A new face for Sclerosaurus – at the base of the Pareiasauria

Sclerosaurus armatus (Meyer 1859) Middle Triassic ~50 cm in length, was originally considered a procolophonid, then a pareiasaurid, then back and forth again and again, with a complete account in Sues and Reisz (2008) who considered it a procolophonid.

FIgure 1. Sclerosaurus face.

FIgure 1. Sclerosaurus face.

Earlier I reconstructed a lower face for Sclerosaurus, a taxon known from a fossil that has been flattened but still has 3D elements. Unfortunately the top of the snout is missing, but most of the rest of the skull is present in 3D or in impressions. A recent review (what I’ve been doing for the last few months) brought new insight and a higher, more box-like face.

Figure 4. Sclerosaurus reconstructed.

Figure 2. Sclerosaurus reconstructed.

Sclerosaurus is a sister to Elginia, the pareiasaur with really big horns! Sclerosaurus is also a sister to Arganaceras, the basal pareiasaur or pareiasaur cousin with vestigial horns.

Arganaceras

Figure 3. Arganaceras, as originally reconstructed and modified. This taxon nests as a sister to Sclerosaurus and together they nest as the sister to the Pareiasauria. See the reduced horn on the suptratemporal?

Classic pareiasaurs, like Anthodon (Fig. 3), don’t have supratemporal horns, but they do raise the tabular and postparietals to the dorsal plane from the ancestral occipital plane. This is a big reversal since anamniotes also have tabulars and post parietals on the dorsal plane and intervening taxa do not.

Anthodon

Figure 4. Anthodon in various views from Lee (1997).

The outgroup taxon for pareiasaurs is the Early Permian giant millerettidStephanospondylus. The skeleton is poorly known, but no horns were present.

These wide-body omnivores/herbivores had to protect themselves from coeval predators. Turtles did this best. The rest went extinct for one reason or another. But these taxa give us the best picture of the many directions evolution took to solve the basic defense question.

References
Sues H-D and Reisz RR 2008. Anatomy and Phylogenetic Relationships of Sclerosaurus armatus (Amniota: Parareptilia) from the Buntsandstein (Triassic) of Europe. Journal of Vertebrate Paleontology 28(4):1031-1042. doi: 10.1671/0272-4634-28.4.1031 online

Sclerosaurus paleocritti

 

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