CONTINUING CONTROVERSIES OR CONFIRMATION THAT CONODONTS ARE VERTEBRATES? RE-EVALUATION OF THE EVIDENCE

Perhaps more than any other group of organisms, conodonts exemplify the difficulties inherent in interpreting incomplete fossil remains. For much of the century-and-a-half since their phosphatic skeletal elements were first described, there was little evidence and little consensus regarding many aspects of their anatomy, phylogenetic affinity, and function. After this prolonged period of almost unconstrained speculation, the discovery in the 1980’s and 1990’s of a series of fossils preserving non-biomineralised parts of the conodont body finally provided the key to establishing what kind of organism conodont elements came from. These remains, from Scotland and South Africa, are exceptionally well preserved and include authigenically phosphatized tissues, yet they also exhibit clear signs of decay and loss of anatomical information. These crucial specimens provide good evidence for the shape and orientation of the conodont body, and the presence of trunk musculature, interior rods and/or tubes, caudal fins and anterior sensory structures. Taken together with their skeletonized elements, the suite of soft-tissue characters evident from these remains indicates that conodonts were jawless vertebrates, broadly comparable to extant hagfish and lampreys. Until recently there was little published scientific disagreement with this hypothesis, but a few recent papers have questioned it, and this has prompted us to reconsider all the available evidence in the light of current understanding of soft tissue taphonomy, relationships among vertebrates and their invertebrate relatives, and alternative approaches to phylogenetic analysis. Our results are clear: only by selectively excluding characters, failing to take anatomical decay into account, and by adopting narrow, character-based definitions of higher taxa (rather than basing assessments of affinity on hypotheses of relationships between clades) can conodonts be excluded from the vertebrates.