Ontogeny in Iguanodontia

In Iguanodontians (“duck billed” dinosaurs) histologic ontogenetic series of two basal taxa ( Dysalotosaurus lettowvorbecki and Tenontosaurus tilletti ) and one derived taxa ( Maiasaura peeblesorum ) are well known. Eolambia caroljonesa is phylogenetically situated between these basal and derived taxa, although it is closer in body size to the more basal taxa. Given this, it provides a good opportunity to begin to be able to understand how body size evolved within the clade. Eolambia is known from the Cedar Mountain Formation (97 Mya). The Cifelli/Eolambia #2 Quarry is, with respect to Dinosauria, monospecific. Growth series of the femur (n=5), tibia (n=9), and humerus (n=8) were sectioned for this study. The smallest specimens of each element are approximately 30% the size of the largest specimens. Upon examination, the largest specimens were found to have fewer than six growth cycles, although the counts do vary between elements. The largest specimens, which are among the largest known for the species, lack an external fundamental system, suggesting that they had not yet reached skeletal maturity. Throughout the series, bone growth is shows signs of being relatively slow (i.e. well organized tissue, small vascular canals), although the youngest specimens do preserve areas of knitted texture which is indicative of rapid growth. Secondary osteons begin appearing in mid‐sized individuals and increase in number with increasing age. This is similar to what is seen in both Dysalotosaurus and Tenontosaurus but not the more derived taxa, Maiasaura . From this we can determine that changes in gross morphology precede changes in growth rate within the clade. This study was funded by the American Association of Anatomists' Visiting Scholars Grant.