Two turtles with soft tissue preservation from the platy limestones of Germany provide evidence for marine flipper adaptations in Late Jurassic thalassochelydians

Abstract

Late Jurassic deposits across Europe have yielded a rich fauna of extinct turtles. Although many of these turtles are recovered from marine deposits, it is unclear which of these taxa are habitually marine and which may be riverine species washed into nearby basins, as adaptations to open marine conditions are yet to be found. Two new fossils from the Late Jurassic of Germany provide unusually strong evidence for open marine adaptations. The first specimen is a partial shell and articulated hind limb from the Late Jurassic (early Tithonian) platy limestones of Schernfeld near Eichstätt, which preserves the integument of the hind limb as an imprint. The skin is fully covered by flat, polygonal scales, which stiffen the pes into a paddle. Although taxonomic attribution is not possible, similarities are apparent with Thalassemys. The second specimen is a large, articulated skeleton with hypertrophied limbs referable to Thalassemys bruntrutana from the Late Jurassic (early Late Kimmeridgian) platy limestone of Wattendorf, near Bamberg. Even though the skin is preserved as a phosphatic film, the scales are not preserved. This specimen can nevertheless be inferred to have had paddles stiffened by scales based on the pose in which they are preserved, the presence of epibionts between the digits, and by full morphological correspondence to the specimen from Schernfeld. An analysis of scalation in extant turtles demonstrated that elongate flippers stiffed by scales are a marine adaptation, in contrast to the elongate but flexible flippers of riverine turtles. Phylogenetic analysis suggests that Thalassemys bruntrutana is referable to the mostly Late Jurassic turtle clade Thalassochelydia. The marine adapted flippers of this taxon therefore evolved convergently with those of later clades of marine turtles. Although thalassochelydian fossils are restricted to Europe, with one notable exception from Argentina, their open marine adaptations combined with the interconnectivity of Jurassic oceans predict that the clade must have been even more wide-spread during that time.

The ecological diversification and evolution of Teleosauroidea (Crocodylomorpha, Thalattosuchia), with insights into their mandibular biomechanics

“…During the Late Jurassic, there was a diverse assemblage of eucryptodiran turtles (Anquetin et al, 2014 ; Joyce et al, 2021 ; Püntener et al, 2015 ), particularly in Europe. Bite marks and embedded teeth suggest that Lemmysuchus and Machimosaurus specialized in macrophagy/durophagy, feeding on larger, armored prey such as turtles and scaled fishes (Meyer, 1988 ; Young et al, 2014 ; Young, Brusatte, Beatty, et al, 2012 ; Young, Brusatte, de Andrade, et al, 2012 ).…”

The integument of pelagic crocodylomorphs (Thalattosuchia: Metriorhynchidae)

“…Integrative analyses have demonstrated their middle Jurassic origin and late Jurassic radiation (Pérez-Losada et al, 2004, 2008, but there is a lack of fossil evidence from the Jurassic (Schweigert, 2015). It appears attractive to link this hypothetical origin with a pelagic shift in turtles (new specimens from Wattendorf and Eichstätt [Joyce et al, 2021] as well as from Painten [pers. obs.…”

An ancestral hard-shelled sea turtle with a mosaic of soft skin and scutes

“…Interestingly, though, none of the six extant cheloniid species has embarked on this evolutionary path, despite being almost exclusively marine. It is possible that the polygonal scales seen in these turtles serves to stiffen the flippers during locomotion, as has been proposed for another lineage of aquatic testudines, namely the Jurassic Thalassochelydia 20 . However, alternative explanations for the heavy scalation in extant hard-shelled sea turtles may also exist.…”

“…In part, this is because fossilized turtle integument is exceedingly rare, with only a handful of specimens showing remnant soft tissues (e.g., Refs. [18][19][20] ; see also Supplementary Information). Recently, however, a partial but largely articulated sea turtle skeleton with associated soft tissues was reported by De La Garza et al 21 (Fig.…”