Anatomy of the fully formed chondrocranium of Podocnemis unifilis (Pleurodira: Podocnemididae)

This study describes the anatomy of the chondrocranium of Podocnemis unifilis (Pleurodira, Podocnemididae), based on recently hatched specimens, and cleared and double‐stained specimens. The orbitotemporal region is dramatically different from those observed for other species of turtles in that the: (1) planum supraseptale is greatly reduced and present only as tiny projections on the posterodorsal margin of the interorbital septum, (2) pila metoptica is free from all neighbouring structures and bifurcates distally, (3) pila antotica is greatly reduced, (4) foramina for optic nerve, ophthalmic artery and oculomotor nerves are open dorsally by virtue of this species lacking the taenia marginalis and taenia medialis, and (5) tectum synoticum is present and invested dorsally by the supraoccipital, despite the fact that this bone forms by replacement of the supraoccipital. The unique morphology of the pila metoptica is explained either as de novo formation of processes on the terminus of this cartilage or by retention of portions of the taenia medialis (anteriorly) and pila antotica or pila accessoria (posteriorly). Variation in the orbitotemporal region presented here is discussed for two other pleurodiran turtles (Phrynops hilarii and Emydura subglobosa) and briefly compared with the anatomy observed in Cryptodira.