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

Walter G Joyce,Matthias Mäuser,Serjoscha W Evers,Thierry Smith

doi:10.1371/journal.pone.0252355

Walter G Joyce, Matthias Mäuser + Show 2 more

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https://doi.org/10.1371/journal.pone.0252355

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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.

Highlights

A rich assemblage of turtles has been recovered from Late Jurassic platy limestone deposits (= plattenkalks) across France and Germany over the course of the last two centuries
These deposits are universally regarded as marine, the rich associated fauna and flora consisting of vascular plants, insects, lepidosaurs, and small dinosaurs, in addition to marine invertebrates and vertebrates and volant animals such as pterosaurs and early birds, universally suggest relatively close proximity to land, either in the form of small, atoll-like islands that may have dotted the landscape in the immediate vicinity of the basins where the platy limestones were deposited, or larger landmasses, such as the Bohemian, Central, or Rhenish massifs, which likely emerged from the oceans close by [2,3,4,5]
Explicit comparison with extant turtles remains invaluable when assessing the paleoecology of fossils, a number of recent morphometric studies have raised cautionary notes regarding a straightforward ecological interpretation of turtle anatomy due to unresolved form-function relationships or the lack of clear correspondence of anatomies with a single ecology

Summary

Introduction

A rich assemblage of turtles has been recovered from Late Jurassic platy limestone deposits (= plattenkalks) across France and Germany over the course of the last two centuries (see [1] for recent summary). While the shape of skull and shell and the relative length of the forelimbs are telling, accessory data is needed to draw forceful conclusions Along these lines, the fossil turtle Eurysternum wagleri was recently argued to have been an inhabitant of reefs, because it appears to have the specialized heart-shaped shell typical of modern marine turtles, and because it is one of the most common turtles to be found in the platy limestones of Germany, because it is numerous even in the basins more distant from land, because it is known from a range of ontogenetic stages, and, because specimens are known with fossilized gut contents consisting of marine invertebrates, echinoderms [12]. All names above the species level used are formally defined clade names [13] that are highlighted as such through the use of italics

Materials and methods

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Discussion