thecachampsa'' is an extinct genus of gavialoid crocodylian, traditionally regarded as a member of the subfamily Tomistominae. Fossils have been found from the eastern United States in deposits of Miocene age. Those named in the 19th century were distinguished primarily by the shape of their teeth, and have since been combined with ''T. antiquus''. More recently erected species were reassigned from other genera, although their assignment to ''Thecachampsa'' has since been questioned.

Description

''Thecachampsa'', like other "tomistomines" of the Oligocene and Miocene, was considerably larger than living crocodilians. Like living gharials, it had a long, slender snout. The teeth were long and recurved. Unlike its living relatives, ''Thecachampsa'' was marine, inhabiting estuaries and shallow coastal waters. Other marine fossils such as sea snail and bivalve shells, shark teeth, and barnacles have been found alongside remains of ''Thecachampsa'' and similar taxa.

Species

In 1852, American paleontologist Joseph Leidy described ''Crocodylus antiquus'' from Miocene deposits in the Lee family's ancestral home in Virginia. The holotype was a tooth found in the Calvert Formation of Virginia. Leidy also described additional material including several teeth and osteoderms, two vertebrae, a rib, and an ungual phalanx or claw bone. The new genus ''Thecachampsa'' was erected by Edward Drinker Cope in 1867, containing two species, ''T. sericodon'' and ''T. contusor'', both also based on teeth, but no type species was designated. Cope (1882) designated ''T. sericodon'' as the ''Thecachampsa'' type species. ''T. sericodon'' was distinguished from ''T. antiqua'' by its slender, curved teeth, each with a sharp edge near the base of the posterior margin (''T. antiqua'' only possessed sharp edges along a small area near the tip of the posterior margin).

Crocodilian material found from Miocene deposits in the eastern United States has often been attributed to ''Thecachampsa'', even isolated teeth with few distinguishable features. In 1869, Cope named a fourth species, ''T. sicaria'', from a jaw fragment and a dorsal vertebra. Unlike those of other species, the teeth of ''T. sicaria'' were lenticular (lens-shaped) in cross section with sharp cutting edges. That year, Othniel Charles Marsh named another species of ''Thecachampsa'', "T. squankensis", after the place of its discovery, Squankum, New Jersey, but that name is a ''nomen nudum'' because Marsh provided no description, diagnosis, or type specimen. Cope named a new species, ''T. fastigiata'', in 1870 as a reassignment of ''Crocodylus fastigiatus'', named by Leidy in 1852. These species were distinguished from one another primarily by differences in the shape of the teeth, the most common material found. The genus was synonymized with ''Crocodylus'' in 1973, but has since been regarded as valid.

In 2001, A.C. Myrick synonymized '' Gavialosuchus americanus'', another "tomistomine" from the eastern United States, with ''T. antiqua''. Myrick also synonymized ''Tomistoma lusitanica'', a "tomistomine" from Portugal, with ''Thecachampsa''. The genus name ''Thecachampsa'' had priority over the other two, as it was erected earlier. However, because the holotype tooth of ''T. antiqua'' has no clear diagnostic features to distinguish it from other species, later studies did not accept the synonymies. Piras ''et al.'' (2007) considered ''G. americanus'' to be a member of the genus ''Thecachampsa'', but placed it as a distinct species, ''T. americanus'', rather than a synonym of ''T. antiqua''. The species was first described as ''Tomistoma americanus'' in 1915, with remains having been found from the Kirkwood Formation in New Jersey, the Calvert Formation in Maryland, the Chesapeake Group of Virginia, and the Pungo River and Yorktown Formations of North Carolina. Remains have also been found from Florida, California, Baja California, and, more recently, Costa Rica. Fossils of the species are present in deposits that range in age from the late Early Miocene to the early Pliocenes, and found relatively strong support for Piras ''et al.'' (2007) suggestion. The cladogram below follows their analysis.

In addition to reassigning ''G. americanus'' and ''G. carolinensis'' to ''Thecachampsa'', Myrick combined all previously named species of ''Thecachampsa'' in a single species, with the oldest name being ''T. antiqua''. The different tooth shapes that distinguished the species were considered variations in the dentition of a single species. However, the variation in dentition could only be seen in complete skulls, all of which had been referred to ''Gavialosuchus'' before the genus was synonymized with ''Thecachampsa''.

Weems (2018) agreed with Piras et al. (2007) and Brochu & Storrs (2012) that ''Tomistoma americanus'' and ''Gavialosuchus carolinensis'' belong to ''Thecachampsa'' rather than ''Gavialosuchus'', but treated ''sericodon'' and ''antiqua'' as distinct species rather than one species, and noted that the ''americanus'' holotype is conspecific with ''T. sericodon'', making ''T. americanus'' a synonym of ''T. sericodon''. In addition, it found ''champsa'' to be an Egyptian word for “Crocodile” via Greek, and is a Masculine noun. Therefore, the spelling ''T. antiquus'' rather than ''T. antiqua'' is preserved.Robert E. Weems (2018). "Crocodilians of the Calvert Cliffs". Smithsonian Contributions to Paleobiology. 100: 213–240. doi:10.5479/si.1943-6688.100.

References

{{Taxonbar, from=Q7777625, from2=Q112844980, from3=Q112845050, from4=Q112845091

Crocodilians
Gavialidae
Prehistoric pseudosuchian genera
Prehistoric marine crocodylomorphs
Oligocene crocodylomorphs
Oligocene reptiles of North America
Taxa named by Edward Drinker Cope