The Odd Swimming Sphenodontids

Updated November 11, 2014 with the Dnesting of pleurosaurs with Megachirella and Marmoretta. And updated December 3, 2014 with the division of the pleurosaurs into two convergent clades. 

Hard to believe that our favorite New Zealand “Living Fossil,” Tuatara (Sphenodon), had some aquatic sisters, but here they are.

Figure 1. Pleurosaurus and Palaeopleurosaurus to scale with sisters.

Figure 1. Pleurosaurus and Palaeopleurosaurus to scale with sisters.

Pleurosaurus goldfussi (Meyer 1831), Late Jurassic. 60 cm in length. The Triassic terrestrial sphenodontians produced a Late Jurassic marine lineage known as the pleurosaurs after the first of these to be discovered, Pleurosaurus and also one of the most derived. Palaeopleurosaurus appears to be a stretched out version of its terrestrial antecedent, Planocephalosaurus, and was a transitional form to later, longer, more streamlined pleurosaurs.

Added Dec 03, 2104: > That was the traditional nesting. New analyses indicate that Pleurosaurus nested between Palaegama, Megachirella and Marmoretta at the base of the Lepidosauria, which radiated in the Middle to Late Permian. Palaeopleurosaurus had a convergent return to an aquatic niche as it nested between Gephyrosaurus and Planocephalosaurus. The similar Ankylosphenodon was a sister taxon.

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

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

Pleurosaurs were Late Jurassic aquatic sphenodontids, characterized by a long, streamlined and elongated body (with a short neck), small limbs and (as in most aquatic reptiles) nares that were displaced from the snout tip to closer to the orbits. The premaxilla of Palaeopleurosaurus was ventrally elongated to form a sharp spike that would have snared prey. They swam with snake-like undulations of the entire body. Their neural spines grew to become large rectangles, as in snakes. Pleurosaurs produced no Cretaceous descendants.

There is not much that is controversial about these lepidosaurs. They are not often studied and they are rarely on anyone’s Top 10 list, so I thought I’d toss them out for a little publicity.

Added Dec. 03, 2014: > Well, perhaps I spoke too soon as the two traditional pleurosaurs are not that closely related to one another. That’s a little bit of news!

References
Carroll RL 1985. A pleurosaur from the Lower Jurassic and the taxonomic position of the Sphenodontids.
Fraser NC and Sues H-D 1997. In the Shadows of the Dinosaurs: early Mesozoic tetrapods. Cambridge University Press, 445 pp. Online book.
Heckert AB 2004. Late Triassic microvertebrates from the lower Chinle Group (Otischalkian-Adamanian: Carnian), southwestern U.S.A. New Mexico Museum of Natural History and Science Bulletin 27:1-170.
Meyer H 1831. IV Neue Fossile Reptilien, aud der Ordnung der Saurier.

wiki/Planocephalosaurus
wiki/Pleurosaurus

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