An early chondrichthyan and the evolutionary assembly of a shark body plan

Research output: Contribution to journalArticlepeer-review

Authors

  • Michael Coates
  • John Finarelli
  • Katherine Criswell
  • Kristen Tietjen
  • Mark Rivers
  • Patrick La Riviere

Colleges, School and Institutes

External organisations

  • University of Cambridge
  • University of Chicago
  • University College Dublin

Abstract

Although relationships among the major groups of living gnathostomes are well established, the relatedness of early jawed vertebrates to modern clades is intensely debated. Here, we provide a new description of Gladbachus, a Middle Devonian (Givetian ~385-million-year-old) stem chondrichthyan from Germany, and one of the very few early chondrichthyans in which substantial portions of the endoskeleton are preserved. Tomographic and histological techniques reveal new details of the gill skeleton, hyoid arch and jaws, neurocranium, cartilage, scales and teeth. Despite many features resembling placoderm or osteichthyan conditions, phylogenetic analysis confirms Gladbachus as a stem chondrichthyan and corroborates hypotheses that all acanthodians are stem chondrichthyans. The unfamiliar character combination displayed by Gladbachus, alongside conditions observed in acanthodians, implies that pre-Devonian stem-chondrichthyans are severely under-sampled and strongly supports indications from isolated scales that the gnathostome crown group originated at the latest by the early Silurian (~440 mya). Moreover, phylogenetic results highlight the likely convergent evolution of conventional chondrichthyan conditions among earliest members of this primary gnathostome division, while skeletal morphology points towards the likely suspension feeding habits of Gladbachus, suggesting a functional origin of the gill slit condition characteristic of the vast majority of living and fossil chondrichthyans.

Details

Original languageEnglish
Article number20172418
JournalRoyal Society of London. Proceedings B. Biological Sciences
Volume285
Issue number1870
Early online date3 Jan 2018
Publication statusPublished - 10 Jan 2018

Keywords

  • chondrichthyes , gill skeleton , scales , Middle Devonian , gnathostome phylogeny, computed tomography