Abstract
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.
Highlights
Mosasauroid reptiles sensu Bell [1] were a diverse and globally distributed clade of lizards that invaded freshwater and marine environments during the Late Cretaceous [1,2,3,4,5]
We provide the first analysis of mosasauroid relationships based on traditional maximum parsimony using two different coding schemes: contingent (Co-UMP) and multistate codings (Mu-UMP)
Despite the change in the outgroup, and minor updates in the ingroup taxa and scorings, the analysis with Co-UMP provided results (Fig 1A) that are generally similar to the most recent analyses of mosasauroid relationships [12, 25, 26]
Summary
Mosasauroid reptiles sensu Bell [1] (mosasaurids + aigialosaurids) were a diverse and globally distributed clade of lizards that invaded freshwater and marine environments during the Late Cretaceous [1,2,3,4,5]. Some of the most relevant aspects of mosasauroid morphology that illustrate their transition to an aquatic lifestyle are concentrated in a set of changes in their pelvic and pedal anatomy These changes, such as loss of contact between the sacral vertebrae and the pelvis followed by a reduction in the number of sacrals, characterize the so called hydropelvic condition [7]. The development of hyperphalangy in the autopodium, which aids in locomotion under water, constitutes the hydropedal condition [8] These two conditions of the pelvic and pedal morphologies as observed in most mosasauroids contrast to the connection between sacrum and ilium (termed plesiopelvic), as well as the typical phalangeal formula (plesiopedal), as seen in most limbed squamates [7, 8]
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