Published online by Cambridge University Press: 24 July 2018
Megacephalosaurus eulerti is a large macropredatory plesiosaur representing one of the last members of the diverse pliosaurid clade Brachaucheninae. The taxon was established upon a nearly complete skull including the mandible and fragments of the postcranial skeleton originating from the lower middle Turonian (Upper Cretaceous) of Kansas, USA. Owing to its age, reasonable completeness and its state of preservation, M. eulerti bears important anatomical details regarding the last brachauchenines. Here we assess the dentition of the taxon, compare the teeth to those of other thalassophonean pliosaurids and comment on the utility of these results for inferences of the phylogenetic relationships of the last brachauchenines. Additionally, we provide remarks on the cranial anatomy of M. eulerti, revise character scores of this taxon used in current phylogenetic studies and address the phylogenetic relationships within Brachaucheninae. Parsimony analyses, aimed to test different character sampling and tree-search strategy, inferred only a single unambiguous synapomorphy uniting a clade formed by mid- to Late Cretaceous brachauchenines: presence of subcircular rather than subtrihedral/trihedral cross-sectional shape of the teeth. Still, the last brachauchenines (Brachauchenius and Megacephalosaurus) can be roughly characterized by a switch from anisodont to subisodont dentition and reduction of their tooth count. Nevertheless, the overall knowledge of the origin, phylogenetic relationships and distinguishability of brachauchenine pliosaurids remains poor and represents a subject for further extensive studies and modifications in taxon and character sampling.