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Title: The anatomy and evolution of "acanthodian" stem-chondrichthyans
Author: Dearden, Richard Peter
ISNI:       0000 0004 8499 4566
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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Acanthodians, an enigmatic grade of Palaeozoic fishes, have loomed large in theories about vertebrate evolution. In recent years evidence has steadily grown that acanthodians, once thought to be a group, are instead a paraphyletic grade comprising the very earliest chondrichthyans (sharks, rays, and elephant sharks). However, the poor preservational potential that once obscured the acanthodians' position in gnathostome phylogeny instead now blights the relationships of stemchondrichthyans to one another and to the chondrichthyan crown-group. This in turn conceals the history of the assembly of the chondrichthyan body plan. In this thesis I aim to address this by describing the anatomy of three acanthodians from the Devonian (419-359 Mya) of the United Kingdom - Ptomacanthus, Diplacanthus, and Vernicomacanthus. Computed microtomographic imaging of a fossil of Ptomacanthus reveals the skeleton of the pharynx, preserved articulated and in three dimensions. Synchrotron microtomographic imaging of a fossil of Diplacanthus reveals a crushed branchial, mandibular, and neurocranial skeleton. Comparison of the tissue forming this endoskeleton with new information on endochondral bone in an acanthothoracid placo- derm allows a reassessment of the evolution of endochondral tissues in gnathostomes. I also redescribe Vernicomacanthus on the basis of new material and histological study, and assess the comparative anatomy of acanthodian shoulder girdles. Finally, I carry out a reanalysis of stem-chondrichthyan phylogeny in the light of this information. This new information helps constrain changes in the chondrichthyan branchial and neurocranial skeleton to specific parts of the chondrichthyan stem-group. It also adds to a growing body of evidence that "acanthodians" form two distinct grades in the chondrichthyan stem-group: a rump grade at the base of the chondrichthyan stem-group, and a paraphyletic grade of "climatiid" taxa.
Supervisor: Brazeau, Martin Sponsor: European Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral