Comparative taphonomy of two juvenile Eolambia caroljonesa (Hadrosauria) bonebeds from the Cedar Mountain Formation of Utah and their implications for juvenile hadrosaurian behavior

Abstract

The Field Museum of Natural History collected a possible juvenile herd assemblage of Eolambia caroljonesa comprised of bone elements at two sites in the Mussentuchit Member of the Cedar Mountain Formation of central Utah. Juvenile herds of dinosaurs have been documented previously but not much is known about their herd structure due to a lack of a complete growth series. Histology and comparisons to another nearby Utah Eolambia caroljonesa site (as well as across Hadrosauria generally) were used to assess individual age composition. Reduced Major Axis regressions and Ordinary Least Squares analyses were used to investigate the validity of hadrosaur limb size relations and size/age ranges in a large data set (one locality’s fossil assemblage of ~500 bones) in order to estimate missing limb lengths (inferencing the size of bones not found at the site relative to other associated bones present) among recovered E. caroljonesa specimens. Based on the findings of this study, the herd of Eolambia caroljonesa at the UT 130904-2 site represent a real biological herd that perished in a catastrophic event and were comprised of individuals spanning multiple, young sizes and ages. This age/size composition of the fossil assemblage supports the likely gregariousness and slower growth of young basal hadrosaurids compared to ones of the Late Cretaceous, providing insight into the evolution of hadrosaurs.