Eocene fossil dermochelyid provides insights into why leatherback turtles “want to become” marine mammals

Leatherbacks constitute a bizarre clade of marine turtles today represented by a single species, Dermochelys coriacea. A series of peculiar physiological adaptations and behaviors make this species particularly reminiscent to some marine mammals. These include particularly advanced skeleton adaptations for swimming, the largest body size among living reptiles, highly elevated growth and metabolic rates, and coldwater tolerance, which enable D. coriacea to lead a truly pelagic, highly migratory, cosmopolitan lifestyle. It is one of the deepest diving animal today, which aids searching for its almost exclusive prey of jellyfishes. Due to their pelagic lifestyle and reduced skeleton, however, the fossil record of leatherbacks is very poor. Here we evaluate the skeletal anatomy of Eosphargis gigas from the Ypresian of Belgium, represented by one of the earliest and most completely preserved fossil dermochelyid. E. gigas already shows several of the anatomical specializations of the extant leatherback but it is primitive in retaining a more ossified shell. The autapomorphic rugose surface decoration of the dermal skull indicates high degree of vascularization, which in turn likely aided regulation of acid–base balance relating to hypercapnia (excess blood carbon dioxide) and/or lactate acidosis based on modern and fossil analogies. Both type of acidosis typically occurs during diving and thus E. gigas likely had deep diving capabilities, which is consistent with its postcranial skeleton. The jaw apparatus also shares many specializations with D. coriacea that may represent adaptation for preying on jellyfish. The emerging hypothesis is that the skeletal and physiological adaptations of leatherbacks are all related to feeding specializations and associated deep diving. Many of these evolved early in the lineage under greenhouse climatic conditions with the likely associated deeper placement of the gelatinous plankton zone compared to icehouse conditions. This study was supported by the SYNTHESYS program (grants AT-TAF 1441, FR-TAF 4290, GB-TAF 1882, BE-TAF 5292); Train2Move-Marie Curie Fellowship awarded to M.R.

RBINS Collection(s), Abstract of an Oral Presentation or a Poster

Paleontology