Observations on the osteology of scutellosaurus lawleri Colbert, 1981 (Ornithischia: Thyreophora) on the basis on new specimens from the Lower Jurassic Kayenta Formation of Arizona

Scutellosaurus lawleri is a basal thyreophoran dinosaur from the Lower Jurassic Kayenta Formation, and it is the most abundantly known ornithischian dinosaur from the Lower Jurassic of North America. Despite its abundance in the fossil record, certain aspects of its anatomy have remained poorly understood due to the incomplete nature of the holotype, paratype, and referred specimens. I report here nearly 30 new specimens of Scutellosaurus lawleri collected along the Adeii Eechii Cliffs of northern Arizona between 1997 and 2000 by field parties from The University of Texas at Austin. Among this new material are two disarticulated associated skeletons, each preserving anatomy that is poorly known or not previously reported for the taxon, including the nasal, maxilla, lacrimal, postorbital, quadratojugal, squamosal, opisthotic, scapula, and ilium. These specimens have both been compressed taphonomically, making their removal from the surrounding matrix in their field jackets difficult without risk of damage to the fossil bone, so the specimens were CT scanned to aid with preparation. A phylogenetic analysis supports the position of Scutellosaurus lawleri as the basalmost member of Thyreophora. New autapomorphies identified include six premaxillary teeth, narrow and elongate frontals, a humerus substantially longer than the scapula, and neural spines of the proximal caudal vertebrae that are greater than 50% taller than the centra.