The phyllolepid placoderm Cowralepis mclachlani: insights into the evolution of feeding mechanisms in jawed vertebrates

Robert K Carr, Zerina Johanson, Alex Ritchie
Journal of Morphology 2009, 270 (7): 775-804
Remarkably preserved specimens of Cowralepis mclachlani Ritchie, 2005 (Proc Linn Soc NSW 126:215-259) (Phyllolepida, Placodermi) represent a unique ontogenetic sequence adding to our understanding of anatomy, function, and phylogeny among basal jawed vertebrates (gnathostomes). A systematic review demonstrates that the Phyllolepida are a subgroup of the Arthrodira. Consideration of visceral and neurocranial characters supports the hypothesis that placoderms are the sister group to remaining gnathostomes. Placoderms possess, as adult plesiomorphic features, a number of characters that are only seen in the development of extant gnathostomes-a peramorphic shift relative to placoderms. Developmental evidence in vertebrates leads to a revised polarity of character transitions. These include 1) hyomandibula-neurocranium and ventral parachordal-palatoquadrate articulations (vertebrate synapomorphies); 2) jointed pharynx, paired basibranchials, anterior ethmoidal-palatoquadrate articulation, short trabeculae cranii, and anterior and posterior neurocranial fissures (gnathostome synapomorphies); and 3) fused basibranchials, dorsal palatoquadrate-neurocranium articulation, loss of the anterior neurocranial fissure, elongated trabeculae cranii, and transfer of the ventral parachordal-palatoquadrate articulation to the trabeculae (crown group gnathostomes). The level of preservation in C. mclachlani provides the basis for a reinterpretation of phyllolepid anatomy and function. Cowralepis mclachlani possesses paired basibranchials allowing the reinterpretation of the visceral skeleton in other placoderms. Mandible depression in C. mclachlani follows an osteichthyan pattern and the ventral visceral skeleton acts as a functional unit. Evidence for hypobranchial musculature demonstrates the neural crest origin of the basibranchials and that Cowralepis was a suction feeder. Finally, the position of the visceral skeleton relative to the neurocranium in placoderms parallels the condition in selachians and osteichthyans, but differs in the elongation of the occiput. The cucullaris fossa of placoderms (interpreted as a site of muscle attachment) is shown to represent, in part, the parabranchial chamber.

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