A new postcranium of Saurichthys from the Early Triassic of Spitsbergen

Abstract

The Saurichthyidae (Actinoperygii, Osteichthyes) are a long-lasting family, ranging from the latest Permian to the Middle Jurassic (~75 million years) and including nearly 50 species, most of which belong to the essentially Triassic genus Saurichthys. At least 14 species occur in the Early Triassic, but many of them are only scarcely known, leading to a gap of knowledge of their early evolution. Four species have been described from the Early Triassic of Spitsbergen (Svalbard archipelago, Arctic Norway): Saurichthys wimani, S. ornatus, S. elongatus and S. hamiltoni. These taxa are based predominantly on cranial material, whereas postcrania, which contain more diagnostic features in Saurichthys, are extremely rare. We present the eighth saurichthyid postcranium from Spitsbergen, which was collected during the 2008 Swiss-Norwegian expedition to Stensiofjellet (Mount Stensio), Sassendalen. The new specimen is the most complete saurichthyid postcranial segment known from Spitsbergen, preserving the portion between the pelvic girdle and the caudal fin, and is ascribed to S. wimani based on the squamation. S. wimani can now be characterized by the following set of postcranial traits: (1) all fins with segmented and branched lepidotrichia and fringing fulcra along their leading edge, (2) rows of large, ornamented scales in mid-dorsal, mid-ventral and mid-lateral position and small, rhombic dorsolateral scales (the ventrolateral scale row being probably absent), and (3) a vertebral column consisting of (dorsal) neural arches with small praeand postzygapophyses and distinct neural spines, relating as 2:1 to the mid-lateral scales, and to the (ventral) haemal arches as 2:1 in the abdominal and as 2:2 in the caudal body portion. Mid-lateral scales of the caudal peduncle bear denticles on their ventrocaudal margin, previously unknown in saurichthyids. At least three species of Saurichthys are distinguishable in the Smithian aged ‘fish horizon’ of Spitsbergen based on postcranial characters, supporting a fast diversification of saurichthyids following the great end-Permian mass extinction event.