fig 2
-Proterochersis porebensis Szczygielski & Sulej, 2016 ZPAL V. 39/480, costal with the front of coarse metaplastic ossification revealing the underlying distal part: A, B, coronal CT scans of the specimen (closer and further away from the exterior) showing a large vascular canal at the interface between the costal bone and overlying coarse dermal ossification; C, closeup of a canal associated with externally visible suture-like groove in polarized light; D, the specimen in external view with marked sectioning lines; E, longitudinal section of the costal showing a possible semiankylosed suture and the large vascular canal; F, cross section of the distal part of the rib; G, cross section of the proximal part with open intercostal suture; H, longitudinal section showing possible remnants of sutures; I, the external cortex of the distal part (section F) in polarized light; J, close up to the large vascular canal in polarized light, showing the tissues surrounding the canal. Arrowheads: red, ISF; yellow, primary canals; black, growth marks; dark blue, large vascular canal between the dermal ossification and the underlying costal; orange, parallel-fibered bone; purple, sutures; purple outlines, areas where sutures could be present as suggested by the bone microstructure. Distal in A-E, H, and J, towards the right; E-H, in normal transmitted light with the external cortex towards the top of the page. See text for discussion. Scale bars: A, B, D-H, 1 cm; C, J, 1 mm; I, 0.5 mm.
Source publication
Shell suture obliteration (ankylosis) was exceptionally frequent in the earliest turtles, in contrast to post-Triassic taxa. Since modern turtles grow mostly along sutures, early ankylosis in Triassic taxa is intriguing. The Triassic turtle Proterochersis porebensis Szczygielski & Sulej, 2016 is known from numerous specimens, allowing observation o...
Citations
Impressions of vertebrate bodies or their parts, such as trace fossils and natural molds of bones, are a valuable source of information about ancient faunas which may supplement the standard fossil record based on skeletal elements. Whereas trace fossils of animal activity are relatively common and actively studied within the field of ichnology, and natural impressions of internal or external surfaces are a frequent preservation mode in fossil invertebrates, natural molds of bones are comparatively rare and less extensively documented and discussed. Among them, internal molds (steinkerns) of turtle shells are a relatively well-known form of preservation, but the mechanisms and taphonomic prerequisites leading to their formation are poorly studied. External shell molds are even less represented in the literature. Herein, we describe a historic specimen of a natural external turtle plastron mold from the Triassic (Norian) Löwenstein Formation of Germany–a formation which also yielded a number of turtle steinkerns. The specimen is significant not only because it represents an unusual form of preservation, but also due to its remarkably large size and the presence of a potential shell pathology. Although it was initially interpreted as Proterochersis sp., the recent progress in the knowledge of proterochersid turtles leading to an increase in the number of known taxa within that group allows us to verify that assessment. We confirm that the specimen is morphologically consistent with the genus and tentatively identify it as Proterochersis robusta , the only representative of that genus from the Löwenstein Formation. We note, however, that its size exceeds the size observed thus far in Proterochersis robusta and fits within the range of Proterochersis porebensis from the Grabowa Formation of Poland. The marks interpreted as shell pathology are morphologically consistent with Karethraichnus lakkos– an ichnotaxon interpreted as a trace of ectoparasites, such as leeches. This may support the previously proposed interpretation of Proterochersis spp. as a semiaquatic turtle. Moreover, if the identification is correct, the specimen may represent a very rare case of a negative preservation of a named ichnotaxon. Finally, we discuss the taphonomy of the Löwenstein Formation turtles in comparison with other Triassic turtle-yielding formations which show no potential for the preservation of internal or external shell molds and propose a taphonomic model for the formation of such fossils.
The Middle Triassic remains a poorly understood time in the evolution of land vertebrates. Here, we report a new Ladinian-age vertebrate assemblage from Miedary (southern Poland). It consists of more than 20 taxa including fish (four
species of Hybodontiformes, cf. Gyrolepis, Redfieldiiformes, ‘Thelodus’, Saurichthys, Serrolepis, Prohalecites, Ptychoceratodus), amphibians (Mastodonsaurus, Gerrothorax, Plagiosternum, chroniosuchian Bystrowiella), and reptiles (Owenettidae, Blezingeria, Nothosaurus, Tanystropheus, an additional, yet unidentified tanystropheid, the doswelliid Jaxtasuchus, and another archosauromorph, as well as eight archosauriform tooth morphotypes). Preliminary comparisons suggest biogeographic and environmental similarities with roughly contemporaneous localities known from the southwestern part of the Germanic Basin. Among differences in these two areas are the presence of a new armored archosauromorph and a surprising abundance of Tanystropheus remains in the new Polish site. Miedary is currently the richest source of three-dimensionally preserved Tanystropheus material in the world, which will be crucial for a better understanding of the preferred environment and lifestyle of this highly specialized reptile.
The turtle locomotor system is heavily modified owing to the severe impact of development of the shell on the body plan of these reptiles. Although limb and girdle osteology of the earliest, Triassic turtles is relatively well understood in general, the exact impacts of variability, ontogeny and preservation (e.g. deformation) on the observed morphologies have rarely been considered in detail. Here, we describe in detail and document the osteology and intraspecific variability of the limbs and girdles of Proterochersis spp., the basalmost true turtles (Testudinata). We also provide a synthesis of currently available data and detailed comparisons with other Triassic stem turtles to gain a better understanding of the diagnostic value of the early turtle appendicular skeleton and to pave the way for future biomechanical and functional studies. Our data suggest that Proterochersis spp. could be at least partly aquatic and could change their preferred habitat during ontogeny, with larger (and, presumably, older) specimens presenting more characters suggestive of a more terrestrial environment.
