The evolutionary relationship between the Russellosaurina and other groups, and thus the identity of "Tylosaurus" as a mosasaur, remains a controversial subject. The traditional view of mosasaur evolution held that all paddle-limbed (hydropedal) mosasaurs originated from a single common ancestor with functional legs (plesiopedal). However, this was shaken with the discovery of "Dallasaurus", a hydropelvic mosasauroid that was found by multiple phylogenetic analyses to be an ancestor of the Mosasaurinae but unrelated to the Russellosaurina, whose basal members included plesiopedal genera (Tethysaurinae) of their own. This meant that the Russellosaurina may have evolved a hydropedal form independently through the tethysaurines, meaning that its placement within the Mosasauridae creates a polyphyly and thus potentially invalid. Caldwell informally proposed in a 2012 publication that the definition of a mosasaur must thus be redefined into one that does not consider russellosaurines like "Tylosaurus" as true mosasaurs, but as an independent group of marine lizards. However, phylogenetic studies of mosasaurs can be fickle, especially given that wild card taxa like "Dallasaurus" remain poorly understood. For example, some studies such as a 2009 analysis by Dutchak and Caldwell instead found that "Dallasaurus" was ancestral to both russellosaurines and mosasaurines, although results were inconsistent in later studies. A 2017 study by Simoes "et al." noted that utilization of different methods of phylogenetic analyses can yield different findings and ultimately found a consistent indication that tethysaurines were a case of hydropedal mosasaurs reversing back to a plesiopedal condition rather than an independent ancestral feature, suggesting that the identity of "Tylosaurus" as a true mosasaur was not as problematic as previously thought.