page_title: Onychodus
section_title: Paleobiology
hierarchical_section_title: Onychodus / Paleobiology
caption_attribution_description: English: Onychodus tooth with carbonized pulp in its pulp cavity; Milwaukee Formation; Berthelet Member; Milwaukee County, Wisconsin; MPM 477; collected by C.E. Monroe; photograph originally published in K.C. Gass, J. Kluessendorf, D.G. Mikulic, and C.E. Brett, 2019. Fossils of the Milwaukee Formation: A Diverse Middle Devonian Biota from Wisconsin, USA. Siri Scientific Press, Manchester, UK.
context_page_description: Onychodus is a genus of prehistoric lobe-finned fish which lived during the Devonian period. It is one of the best known of the group of onychodontiform fishes. Scattered fossil bones of Onychodus were first discovered in 1857, in North America, and described by John Strong Newberry. Other species were found in Australia, England, Norway and Germany showing that it had a widespread range.
  Onychodus was about 2 to 4 meters in length and was a pelagic animal. Like other onychodontiformes, it had a pair of tooth spirals bearing tusk-like teeth.
  The most well-preserved specimen of Onychodus has been found at the Gogo Formation of Western Australia giving palaeontologists more information about the structure of the fish. Other species of Onychodus are known only from poor material based on isolated tusks, teeth and scales.
context_section_description: Many features of onychodont anatomy are known only from Onychodus itself. As other onychodontiformes, it has a highly kinetic and flexible skull. This unusual characteristic was due to the loose attachment of the skull bones, which sometimes overlapped, and were connected only by soft tissue and cartilage. Even the braincase was only partially made of bone.
  Other distinctive traits are related to the tusk whorls on the lower jaw. Because they could rotate, this was a different method of jaw articulation which did not compare with primitive ray-finned fish. The lower jaw was entirely articulated with cartilage, without an intermediate structure between the opposite sides, allowing the separation of the bones when prey was struck. Moreover, the loose articulation caused lateral movement, making the tusk walls move out of alignment. The parts of the lower jaw would have rotated inwards upon closing in order for the tusk whorls to fit exactly into the hollow spaces in the upper jaw. It is suggested that a ligament attachment and retractor mechanism existed in a pit under the tusk whorl, a unique condition in vertebrates. The capacity of the teeth on the lower jaw to fit with the tooth rows in the upper jaw, upon closure, kept the tusk whorls in place.
caption: Onychodus tooth with carbonized pulp in its pulp cavity, from the Middle Devonian of Wisconsin
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