Skull roof anatomy of the Early Jurassic (Toarcian) acipenseriform †Gyrosteus mirabilis Woodward ex Agassiz, from Yorkshire, England, elucidates diversity of †Chondrosteidae

Abstract

An articulated and mostly complete skull roof of the enigmatic Early Jurassic chondrosteid fish, ^†Gyrosteus mirabilis Woodward ex Agassiz is described from the Whitby Mudstone Formation (Toarcian) of Whitby, Yorkshire. Known since the 1830s, isolated bones attributable to this taxon are frequently found along the Whitby coast, although articulated remains are rarely reported. The fragmentary nature of Gyrosteus mirabilis, the largest non-reptilian animal from the British Lower Jurassic, has led some researchers to propose it as a subjective synonym of ^†Chondrosteus Egerton ex Agassiz, from the Lower Lias of Dorset, and others to suggest synonymy with the similarly massive ^†Strongylosteus Jaekel from the Upper Lias (Toarcian) of Germany. Here, we compare the skull roof anatomies of ^†Gyrosteus mirabilis with ^†Chondrosteus acipenseroides and ^†Strongylosteus hindenburgi and present new criteria to differentiate between these taxa. We find the skull roofs of Chondrosteus and Strongylosteus to be vastly different with Gyrosteus representing something of an ‘intermediate’ morphology with closer affinities to Strongylosteus. The presence of a medial parietal and rostral bones is shared between Gyrosteus and Strongylosteus but these are absent in Chondrosteus. Our results support the taxonomic validity of both Gyrosteus mirabilis and Strongylosteus hindenburgi as monotypic genera distinct from Chondrosteus acipenseroides.