Endocast and Bony Labyrinth of a Stem Gnathostome Shed Light on the Earliest Diversification of Jawed Vertebrates

Our understanding of the earliest evolution of jawed vertebrates depends on a credible phylogenetic framework for the jawed stem gnathostomes collectively known as ‘placoderms’. However, their relationships, and even whether ‘placoderms’ represent a single radiation or a paraphyletic array, remain contentious. Here we describe the endocranial cavity and bony labyrinth of Brindabellaspis stensioi, commonly recovered as a taxon of uncertain affinity branching near the base of ‘placoderms’. While some features of its braincase and endocast resemble those of jawless vertebrates, its inner ear displays a repertoire of crown gnathostome characters. Both parsimony and Bayesian analyses suggest that prevailing hypotheses of ‘placoderm’ relationships are unstable, with newly-revealed anatomy pointing to a radical revision of early gnathostome evolution. Our results call into question the appropriateness of arthrodire-like ‘placoderms’ as models of primitive gnathostome anatomy and raise questions of homology relating to key cranial features.