A New Species of Docodon (Mammaliaformes: Docodonta) from the Upper Jurassic Morrison Formation and a Reassessment of Selected Craniodental Characters in Basal Mammaliaforms

摘要

We describe a new species of the basal mammaliaform Docodon, D. apoxys, sp. nov., represented by three nearly complete dentaries from the Upper Jurassic Morrison Formation of central Colorado. This species differs from other known species of Docodon in the relative heights of the principal molar cusps and in the size of the distalmolars. In addition, we attribute a partial rostrum preserving much of the maxilla and most of the premaxilla, bearing a complete incisor and premolar count, to Docodon sp. These materials supply previously unknown morphology for this classic North American Jurassic taxon, and help establish a common morphology for Docodonta as a whole. While Docodon differs from the Portuguese docodont Haldanodon in the presence of only five upper incisors (none of which is entirely within the maxilla), an internarial bar is present in both taxa. The new specimens also unambiguously preserve a complete and undistorted angular process that differs conspicuously from the classical interpretation of the holotype of Docodon victor; this structure in docodonts closely resembles the angular process in australosphenidans and trechnotheres. We abandon the concept of a "pseudangular" process and we consider the angular process to be a homologous feature (where present) across Mammaliaformes.