Psephophorus, MEYER, 1847

Fallon, Bailey R. & Boessenecker, Robert W., 2019, First record of a leatherback sea turtle (Dermochelyidae) from the Mio-Pliocene Purisima Formation of northern California, USA, PaleoBios 36, pp. 1-8 : 2-3

CF. PSEPHOPHORUS MEYER, 1847

FIG. 2

Referred specimen— UCMP 293839, an isolated carapacial ossicle.

Occurrence— Bonebed 5 of the Purisima Formation near Santa Cruz, California.

Description— UCMP 293839 is polygonal, slightly longer than wide, and measures 22.61 mm long, 16.35 mm wide, and 5.56 mm thick. It is approximately tabular in cross section, but has a very slight concavity and convexity on the dorsal and visceral surfaces, respectively. The dorsal surface is scattered with shallow, circular pits ranging in size from ~ 0.50–0.91 mm in diameter, which are weakly radially oriented ( Fig. 2A). The pits become increasingly elongated toward the edges of the dorsal surface, ranging in size from ~ 2.24–2.62 mm in length, and are also radially oriented.The visceral surface is scattered with slightly larger, more irregularly-shaped pits ranging in size from ~ 0.68–1.67 mm and are not radially oriented ( Fig. 2B). Two different textures comprise the sutural surface. A groove with rounded, worn projections runs parallel to the dorsal and visceral surfaces, essentially dividing the ossicle in half. The groove ends with an X-shaped depression. The non-groove sutural surface is rugose with peaks and troughs that potentially indicate the original presence of finger-like projections ( Fig. 2C). The internal structure of the ossicle, as revealed by a fracture, is comprised of a compact layer extending from the dorsal surface to just over half the thickness of the ossicle. A porous, more vascularized layer extends from this point to the visceral surface ( Fig. 2D).

Remarks —UCMP 293839 is tentatively referred to Psephophorus based on its morphological and internal structural differences as compared to extant Dermochelys and other Cenozoic dermochelyids. Its stratified internal structure is characterized by dense dorsal bone and porous visceral bone when viewed in cross section. These features are unlike the thinner, non-stratified bone structure found in the ossicles of extant Dermochelys . UCMP 293839 is thinner than the ossicles of other Cenozoic dermochelyids (e.g., Natemys , Egyptemys ), and thus may be distinguished from these genera.