Ornithopods were obligate bipeds, facultative bipeds, or quadrupeds, as suggested by their appendicular skeletons and verified by their trackways. The appendicular skeletons of heterodontosaurids and other non-iguanodontians imply that these lighter and smaller ornithopods were quicker moving than iguanodontians. Of course, ornithopod trackways, particularly from the Cretaceous, are very common in some parts of the world and constitute an excellent fossil record of their locomotion, which can be used in conjunction with body fossils. In contrast, tracks and trackways attributable to heterodontosaurids and other non-iguanodontians are still not well documented, although some 10-15 cm long ornithopod tracks in Lower Cretaceous strata of Spain are interpreted as belonging to such smaller ornithopods.
One difficulty in distinguishing the tracks of these generally smaller ornithopods in deposits worldwide lies in their possible morphological and dimensional similarity to theropod tracks in same-age strata. For example, a typical heterodontosaurid pes is close in size and shape to the feet of some small theropods that lived during the Early Jurassic. Likewise, the pes of Hypsilophodon also has a narrow-toed three-toed outline that probably produced elongate tracks, which can be easily confused with those of some theropods or bipedal prosauropods (Chapter 14). Because many three-toed Jurassic tracks have been described from the geologic record, it is reasonable to predict that at least some of those currently correlated with theropods were actually made by smaller ornithopods. If more accurate distinctions can be made, more detailed analyses of non-iguanodontian locomotion can be used to test the conceptions derived from skeletal data.
One example of how speedy, smaller ornithopods have been interpreted from trackway evidence is the Early Cretaceous tracks associated with a so-called "dinosaur stampede" preserved in a Lower Cretaceous stratum in Queensland, Australia (Chapter 14). The tracks indicate that the smaller ornithopods and some theropods mostly ran in the opposite direction of a large theropod at top speeds of 4-5 m/s (14-18 km/hour). However, whether these ornithopods were non-iguanodontians is unknown; skeletal data from strata in the same area are lacking in this respect.
Iguanodontian tracks show that iguanodontians were either bipedal or quadrupedal, and seemingly all were diagonal walkers. Thus far, no iguanodontian running trackways have been found; their calculated top speeds, based on track and hip-height measurements, are about 2 m/s (7 km/hour). This trackway evidence agrees with hypotheses of iguano-dontian locomotion based on skeletal proportions, which also indicate relatively slow-moving animals. For example, iguano-dontian fore limbs typically are only about 70% the length of the hind limbs, so some paleontologists argue that these ornithopods were not capable of running quadrupedally. To visualize this biomechanical concept, think of how horses or deer have nearly equal fore limb and hind limb lengths, which helps them to run more efficiently. The simplest solution to running, or any other movement at higher speeds, was thus achieved bipedally. Consequently, bipedal trackways made by iguano-dontians can be tested independently through examination of pace length, footprint length versus hip height, and pressure-release structures to see whether they indicate running speeds (Chapter 14).
One of the best-documented examples of iguanodontian trackways is in the area of Morrison, Colorado, in the same area where "The Great Dinosaur Rush" began in the 1870s (Chapter 3). About 50 years after that, road construction removed Early Cretaceous rocks, unveiling dozens of iguanodontian tracks mixed with some theropod tracks (probably from ornithomimids) on a bedding plane (Fig. 11.11).
The equant, ree-toed, and occasionally very large feet of most iguanodontians are distinctive and correlate easily with numerous tracks in Cretaceous deposits.
The iguanodontian tracks represent numerous individuals that walked quad-rupedally along a shallow marine shoreline. Some were apparently traveling alone, but others were walking together in the same direction. At least two trackways show a smaller iguanodontian walking parallel to a larger one, which is evocative of a parent and juvenile moving together. This information, along with ornithopod trackway data from other areas, argues for sociality and herd behavior in at least some iguanodontians.
Probably the most enduring antiquated concept of ornithopod locomotion is that some of them were either fully or semi-aquatic. This hypothesis was first proposed in the early part of the twentieth century with the discovery by the Sternberg family (Chapter 3) of hadrosaurids that were "mummified." These hadrosaurids were skeletons preserved with skin impressions nearly all the way around their bodies. This mode of preservation was most certainly a result of arid conditions and desiccation soon after death (Chapter 7), which caused an interesting look: the skin impressions between the phalanges seemed stretched, giving the feet a "webbed" appearance. This apparent condition, interpreted without its taphonomic subtext, was originally considered as evidence of an aquatic adaptation. As a result, other features of hadrosaurids (and iguanodontians in general) then were fitted to this hypothesis. For example, the lack of dermal armor and other obvious defenses against predation led early researchers to propose that these large ornithopods used the supposed safety of water bodies to swim away from voracious theropods. Of course, this hypothesis did not take into account some of the 10-meter long crocodiles, such as Deinosuchus, that lived in those water bodies and ate hadrosaurids during the Late Cretaceous (see Fig. 7.1). The mistaken assumption of aquatic iguanodontians was similar to the one made with sauropods, which were deemed too large to have lived on dry land (Chapter 10). Thus, the artistic recreations of large ornithopods and sauropods from earlier in the twentieth century show them in or near bodies of water (Chapter 1).
The aquatic-habitat hypothesis is unsupported, not only by trackway evidence showing that iguanodontians traveled on emergent land, but also by anatomical details such as the following:
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