Endocast and bony labyrinth of a Devonian ‘placoderm’ challenges stem gnathostome phylogeny

You-an Zhu, Sam Giles, Gavin Young, Yuzhi Hu, Mohamad Bazzi, Per Ahlberg, Min Zhu, Jing Lu

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Abstract

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.
Original languageEnglish
JournalCurrent Biology
DOIs
Publication statusPublished - 25 Jan 2021

Keywords

  • early vertebrates
  • Jawed Vertebrates
  • crown-group gnathostomes
  • phylogenetic analysis
  • placoderms
  • devonian fish
  • evolution of brain
  • evolution of inner ear
  • evolution of endolymphatic system

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