Sapheosaurus Bridges the Sphenodontid/Trilophosaur/Rhychosaur gap

Sapheosaurus (Meyer 1850, Kimmeridgian Late Jurassic, France) has been known and largely ignored for a long time. Two species of this large sphenodontian are identified, S. laticeps with 22 presacrals and S. thiollierei with 26.

Figure 1. Sapheosaurus, a complete and articulated sphenodontian (rhynchocephalian) preserved on its back.  Plus the related Kallimodon in palatal view.

Figure 1. Sapheosaurus, a complete and articulated sphenodontian (rhynchocephalian) preserved on its back. Plus the related Kallimodon in palatal view. Note the narrow upper temporal fenestrae and narrow parietal. Kallimodon was found to be a sister to Sapheosaurus by Rauhut et al. 2012. Note the parallel rows of dentition also seen in Sphendon and rhynchosaurs.

 

Sapheosaurus nests between rhynchocephalians and rhynchosaurs.

Sapheosaurus nests between rhynchocephalians and rhynchosaurs.

Adding Sapheosaurus to the large reptile tree nests it between Brachyrhinodon and the higher rhynchocephalians, Trilophosaurus, Mesosuchus and the rhynchosaur, Hyperodapedon. Conventinal thinking nests rhynchosaurs and trilophosaurs at the base of the archosauriformes, close to Prolacerta, but no series of archosauromorph taxa share so many traits.

Sapheosaurus tree from Rauhut  et al. 2012

Figure 3. Sapheosaurus tree from Rauhut et al. 2012

Of course, there’s a time problem.
Rhynchosaurs and trilophosaurs were Triassic and Sapheosaurus is Late Jurassic. Of course, sphenodontians are well know for their phylogenetic longevity, with New Zealand’s own Sphenodon outlasting all of its former contemporaries.

In the Rauhut et al. (2012) tree (Fig. 3) they include many more sphenodontians than in the large reptile tree. Unfortunately they used the archosauromorph, Youngina, as an outgroup. It’s not related. Using Squamata was also ill-advised. After tall, which traits from which taxa would you cherry-pick? As in the large reptile tree, Brachyrhinodon and Sapheosaurus are close to one another. No rhynchosaurs or trilophosaurs are included in the Rauhut et al. tree, so, unfortunately, we will never see how they would nest there.

The palate of Kallimodon is interesting
because it demonstrates the side-by-side teeth that make the higher (but earlier) taxa so interesting (see Fig. 3 caption). Brachyrhinodon has similar parallel palatine teeth.

Figure 1. Rhynchocephalian and Rhynchosaur palates. That's Priosphenodon in the middle leading to Mesosuchus and Howesia, to Trilophosaurus and Azendohsaurus and rhynchosaurs. That's where the palatine grows as large as and alongside the maxilla. In derived taxa these two bones fuse creating the illusion that the maxilla has the entire tooth pad. Look at those palatine stems on Priospbenodon, which really come out on rhynchosaurs.

Figure 1. Rhynchocephalian and Rhynchosaur palates. That’s Priosphenodon in the middle leading to Mesosuchus and Howesia, to Trilophosaurus and rhynchosaurs. That’s where the palatine grows as large as and alongside the maxilla. In derived taxa these two bones fuse creating the illusion that the maxilla has the entire tooth pad. Look at those palatine stems on Priospbenodon, which really come out on rhynchosaurs.

References
Apesteguía S and Novas FE 2003. Large Cretaceous sphenodontian from Patagonia provides insight into lepidosaur evolution in Gondwana. Nature 425:609-612
Apesteguia S,  Gomez  RO, and Rougier GW 2012. A basal sphenodontian (Lepidosauria) from the Jurassic of Patagonia: new insights on the phylogeny and biogeography of Gondwanan rhynchocephalians. Zoological Journal of the Linnean Society 166:342-360
Benton MJ 1985. Classification and phylogeny of the diapsid reptiles. Zoological Journal of the Linnean Society 84(2):97-164
Rauhut OWM, Heyng AM, López-Arbarello A and Hecker A. 2012. A new rhynchocephalian from the Late Jurassic of Germany with a dentition that is unique amongst tetrapods. PLoS ONE 7(10):e46839
Reynoso VH 2000. An unusual aquatic sphenodontian (Reptilia: Diapsida) from the Tlayua Formation (Albian), central Mexico. Journal of Paleontology 74:133-148
von Meyer H 1850. Mittheilungen an Professor Bronn gerichtet: Neües Jahrbuch fur Mineralogie, Geologie und Palaontologie, Bd 18, p. 195-204.

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