A well-preserved ‘placoderm’ (stem-group Gnathostomata) upper jaw from the Early Devonian of Mongolia clarifies jaw evolution

Author:

Brazeau Martin D.12ORCID,Yuan Haobo1,Giles Sam23ORCID,Jerve Anna L.1,Zorig E.4,Ariunchimeg Ya.5,Sansom Robert S.6ORCID,Atwood Robert C.7

Affiliation:

1. Department of Life Sciences, Imperial College London, Silwood Park Campus, Ascot SL5 7PY, UK

2. The Natural History Museum, London SW7 5BD, UK

3. School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK

4. Institute of Paleontology, Mongolian Academy of Sciences, Ulaanbaatar 15160, Mongolia

5. The Natural History Museum, Ulaanbaatar 1420, Mongolia

6. Department of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PT, UK

7. Diamond Light Source, Didcot OX11 0DE, UK

Abstract

The origin of jaws and teeth remains contentious in vertebrate evolution. ‘Placoderms’ (Silurian-Devonian armoured jawed fishes) are central to debates on the origins of these anatomical structures. ‘Acanthothoracids’ are generally considered the most primitive ‘placoderms’. However, they are so far known mainly from disarticulated skeletal elements that are typically incomplete. The structure of the jaws—particularly the jaw hinge—is poorly known, leaving open questions about their jaw function and comparison with other placoderms and modern gnathostomes. Here we describe a near-complete ‘acanthothoracid’ upper jaw, allowing us to reconstruct the likely orientation and angle of the bite and compare its morphology with that of other known ‘placoderm’ groups. We clarify that the bite position is located on the upper jaw cartilage rather than on the dermal cheek and thus show that there is a highly conserved bite morphology among most groups of ‘placoderms’, regardless of their overall cranial geometry. Incorporation of the dermal skeleton appears to provide a sound biomechanical basis for jaw origins. It appears that ‘acanthothoracid’ dentitions were fundamentally similar in location to that of arthrodire ‘placoderms’, rather than resembling bony fishes. Irrespective of current phylogenetic uncertainty, the new data here resolve the likely general condition for ‘placoderms’ as a whole, and as such, ancestral morphology of known jawed vertebrates.

Funder

Imperial College London

Royal Society

National Geographic Society

Smålands Nation, Uppsala University

University of Manchester

FP7 Ideas: European Research Council

Publisher

The Royal Society

Subject

Multidisciplinary

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