Mosasauroids were a successful lineage of squamate reptiles (lizards and snakes) that radiated during the Late Cretaceous (95–66 million years ago). They can be considered one of the few lineages in the evolutionary history of tetrapods to have acquired a fully aquatic lifestyle, similarly to whales, ichthyosaurs and plesiosaurs. Despite a long history of research on this group, their phylogenetic relationships have only been tested so far using traditional (unweighted) maximum parsimony. However, hypotheses of mosasauroid relationships and the recently proposed multiple origins of aquatically adapted pelvic and pedal features in this group can be more thoroughly tested by methods that take into account variation in branch lengths and evolutionary rates. In this study, we present the first mosasauroid phylogenetic analysis performed under different analytical methods, including maximum likelihood, Bayesian inference, and implied weighting maximum parsimony. The results indicate a lack of congruence in the topological position of halisaurines and Dallasaurus. Additionally, the genus Prognathodon is paraphyletic under all hypotheses. Interestingly, a number of traditional mosasauroid clades become weakly supported, or unresolved, under Bayesian analyses. The reduced resolutions in some consensus trees create ambiguities concerning the evolution of fully aquatic pelvic/pedal conditions under many analyses. However, when enough resolution was obtained, reversals of the pelvic/pedal conditions were favoured by parsimony and likelihood ancestral state reconstructions instead of independent origins of aquatic features in mosasauroids. It is concluded that most of the observed discrepancies among the results can be associated with different analytical procedures, but also due to limited postcranial data on halisaurines, yaguarasaurines and Dallasaurus.

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Tylosaurinae (Williston, 1897), is reconstructed in most analyses as the sister group of the Plioplatecarpinae (Dollo, 1884). The most distinctive characteristic of the group is an elongated edentulous rostrum on the premaxilla. Members of the tylosaurine subfamily are divided into two genera: Tylosaurus (Marsh, 1874), and Taniwhasaurus (Hector, 1874). When all arguably valid tylosaurine species are included in a single phylogenetic analysis, some clades are well supported, i.e., the clade formed by T. proriger (Cope, 1869) + T. bernardi (Dollo, 1885), or the clade T. pembinensis (Nicholls, 1988) + T. saskatchewanensis. In contrast, clade relationships for other species remain unresolved, i.e., T. gaudryi (Thevenin, 1896), T. nepaeolicus (Cope, 1874), and the several species within the genus Taniwhasaurus. When T. gaudryi (Thevenin, 1896), Ta. 'mikasaensis' (Caldwell et al., 2008), and 'T'. capensis (Broom, 1912) were removed from the analysis, T. nepaeolicus appeared as the basal member of the genus. The relationship within the genus Taniwhasaurus remains unresolved; however, when the problematic taxa was removed, the genus became monophyletic, with Ta. oweni as the sister group of Ta. antarcticus. Based on morphological characters present in the holotype of 'T'. capensis Broom, 1912, we suggested a re-assignment to the genus Taniwhasaurus, based on the flutes and facets in the crown of the two preserved replacement teeth. The lack of a clear diagnosis of Hainosaurus/Tylosaurus neumilleri leaves this taxon as a nomen dubium; the few characters visible on the specimen show strong similarities to both T. pembinensis and T. saskatchewanensis; therefore, the specimen cannot be identified beyond Tylosaurus sp. Reassessment of the known materials of the Japanese species Ta. 'mikasaensis' suggests that the various specimens do not display sufficient diagnostic characters to support 'mikasaensis' as distinct from Taniwhasaurus oweni. The hypothesis for a North Atlantic Circle Basin distribution for a species of the genus Tylosaurus, from the Coniacian to the Maastrichtian is supported, and a more cosmopolitan distribution is suggested for the genus Taniwhasaurus (Santonian to Maastrichtian) with species present along the margins of the Pacific, Indian, and Antarctic Ocean Basins.

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