HYDRODYNAMIC PROPERTIES OF PACHYCEPHALOSAURID CRANIA

The hydrodynamic processes of isolated dinosaur bones in general have been overlooked in scientific research. Pitting and erosive structures on pachycephalosaurid frontoparietal domes have been attributed to taphonomic processes, such as weathering and erosion. Furthermore, the high frequency of isolated domes characterizing the pachycephalosaurid fossil record suggests a unique taphonomic history. In order to investigate the hydrodynamic properties and potential roles of taphonomic processes on pachycephalosaurid domes, a series of transport experiments were conducted to assess the competent velocities and settling orientations of a collection of pachycephalosaurid specimens. Casts of three pachycephalosaurid domes and skulls were composed of a urethane resin with a comparable average density to compact and cancellous bone (r = 1500 kg/m³), and placed in a flume with manual velocity control for transport experiments. Data was recorded for competent velocity, transport distance, and settling orientations upon resting and burial of specimens for 35 trials per cast. Though specimens vary considerably in mass, the results suggest specimen shape has a greater influence on transport and hydrodynamic behavior than size; significantly lower velocities are required to transport complete skulls than isolated domes. Resting and burial orientations of specimens vary significantly for domes and skulls. Isolated domes generally settle and rest on a dorsal or ventral side with their lateral sides facing upstream. Alternatively, complete skulls have a higher tendency to rest on a lateral side compared to isolated domes. These results suggest domes are likely deposited as lag following disarticulation from other cranial and skeletal elements. The near-equal occurrences of domes resting on dorsal and ventral sides is at odds with the exclusively dorsal distribution of pitting and “erosive” features on frontoparietal domes, suggesting taphonomic processes are not a primary cause of these features. The high variability of transport velocities and settling orientations such as these offer significant implications for future taphonomic studies on large vertebrate remains.