Thursday 21 April 2022

Dortoka vremiri: A new species of Dortokid Turtle from the Late Cretaceous of the Hațeg Basin, Romania.

The Cretaceous deposits of the Hațeg Basin of Romania are noted for the production of a rich diversity of endemic fossil Vertebrates, interpreted as an island biota, with members of many taxa exhibiting dwarfism compared to relatives elsewhere in the world. The fossils recovered from the Hațeg Basin include Fish, Amphibians, Squamates, Turtles, Crocodyliforms, Pterosaurs, non-Avian Dinosaurs, Birds and Mammals, with Turtles being among the most common. Despite this abundance, only two types of Turtle are known from the Hațeg, Kallokibotion bajazidi, a stem-group Turtle (i.e. a member of a group of Turtles that are not descended from the last common ancestor of all living Turtles), and an unnamed Dortokid.
In a paper published in the Journal of Systematic Palaeontology on 8 February 2022, Felix Augustin of the Institut für Geowissenschaften at Eberhard Karls Universität Tübingen, Zoltán Csiki-Sava of the Faculty of Geology and Geophysics at the University of Bucharest, Andreas Matzke, also of the Institut für Geowissenschaften at Eberhard Karls Universität Tübingen, Gábor Botfalvai of the Department of Palaeontology and Geology at the Hungarian Natural History Museum and the Department of Paleontology at Eötvös Lorand University, and Márton Rabi, again of the Institut für Geowissenschaften at Eberhard Karls Universität Tübingen, and of the Natural Sciences Collections at Martin-Luther University Halle-Wittenberg, formally describe the Hațeg Dortokid as a new species.

The Dortokidae are interpreted as an extinct, early-branching group of Pleurodire (side-necked) Turtles, likely to have been semi-aquatic and to have lived in freshwater environments. Sadly, no known example of the group has a preserved group, preventing the exact relationship of these Turtles to other Pleurodires being established with any confidence, but members of the group can be confidently identified by their micro-reticulate shell ornamentation. The earliest known members of the group date from the Early Cretaceous, while the latest date from the Early Eocene.

The new species is placed within the established genus Dortoka, which gives its name to the whole group, and given the specific name vremiri, in honour of the late Matyas Vremir, who prepared the specimen from which the species is named, and who previously worked upon Dortokid Turtles from the Hațeg Basin. The specimen is a partial skeleton with most of the carapace and the complete plastron preserved in contact, together with the exposed in situ right scapula and right pubis; the left-side counterparts of these girdle elements are potentially also preserved inside the shell.

Holotype carapace of Dortoka vremiri, LPB (FGGUB) R.2297, from the Upper Cretaceous Sînpetru Formation of the south-central Hațeg Basin, near Sânpetru. (A) Photograph and (B) drawing of the carapace in dorsal view, both to the same scale. Abbreviations: c, costal; M, marginal scale; n, neural; nu, nuchal; p, peripheral; PL, pleural scale; VE, vertebral scale. Augustin et al. (2022).

Dortoka vremiri is a medium-sized Dortokid Turtle which the first pair of costal plates meet behind the first neural plate, and the last pair of costal plates meet behind the eighth neural plate. This last pair of costal plates has a well defined suture, and prevents the eighth neural plate from making contact with the suprapygal bone. The specimen does not have a cervical plate. The first pair of pleural plates connect only to the first pair of costal plates, not the second. The second pair of pleural plates stops before making contact with the fifth pair of costal plates. The sulcus between the fourth and fifth vertebra plates is located towards the rear of the last neural plate, with the fifth vertebral having no contact with the costal plates. The entoplastron is at its greatest width towards its front. The Pectoral scales contact the entoplastron, and the extragulars are less than a third the size of the gulars.

Dortokid Turtles are known from the Hațeg Basin both before and after the End Cretaceous Extinction event, whereas Kallokibotion bajazidi, a species interpreted as having been more terrestrial in nature, disappears at the end of the Cretaceous, leaving no known close relatives. Thus, it is possible that the extinction favoured more aquatic forms living in this area, but also that all life here was wiped out, and that semi-aquatic species such as Dortokid Turtles were able to recolonise the Hațeg island from elsewhere.

A phylogenetic analysis of Dortokid Turtles carried out by Augustin et al. found that Dortoka vremiri was the closest known relative of Dortoka botanica, the species found in the post-Cretaceous Hațeg. This supports the idea that occupation of this area by Dortokid Turtles was continuous, and that therefore the End Cretaceous Extinction event was more lethal to terrestrial forms than semi-aquatic ones in the Hațeg Basin.

Phylogenetic relationships, as well as temporal and palaeogeographical position of Dortoka vremiri within the Dortokidae. (A) Strict consensus tree of the in-group phylogenetic analysis of the Dortokidae. There are two distinct lineages of derived Dortokids, a western lineage comprising Dortoka vasconica from the Late Cretaceous of Spain and France as well as an eastern European lineage comprising Dortoka vremiri from the Late Cretaceous of Romania and Dortoka botanica from the uppermost Paleocene of Romania. (B) Palaeogeographical map of the Late Cretaceous European Archipelago depicting the distribution of the different Dortokids. Abbreviations: Do, Dortoka vasconica from the Late Cretaceous of Laño and Armuña, Spain; Ds, Dortoka sp. (Dortoka vasconica?) from the Late Cretaceous of southern and south-eastern France; El, indeterminate Dortokid from the Early Cretaceous of El Castellar, Spain; Eo, Eodortoka morellana from the Early Cretaceous of Morella, Spain; Ha, Dortoka vremiri from the Late Cretaceous of the Hat¸eg Basin, Romania; Ih, indeterminate Dortokid from the Late Cretaceous of Iharkut, Hungary; Mu, indeterminate Dortokid from the Late Cretaceous of Muthmannsdorf, Austria; Ro, Dortoka botanica from the uppermost Paleocene of Rona, Romania; Sb, indeterminate Dortokid from the lower Eocene of the Şimleu Basin, Romania; Va, indeterminate Dortokid from the Early Cretaceous of Vallipon, Spain. Augustin et al. (2022).

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