Moving Rhynchosaurs and Trilophosaurs Back into the Rhynchocephalia (Sphenodontia)

Rhynchosaurs are among the strangest reptiles that ever lived.
Characterized by a weird wide skull and protruding toothless, beak-like premaxillae, rhynchosaurs had rows of crushing teeth and giant jaw muscles for grinding food before swallowing to hasten digestion. Although the body was nothing special, no other reptile had such a skull. And evidently THAT is the cause of the current lack of a solid phylogenetic nesting.

Pre-rhynchosaurs, like Mesosuchus, appeared in the late Early Triassic to early Middle Triassic. All rhynchosaurs, including Hyperodapedon, disappeared in the Late Triassic.

Hyperodapedon in various views.

Figure 1. Hyperodapedon in various views. Note the extreme width of the skull and multiple rows of grinding teeth.

Where Did Rhynchosaurs Fit In?
Romer (1956) considered rhynchosaurs and sphenodontians to be related, but Cruickshank (1972), Benton (1983), Carroll (1988) and Dilkes (1998) split them apart, perhaps by placing too much emphasis on the lack of fusion in the tarsus and lack of acrodont teeth (see below). Caroll (1988) placed Trilophosaurus and rhynchosaurs with Prolacerta, Tanystropheus, Proterosuchus and Euparkeria. Unfortunately, no phylogenetic analysis has yet tested this nesting against a large gamut of reptiles, other than the large study.

rhynchosaurs588

Priosphenodon and its sphenodontid sisters, including Trilophosaurus and the rhynchosaur Hyperodapedo

Let’s Look at the Candidates
Above a selection of several rhynchocephalians is compared to three candidate diapsids. Overall Hyperodapedon, Mesosuchus and Trilophosaurus share more traits with Brachyrhinodon than with Youngina, Prolacerta or Proterosuchus. This is also demonstrated by several hundred characters and taxa in the large reptile tree. No other terrestrial reptile had such a wide skull, but Mesosuchus comes close. Brachyrhinodon and Priosphenodon come close to MesosuchusProlacerta and Proterosuchus were known for their narrow skulls filled with sharp teeth.

Here’s the Hump We Have to Get Over
According to the textbooks, lepidosaurs all have a fused astragalus and calcaneum and derived characters of bone growth with epiphyses. The problem is Trilophosaurus and rhynchosaurs don’t fuse those proximal ankle bones.

Benton (1983) reported, “Rhynchosaurs have no special relationship with the sphenodontids. The supposed shared characters are either primitive (e.g. complete lower temporal bar, quadratojugal, akinetic skull, inner ear structure, 25 presacral vertebrae, vertebral shape, certain character of limbs and girdles) or incorrect (e.g. rhynchosaurs do not have acrodont teeth, the ‘beak-like’ premaxilla of both groups is quite different in appearance, the ‘tooth plate’ is wholly on the maxilla in rhynchosaurs but on maxilla and palatine in sphenodontids).”

These nits and picks are important, but taken as a whole (which is what we must do) currently there are no taxa more closely related to rhynchosaurs than rhynchocephalians (sphenodontians) and the trilophosaurs. Granted, all other rhynchocephalians had fused ankle bones, but having an unfused ankle is simply a matter of not fusing those bones, which develop separately in embryos. Acrodont teeth also form with fusion. Again, this would be a simple matter of switching off a gene.

Some of the Strangest Teeth You’ll Ever See
Benton (1983) discussed the placement of teeth wholly on the maxilla in rhynchosaurs. Let’s see what that looks like. The palatine (in orange) is the key bone in this controversy. In Mesosuchus the palatine is reduced and has lost its teeth. In Hyperodapedon the palatine retains teeth and extends lateral to the choanae to contact the premaxilla. In Howesia the palatine fuses to the maxillary tooth plate. In Trilophosaurus the palatine likewise fused to the maxillary tooth plate and the palatine teeth fused to the maxillary teeth, creating laterally elongated teeth with three lateral cusps. Click here for enlargement.

The palates of several rhynchocephalians, including rhynchosaurs

Figure 3. The palates of several rhynchocephalians, including rhynchosaurs. Click to enlarge.

Summary
Romer was right. Rhynchosaurs are closer to rhynchocephalians (sphenodontians). The differences noted by Benton (1983) are insufficient to outweigh a larger suite of characters that nest these taxa together.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

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.
Case EC 1928. A cotylosaur from the Upper Triassic of western Texas: Journal of Washington Academy of Science 18:177-178.
Cruickshank ARI 1972. 
The proterosuchian thecodonts. In Studies in Vertebrate Evolution (ed. Jenkins KA and Kemp TS) 89-119. Edinburgh: Oliver and Boyd.
Dutuit J-M 1972. Découverte d’un Dinosaure ornithischien dans le Trias supérieur de l’Atlas occidental marocain. Comptes Rendus de l’Académie des Sciences à Paris, Série D 275:2841-2844. 
Flynn JJ, Nesbitt, SJ, Parrish JM, Ranivoharimanana L and Wyss AR 2010. 
A new species of Azendohsaurus (Diapsida: Archosauromorpha) from the Triassic Isalo Group of southwestern Madagascar: cranium and mandible”. Palaeontology 53 (3): 669–688. doi:10.1111/j.1475-4983.2010.00954.x
Fraser NC and Benton MJ 1989.
 
The Triassic reptiles Brachyrhinodon and  Polysphenodon and the relationships of the sphenodontids. Zoological Journal of the Linnean Society 96:413-445.
Gregory JT 1945. Osteology and relationships of Trilophosaurus: University of Texas, Publication 4401:273-359.
Heckert AB et al. 2006. Revision of the archosauromorph reptile Trilophosaurus, with a description of the first skull of Trilophosaurus jacobsi, from the upper Triassic Chinle Group, West Texas, USA: Palaeontology 4(3):1-20.
Huxley TH 1859.
 Postscript to, R. I. Murchinson. On the sandstones of Morayshire (Elgin & c.) containing reptile remains; and their relations to the Old Red Sandstone of that country. Quarterly Journal of the Geological Society, London, 15, 138-152.
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. Parks P 1969. Cranial anatomy and mastication of the Triassic reptile,  Trilophosaurus [M.S. thesis]: University of Texas, 100 pp.
Romer AS 1956. Osteology of the Reptiles. University of Chicago Press, Chicago.

wiki/Hyperodapedon
wiki/Rhynchosaur
wiki/Brachyrhinodon
wiki/Trilophosaurus
wiki/Azendohsaurus

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