reported so far in bones of other dinosaurs and Mesozoic vertebrates are correlated with theropod tracemakers (Chapter 14). This evidence at least supports the assertion that, if any given dinosaur was carnivorous, it was a theropod. As consumers in most terrestrial environments from the Late Triassic through to the Late Cretaceous, many of them fulfilled important ecological roles near the top of their food chains.
In the abbreviated list of characters that distinguish theropods from other dinosaurs, most items relate to their development and refinement of bipedalism. As mentioned previously, bipedalism is probably a plesiomorphic trait in theropods, and only a few theropod species show evidence of having been quadrupedal. This obligate bipedal posture for the vast majority of theropods is interpreted on the basis of skeletal features, such as leg lengths considerably exceeding arm lengths. Theropod bipedalism is also verified by trackway evidence, which shows diagonal walking patterns that involved only two alternating pes impressions (Chapter 14). Moreover, the narrow straddle of interpreted theropod trackways correlates with anatomical characteristics that indicate theropod legs were positioned proximal to the midline of their bodies, an adaptation that aided in their efficient two-legged movement. Finally, manus imprints attributed to theropods are exceedingly rare, suggesting that they spent most of their time in an upright position, supported by their hind limbs. However, exceptions are shown by so-called "sitting traces," where theropods sat back on their haunches (metatarsals and ischiadic wKfa
FIGURE 9.3 Right manus of Late Jurassic Allosaurus fragilis with digits I to III (from top to bottom) and right human hand for scale. Dinosaur National Monument, Vernal, Utah.
region) and left visible impressions of their hands, too. In Alberta, Canada, one trackway of a large theropod also shows probable manus impressions where the theropod scraped the ground in front of it as it moved. This evidence also indicates a low, horizontal posture for that theropod, a condition also inferred for other theropods.
A bipedal habit means that hands were free to grasp, and a typical theropod manus with digits I through to III indeed exhibits such a capability (Fig. 9.3). For example, grasping in a human hand can still be accomplished using only digits I through to III because the first phalanx, carpal, and metacarpal of the thumb are positioned posterior to the other two digits. This arrangement allows the thumb to meet with the ventral surfaces of the other digits, and theropods were apparently capable of the same motion. Their grasping ability was more than sufficient for holding on to food items or mates. However, digit III of some large theropods of the Cretaceous, such as abelisaurids and tyrannosaurids, was so reduced that they only had a two-fingered manus. This circumstance corresponded with a relatively small humerus, radius, and ulna in each arm. Such seemingly odd traits provoke speculation about the functionality of such minimal appendages in these large theropods. Were they vestigial organs or did they serve some other, as yet, unknown purpose?
Theropods maintained their bipedalism with the aid of caudal vertebrae that were stiffened distally by long processes. These structures probably acted as a counterbalance to large skulls and consequently caused a more or less horizontal alignment of the vertebral column. Recognition of this alignment, in conjunction with other skeletal features, resulted in revisions of how theropods were depicted in both museums and textbooks. At the beginning of the twentieth century, mounts of thero-pod skeletons and artists' restorations put theropods in near-vertical, kangaroo-like poses. On the basis of the large amount of scientific data gained since then, from both skeletal and track data, museums now show them closer to and parallel to the ground. Of course, such a posture relates to predation, an important life habit for most theropods. A horizontally-aligned theropod conveyed greater ease of movement, and placed their arms and often large and sharp teeth at the same level as their potential prey animals.
Another unique characteristic of theropods versus other dinosaurs was the tendency of some of their members toward increased "braininess," as measured roughly by the brain-mass/body-mass ratio, and more precisely by the encephalization quotient (EQ), first discussed in Chapter 8. EQ is the cerebral-cortex-mass/total-brain-mass ratio, but the more easily calculated measurement is the brain-mass/body-mass ratio. This can be approximated for any given fossil vertebrate by measuring the volume of the braincase versus the volume of the entire body:
where Br is brain/body ratio, Ve is endocranial volume (normally measured in cubic centimeters, or cm3), and Vb is body volume. Another way to view this ratio from the standpoint of mass is to compare the brain mass to the total body mass as a percentage:
where Bp is brain/body percentage, Me is encephalic mass, and Mb is body mass. For example, the brain of a modern African elephant (Loxodonta africanus) is about 7500 g and its body mass is 5.0 metric tons (5000 kg). In contrast, a typical adult human brain is 1500 g with a body mass of about 0.07 metric tons (70 kg). The elephant has a brain five times more massive than the human, but a striking difference is apparent when the two species are compared using Equation 9.2:
Bp (elephant) = 7.5 x 103 g/5.0 x 106 g x 100 = 0.15%
Through this comparison, a typical human has 14 times the brain mass in proportion to its body size when matched against an elephant.
For a dinosaur example, recall that the volume of the Tyrannosaurus model measured in Chapter 1 (Eqns 1.3 and 1.4) was 235 cm3, but was a scale model at 1 : 30. This means that its "life-size" volume was 235 cm3 x 30 x 30 x 30 = 6.35 x 106 cm3. Using an estimated endocranial volume for Tyrannosaurus of 500 cm3, the brain/body ratio would have been about
which, if 1.0g/cm3 is assumed for the density of both the brain and body for Tyrannosaurus, is about 19 times less than the percentage calculated for an elephant. However, the tyrannosaurid is still larger than that for its possible contemporaneous prey animal, Triceratops. Using 300 cm3 as an endocranial volume and an estimated body volume of 8.5 x 106 cm3, Triceratops would have had the following brain/body ratio:
which means that a typical adult Tyrannosaurus still had twice the brain size relative to its body size in comparison to Triceratops. Thus, the rhetorical question of "How large did theropod brains need to be?" is apparently answered by "Larger than those of their intended meals." Along those lines, other researchers have noticed an increase in brain/body ratios for carnivorous mammals with respect to their probable prey animals throughout the Cenozoic Era. This is a possible example of a co-evolutionary process in action (Chapter 6).
The EQs for many modern species of vertebrates have been calculated, and a plot called an allometric line can be drawn to best describe the average EQ of closely-related or otherwise similar modern groups. This graph can then be used to compare EQs of modern and extinct animals. For example, the average EQ of modern crocodilians can be used as a standard of 1.0 for a line to compare to dinosaur EQs. If the plot for the EQ of a particular dinosaur falls below the line, that dinosaur had a smaller brain than should be expected for an animal of similar size, and vice versa for any dinosaurs with EQs that plot above the line. When compared to the EQs of crocodilians, theropods plot above the line. Some small Late Cretaceous
Was this article helpful?