File:Mosasaur aquatic origins hypotheses.svg

Sources and notes:

• Old hypothesis: The traditional single-origin view, with intermediate plesiopedal forms like Tethysaurus and hydropedal but primitive-limbed Halisaurines assumed as successive transitional taxa. Based on phylogeny recovered by DeBraga and Carroll (1993) for placement of Aigialosaurus and hydropedal taxa. Placement of Tethysaurus follows phylogeny recovered by Bardet et al. (2003).
• Convergent evolution hypothesis: Competing modern view of multiple independent aquatic lineages first postulated by Bell and Polcyn (2005) based on revised placements of Dallasaurus and Tethysaurus among other plesiopedal taxa. In its present iteration, it rests on the assumptions that the consistent recovery of the plesiopedal Dallasaurus as a basal mosasaurine is accurate and that all of its mosasauroid ancestors were plesiopedal. Most phylogenetic analyses in support of this hypothesis (e.g. Caldwell & Palci, 2007; Makádi et al., 2012; Palci et al., 2014) recover two or three independent lineages, although up to seven possible lineages have been recovered (Dutchak & Caldwell, 2009). The diagram here depicts four lineages, with two lineages within the Russellosaurina following Polcyn et al. (2023) and the specific view of Tethysaurus as ancestral to Platecarpus based on the study's Varanus-outgroup phylogeny. The placement of the Halisaurinae and its status as an independent aquatic lineage is uncertain. Its depiction as sister to the Russellosaurinae here is consistent with Bell and Polcyn (2005) but ultimately arbitrary.
• Reversal hypothesis: Competing modern view of a single aquatic origin first supported by Simões et al. (2017). It mostly follows the phylogenies of hydropedal mosasaurs recovered by supporters of the convergent evolution hypothesis, but differs in interpreting Tethysaurus and kin as reversions to the plesiopedal state rather than transitional forms to the hydropedal state. Its validity also rests on assumptions about Dallasaurus, namely that either (a) its placement as a basal mosasaurine is inaccurate due to incomplete fossils, or (b) represents another plesiopedal reversion. This hypothesis' support is largely statistical based on ancestral state reconstruction (Simões et al, 2017; Cross et al, 2022). Reversal of Tethysaurus was also supported in a mosasaur-outgroup phylogeny in Polcyn et al. (2023), although they interpreted this result to be an artifact of implied polarization of unordered characters (in which the phylogenetic algorithm erroneously classified derived mosasaur traits in the outgroup as ancestral).