Historical debate The specific placement of Mosasauria within the Squamata has been controversial since its inception, with early debate focusing on the classification of the mosasaurs. Cuvier was the first scientist to deeply analyze their possible taxonomic placement through
Mosasaurus. While his original 1808 hypothesis that the genus was a lizard with affinities to monitor lizards remained the most popular, Cuvier was uncertain, even at the time, about the accuracy of this placement. He simultaneously proposed a number of alternative hypotheses, with one such alternative suggesting that
Mosasaurus instead had closer affinities with
iguanas due to their shared presence of pterygoid teeth. With the absence of sufficient fossil evidence, researchers during the early and mid-19th century had little to work with. Instead, they primarily relied on stratigraphic associations and Cuvier's 1808 research on the holotype skull. Thus, in-depth research on the placement of
Mosasaurus was not undertaken until the discovery of more complete mosasaur fossils during the late 19th century, which reignited research on the placement of mosasaurs among squamates.],
Mosasaurus, and
Platecarpus). However, a close relationship between
mosasaurs and
snakes was rejected by most 20th-century
herpetologists and paleontologists, who sought, instead, to demonstrate a close relationship between mosasaurs and
varanid (monitor) lizards and who generally considered snakes to have evolved from
terrestrial, burrowing lizards (see, for example, ). Cope's Pythonomorpha was later resurrected by a number of paleontologists (Lee, 1997; Caldwell et Lee, 1997) who had conducted
cladistic analyses that seemed to show that snakes and mosasaurs may have been more closely related to one another than either were to the varanid lizards, and that snakes more likely arose from aquatic ancestors. As redefined by Lee (1997), the
monophyletic Pythonomorpha consists of "the most recent common ancestor of mosasauroids and snakes, and all its descendants." This would include the
aigialosaurs,
dolichosaurs,
coniasaurs, mosasaurs, and all snakes. Lee (1997) was able to show no less than 38
synapomorphies supporting Pythonomorpha. If Pythonomorpha is valid, it contains not only mosasauroids but the
Ophidiomorpha, which was defined as a node-based clade containing the most recent common ancestor of dolichosaurs, adriosaurs,
Aphanizocnemus, and fossil and extant
Ophidia and all of its descendants. The validity of Pythonomorpha is still debated; there is no consensus about the relationships of snakes or mosasaurs to each other, or to the rest of the lizards. An analysis by Conrad (2008) placed mosasaurs with varanoid lizards, and snakes with
skinks, while an analysis by Gauthier, et al., (2012) suggested that mosasaurs are more primitive than either snakes or varanoids. A combined morphological and molecular analysis by Reeder, et al., (2015) recovered Mosasauria and Serpentes as sisters, consistent with Pythonomorpha. A 2022 analysis found that mosasaurs were most closely related to
Varanoidea, and stated that they "consider most characters previously reported as supporting the Pythonomorph Hypothesis to be problematic, because of incomplete fossil preparation, artefacts of
taphonomy, limited comparisons, misinterpretations of anatomy, incomplete taxon sampling, or inadequate character formulation and/or scoring". Therefore, Pythonomorpha could be synonymous with Toxicofera according to the definition.
Ophidiomorpha Ophidiomorpha is a proposed
clade composed of
snakes and a number of extinct
squamate groups. The clade was defined by Palci and Caldwell in 2007 as a
node-based clade containing the most recent common ancestor of
dolichosaurs,
adriosaurs,
Aphanizocnemus, and fossil and extant
Ophidia and all of its descendants. ==References==