A juvenile Elginia and early growth in pareiasaurs

Abstract

Although pareiasaurs are one of the most abundant and conspicuous elements of Late Permian terrestrial ecosystems, small individuals of these reptiles (either small species or juveniles of large species) are extremely rare. Until now, the smallest known pareiasaur is the type of the late, heavily armored form Anthodon (=Nanoparia) pricei (Bernard Price Institute of Palaeontological Research, Johannesburg 1/6), with a skull length of 10 cm and an inferred snout-vent length of approximately 50 cm. This is presumably an adult of a dwarf form, since all elements of the skull and postcranial skeleton are fully ossified, sutures are closed, and the dermal armor is more highly developed than in any other pareiasaur (Broom and Robinson, 1948; Brink, 1955; Findlay, 1970; Lee, 1997). A second and as yet undescribed specimen (Geological Survey, Pretoria CM86/544) is approximately the same size, being only very slightly larger (Lee, 1997). No other specimens of this taxon are known. An unnumbered humerus in the Amalitsky collection of the Palaeontological Institute, Moscow, is from a pareiasaur of similar size to the second specimen of A. pricei. However, this specimen is presumably a juvenile of a large form: the ends of the humerus are unossified, and the specimen comes from the North Dvina bone beds, which have yielded numerous specimens of the large pareiasaur Scutosaurus but no other pareiasaur taxon. Here, we re-evaluate a specimen originally described as a dicynodont tail, and later as a procolophonoid, and demonstrate that it is actually a juvenile of the pareiasaur Elginia mirabilis. It is also by far the smallest pareiasaur so far known, being approximately half the dimensions of the next smallest individual, the type specimen of Anthodon pricei. The newly recognized specimen provides new information on the ontogeny of pareiasaurs and the homology of some problematic skull elements.