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

  • ️Fri Jan 01 2021

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 et al. PLoS One. 2021.

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.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. JME 3995, Thalassochelydia indet., Late Jurassic (early Tithonian) of Birkhof, Bavaria, Germany.

Photograph (top) and interpretive line drawing (bottom) in ventral view. Hatched areas denote bones. Roman numerals denote digit identity. Note large patches of scaly skin preserved across the specimen. For additional images, please see S1 Appendix. Abbreviations: fe, femur; ti, tibia.

Fig 2
Fig 2. NKMB Watt18/211, Thalassemys bruntrutana, Late Jurassic (early Late Kimmeridgian) of Wattendorf, Bavaria, Germany.

Photograph (left) and interpretive line drawing (right) of specimen in ventral view. Roman numerals denote digit identity, Arabic numerals vertebral identity. Abbreviations: Ab, abdominal scute; ce, cervical vertebra; cen, centrale; co, coracoid; col, cololite (cololith); cr, costal rib; epi, epiplastron; Fe, femoral scute; fe, femur; fi, fibula; hu, humerus; hyo, hyoplastron; hyp, hypoplastron; il, ilium; Im, inframarginal scute; int, intermedium; is, ischium; nu, nuchal; Pe, pectoral scute; per, peripheral; pu, pubis; py, pygal; ra, radius; sc, scapula; spy, suprapygal; ti, tibia; tr, thoracic rib; ul, ulna; uln, ulnare; xi, xiphiplastron.

Fig 3
Fig 3. NKMB Watt18/211, Thalassemys bruntrutana, Late Jurassic (late Kimmeridgian) of Wattendorf, Bavaria, Germany.

Photographs taken under UV light. (A) right forelimb, (B) skull and neck, (C) left forelimb, (D) right hind limb, (E) tail, and (G) left hind limb in ventral view. White arrows highlight soft tissue. Yellow arrows indicate epibionts. For anatomical interpretation of vertebral column and limb, please refer to Fig 2. Abbreviations: cb, ceratobranchial; ch, ceratohyal; de, dentary; ex, exoccipital; ju, jugal; mx, maxilla; pa, parietal; qj, quadratojugal; so, supraoccipital; sur, surangular.

Fig 4
Fig 4. Strict consensus of 5940 MPTs of 1743 steps showing the phylogenetic placement of Thalassemys bruntrutana as well as important clades and taxa discussed in the text.

Note that terminal clades in bold were been collapsed to save space (see S4 Appendix for full topology). Important internal nodes are labelled. ‘Plesiochelydidae’ is placed in single quotes to indicate that is does not have a formal phylogenetic definition. ‘Macrobaenidae’ is also placed in single quotes because the specifier for this clade, Macrobaena mongolica, is not included. However, the respective taxa are commonly labelled as ‘macrobaenids’ in the literature.

Fig 5
Fig 5. The relative length of the humerus, ulna, and hand of Thalassemys bruntrutana relative to that of extant turtles.

In the diagram to the front, extant turtles are color coded for taxonomic groups, in the diagram to the back, extant turtles are color coded for habitat preferences. All data of extant turtles are taken from Joyce and Gauthier (2004) [11]. Note that Thalassemys bruntrutana has relatively longer hand that extant trionychids, but shorter hands that extant carettochelyids and chelonioids.

Fig 6
Fig 6. The comparative anatomy of the skin of the limbs of extant turtles.

(A) the testudinid tortoise CAS 165598 Psammobates oculifera, an inhabitant of desserts. (B) the trionychid turtle CAS 65705 Apalone spinifera, an inhabitant of rivers. (C) the emydid turtle CAS 13889 Chrysemys picta, an inhabitant of ponds. (D) the cheloniid turtle CAS 8478 Chelonia mydas, an inhabitant of oceans.

Fig 7
Fig 7. Taphonomic preservation of freshwater adapted turtle forelimbs in dorsal view.

(A) the carettochelyid Allaeochelys crassesculpta from the Eocene Messel Formation of Germany (IRSNB AFR34). (B) the pan-trionychid Perochelys lamadongensis from the early Cretaceous Jehol Biota of China (IVPP V 18048). (C) the testudinoid turtle Echmatemys sp. (FOBU 14014) from the Green River Formation of the USA. (D) Parachelys sp. (NKMB Watt05/202) from the Late Jurassic of Wattendorf, Germany. Note that the forelimb is slightly adducted in all cases and that the digits are stacked on top of one another. The images are not to scale.

Fig 8
Fig 8. Comparisons of forelimbs of marine adapted turtles.

(A) photograph and (B) line drawing of reflected left flipper of the extant leatherback turtle Dermochelys coriacea (QM J47453) in dorsal view. (C) photograph and (D) line drawing of right flipper of the extinct chelonioid Allopleuron hofmanni (NHMUK PV 42893) in dorsal view. (E) photograph and (F) line drawing of the right flipper of the extant cheloniid Natator depressus (QM J14463) in dorsal view. (G) photograph and (H) line drawing of right flipper of the extinct protostegid Rhinochelys nammourensis (MSNM V3933; picture courtesy of Ren Hirayama) in dorsal view. (I) photograph and (J) line drawing of reflected left flipper of the extinct thalassochelydian Thalassemys bruntrutana (NKMB Watt18/211) in ventral view. Roman numerals denote digit identity. Abbreviations: c, central carpal; dc, distal carpal; i, intermedium; mc, metacarpal; p, pisiform; r, radiale; ra, radius; u, ulnare; ul, ulna.

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Grants and funding

This contribution was funded by a grant from the Swiss National Science Foundation to WGJ (SNF 200021_178780/1) and by funds from the Bürgerverein Bamberg Mitte e.V., the Freunde des Naturkunde-Museums Bamberg e.V., the Landesstelle für die nichtstaatlichen Museen in Bayern, the Oberfrankenstiftung, the Stiftung der Sparkasse Bamberg, the Rotary Club Bamberg, and the VR-Bank Bamberg-Forchheim eG to MM. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.