A Multi-Evidence Approach to the Affinity of Tylodus deltoides Rohon 1893

Abstract:

Early gnathostome evolution has recently undergone revision due to newly published phylogenies. Within this new framework, early gnathostomes in the fossil record can be revised, particularly the subject of this paper Tylodus deltoides (Rohon, 1893), vertebrate microremains from the Silurian Ohesaare Formation of Estonia. Multiple analytical approaches are performed in order to offer insight into a phylogenetic position for the enigmatic T. deltoides. A three-dimensional model, based on synchrotron x-ray phase contrast microtomography of a small polyodontode morphotype and virtual thin sections show a unique palaeohistology. A survey of wear patterns shows the presence of tooth plates, comprising the larger cusp and plate morphotypes. Histological observations show a mosaic of vertebrate hard-tissues and organisations, including a tripartitie layered structure, descending rows of odontodes on a large primary odontode, an osteodentine or mesodentine base, pleromic dentine middle layer and capping layer of unknown tissue type.Two interpretations of this collection of observations are given. Both assume the large morphotypes are teeth, one that the smaller polyodontodes are scales, the other, that they are developing teeth. Histology alone demonstrates the remains are the teeth of a holocephalan, but with some uncertainty. A model for attachment by acrodonty and developmental model for conserved lyodont development are given to help explain the unusual histology and morphology of T. deltoides, but greater certainty in the phylogenetic position of T. deltoides requires a more detailed histological analysis based on thin sections and a broader comparison with 3D data of relevent taxa. This willnecessarily be improved by formal phylogenetic analysis.