SCANNELLA, J.B. & FOWLER, D.W. (2009) Anagenesis in Triceratops: evidence from a newly resolved stratigraphic framework for the Hell Creek Formation, North American Paleontological Convention, abstracts volume: 130

ABSTRACT

The dinosaur fauna of the Hell Creek Formation presents the clearest view of dinosaur diversity and evolutionary trends just prior to the terminal Cretaceous extinction event. However, our understanding has been hindered by poor internal stratigraphic resolution of this unit. We have used a newly resolved stratigraphic framework for the Hell Creek Formation to study Triceratops cranial variation, placing variant morphologies into temporal context at the end of the Cretaceous Period. Triceratops is the most commonly recovered dinosaur in the formation and though it has long been the subject of cranial studies, the lack of reliable stratigraphic data has obscured evolutionary trends within the genus. A temporal context for specimens reveals an increase in the length of the nasal horn core over time as well as trends in the shape of the post orbital horn cores, elongation of the rostrum, and closure of the frontal fontanelle. These features have all previously been considered diagnostic of the two currently recognized Triceratops species - Triceratops horridus and Triceratops prorsus. There is no stratigraphic overlap of these species; rather the new temporal framework suggests that the morphologies which define them fall within an anagenetic lineage of Triceratops.