Reappraisal of the braincase anatomy of the ornithopod dinosaurs Telmatosaurus and Zalmoxes from the Upper Cretaceous of the Haţeg Basin (Romania) and the taxonomic reassessment of some previously referred specimens

Abstract

The hadrosauroid Telmatosaurus and the rhabdodontid Zalmoxes were the first and second dinosaur taxa that were described in detail from the famous Upper Cretaceous continental deposits of the Haţeg Basin by Franz Baron Nopcsa at the beginning of the twentieth century. Although they are among the most common and best-known dinosaurs discovered from these deposits, there are still many open questions as to their taxonomy and anatomy. Here, we re-describe two partial braincases from the uppermost Cretaceous of the Haţeg Basin that have been recently referred to the rhabdodontid Zalmoxes and re-assign them to hadrosauroids, possibly to Telmatosaurus. These specimens both exhibit basicranial features that are characteristic of derived hadrosauroids but are absent in more basal iguanodontians. These include an antero-posteriorly short basioccipital lacking a distinct neck, the presence of two well-developed sphenoccipital tubercles on the ventral aspect of the braincase and that are directly positioned anterior to the basioccipital, as well as a deep depression on the ventral aspect of the braincase between the sphenoccipital tubercles. The comparison provided herein demonstrates several important differences between the basicranium of hadrosauroids and that of rhabdodontids, which allows for the confident identification of even isolated and incomplete specimens. Moreover, the removal of the only basicranium that has been referred to Zalmoxes shqiperorum prompts a revised diagnosis of that species.