Pelvic girdle morphology in Stagonolepis, with remarks on aetosaur systematics

In recent years, the Upper Triassic deposits at Krasiejów (south-west Poland) have yielded several tetrapod taxa, both aquatic and terrestrial. Stagonolepis olenkae is one of the terrestrial vertebrates recovered there; a quadrupedal, armoured aetosaur, which belonged to the crocodile-line archosaurs with a characteristic shovel-shaped snout. Several previous studies (osteological, histological and taphonomic) have attempted to understand the mode of life, growth pattern and possible dimorphism of this species and on this basis, to interpret palaeoecological, palaeoclimatic and stratigraphical implications. So far, the pelvic girdle of S. olenkae from Krasiejów remained undescribed. Here, the authors record stagonolepid ilia and pubes and a single ischium from collections housed at the University of Opole, and compare these with the pelvic girdles of other aetosaurian taxa. These well-preserved bones have a typical aetosaurian general outline, but also show some peculiar features. For instance, the preacetabular blade of the ilium is short and flattened and does not exceed the pubic peduncle; several small foramina occur dorsally of the supracetabular crest; the number of pubic foramina is two and the pubic symphysis is less than half the length of the pubis. The material is similar to previously known Stagonolepis robertsoni from the Elgin area, except for the length of the symphysis. The bones presented here differ between each other in thickness, morphology of the preacetabular blade or attachment of sacral ribs, which may be connected with sexual dimorphism. The pelvic girdle of most aetosaurs is not well known. This is unfortunate, because it is an important element in the study of the evolution of the pelvic girdle and in phylogenetic analyses. Thus, the ilia, pubes and ischium of the present study are valuable examples that may contribute to the discussion of the ontogeny and sexual dimorphism in Stagonolepis, as well as to our general knowledge of the Aetosauria.