Recognizing a lost nesting ground: First unambiguous Testudines eggshells from the Eocene, associated with the pleurodiran Eocenochelus (Huesca, Northern Spain)

Abstract

The Eocene record of turtle eggshells is scarce, with a single unconfirmed report from France. This scarcity contrasts with the great abundance of osteological remains, distributed over a wide palaeogeographical range as a result of climatic warmth. In this paper, we describe the first definitive Eocene Testudoolithidae eggshell fragments attributable to Testudines, most likely pleurodiran turtles, from the Eocene Sobrarbe Formation at the Castejón de Sobrarbe fossil site (CS-41) in northern Spain. The eggshells were found in association with osteological remains of at least four individuals of Eocenochelus eremberti (Pleurodira, Erymnochelyini) in an otherwise sirenian-dominated bonebed. Analysis of eggshell ultra- and microstructure allows comparison with eggshells from fossil and extant turtles. The eggshells are highly recrystallized but preserve relics of their original aragonitic radial ultrastructure as crystal phantoms. The barrel-shaped shell units, which are taller than wide, with compactituberculate ornamentation and funnel-shaped pore openings are similar to those of the Palaeocene ootaxon Haininchelys curiosa, whose holotype and paratype have been lost. The CS-41 eggshells are most similar to those of the pleurodiran Erymnochelys madagascariensis from Madagascar, the closest living relative to Eocenochelus, further supporting our attribution. Sedimentological and taphonomic analysis of the assemblage supports the hypothesis that the CS-41 fossil site was formed as the infilling of an abandoned tributary channel in the deltaic plain by an overbank or debris-flow deposit associated with a storm event. This storm resulted in the exhumation and subsequent re-sedimetation of the remains of a nesting ground of Eocenochelus eremberti, an otherwise coastal turtle, which would have entered inland streams to nest in the sand bars at the mouth of the Sobrarbe Deltaic Complex.