THE UNIVERSITY OF CALGARY Eric Snively A ... - Ohio University

to its right foot to prepare for a change in direction. The force on the metatarsus
is therefore higher than if the leg was extended to the same angle dunng Iinear
locomotion, but lower than if the animal was stopping up short or propelling itself
from a standstill.
The magnitude of the force can be set to that just before the animals uses
muscular forces to decelerate from a tinear speed or to change direction. For
simplicity the same duty factor may be assumed as for the linear progression
above, but the angle will be less than 90 degrees. In walking humans, the forces
of the phalanges on the metatarsals are a small percentage of the vertical ground
force on these elements (Stokes et al. 1979). The stance modeled here for
Gorgosaurus libratus entails the metatarsus at 50 degrees to the substrate, with
the toes flat on the ground. This approximates the condition in humans near the
push-off phase of the step. Therefore, horizontal forces of the phalanges on the
metatarsals of G. libratus are neglected under this loading regime, although they
are conceivably very important if the animal was decelerating.
Figure 4.2 is a diagram of forces on MT 111 experienced during this loading
regime. When the metatarsus is at an angle of 50 degrees to the horizontal, ail
three metatarsals are in contact with the substrate through their footpads.
Footpad area would be a better approximation of the proportion of mass each
metatarsal bore. Probable Campanian tyrannosaurid tracks are known from the
St Mary's River region of Alberta, but unfortunately were not accessible in time
for this analysis. Therefore the area of ground undemeath each metatarsal was