Psephophorus polygonus Meyer, 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 with Dermochelys
coriacea, the only living dermochelyid turtle. The gross morphology is congruent with
that already described for P. polygonus, but with two significant exceptions: the
ridged ossicles of P. polygonus may 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 of D. coriacea. Both ridged and non-ridged ossicles of P. polygonus
are characterized by compact diploe structures with an internal cortex consisting
of a coarse fibrous meshwork, whereas the proportionately thinner ossicles of D. coriacea
tend to lose the internal cortex, and thus their diploe, during ontogeny. The ossicles
of both P. polygonus and D. coriacea differ 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 of
P. polygonus allows us to preliminarily re-evaluate the taxonomy of some of the Psephophorus-like
species. Despite some macro-and micromorphological differences, it seems likely that
Psephophorus was as cosmopolitan as extant Dermochelys and had a broadly similar ecology,
with a possible difference concerning the dive depth.
