A new Trachyaspis-like pan-cheloniid turtle from the Bartonian of Kazakhstan

Abstract

The article describes a fossil pan-cheloniid Protrachyaspis shorymensis gen. et sp. nov. from the Karakeshi, Kert, Kuyulus, and Monata localities of the Shorym Formation (Bartonian, middle Eocene), as well as unknown localities of the Mangyshlak Peninsula, Kazakhstan. In addition, the shell bones of small pan-cheloniids from the Kuyulus and Tuzbair localities of the Shorym Formation with some traits of P. shorymensis are described, which probably represent remains of juvenile specimens of this species. The new taxon is characterized by a number of features rarely found in pan-cheloniids, including serrated dentaries, distally displaced lateral process of the humerus, and deeply sculptured external carapace surface. These features indicate a likely herbivorous diet and pelagic lifestyle of the new pan-cheloniid. According to the results of the cladistic analysis, P. shorymensis is sister to the Neogene species Trachyaspis lardyi Meyer, 1843, from which it differs in the absence of ridges on the carapace in adult individuals, the configuration of the scutes on the parietal bone, and the structure of the plastron. The stratigraphic gap between the appearance of P. shorymensis and its sister T. lardyi suggests a long ghost lineage of members of this clade throughout the Bartonian – Aquitanian (ca. 20.7 million years) preceding the appearance of T. lardyi. In most trees, the P. shorymensis + T. lardyi clade is located within the Chelonini clade, which, taking into account the middle Eocene age of P. shorymensis, indicates the early divergence of crown cheloniids, previously established based on molecular data. The new taxon is similar to the fragmentary remains of pan-cheloniids with a sculptured external surface of the shell, previously described from four localities of the upper part of the Buchak and lower part of Kiev formations in the south of European Russia and Ukraine, which makes it possible to determine these materials as cf. Protrachyaspis sp., and probably extends the appearance of the P. shorymensis + T. lardyi clade back to the middle Lutetian.