Now that I’m back in Martinsville, with our five Carmel Church jackets safely stowed in the museum, I can begin to follow-up on some of our finds. On our first day Brett found the small bone shown above, which I tentatively attributed to a leatherback turtle. Leatherbacks, as their name implies, have a shell composed of a leathery hide with large numbers of embedded bony plates, which have a highly variable and irregular polygonal shape.
We have no recognized leatherbacks in the VMNH collection (modern or fossil), so I had to turn to the literature for the time being. Actually, I turned to Mary Catherine Santoro, VMNH’s librarian, who found a several papers on Miocene sea turtles for me in a matter on minutes. One of these papers, by Chesi, Delfino, Varola, and Rook (available online as a pdf) includes drawings of parts of the shell of a leatherback turtle from the Miocene of Italy:
It seems that no two leatherback plates are ever exact matches, and given that, our plate seems a pretty good match. The turtle figured in Chesi et al. is Psephophorus polygonus. There is a similar leatherback known from the Calvert Formation, Psephophorus calvertensis. It’s not yet clear to me how the different named species of Psephophorus (there are 8) are distinguished from each other; Köhler (1995) lists the named species (pdf available), and at least some of their diagnostic features, but without figures it’s difficult to interpret them. I am dubious about some of these taxa, considering the wide-ranging habits of modern leatherbacks; P. polygonus and P. calvertensis are the same age, from opposite sides of the Atlantic Ocean.
I also mentioned the scapula-like bone found last fall as possibly representing the same turtle. Weems (1974) referred an isolated scapula fragment to Psephophorus (I assume based on comparison to the modern leatherback). At least in the published photo, this is not an especially good match for the Carmel Church specimen, so if Weems’ referral is correct it seems likely that the Carmel Church bone is something else.
One final note: Weems mentions that Psephophorus is quite rare in the Calvert, and it seems that it’s only known from Calvert Bed 10. This is lower in the Calvert than Carmel Church, so it’s possible our specimen may also be the first leatherback from all of Bed 14. Confirming this will require going through the extensive unpublished Calvert material in various museums.