Exploring orodromine variation in the Two Medicine-Judith River-Belly River clastic wedge in Montana and Alberta

Abstract

Quantifying intraspecific variation within dinosaur assemblages is a quagmire that's vexed by limited samples and poor temporal constraints for many dinosaur taxa. This thesis attempts to explore variation within a large sample of early diverging ornithischian dinosaurs that spans across the Two Medicine-Judith River-Belly River clastic wedge in Montana, USA and Alberta, Canada. Orodromines were a group of small-bodied ornithischians that rapidly diversified in the Campanian of western North America and are disproportionally abundant within the Two Medicine Formation. Previous workers assigned all Two Medicine orodromine material to the monospecific genus Orodromeus makelai. This thesis reevaluates orodromine specimens within a newly updated chronostratigraphic framework. These new temporal constraints facilitate accurate comparisons with other orodromines elsewhere in the clastic wedge. Comparative morphological analyses reveal at least three previously unrecognized morphotypes within the Two Medicine-Judith River-Belly River clastic wedge with minimal temporal overlap. Data suggests these morphotypes are indicative of interspecific variation and do not support an intraspecific interpretation. This thesis proposes three new orodromine taxa and creates new systematic divisions between specimens previously considered to all be Orodromeus makelai.