The three-dimensionally articulated oral apparatus of a Devonian heterostracan sheds light on feeding in Palaeozoic jawless fishes

Richard P. Dearden*, Andy S. Jones, Sam Giles, Agnese Lanzetti, Madleen Grohganz, Zerina Johanson, Stephan Lautenschlager, Emma Randle, Philip C. J. Donoghue, Ivan Sansom

*Corresponding author for this work

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Abstract

Attempts to explain the origin and diversification of vertebrates have commonly invoked the evolution of feeding ecology, contrasting the passive suspension feeding of invertebrate chordates and larval lampreys with active predation in living jawed vertebrates. Of the extinct jawless vertebrates that phylogenetically intercalate these living groups, the feeding apparatus is well-preserved only in the early diverging stem-gnathostome heterostracans. However, its anatomy remains poorly understood. Here, we use X-ray microtomography to characterize the feeding apparatus of the pteraspid heterostracan Rhinopteraspis dunensis (Roemer, 1855). The apparatus is composed of 13 plates arranged approximately bilaterally, most of which articulate from the postoral plate. Our reconstruction shows that the oral plates were capable of rotating around the transverse axis, but likely with limited movement. It also suggests the nasohypophyseal organs opened internally, into the pharynx. The functional morphology of the apparatus in Rhinopteraspis precludes all proposed interpretations of feeding except for suspension/deposit feeding and we interpret the apparatus as having served primarily to moderate the oral gape. This is consistent with evidence that at least some early jawless gnathostomes were suspension feeders and runs contrary to macroecological scenarios that envisage early vertebrate evolution as characterized by a directional trend towards increasingly active food acquisition.
Original languageEnglish
Article number20232258
Number of pages12
JournalProceedings of the Royal Society B: Biological Sciences
Volume291
Issue number2019
DOIs
Publication statusPublished - 27 Mar 2024

Bibliographical note

Funding:
This work was supported by the Leverhulme Trust (RPG-2021-271 ‘Feeding without jaws: innovations in early vertebrates'). R.P.D. is currently supported by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 101062426 (DEADSharks). S.G. was also supported by a Royal Society Dorothy Hodgkin Research Fellowship (DH160098). M.G. is funded by the Natural Environment Research Council (GW4+ DTP studentship NE/S007504/1) while P.C.J.D. is funded by the Leverhulme Trust (RF-2022-167) and the John Templeton Foundation (JTF 62220; JTF 62574).

Keywords

  • heterostracan
  • pteraspid
  • feeding
  • Devonian
  • Palaeozoic
  • ostracoderma

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