First report of desmatosuchine aetosaur (Pseudosuchia, Aetosauriformes) osteoderms from the Upper Triassic Tiki Formation of India: Their complex internal vascular system, functional significance and biostratigraphy.

Abstract

The Late Triassic Tiki fauna from India represents one of the richest and most diverse vertebrate faunal assemblages, essentially known for archosauromorphs, temnospondyls, fishes, lizards, and trace fossils like coprolites. In this paper, we provide a detailed morphological description of isolated lateral osteoderms from a multitaxic bonebed of the Tiki Formation. The osteoderms bear a spike-like eminence formed by two asymmetric flanges, radial ornamentation, and a thick, indented medial margin suggesting that these pertained to a pseudosuchian archosaur. With the purpose of demonstrating the internal anatomy, X-ray microCT was used to reveal a compact diploe structure with a highly vascularised core characterising most archosaurian osteoderms. Such a high degree of compactness with an internal vascular network and open vascular canals can be hypothesised to serve as protection from predation and for thermoregulation. A comparative study with different Late Triassic armoured archosauromorphs revealed morphological resemblance of the osteoderms to lateral osteoderms of desmatosuchine aetosaurs and the aetosauriform Acaenasuchus geoffreyi. A phylogenetic analysis comprising all the Late Triassic archosauromorphs placed the Tiki taxon within Aetosauriformes. However, an additional analysis including all aetosaurs positioned the new taxon deeply nested within Desmatosuchini as sister taxon to Desmatosuchus. Better understanding of the phylogenetic position can only be achieved by recovering more cranial and post-cranial materials. This contribution demonstrates the first occurrence of a Desmatosuchus-like taxon from the Tiki Formation and bolsters the significance of the horizon in global correlation and also contributes to our understanding of the dispersal of aetosaurs or similar taxa in different parts of Pangaea during the Late Triassic.