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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 of shell microstructure changes during ontogeny. Shell ankylosis occurred seemingly randomly in individuals of variable size, including small and morphologically juvenile, and completely obscured the initial bony composition. We propose that this phenomenon in the Triassic turtles can be an effect of early evolutionary stages of shell histogenesis and physiological mechanisms still used in shell regeneration in modern species. We also describe some parallels between the unusual peripheral microstructure of another Triassic turtle, Waluchelys cavitesta Sterli, Martínez, Cerda & Apaldetti, 2020, and Proterochersis porebensis. Microstructural changes imply that Proterochersis porebensis could change habitat during ontogeny, small individuals appearing more aquatic and larger more terrestrial.

Although Western Europe has yielded numerous Jurassic turtle taxa, several represented by cranial material or complete skeletons, the fossil record of the Jurassic turtles remains scarce to the north and east from Germany. Although some Late Jurassic testudinates were historically described from Poland, they were, thus far, represented by fragmentary remains that never were properly figured or described in detail. Therefore, very little is known about the mid-Mesozoic diversity of turtles in that region of the continent. A new pancryptodiran turtle genus and species, Owadowia borsukbialynickae, is described from the uppermost Jurassic (Tithonian, ca. 148 Ma) carbonate sediments of the Kcynia Formation in Owadow-Brzezinki Quarry, near Tomaszow Mazowiecki in central Poland. The lower jaw morphology and palaeoecological setting inhabited by the new genus and species, together with the trophic relationships of the Jurassic pancryptodiran turtles, are discussed in an attempt to determine the potential range of mode of life of O. borsukbialynickae. We propose that the new specimen belongs to a new durophagous pancryptodiran turtle taxon. O. borsukbialynickae might have spent considerable time in the marine environment and specialized on eating hard-shelled invertebrates like bivalves and decapod crustaceans, common to that setting.

A recently discovered Norian outcrop in Poreba, Poland, has yielded numerous well-preserved turtle remains. These, together with historical materials from Germany, enabled the identification of two new proterochersid taxa: Proterochersis porebensis sp. nov. from Poland and Keuperotesta limendorsa gen. et sp. nov. from Germany. Moreover, two problematic taxa, Proterochersis intermedia Fraas, 1913; and Murrhardtia staeschei Karl & Tichy, 2000; are shown to be conspecific with Proterochersis robusta Fraas, 1913. New diagnoses for the family Proterochersidae Nopcsa, 1923 and all included taxa are provided. Proterochersids are of great importance to turtle phylogeny because of their age (they comprise the oldest fully shelled turtle species known to date) and their still-debated phylogenetic position (classically they are considered the basalmost Pleurodira, but in some analyses they are placed on the stem of Testudinata). Newly discovered plesiomorphic aspects of the anatomy of these three species included in the phylogenetic analysis demonstrate that they are not only the oldest, but are also the most basal fully shelled turtles. The unique features of their shell (e.g. the first thoracic rib unreduced and costal-bearing, and the osseous contact between the carapace and sacral vertebra) are of special significance for future research of the earliest stages of turtle shell evolution. This is the first contribution focused solely on the new, phylogenetically informative, and important characters of this group since the initial description of P. robusta more than 100 years ago.

