Palaeopleurosaurus and Pleurosaurus are not sisters. >GASP!<

Earlier we looked at the phylogenetic connection between Marmoretta, Megachirella, Ankylosphenodon and the aquatic rhynchocephalians, the pleurosaurs (Figs. 1, 2). Continuing work with the large reptile tree (still not updated) illuminates new relationships among Palaeopleurosaurus and Pleurosaurus (Figs. 1, 2), which turns out to be pretty obvious on hindsight.

Figure 1. Pleurosaurus and Palaeopleurosaurus to scale with sisters.

Figure 1. Pleurosaurus and Palaeopleurosaurus to scale with sisters along with arrows indicating evolutionary pathways. 

The distinct skulls
of conventional pleurosaurs give them away (Fig. 2). Palaepleurosaurus looks like many basal rhynchocephalians. Pleurosaurus looks more streamlined. By convention that meant it was more derived, better adapted to an aquatic niche.

Figure 2. Pleurosaurus and Palaeopleurosaurus skulls compared to those of sister taxa.

Figure 2. Pleurosaurus and Palaeopleurosaurus skulls compared to those of sister taxa. They are more closely related to the graphically associated taxa than to each other. 

Actually just the opposite is true,
according to phylogenetic analysis. Pleurosaurus has a low, long rostrum because its closest sisters, Palaegama, Megachirella and Marmoretta share this skull shape. Elsewhere on the tree, Palaeopleurosaurus nests with rhynchocephalians with a similar skull morphology, including Ankylosphenodon, succeeding Gephyrosaurus and preceding Planocephalosaurus.

In case you didn’t notice…
Long-legged Palaegama is a VERY basal lepidosauriform. One branch of descendants, the lepidosaurs, retained a terrestrial niche. These included basal tritosaurs, like Tijubina, and rhynchocephalians. Another branch produced the Permian to Cretaceous so-called ‘rib’ gliders, like Icarosaurus. A third branch produced aquatic forms in the form of Pleurosaurus.

Figure 3. Palaegama, close to the origin of all Lepidosauriformes.

Figure 3. Palaegama, close to the origin of all Lepidosauriformes. This generalized taxon was close to the origin of arboreal climbers and gliders, terrestrial lizard-like forms and aquatic pleurosaurs.

Earlier I mistakenly nested the Marmoretta, Megachirella, Palaeopleurosaurus, Pleurosaurus clade between Gephyrosaurus and Planocephalosaurus. Less reliance on published drawings and more reliance on specimen photographs was helpful in working out the problems that arose with the data from the drawings. This new nesting sheds light on the origin of the Lepidosauria and requires the redefinition of the Pleurosauridae because the two former pleurosaur genera are more closely related to other taxa than to each other.

References
Carroll RL 1985. A pleurosaur from the Lower Jurassic and the taxonomic position of the Sphenodontida. Palaeontographica Abteilung A, 189:1-28.
Dupret V 2004. The pleurosaurs: anatomy and phylogeny. Revue de Paléobiologie, Geneve 9:61-80.
Meyer H 1831. IV Neue Fossile Reptilien, aud der Ordnung der Saurier.
Reynoso VH 1996. Early Cretaceous Lepidosaurs (Reptilia: Diapsida) from Central Mexico and the Phylogeny of Lepidosauromorphs. 369 pp. Unpublished Ph.D. Thesis, McGill University, Montreal, Canada.

wiki/Pleurosaurus

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