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Presenting Author(s) Michael Coates
Abstract Title Limbs, fins, and homoplasy.
Full author List Michael Coates
Text of abstract

The recently reported humerus from the Late Devonian of Pennsylvania adds to an increasingly detailed picture of the evolutionary transformation from fish fins to tetrapod limbs. Although isolated, this primitive humerus is 'character-rich' and occupies a stable position in comparative analyses (fragmentary data are not necessarily noisy when used in phylogeny reconstruction). The form of this bone highlights conservatism in humeral architecture throughout large parts of the tetrapod 'stem-group' membership. It also indicates hitherto unknown aspects of soft-tissue remodelling in paired appendages at, or close to, the point of fin-ray loss and digit origin. Muscles on dorsal and ventral humeral surfaces become increasingly compartmentalised, with greater elaboration distally (evolving modularity?) as well as conforming to an overall change in the 'neutral' orientation of the forelimb relative to the main, rostro-caudal, body-axis.

These new data suggest a plausible alternative role for early forelimbs, between the steering and braking function of paired fins in fishes and walking gaits in tetrapods.

Pattern and function-related changes in tetrapod appendages parallel specialisations (out-turning, basal radial elaboration) seen elsewhere in fish paired fins; notably among Recent orectolobiform sharks. Although tangential to the first part of this talk, instances of convergence (homoplasy) between the fins of fossil and Recent chondrichthyans provide an informative perspective on the fin-to-limb transition. Early and independent instances of fin morphologies otherwise associated with modern taxa provide some basis for speculation about the evolutionary antiquity of patterning mechanisms in extant taxa. The key issue here is the extent to which we are able to rely on morphology as a 'phenotypic proxy', indicating the recruitment of similar or identical developmental genetic phenomena.
Which session is your work most relevant to: Limb evolution