HINDLIMB PROPORTIONS AND RELATIVE STRIDE LENGTH IN BIPEDAL AND POTENTIALLY BIPEDAL DINOSAURS

During ontogeny of American alligators and domestic chickens, the pes shows negative allometry with respect to the length of the rest of the hindlimb: the animals grow into their feet. In contrast, in interspecific samples of large non-avian theropod dinosaurs, the portion of the foot that routinely touched the ground (as represented by digit III length) becomes relatively longer, relative to the remainder of the hindlimb, with increasing animal size. This prompted a look at hindlimb proportions across bipedal (or at least potentially bipedal) theropod and ornithischian dinosaurs, and comparison with a trackway parameter likely to be affected by hindlimb proportions, the stride length to footprint length ratio (S/FL).

In theropods, the (femur + tibia + metatarsal III) length/digit III length ratio, plotted against total hindlimb length (femur + tibia + metatarsal III + digit III), increases interspecifically from the smallest forms to dinosaurs with total hindlimb lengths of about 1 m, consistent with the ontogenetic pattern in alligators and chickens. For still larger theropods the ratio then declines with increasing total hindlimb length. In ornithischians the ratio is comparable to that of theropods at smaller sizes, but continues to increase among large ornithopods.

Dinosaur trackways (sample size > 2000) were assigned to size classes on the basis of footprint length. In all size classes, the median S/FL of trackways attributed to theropods is greater than the median S/FL of trackways attributed to ornithischians (mainly ornithopods). For walking dinosaurs (S/FL = 10 or less), the median S/FL questionably increases from very small dinosaurs to animals with footprint lengths of 10-20 cm, but then unambiguously declines at larger sizes. This pattern is consistent with size-related changes in the limb/digit III ratio in theropod dinosaurs, but not ornithischians--possibly because the latter were mainly quadrupedal? The maximum S/FL (associated with running dinosaurs?) shows the same trend with respect to footprint size class as the median S/FL, possibly increasing from the smallest to mid-sized dinosaurs, and then declining among larger forms. Mid-sized dinosaurs seem to have been “bouncier” than their bigger relatives, both interspecifically and possibly intraspecifically.