Gross morphology and microstructure of type locality ossicles of Psephophorus polygonus Meyer, 1847 (Testudines, Dermochelyidae)

Psephophorus polygonusMeyer, 1847, the first fossil leatherback turtle to be named, was described on the basis of shell ossicles from the middle Miocene (MN6–7/8?) of Slovakia. The whereabouts of this material is uncertain but a slab on display at the Naturhistorisches Museum Wien is considered the neotype. We rediscovered further type locality ossicles in four European institutions, re-evaluated their gross morphology and described for the first time their microstructure by comparing them withDermochelys coriacea, the only living dermochelyid turtle. The gross morphology is congruent with that already described forP. polygonus, but with two significant exceptions: the ridged ossicles ofP. polygonusmay have a distinctly concave ventral surface as well as a tectiform shape in cross-section. They do not develop the external keel typical of many ossicles ofD. coriacea. Both ridged and non-ridged ossicles ofP. polygonusare characterized by compact diploe structures with an internal cortex consisting of a coarse fibrous meshwork, whereas the proportionately thinner ossicles ofD. coriaceatend to lose the internal cortex, and thus their diploe, during ontogeny. The ossicles of bothP. polygonusandD. coriaceadiffer from those of other lineages of amniotes whose carapace is composed of polygonal ossicles or platelets, in having growth centres situated at the plate centres just interior to the external bone surface and not within the cancellous core or closer to the internal compact layer. The new diagnosis ofP. polygonusallows us to preliminarily re-evaluate the taxonomy of some of thePsephophorus-like species. Despite some macro- and micromorphological differences, it seems likely thatPsephophoruswas as cosmopolitan as extantDermochelysand had a broadly similar ecology, with a possible difference concerning the dive depth.