Eohyosaurus – a new basal rhynchosaur

Eohyosaurus wolvaardti, SAM-PK-K-10159 (Butler 2015, Fig. 1) is a new basal rhynchosaur from the early Middle Triassic (Anisian) of the Karroo supergroup, known from a single skull. It is similar to Mesosuchus.

Figure 1. Eohyosaurus reconstructed. This taxon nests between, Trilophosaurus + Azendohsaurus and the Rhychosauridae.

Figure 1. Eohyosaurus reconstructed from several views of a single specimen. This taxon nests between, Trilophosaurus + Azendohsaurus and the Rhychosauridae (Figs. 2, 3).

Butler et al. did a thorough and excellent job
of describing their specimen. They nested it accurately.

Unfortunately,
Butler et al. added two non-rhynchosaurian outgroups (Prolacerta broomi and Protorosaurus speneri) to their cladistic analysis and omitted many others (Figs. 2, 3).

Figure 2. Rhynchosaur tree from Butler et al. Color area added for rhynchosauridae.

Figure 2. Rhynchosaur tree from Butler et al. Color area added for rhynchosauridae.

In the large reptile tree (Fig. 3 subset) the protorosaurs are not related to the rhynchosaurs. And rhynchosaurs are derived from sphenodontians. That was the original assessment, but the lack of fusion in the ankles of rhynchosaurs caused Cruickshank (1972) and Benton (1983) to consider rhynchosaurs close to protorosaurs and archosaurs, like Prolacerta and Proterosuchus. Carroll (1988) considered this valid in his landmark textbook and Dilkes (1998) agreed. Details here, here and here.

They’re all wrong,
if you include the following taxa (Fig. 3) and all the 556 intervening taxa.

Figure 3. Here is where Eohyosaurus fits on the large reptile tree.

Figure 3. Here is where Eohyosaurus fits on the large reptile tree.

Butler et al. considered
Noteosuchus the earliest known rhynchosaur (Early Triassic). Actually it’s a transitional clade member bridging Clevosaurus, a sphenodontian, to Eohyosaurus and Mesosuchus, basal rhynchosaurs.

All you young and old scientists (paleontologists)
keep adding taxa and see what your tree recovers.

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.
Butler R, Ezcurra M, Montefeltro F, Samathi A, Sobral G 2015. A new species of basal rhynchosaur (Diapsida: Archosauromorpha) from the early Middle Triassic of South Africa, and the early evolution of Rhynchosauria. Zoological Journal of the Linnean Society 10.1111/zoj.12246.
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.

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