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The Origin of Rhynchosaurs Revisited

Monday, February 17, 2014 13:27

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(Before It's News)

Earlier we looked at the origin of rhynchosaurs. Today, another look.

Figure 1. The best data I have been able to found to document the origin of rhynchosaurs like Scaphonyx and Hyperodapedon. Despite their apparent (from the literature) commonality, there is precious little in the literature about rhynchosaurs.

Figure 1. The best data I have been able to found to document the origin of rhynchosaurs like Scaphonyx and Hyperodapedon, to scale. Despite their apparent (from the literature) commonality, there is precious little in the literature about rhynchosaurs. Lower images from Evans and Jones 2010. I’m a little unsure about the lacrimal on Scaphonyx. Help will be appreciated.

This post was inspired
by learning that Kaikaifilusaurus was conspecific with Priosphenodon (Fig. 1). I was also reviewing the Gauthier et al. (1988) of archosaur traits not shared by lepidosaurs, including the postnarial connection of the nasal and premaxilla. (BTW, Brachyrhinodon and Pleurosaurus (lower right, fig. 1) has this connection while pterosaurs do not.) Here bone colors help tell the story of rhynchosaur origins better than any 1000 words can. The reappearance of the lacrimal, quadratojugal and socketed teeth are all part of the story. There may also have been a love child produced by the illicit mating of a Priosphenodon with a Mesosuchus. Well, maybe that mystery will be solved when a taxon is found that nests with them.

The reappearance of teeth
on the premaxilla of Mesosuchus documents some sort of legacy genetic code reappearing. So, if this is the case, there was evidently something in the water that permitted the reappearance of several other previously lost traits.

The infilling of the squamosal
on Priosphendon is also an autapomorphy not shared with other rhynchocephalians.

You can look from here to there,
but you won’t find a closer sister taxa among the archosauromorpha that nests more parsimoniously with rhynchosaurs than the rhynchocephalians. If you do, please let me know. In the meantime, the most comprehensive family tree on this subject can be found here.

References
Benton MJ 1983. The Triassic reptile Hyperodapedon from Elgin, functional morphology and relationships. Philosophical Transactions of the Royal Society of London, Series B, 302, 605-717.
Benton MJ 1990. The Species of Rhynchosaurus, A Rhynchosaur (Reptilia, Diapsida) from the Middle Triassic of England. Philosophical transactions of the Royal Society, London B 328:213-306. online paper
Benton MJ 1985. Classification and phylogeny of diapsid reptiles. Zoological Journal of the Linnean Society 84: 97-164.
Carroll RL 1988. Vertebrate Paleontology and Evolution. WH Freeman and Company.
Cruickshank ARI 1972. The proterosuchian thecodonts. In Studies in Vertebrate Evolution (ed. Jenkins KA and Kemp TS) 89-119. Edinburgh: Oliver and Boyd.
Dilkes DW 1995. The rhynchosaur Howesia browni from the Lower Triassic of South Africa. Paleontology 38(3):665-685.
Evans S. E. & Jones M. E. H. 2010. The Origin, Early History and Diversification of Lepidosauromorph Reptiles. In Bandyopadhyay S (ed.) New Aspects of Mesozoic Biodiversity, Lecture Notes in Earth Science 132, 27-44.
GauthierJ, Kluge, AG & Rowe T 1988. The early evolution of the Amniota. pp. 103–155 in Benton, M.J. (ed.), The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds. Oxford: Clarendon Press.
Huxley TH 1869. On Hyperodapedon. Quarterly Journal of the Geological Society, London, 25, 138-152.
Huxley TH 1887. Further observations upon Hyperodapedon gordoni. Quarterly Journal of the Geological Society, London, 43, 675-694.