BackgroundTurtle shells tend to exhibit frequent and substantial variability, both in bone and scute layout. Aside from secondary changes, caused by diseases, parasites, and trauma, this variability appears to be inherent and result from stochastic or externally induced flaws of developmental programs. It is, thus, expected to be present in fossil turtle species at least as prominently, as in modern populations. Descriptions of variability and ontogeny are, however, rare for fossil turtles, mainly due to rarity, incompleteness, damage, and post-mortem deformation of their remains. This paper is an attempt at description and interpretation of external shell variability in representatives of the oldest true turtles,Proterochersis robustaandProterochersis porebensis(Proterochersidae, the sister group to all other known testudinatans) from the Late Triassic (Norian) of Germany and Poland.MethodsAll the available shell remains ofProterochersis robusta(13 specimens) andProterochersis porebensis(275 specimens) were studied morphologically in order to identify any ontogenetic changes, intraspecific variability, sexual dimorphism, and shell abnormalities. To test the inferred sexual dimorphism, shape analyses were performed for two regions (gular and anal) of the plastron.ResultsProterochersisspp. exhibits large shell variability, and at least some of the observed changes seem to be correlated with ontogeny (growth of gulars, extragulars, caudals, and marginals, disappearance of middorsal keel on the carapace). Several specimens show abnormal layout of scute sulci, several others unusual morphologies of vertebral scute areas, one has an additional pair of plastral scutes, and one extraordinarily pronounced, likely pathological, growth rings on the carapace. Both species are represented in a wide spectrum of sizes, from hatchlings to old, mature individuals. The largest fragmentary specimens ofProterochersis porebensisallow estimation of its maximal carapace length at approximately 80 cm, whileProterochersis robustaappears to have reached lower maximal sizes.DiscussionThis is the second contribution describing variability among numerous specimens of Triassic turtles, and the first to show evidence of unambiguous shell abnormalities. Presented data supplement the sparse knowledge of shell scute development in the earliest turtles and suggest that at least some aspects of the developmental programs governing scute development were already similar in the Late Triassic to these of modern forms.

All derived turtles are characterized by one of the strongest reductions of the dorsal elements among Amniota, and have only 10 dorsal and eight cervical vertebrae. I demonstrate that the Late Triassic turtles, which represent successive stages of the shell evolution, indicate that the shift of the boundary between the cervical and dorsal sections of the vertebral column occurred over the course of several million years after the formation of complete carapace. The more generalized reptilian formula of at most seven cervicals and at least 11 dorsals is thus plesiomorphic for Testudinata. The morphological modifications associated with an anterior homeotic change of the first dorsal vertebra towards the last cervical vertebra in the Triassic turtles are partially recapitulated by the reduction of the first dorsal vertebra in crown-group Testudines, and they resemble the morphologies observed under laboratory conditions resulting from the experimental changes ofHoxgene expression patterns. This homeotic shift hypothesis is supported by the, unique to turtles, restriction ofHox-5expression domains, somitic precursors of scapula, and brachial plexus branches to the cervical region, by the number of the marginal scute-forming placodes, which was larger in the Triassic than in modern turtles, and by phylogenetic analyses.

Background. The origin of turtles, their earliest evolution and the homologies of the elements building their shell are still enigmatic and remain an object of ongoing discussion and research. Although the oldest fully shelled turtle – Proterochersis robusta from the Lower Stubensandstein (Norian) of Germany – was described more than a century ago, until recently it was mostly ignored by researchers. This is surprising, not only because of its notable stratigraphic position, but also due to the critical significance of this taxon in two competing hypotheses of turtle interrelationships. The divergence time of two main branches of Testudines crown group depends on whether Proterochersis is a basal pleurodire or a stem turtle. Methods. A detailed study of the German material of Proterochersis and Murrhardtia was performed by the author and the available specimens were compared with still growing collection of proterochersid remains from the Norian location in Poręba (Poland). Results. Two controversial taxa from Germany, Proterochersis intermedia and Murrhardtia staeschei are proved to be the synonyms of P. robusta. Establishment of another two proterochersid taxa is proposed and new primitive postcranial characters are recognized, supporting the stem position of Proterochersidae on the turtle phylogenetic tree. Discussion. Two general hypotheses concerning the split time of Cryptodira and Pleurodira are functioning in the literature, and Proterochersis plays a crucial role in these considerations. According to traditional view, this taxon is the oldest side-necked turtle, based on the sutural connection between its pelvis and shell. Numerous recent analyses tended to allocate the Pleurodira much higher on the tree, resulting in stem position of Proterochersis, though such placement usually lacked a strong support. This was caused by an incompletness of Proterochersis material (only shells) and errors in interpretation of some characters. New data, gathered mostly from the Polish – much more complete and well preserved – specimens, supports the basal position of Proterochersidae and provides new insights in the ancestral structure of the turtle shell.