FIG. 1 in The Serrialis Bone, Interparietals, "X" Elements, Entotympanics, And The Composition Of The Notoungulate Caudal Cranium

FIG. 1. Oldfieldthomasia cf. debilitata AMNH-VP 28600, left hemicranium in ventral (A; above), oblique ventrolateral (B; opposite), and caudal (C; opposite) aspects (after Simpson [1936], slightly modified and relabeled). Scale bar in C = 0.5 cm (all to same scale). Most but not all of the features identified in Simpson's (1936) original illustrations are included in the following key (updated with modern nomina where necessary): 1,?foramen; 2, part of (preotic) squamoso-alisphenoid suture; 3, apparent "suture" (in fact a fracture) delimiting element Xa according to Simpson (see text); 4, exposed cellule (not stylomastoid foramen, contra Simpson); 5, fractured area in tympanic roof; 6, broken edge of bulla; 7, retroarticular sinus, exposed by breakage; 8, floor of epitympanic sinus (not posttemporal canal/"mastoid foramen," contra Simpson). The specimen is obviously in poor condition, with many broken (br) structures. In the drawings as originally published, bone was not properly differentiated from matrix still adherent to inner walls of epitympanic sinus. In C, Simpson evidently assumed that the matrix-filled area (asterisk, *) incorporated the actual caudal wall of the theca, and that the declivity medial to it (8) was therefore the "mastoid foramen" (= posttemporal canal). Declivity is in fact curved floor of epitympanic sinus, not an aperture; true posttemporal canal lies, as indicated, more medially and dorsally within a groove appearing on original drawing but not called out as such. Stylomastoid and hypoglossal foramina originally drawn slightly out of place (now corrected); hyoid recess cannot be seen from this angle (thus omitted); and leader for Xp points to nothing interpretable as an independent element (as in original drawing). In A, small arrows with italicized figure numbers indicate approximate planes of virtual sections (figs. 6, 7).