TOOTH CYCLE AND REPLACEMENT PATTERNS IN ICHTHYORNIS (AVES, ORNITHUROMORPHA)

Ichthyornis is a stem, toothed Mesozoic bird known since the 19th century. As an ornithurine, a clade that includes modern birds, Ichthyornis stands at an important phylogenetic position to understand the dentition evolution of Aves. Former studies have inferred a labial migration of the developing tooth during the process of dental replacement of Ichthyornis through 2D X-ray and synchrotron imaging data. However, due to limitations on the data and fossil preservation, we still lack knowledge of the dental development process during the tooth cycle and the replacement pattern throughout the jaw. Here we present micro-CT data of two Ichthyornis specimens (FHM 2503 and AMNH FARB 32773) that shed new light to the tooth cycle and replacement patterns of this toothed bird. The CT images show that the neighboring tooth positions are at different stages of the tooth cycle. Replacement teeth developing in the alveoli show an alternating replacement pattern along the jaw. These specimens also preserved teeth from early to late developmental stages, which not only show a labial migration of the new teeth, but also present a resorption pattern during development that differs from non-avian theropods (i.e., Coelophysis, Allosaurus, and Gorgosaurus) and crocodilians. In addition, the positions of the early-stage teeth vary from mid- to deep alveoli, which may indicate a transitory position of the dental lamina. The shape of the developing teeth in the alveoli also suggest a volatile morphogenetic process that can modify the morphology of the functional tooth through the regeneration process of tooth cycles. These new findings suggest that a generally conserved genetic regulatory mechanism underlies reptilian (including birds) tooth cycles and replacement patterns with modifications in separate lineages.