Davis, Samuel Patrick; (2002) Comparative anatomy and relationships of the acanthodian fishes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The acanthodian fishes constitute the earliest known group of vertebrate gnathostomes. Systematic studies started in the early 19th century, however until now, there have been no coherent hypotheses of inter- and intrarelationships. The comparative anatomy of the members comprising the orders: Climatiiformes, Diplacanthiformes, and Acanthodiformes is examined and described in detail, many aspects for the first time. These data were used to formulate character descriptions and statements for computerised phylogenetic analysis. Due to specimen preservation, character formulation focused mainly on the braincase, mandibular and hyoid arches, scapulocoracoid, opercular exoskeleton, and fin-spines. Cladistic analyses to determine the phylogenetic patterns of 28+ acanthodian genera was performed using 113 multistate and binary characters with multiple outgroup comparison. At present, the acanthodians are considered to be a monophyletic group supported by a single, completely-ribbed spine preceding the anal fin, and possibly, the presence of a pectoral spine which is not surrounded completely at the base by dermal bone. Data from the phylogenetic analysis places the Acanthodii into five major clades instead of the three historically-accepted ordinal groups, thus a new nodal classification for the class Acanthodii is hereby erected. Although not fully resolved at this time, it appears that the acanthodians may be considered as the putative sister group to the Osteichthyes based primarily on the shared braincase characters of Acanthodes. This relationship hypothesis is far from definitive and requires further congruence testing with different outgroup configurations. Re-evaluation of the most problematic acanthodian order: the Climatiiformes, is further resolved by examining and re-describing type and new specimens of selected informative taxa such as Brachyacanthus and Vernicomacanthus. Based on mostly complete fossils from the MOTH Formation, Canada, a comprehensive re-examination of the problematic diplcanthid Gladiohranchus probaton and MORS Diplacanthus crassissimus was conducted. With the advent of specialised silicone casting materials and techniques, important new information from the braincase of Acanthodes bronni can now be revealed. For the first time many novel features, e.g. dorsal ossification, orbit, and features of the otic region, can be more accurately mapped and described.

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