INTRODUCTION

In 1991, the most complete dinosaur known from Wyoming was excavated from the Upper Jurassic Morrison Formation in the eastern Bighorn Basin near the town of Shell (Breithaupt 2001). Although the theropod Allosaurus has been known for over 100 years, this specimen (nicknamed 'Big Al') had one of the most complete skulls and skeletons of this genus yet to be found. This specimen is particularly important as it represents a partially articulated, 95% complete, pathologic (i.e., with broken, fractured, and infected bones) skeleton of a sub-adult Allosaurus fragilis (although it may represent a new species). Consequently, MOR 693 has gained international recognition, as the scenarios of its painful life, early death, and rapid burial have been determined through various paleontological analyses. The Museum of the Rockies molded MOR 693 and provided a permanent display cast to the University of Wyoming Geological Museum.

Such complete skeletons provide an opportunity to investigate the mass properties of extinct animals by allowing accurate physical or digital volumetric models to be produced (Colbert 1962; Alexander 1985, 1989; Farlow et al. 1995; Henderson 1999; Hutchinson et al. 2007; Bates et al. 2009). These models can be used to characterize the mass, centre of mass (CM) and inertial properties of body segments, providing information necessary for numerical biomechanical assessments of functional morphology (e.g., Hutchinson et al. 2007). Bates et al. (2009) used laser scanning and computer modelling software to construct volumetric models of five specimens of non-avian dinosaurs. A long-range laser (LiDAR) scanner was used to digitize mounted skeletons allowing the reconstruction of body volumes and respiratory structures around and within the 3D skeletal model. The digital medium offered the facility to modify model properties non-destructively, such that a detailed sensitivity analysis could be conducted to quantify the effect of the many unknown parameters involved in such reconstructions. By varying the volume of body segments and respiratory structures, it was possible to constrain the maximum plausible range of mass set values within broad limits. In this study the same approach is used to constrain the maximum plausible range in mass values for Allosaurus MOR 693 and by inference specimens of similar skeletal proportions. Sensitivity analyses have only recently begun to be applied in dinosaur body volumetric reconstructions and the few previous studies (Henderson and Snively 2003; Hutchinson et al. 2007; Bates et al. 2009) have not directly addressed the affects of ambiguities in the articulation or mounting of dinosaur skeletons on volumetric reconstructions. In this study the sensitivity analysis is extended to include additional uncertainties regarding osteological articulations in non-avian dinosaurs, with specific focus on the effects of inter-vertebral spacing (i.e., the unknown volume of inter-vertebral discs) and the orientation or 'flare' of the rib cage in MOR 693.