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

Delfino, Massimo, Scheyer, Torsten M., Chesi, Francesco, Fletcher, Tamara, Gemel, Richard, Macdonald, Stewart, Rabi, Márton, and Salisbury, Steven W. (2013) Gross morphology and microstructure of type locality ossicles of Psephophorus polygonus Meyer, 1847 (Testudines, Dermochelyidae). Geological Magazine, 150 (5). pp. 767-782.

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Abstract

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.

Item ID: 30176
Item Type: Article (Research - C1)
ISSN: 1469-5081
Keywords: Dermochelys coriacea, bone histology, middle Miocene, palaeoecology, petrographic thin-sections, taxonomy
Funders: Swiss National Science Foundation, Spanish Ministerio de Economia y Competitividad, Italian MIUR PRIN, Synthesys Programme
Projects and Grants: Swiss National Science Foundation Grant no. 31003A_127053/1, Spanish Ministerio de Economia y Competitividad CGL2011-28681, Italian MIUR PRIN 2009MSSS9L_002, Synthesys Programme AT-TAF-1281, Synthesys Programme HU-TAF-1894
Date Deposited: 13 Nov 2013 05:42
FoR Codes: 06 BIOLOGICAL SCIENCES > 0608 Zoology > 060809 Vertebrate Biology @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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