AN OSTEOHISTOLOGIC ANALYSIS OF GROWTH IN ORNITHOPODA

[摘要] In Iguanodontian dinosaurs, histologic ontogentic series of two basal taxa (Dysalotosaurus lettowvorbecki and Tenontosaurus tilletti) and one derived taxon (Maiasaura peeblesorum) are known. Two of the ontogenetic series of taxa sectioned for this study (Eolambia caroljonesa and Iguanacolossus forti) are phylogenetically situated between these basal and derived taxa. The third ontogenetic series of taxa sectioned for this study (Hypacrosaurus stebingeri) is a derived taxon, similar to Maiasaura; however, Hypacrosaurus is a lambeosaurine, while Maiasaura is a hadrosaurine. This sample provides a good opportunity to begin to understand how small, basal members of the iguanodontian clade evolved into enormous derived members. By comparing the histology of humeri with that of femora and tibiae in each series, we can begin to understand how bipediality in basal members of the clade evolved into the facultative bipediality of derived members.Eolambia exhibits growth similar to Dysalotosaurus and Tenontosaurus. Bone growth is 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. The first annual growth marks (AGMs) appear in small sized individuals; secondary osteons begin appearing in mid-sized individuals and increase in number with increasing age. Iguanacolossus and Hypacrosaurus exhibit growth more similar to Maiasaura. Bone growth is relatively fast (i.e., poorly organized tissue, large vascular canals) until specimens reach mid-size. The first AGMs appear in mid-sized individuals, and secondary osteons begin appearing soon thereafter and increase in number with increasing age.In all taxa under consideration in this study, growth is similar between forelimb and hind limb elements throughout the respective ontogenetic series. This indicates that even as juveniles, these taxa were likely facultative bipeds.

[发布机构] Johns Hopkins University

[效力级别] ontogeny

[关键词] Ornithopoda;ontogeny;histology;not listed