A NEW SILESAURID DINOSAURIFORM FROM THE UPPER TRIASSIC CHINLE FORMATION OF NORTHERN COLORADO: INSIGHTS INTO THE EARLY EVOLUTION OF DINOSAURIFORM HERBIVORY

The Triassic Period saw the re-establishment of terrestrial vertebrate ecosystems across Pangea, in which non-dinosaurian dinosauriforms evolved in tandem with early herbivorous dinosaurs. While the Upper Triassic Chinle Formation and Dockum Group of the Colorado Plateau and Texas contain one of the best-studied Late Triassic terrestrial vertebrate faunas in the world, the Eagle Basin of northern Colorado is a much less studied ecosystem, separated from the southern Chinle-Dockum depocenter by the Ancestral Rocky Mountains, that contains the northernmost Triassic dinosauromorph fauna in North America. Localities in the Eagle Basin have produced the small lagerpetid dinosauromorph Dromeron romeri, the new silesaurid dinosauriform Kwanasaurus williamparkeri, and a coelophysoid theropod indistinguishable from Coelophysis bauri. This dinosauromorph fauna probably falls within the Revueltian or Apachean estimated biochronozone (Norian or Rhaetian, 215-202 Ma), making the lagerpetids and silesaurids among the youngest known globally. Our phylogenetic analysis of silesaurids is broadly consistent with previous analysis, but clarifies patterns of silesaurid diversification. The earliest silesaurids in Gondwana were probably faunivorous with ziphodont teeth, but developed critical modifications in the mandible and dentition through the Middle and Late Triassic indicating a dietary shift. Forms with conical teeth and an edentulous beak on the dentary that first appeared in the Middle Triassic (e.g. Asilisaurus) may have been developing a more omnivorous diet, and most members of the derived Late Triassic clade Sulcimentisauria (including Silesaurus, Sacisaurus, Diodorus, Eucoelophysis, and Kwanasaurus) developed broad and coarsely denticulate folidont teeth suggesting a stronger reliance on herbivory. These dietary adaptations seem to coincide with a radiation of sulcimentisaurians across both Gondwana and Laurasia. Indeed, Kwanasaurus, one of the latest known silesaurids, has a remarkably robust maxilla and dentary that may have evolved to cope with tough and fibrous vegetation. These trends in silesaurid dental evolution parallel those seen in herbivorous dinosaurs, and suggest that silesaurids were the most widespread and successful small herbivores across Pangea during the Late Triassic.