The origin of the Mosasauria in the LRT

Figure 1. Tylosaurus represented by three species. T. kansasensis, T. proriger and T. dyspelor. Note the differences in flipper size.

Figure 1. Tylosaurus represented by three species. T. kansasensis, T. proriger and T. dyspelor. Note the differences in flipper size.

Wikipedia reports,
Mosasauria: “The exact phylogenetic position of the clade containing mosasaurids and their closest relatives (aigialosaurids and dolichosaurs) within Squamata remains uncertain. Some cladistic analyses recovered them as the closest relatives of snakes, taking into account similarities in jaw and skull anatomies; however, this has been disputed[ and the morphological analysis conducted by Conrad (2008) recovered them as varanoids closely related to terrestrial monitor lizards instead. Longrich, Bhullar and Gauthier (2012) conducted a morphological analysis of squamate relationships using a modified version of the matrix from the analysis of Gauthier et al. (2012); they found the phylogenetic position of the clade containing mosasaurs and their closest relatives within Squamata to be highly unstable, with the clade “variously being recovered outside Scleroglossa (as in Gauthier et al., 2012) or alongside the limbless forms”.

By contrast
the large reptile tree (LRT. 1006 taxa) nests the mosasaurs Tethysaurus and Tylosaurus with Aigialosaurus and this clade is a sister to the clade Saniwa + (Estesia + Varanus). All have a common ancestor that is a sister to Bahndwivici and Yabeinosaurus + Sakurasaurus. In other words, not close to snakes, earless monitors, Gila monsters or amphisbaenids.

Figure 2. Tylosaurus and other mosasaurs. Boxed: The shoulder girdle, left paddle and cartilaginous sternal cartilages of Tylosaurus in dorsal view. These broad ribs and sternum anchored powerful pectoral muscles to the front paddles.

Figure 2. Tylosaurus and other mosasaurs. Boxed: The shoulder girdle, left paddle and cartilaginous sternal cartilages of Tylosaurus in dorsal view. These broad ribs and sternum anchored powerful pectoral muscles to the front paddles.

Tylosaurus proriger (Marsh 1872, Late Cretaceous) was a giant mosasaur, a clade that traditionally and currently nests with small Aigialosaurus. Mosasaurs were sea-going giant varanoid lizards that gave live birth and had extra phalanges on the medial digits as the hands and feet were transformed into paddles. Large teeth appear on the pterygoids, but these are convergent with those found on basal marine snakes like Pachyrhachis. The nasals were fused together and fused to the premaxilla ascending process.

Figure 3. Aigialosaurus, a small mosasauroid, compared to Coniasaurus, an even smaller mosasauroid, not related to Dolichosaurus and snakes.

Figure 3. Aigialosaurus, a small mosasauroid, compared to Coniasaurus, unrelated to mosasaurs, but related to  Dolichosaurus and snakes.

Other much more distantly related aquatic squamates include
dolichosaurs like Dolichosaurus, Adriosaurus and Pontosaurus. Those are in the lineage of pre-snakes, like Tetrapodophis in the LRT.

On a side note:
Caldwell 2012 asked, “What, if anything is a mosasaur?”

From the Caldwell abstract:
“This treatise critically assesses Camp’s [1923] diagnostic characters for Anguimorpha, Platynota, Varanoidea, and Mosasauroidea, concluding that Camp’s data permit mosasaurs to be viewed only as anguimorphans, not platynotans nor varanoids. A similar critical assessment is given for the characters used to diagnose anguimorphans and varanoids in Estes et al. [1988], concluding here that not a single character out of twenty-two is shared between varanoids and mosasaurs… It is concluded here that there is no character-based evidence to support phylogenetic hypotheses that mosasaurs are derived aquatic varanoid lizards. It is recognized that the concept and term “mosasaur” has ceased to exist in any biologically meaningful way, and that the future requires the construction of a new suite of terms and concepts to convey what we now think we know about these animals.”

The LRT agrees
with the above hypothesis (Fig. 4) to an extent. Mosasaurs and varanids shared a common ancestor, probably as early as the Permian. TA1045 (Rößler et al. 2012; yet unnamed) is one of the earliest known lepidosaurs. It lived during the Permian. Saniwa lived during the Eocene much later than the Cretaceous mosasaurs. I could not locate any Mesozoic varanids, though they were undoubtedly present.

Estesia is not considered a varanid, but it nests with them in the LRT. Rather, Wikipedia reports that Estesia is close to the living Gila monster, Heloderma, a desert lizard.

Figure 4. Mosasaur cladogram showing related taxa.

Figure 4. Mosasaur cladogram showing related taxa.

On an another note
I just learned about this website: http://homepages.vodafone.co.nz/~jollyroger_wave/DinoNew/links2.htm
listing blogs and other web pages related to prehistoric topics. Both PterosaurHeresies and ReptileEvolution are listed. Thank you Vodphone/JollyRoger/DinoNews!

References
Caldwell M 2012. A challenge to categories: “What, if anything, is a mosasaur?” Bulletin de la Société Géologique de France 183(1): DOI: 10.2113/gssgfbull.183.1.7
Marsh OC 1872. Note on Rhinosaurus. American Journal of Science 4 (20):147.
Rößler R, Zierold T, Feng Z, Kretzschmar R, Merbitz M, Annacker V and Schneider JW 2012. A snapshot of an early Permian ecosystem preserved by explosive volcanism: New results from the Chemnitz Petrified Forest, Germany. PALAIOS, 2012, v. 27, p. 814–834

wiki/Mosasaur
wiki/Tylosaurus
wiki/Tethysaurus
wiki/Saniwa

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2 thoughts on “The origin of the Mosasauria in the LRT

  1. Huh. Why do you think Pachyrhachis was terrestrial?

    Mosasaurs and varanids shared a common ancestor, probably as early as the Permian.

    Whoa. That’s a lot of ghost lineage you’re postulating. Are the characters that place the mosasaurs elsewhere in other matrices all in your matrix, too?

  2. re: Pachyrhachis. My bad. The dividing line happens between Pachyrhachis and Dinilysia. Will fix.
    re: Ghost lineage. I would not say that but for that Permian provisional varanid with no name. That may change when and if the paper comes out and details become crisper.

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