DIVERSIFICATION OF JAW GEOMETRY DURING THE INITIAL RADIATION OF LOBE-FINNED FISHES (OSTEICHTHYES: SARCOPTERYGII)

The origin of crown sarcopterygians around 420 Ma proceeds the emergence of major lineage specializations, including contrasts relating to morphology of the feeding apparatus. Key examples include dental plates in lungfishes, reduction of marginal jaw bones in coelacanths, and elaboration of fangs in a variety of porolepiforms and tetrapodomorphs. Such putative ecological divergence, alongside evidence that individual lineages show high rates of evolutionary change early in their history, mark this event as a possible episode of adaptive radiation. Here, we explicitly test this hypothesis using mandibles as a taphonomically robust, taxonomically diagnostic trait intimately associated with ecology. We assembled a dataset of 30 three-dimensionally preserved jaws of total-group Sarcopterygii obtained by CT scanning or photogrammetry. Most are dipnomorphs or tetrapodomorph fishes, plus a smaller sample of stem sarcopterygians and taxa of less certain placement. We developed a basic landmarking scheme (6 fixed landmarks, 8 curves with sliding semilandmarks) capturing overall jaw shape and orientation, including aspects of the glenoid and adductor fossa. We paired these data with a composite phylogenetic tree with branch durations informed by the ages of fossil tips. We examined the fit of three basic models of trait evolution to these shape data in a multivariate framework: Brownian motion (BM; diffusive evolution at a constant rate), early burst (EB; diffusive evolution with a declining rate over time, corresponding to theoretical predictions for adaptive radiation), and Ornstein-Uhlenbeck (OU; constant rates of change with a central tendency limiting the accumulation of variation over time). Our results indicate that EB is the best-supported model for jaw shape evolution, corroborating our hypothesis. This implies that high initial rates of phenotypic evolution complemented the extensive exploration of shape space early in the history of lobe-finned fishes.