High diversity of pancryptodiran turtles in the Lower Cretaceous of Europe

Abstract

Turtles traditionally identified as basal Eucryptodira members were very abundant and diverse in the European Upper Jurassic. However, the European Cretaceous record of these turtles is poorly known. Brodiechelys brodiei, from the Wealden of the UK, is a turtle traditionally assigned to Eucryptodira. This taxon, recognized on the basis of very little material, has been poorly described and figured. More than a dozen shells are here attributed to B. brodiei. This allows a new diagnosis to be proposed for the taxon, and recognizes that their shells are subject to great intraspecific variability. This conclusion affects many characters used in the diagnosis of taxa defined by scarce material. The analysis of Hylaeochelys, a European Lower Cretaceous taxon traditionally identified as another basal Eucryptodira member, also allows high variability to be observed and several synonyms to be recognized. Hylaeochelys belli is identified here as the only valid species of this genus. Brodiechelys brodiei, H. belli and other European and Asian Jurassic and Lower Cretaceous taxa, previously identified as basal members of Eucryptodira, are incorporated into two hypotheses of phylogenetic relationships. A basal dichotomy in Pancryptodira is obtained. In one of the nodes, taxa traditionally assigned to Paracryptodira, Plesiochelyidae and H. belli are included. The other node groups B. brodiei, taxa traditionally assigned to “Xinjiangchelyidae”, and the Cryptodira members. This study shows a high diversity of Pancryptodira members in the Lower Cretaceous of Europe, assigned to several nodes. Representatives of Paracryptodira, other members of the node grouping the traditional members of Plesiochelyidae and Paracryptodira, and several members of the node grouping Xinjiangchelys latimarginalis and Cryptodira are identified.