Assessing Social Behaviour, Ontogenetic Change and Taxonomic Status in a Juvenile Gorgosaurus Libratus (Dinosauria; Theropoda; Tyrannosauridae) PDF Download

Author: Gavin Bradley
Publisher:
ISBN:
Category : Albertosaurus
Languages : en
Pages : 208

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Book Description
The sparseness of the fossil record and the subjectivity of interpreting behaviour from morphological and taphonomic evidence have impeded studies on the behaviour of juvenile theropod dinosaurs. Most evidence for social behaviour in juvenile dinosaurs comes from multi individual bone beds or parent dinosaurs preserved while brooding on eggs or young. There is therefore a desire for alternative methods of assessing social behaviour, leading to the two key questions of this thesis: 1) can inferences about social behaviour be made using isolated specimens?, and 2) does gregariousness change with ontogeny in Gorgosaurus? A multidisciplinary study using isolated specimens of Gorgosaurus libratus, including a newly described juvenile specimen from Dinosaur Provincial Park in southern Alberta, was carried out in the hopes of answering these two questions. A review of social behaviour in modern animals suggests that gregariousness is gradational and varies from taxon to taxon, even within closely related groups such as Felidae. Inferences of social behaviour for dinosaurs based solely on phylogenetic bracketing is therefore not recommended. However, numerous analyses can be performed on isolated specimens in order to infer social behaviours including parental care, group living, sexual display and combat. For example, palaeopathologies may indicate intraspecific combat. Ontogenetic changes, such as growth curves, allometry of horns and crests, and changes in stable isotopes because of dietary changes may also indicate changing behavioural as well as ecological roles during development. Femoral circumferences are useful in inferring body mass of theropods, and in bone bed aggregations this can aid in studies of growth rates, which can inform social behaviour. Taphonomic damage can reduce sample sizes and make such studies problematic. Statistical analyses, however, suggest that three femoral-diameter-based estimation models may be used to predict femoral circumference measurements in tyrannosaurids. Ontogenetic morphological changes in Gorgosaurus may also inform inferences about social behaviour. Positive allometric growth of the lacrimal horns may imply a display function, as seen in modern bovid and cervid mammals. Slow maximum growth rates of juvenile Gorgosaurus, compared to other tyrannosaurids, calculated using lines of arrested growth and body mass estimations, may indicate social aggregation during early ontogeny in order to survive alongside faster growing and larger predators, such as Daspletosaurus or, alternatively, reduced growth due to nutritional stresses on juveniles. Macrowear patterns in the teeth of UALVP 49500 and UALVP 10, an adult specimen, exhibit four major types of tooth wear: enamel spalling, longitudinal facets, tip wear, and barrel-shaped puncture marks. Adult teeth were typified by tip wear, and juvenile teeth were typified by longitudinal wear facets. This is hypothesised to reflect a change in feeding behaviour during ontogeny, from shearing and slicing of meat, with high levels of tooth occlusion in the young, to the "puncture and pull" method previously hypothesized for adults. A slicing feeding method for juveniles is further supported by thinner teeth with higher denticle densities, smaller bite forces, and a more circular orbit shape that is less resistant to the strain of high bite forces on the skull. This multidisciplinary analysis shows that substantial ontogenetic change occurred in Gorgosaurus, and demonstrates that social behaviour may be inferred from isolated specimens. As well, a description of a juvenile Gorgosaurus libratus, UALVP 49500, presents the first examination of post cranial material in such a specimen, and supports a genus-level distinction between Albertosaurus sarcophagus and Gorgosaurus libratus.