John R Horner

A well-preserved skull from a previously unknown growth stage for the lambeosaurine hadrosaurid Hypacrosaurus stebingeri from the Campanian of western North America is described. This skull is equivalent in size to the smallest known growth stages for Corythosaurus, Hypacrosaurus altispinus and Lambeosaurus, and allows for a direct comparison of the juvenile growth stage of all four taxa for the first

Predator confrontation or predator evasion frequently produces bone fractures in potential prey in the wild. Although there are reports of healed bone injuries and pathologies in non-avian dinosaurs, no previously published instances of biomechanically adaptive bone modeling exist. Two tibiae from an ontogenetic sample of fifty specimens of the herbivorous dinosaur Maiasaura peeblesorum (Ornithopoda: Hadrosaurinae) exhibit exostoses. We show that these outgrowths are cases of biomechanically adaptive periosteal bone modeling resulting from overstrain on the tibia after a fibula fracture. Histological and biomechanical results are congruent with predictions derived from this hypothesis. Histologically, the outgrowths are constituted by radial fibrolamellar periosteal bone tissue formed at very high growth rates, as expected in a process of rapid strain equilibration response. These outgrowths show greater compactness at the periphery, where tensile and compressive biomechanical constraints are higher. Moreover, these outgrowths increase the maximum bending strength in the direction of the stresses derived from locomotion. They are located on the antero-lateral side of the tibia, as expected in a presumably bipedal one year old individual, and in the posterior position of the tibia, as expected in a presumably quadrupedal individual at least four years of age. These results reinforce myological evidence suggesting that Maiasaura underwent an ontogenetic shift from the primitive ornithischian bipedal condition when young to a derived quadrupedal posture when older.

ABSTRACT The evolution of scutes in thyreophoran dinosaurs, based on Scutellosaurus, Scelidosaurus, Stegosaurus, and several ankylosaurs, began with small rounded or ovoid structures that typically had slight, anteroposteriorly oriented keels. These scutes were elaborated in two general and overlapping ways: they could flare laterally and asymmetrically beneath the keels that mark the anteroposterior axis, and they could be hypertrophied in their distal growth to produce plates, spikes, and other kinds of ornamentation. Stegosaurus plates and spikes are thus primarily hypertrophied keels of primitive thyreophoran scutes, sometimes with elaboration of dermal bone around their pustulate bases. Histologically, most thyreophoran scute tissues comprise secondary trabecular medullary bone that is sandwiched between layers of compact primary bone. Some scutes partly or mostly comprise anatomically metaplastic bone, that is, ossified fibrous tissue that shows incremental growth. The "plumbing" of Stegosaurus plates was not apparently built to support a "radiator system of internal blood vessels that communicated with the outside of the plates and coursed along their external surfaces to return heated or cooled blood to the body core. Possibly a purely external system supported this function but there is no independent evidence for it. On the other hand, many of the vascular features in stegosaurian plates and spikes reflect bautechnisches artifacts of growth and production of bone. Surface vascular features likely supported bone growth and remodeling, as well as the blood supply to a keratinous covering. When the gross and microstructural features of the plates and spikes are viewed in phylogenetic context, no clear pattern of thermoregulatory function emerges, though an accessory role cannot be eliminated in certain individual species. It seems more likely, as in other groups of dinosaurs, that the variation of dermal armor form in stegosaurs was primarily linked to species individuation and recognition, perhaps secondarily to interand intraspecific display, and rarely to facultative thermoregulation.

... A second, lower nesting horizon, at the Willow Creek Anti-cline has yielded the nests of juvenile hadrosaurs (Horner and Makela, 1979; Horner, 1982), and a few egg ... 1997. Couvée, oeufs et embryons d'un Dino-saure Théropode du Jurassique supérieur de Lourinha (Portugal ...

ABSTRACT The oldest well-known hadrosaurid fauna (from Iren Dabasu, People's Republic of China) includes undescribed cranial material, primarily the prefrontals, which indicates the presence of a true lambeosaurine that bore a hollow supracranial crest. The lambeosaurine taxon Bactrosaurus johnsoni Gilmore (1933) is redefined based on this material, while the hadrosaurine skull elements formerly attributed to this form are now assigned to Gilmoreosaurus mongoliensis.

ABSTRACT An external fundamental system (EFS) is a form of bone microstructure present in the outermost cortex of long bones in animals that have attained skeletal maturity. It indicates an effective cessation of any significant periosteal growth (i.e., growth in circumference or girth). Although an EFS has been noted in several reptile taxa, the idea that reptiles grow continually throughout their lives remains popular. Examination of femoral bone microstructure from captive American Alligators (Alligator mississippiensis) reveals parallel-fibered tissue terminating periosteally in an EFS, thus confirming determinate growth in another reptile taxon. The results of this study have several important implications for both modern and fossil tetrapods: first, because many birds, nonavian dinosaurs, pterosaurs, and basal pseudosuchians all produce an EFS, it can be concluded that determinate growth is a shared characteristic of Archosauria; second, because the captive alligators were not senescent, an EFS should not be associated with “old age” when interpreting growth histories of extinct animals; third, if no EFS is present, this should not immediately suggest indeterminate growth but rather that skeletal maturity was not attained prior to death. In addition, this study highlights the need for more osteohistological studies to establish exactly how widespread determinate growth is within both extinct and extant members of Sauropsida, because this form of growth may be the rule rather than the exception.