Frontiers in Earth Science (Mar 2019)
A New Hypothesis of the Phylogenetic Relationships of the Tylosaurinae (Squamata: Mosasauroidea)
Abstract
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’. capensisBroom, 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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