THE KAIPAROWITS FORMATION OF UTAH: PROVIDING NEW INSIGHT INTO THE PALEOBIOLOGY AND PALEOECOLOGY OF NORTH AMERICAN BASAL NEORNITHISCHIANS

The Kaiparowits Formation of southeastern Utah is providing important insights into basal neornithischian dinosaur evolution within the Western Interior Basin of North America during the Late Cretaceous. Recently recovered specimens provide a wealth of morphological information, facilitating the recognition of a previously undescribed taxon based on three autapomorphic traits present in the skull and forelimb. A phylogenetic analysis places the Kaiparowits taxon in a clade with the North American taxa Orodromeus, Zephyrosaurus, and Oryctodromeus based on the unambiguous presence of two synapomorphies. This new taxon represents the southernmost occurrence of a basal neornithischian during the Late Cretaceous in North America and provides important insights into morphological evolution within this clade, particularly with regards to those traits previously proposed to be linked to fossorial habits.

Material referred to this taxon spans a wide range of sizes (femoral circumference: 31.5 - 77 mm), with smaller material being more common than larger material. At one locality (UMNHVP 12665) material from at least three small individuals is present within a fine sand to silt dominated crevasse splay deposit, as indicated by the presence of three differently sized left femora. Feeding traces are present on two elements recovered from this site: a proximal left scapula and a distal left femur. A tooth crown fragment is embedded within a circular puncture in the anterior surface of the distal femur, the cross-sectional shape of which is consistent with a crocodyliform trace-maker. Teeth are rarely found still embedded in their corresponding feeding traces, and this marks the first time that such tangible evidence linking a crocodyliform to a feeding event has been found. On the lateral surface of the proximal scapula an ovoid bite mark is present that is divided in half about its long axis by a thin subscore that extends slightly beyond the margin of the bite mark. This bisected pit is diagnostic of a crocodyliform trace-maker based on actualistic experiments examining the feeding traces of extant taxa. These bite marks provide fascinating insight into the trophic interactions between the small crocodyliforms and dinosaurians of the Kaiparowits Formation.