THE ECOMORPHOLOGICAL EVOLUTION OF LOBE-FINNED FISHES (SARCOPTERYGII) DURING THE PALEOZOIC

Extant lobe-finned fish (Sarcopterygii) include terrestrial tetrapods (~7,000 amphibians, ~5,000 mammals, ~20,000 reptiles) and aquatic sarcopterygian fish (six lungfish and two coelacanth species). This ratio of tetrapods to fish was reversed during one of the earliest and most important stages of lobe-fin evolution: the Devonian (418-359 MYA). More than 100 studies over the past decade have examined the diversification patterns of lobe-fins but these have focused primarily on later tetrapods or lungfishes. Thus, we know remarkably little about the mode and tempo of early sarcopterygian evolution despite an excellently preserved and abundant Devonian fossil record. Sarcopterygians might have undergone multiple adaptive radiations early in the Devonian in response to new resources and new dominance, late in the period in response in increased ecosystem complexity, cosmopolitanism or invasions of new areas, and after various biotic crises. In addition, previous research has shown that the Hangenberg mass extinction (unlike the preceding Kellwasser “Big Five” mass depletion) reshaped ecosystems such that terrestrial sarcopterygians became dominant while aquatic sarcopterygians declined, setting the pattern of modern lobe-fin biodiversity. However, patterns of selectivity and the mode and extent of ecological recovery have remained unclear. Here, we present results from new databases on over 400 Devonian sarcopterygian species, involving biogeographical, ecomorphological, phylogenetic, shape, and temporal data. This data is being used to test multiple hypotheses of Devonian vertebrate evolution. We are applying phylogenetic comparative methods (PCM) and geometric morphometrics to test major hypotheses: 1) the intervals before and after the Hangenberg mass extinction, the Kellwasser mass depletion and other intervals are adaptive radiations marked by ecological release, 2) diversification of cranial and post-cranial traits, which are related to resource-use and habitat preference, is not linked and follows a predictable temporal pattern, 3) the trajectory and ecology of tetrapods and their relatives is significantly different from other Devonian lobe-finned fishes, and 4) modes of evolution reconstructed from extant sarcopterygians apply to the early fossil record.