page_title: Mosasaurus
section_title: Phylogeny and evolution of the genus
hierarchical_section_title: Mosasaurus / Classification / Systematics and evolution / Phylogeny and evolution of the genus
caption_attribution_description: Italiano: Fossil - Took the picture at Musee des Confluences, Lyon
context_page_description: Mosasaurus is the type genus of the mosasaurs, an extinct group of aquatic squamate reptiles. The genus lived from about 82 to 66 million years ago during the Campanian and Maastrichtian stages of the Late Cretaceous. The earliest fossils known to science were found as skulls in a chalk quarry near the Dutch city of Maastricht in the late 1700s, which were initially thought to have been the bones of crocodiles or whales. One particular skull discovered at around 1780, and which was seized during the French Revolutionary Wars for its scientific value and transported to Paris, was famously nicknamed the "great animal of Maastricht". In 1808, naturalist Georges Cuvier concluded that it belonged to a giant marine lizard with similarities to monitor lizards but otherwise unlike any animal known today. This concept was revolutionary at the time and helped support the then-developing ideas of extinction. However, Cuvier did not designate a scientific name for the new animal; this task was completed by William Daniel Conybeare in 1822 when he named it Mosasaurus in reference to its origin in fossil deposits near the Meuse River.
context_section_description: One of the earliest relevant attempts at an evolutionary study of Mosasaurus was done by Russell (1967), which proposed that Mosasaurus evolved from a Clidastes-like mosasaur, and diverged into two lineages. Of these two hypothesized lineages, one gave rise to M. conodon and another led to a chronospecies sequence which contained in order of succession M. ivoensis, M. missouriensis, and M. maximus/hoffmanni. However, Russell used a more primitive method of phylogenetics in his study as at the time the works giving rise to the modern field of cladistics had yet to be widely established.
  In 1997, Bell published the first cladistical study of North American mosasaurs. In it, he incorporated the species M. missouriensis, M. conodon, M. maximus, and an unnamed species identified as Mosasaurus sp.. Some findings of the study were in agreement with Russell (1967), such as Mosasaurus descending from an ancestral group which Clidastes is a member of and M. conodon being the most basal of the genus. Bell also found that Mosasaurus formed a sister relationship with another group that included Globidens and Prognathodon, and that M. maximus formed a sister relationship with Plotosaurus. These findings are contrary to Russell (1967), which hypothesized that Prognathodon and Plotosaurus arose from different lineages. The recovery of M. maximus as a sister species of Plotosaurus rendered Mosasaurus paraphyletic, but Bell (1997) nevertheless recognized Plotosaurus as a distinct genus. Although some later studies have recovered the basal sister group of Mosasaurus to instead be Eremiasaurus or Plesiotylosaurus depending on the method of data interpretation used, with at least one study also recovering M. missouriensis to be the most basal species of the genus instead of M. conodon, Bell's study served as a precedent for later studies that have mostly left the systematics of Mosasaurus unchanged. However, a number of issues from this have been pointed out: first, the genus was severely underrepresented by incorporating only the three North American species M. hoffmannii/M. maximus, M. missouriensis, and M. conodon; by doing so, other important species such as M. lemonnieri, which is one of the most completely known in the genus, were neglected, which affected the results of the phylogenetic analyses. Second, the studies relied on a still unclean and shaky taxonomy of the Mosasaurus genus due to the lack of a clear holotype diagnosis, which may be behind the paraphyletic status of Mosasaurus. Third, there was still a lack of osteological comparative studies of large mosasaurines including Mosasaurus at the time, which also hampered accurate results. These problems were addressed in the phylogenetic study in Street's doctoral thesis, which yielded different results.
  Conrad (2008) uniquely utilized only M. hoffmannii and M. lemonnieri in its phylogenetic analysis, which recovered M. hoffmannii as basal to a multitude of descending clades containing (in order of most to least basal) Globidens, M. lemonnieri, Goronyosaurus, and Plotosaurus. This result indicated that M. hoffmannii and M. lemonnieri are not related by genus and that M. lemonnieri did not belong in Mosasaurus. However, the study utilized a method completely foreign to traditional phylogenetic studies on mosasaur species, namely the use of an artificial entity rather than a real taxon as the operational taxonomic unit outgroup because its focus was on higher rather than lower classification. As a result, it has been cautioned that lower-classification results such as the specific placement of Mosasaurus within the Mosasauria may contain technical problems that can affect its accuracy.
  The following cladogram on the left (Topology A) is modified from a maximum clade credibility tree inferred by a Bayesian analysis from the most recent major phylogenetic analysis of the Mosasaurinae subfamily by Madzia & Cau (2017), which has been self-described as a refinement of a larger phylogenetic study on mosasaurs by Simões et al., (2017). The cladogram on the right (Topology B) is modified 
caption: Skeleton of M. beaugei
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