Oldfieldthomasia

MacPhee, R. D. E., 2014, The Serrialis Bone, Interparietals, “ X ” Elements, Entotympanics, And The Composition Of The Notoungulate Caudal Cranium, Bulletin of the American Museum of Natural History 2014 (384), pp. 1-69 : 1-69

Oldfieldthomasia

As already noted, Oldfieldthomasia AMNH- VP 28600 was not a happy choice for Simpson’s (1936) purpose, not only because of its general condition, but because it was not one of the taxa that Roth (1903) specifically noted as possessing the serrialis. Despite its greater geological age compared to that of most other notoungulates known to Simpson in the early 20th century, in its caudal cranial construction Oldfieldthomasia cannot be described as especially primitive (as compared, for example, to the definitely more primitive henricosborniid Simpsonotus [Late Paleocene; see Billet, 2011]).

Although Simpson (1936) maintained in his text that the thecal walls (his “pars epitympan-

2014 MACPHEE: NOTOUNGULATE CAUDAL CRANIUM 49

ica”) of Oldfieldthomasia were squamosal, some section drawings show apparent continuity between thecal walls and bone that is surely ectotympanic in origin; others show a connection with his putative adventitious elements (see The X Elements). However, ignoring these, there is no reason to doubt that the squamosal contributed most of the thecal covering in Oldfieldthomasia , with the petrotympanic contributing modestly to the adital area where bulla, theca, and tegmen tympani anatomically meet and where intense remodeling probably first began during ontogeny. Evidence for a serrialis is accordingly completely lacking.