Styracopterid (Actinopterygii) ontogeny and the multiple origins of post-Hangenberg deep-bodied fishes

Abstract

The Carboniferous fish family Styracopteridae (Actinopterygii) originated as part of the initial radiation of ray-finned fishes following the end-Devonian Hangenberg extinction. Specimens of Styracopterus fulcratus (Traquair, 1890) have been collected from post-extinction Tournaisian and Visean Scottish sediments for over 100 years, including sites containing some of the earliest ‘Romer's Gap’ tetrapods. Re-examination of this supposedly long-lived, static species has revealed two genera, Styracopterus and Fouldenia White, 1927, divergent from each other and previous descriptions. Here, we show that styracopterids are among the earliest actinopterygians with durophagous dentition and toothplates, the latter likely to have derived from the ectopterygoids and coronoids. On the basis of this and other traits, such as the presence of an enameloid ‘beak’, the fusiform styracopterids are linked to some, but not all, the deep-bodied actinopterygians previously placed in the suborder Platysomoidei. A new plesion, Eurynotiformes, is erected to contain the styracopterids, the deeply fusiform Eurynotus and the widespread Amphicentrum, among other laterally compressed fishes. This implies that platysomoids are polyphyletic: deep-bodied and/or durophagous fishes evolved multiple times following the Hangenberg event. Reconstructed styracopterid growth series show that trunk depth increased during maturation, mirroring the shape variation observed among the Eurynotiformes. Other ontogenetic changes involve fin-ray differentiation, jaw form, dermal bone ornamentation, and scale morphology; all of these are widely used as actinopterygian diagnostic characters. Further investigation of Eurynotiformes should reveal the extent of evolutionary and ontogenetic change within the earliest actinopterygian radiation, and are likely to rewrite their phylogeny. © 2013 The Linnean Society of London