Abstract.-Fossil bone microanalyses reveal the ontogenetic histories of extinct tetrapods, but incomplete fossil records often result in small sample sets lacking statistical strength. In contrast, a histological sample of 50 tibiae of the hadrosaurid dinosaur Maiasaura peeblesorum allows predictions of annual growth and ecological interpretations based on more histologic data than any previous large sample study. Tibia length correlates well (R 2 > 0.9) with diaphyseal circumference, cortical area, and bone wall thickness, thereby allowing longitudinal predictions of annual body size increases based on growth mark circumference measurements. With an avian level apposition rate of 86.4 μm/day, Maiasaura achieved over half of asymptotic tibia diaphyseal circumference within its first year. Mortality rate for the first year was 89.9% but a seven year period of peak performance followed, when survivorship (mean mortality rate = 12.7%) was highest. During the third year of life, Maiasaura attained 36% (x = 1260 kg) of asymptotic body mass, growth rate was decelerating (18.2 μm/day), cortical vascular orientation changed, and mortality rate briefly increased. These transitions may indicate onset of sexual maturity and corresponding reallocation of resources to reproduction. Skeletal maturity and senescence occurred after 8 years, at which point the mean mortality rate increased to 44.4%. Compared with Alligator, an extant relative, Maiasaura exhibits rapid cortical increase early in ontogeny, while Alligator cortical growth is much lower and protracted throughout ontogeny. Our life history synthesis of Maiasaura utilizes the largest histological sample size for any extinct tetrapod species thus far, demonstrating how large sample microanalyses strengthen paleobiological interpretations.Holly N. Woodward*. Museum of the Rockies,

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Bone microanalyses of extant vertebrates provide a necessary framework from which to form hypotheses regarding the growth and skeletochronology of extinct taxa. Here, we describe the bone microstructure and quantify the histovariability of appendicular elements and osteoderms from three juvenile American alligators (Alligator mississippiensis) to assess growth mark and tissue organization within and amongst individuals, with the intention of validating paleohistological interpretations. Results confirm previous observations that lamellar and parallel fibered tissue organization are typical of crocodylians, and also that crocodylians are capable of forming woven tissue for brief periods. Tissue organization and growth mark count varies across individual skeletal elements and reveal that the femur, tibia, and humerus had the highest annual apposition rates in each individual. Cyclical growth mark count also varies intraskeletally, but data suggest these inconsistencies are due to differing medullary cavity expansion rates. There was no appreciable difference in either diaphyseal circumference or cyclical growth mark circumferences between left and right element pairs from an individual if diaphyses were sampled from roughly the same location. The considerable intraskeletal data obtained here provide validation for long-held paleohistology assumptions, but because medullary expansion, cyclical growth mark formation, and variable intraskeletal growth rates are skeletal features found in tetrapod taxa living or extinct, the validations presented herein should be considered during any tetrapod bone microanalysis.

Modern birds have markedly foreshortened tails and their body mass is centred anteriorly, near the wings. To provide stability during powered flight, the avian centre of mass is far from the pelvis, which poses potential balance problems for cursorial birds. To compensate, avians adapted to running maintain the femur subhorizontally, with its distal end situated anteriorly, close to the animal's centre of mass; stride generation stems largely from parasagittal rotation of the lower leg about the knee joint. In contrast, bipedal dinosaurs had a centre of mass near the hip joint and rotated the entire hindlimb during stride generation. Here we show that these contrasting styles of cursoriality are tightly linked to longer relative total hindlimb length in cursorial birds than in bipedal dinosaurs. Surprisingly, Caudipteryx, described as a theropod dinosaur, possessed an anterior centre of mass and hindlimb proportions resembling those of cursorial birds. Accordingly, Caudipteryx probably used a running mechanism more similar to that of modern cursorial birds than to that of all other bipedal dinosaurs. These observations provide valuable clues about cursoriality in Caudipteryx, but may also have implications for interpreting the locomotory status of its ancestors.

The Late Cretaceous Oldman Formation comprises sediments that were deposited along the margin of a great inland sea that covered much of the western interior of North America. The environment of deposition appears to have been tracts of fluvial marshes that separated "islands" of higher, drier ground. The climate was probably warm-temperate, and it is suggested that upland plant communities were parkland-like in aspect.The large dinosaurs of this community comprised animals that were between a hippopotamus and a large African elephant in adult weight. Some workers have suggested that dinosaurs had metabolic rates comparable to those of living birds or mammals. By extrapolating from the food consumption rates of these living endotherms it is possible to obtain crude estimates of the ingestion rates of endothermic dinosaurs. Similar extrapolations from the ingestion rates of living reptiles and amphibians provide estimates of the ingestion rates of ectothermic dinosaurs. By deriving an empirical equation relating the ratio of annual secondary productivity/average annual biomass to adult weight in living mammals, and employing estimates of adult weight and biomass for the herbivorous dinosaur populations, it is possible to estimate the annual secondary production of endothermic Oldman herbivorous dinosaurs. If the body weight vs. production/biomass relation derived for mammals can be applied to ectothermic tetrapods, it is possible to estimate annual secondary production of ectothermic dinosaur populations.These calculations suggest that the annual secondary production of endothermic herbivorous dinosaurs would have been insufficient to meet the food requirements of an endothermic carnivorous dinosaur population as large as is preserved in the Oldman Formation. However, ectothermic carnivorous dinosaurs would have been easily able to make energetic ends meet. Unfortunately, the situation is complicated by the possibility that carnivores are overrepresented in collections from the Oldman. Because of this, I cannot presently decide between ectothermy and endothermy in dinosaurs on the basis of methods presented in this paper. Alternative methods that may be more successful in this regard are discussed. It is hoped that as paleontologists collect fossils from an ecological point of view the methods presented in this paper can be employed to make realistic statements about the trophic dynamics of ancient vertebrate communities.

The skeletal record of tyrannosaurids is well-documented, whereas their footprint record is surprisingly sparse. There are only a few isolated footprints attributed to tyrannosaurids and, hitherto, no reported trackways. We report the world’s first trackways attributable to tyrannosaurids, and describe a new ichnotaxon attributable to tyrannosaurids. These trackways are from the Upper Cretaceous (Campanian - Maastrichtian) of northeastern British Columbia, Canada. One trackway consists of three tridactyl footprints, and two adjacent trackways consist of two footprints each. All three trackways show animals bearing southeast within an 8.5 meter-wide corridor. Similarities in depth and preservation of the tyrannosaurid tracks indicate that these three trackways were made by track-makers walking concurrently in the same direction. These trackways add significantly to previous osteology-based hypotheses of locomotion and behavior in Tyrannosauridae by providing ichnologic support for gregariousness in tyrannosaurids, and the first record of the walking gait of tyrannosaurids.