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The last of these caveats can be tested independently through a track census, which looks at the proportion of tracks attributable to theropods versus all other dinosaurs, then normalizes the data for biomass. Footprint length gives an estimate of the size of the track-making dinosaur (Chapter 14), which is used to calculate a biomass for each track-making dinosaur. These biomasses are then totaled for theropods and other dinosaurs, represented on a track horizon to derive a predator/prey ratio. Using such methods, dinosaur ichnologists calculated predator/prey ratios from a Late Jurassic tracksite of North America of about 1 : 7 (about 15% theropods). Interestingly, this ratio is intermediate compared to those calculated for predator/prey ratios in endothermic and ectothermic populations (Fig. 8.7). The advantage of this independent measure is that, in most instances, tracks from a given horizon are more likely to be contemporaneous samples of dinosaur populations than a bone bed. The disadvantages are that this analysis gives more of an assessment of the physiology of the predators, not the prey, and that tracks hold their own distinctive biases (Chapter 14).

A similar method that would compare the ratio of herbivorous dinosaur biomass to plant biomass has yet to be carried out. This approach operates on the similar assumption that endothermic herbivores would have had much greater food needs than ectothermic herbivores in a given terrestrial ecosystem. As a result, the biomass of large herbivorous dinosaurs should have been limited by the biomass and caloric quality of the available plants. Some paleontologists have surmised on

FIGURE 8.7 Estimates of dinosaur biomass in two different Mesozoic deposits as determined by predator/prey ratios and using different sources of data. (A) Bone data (based on a Late Creteceous deposit). (B) Track data, normalized for biomass (based on a Late Jurassic tracksite). Data from Lockley (1990).

FIGURE 8.7 Estimates of dinosaur biomass in two different Mesozoic deposits as determined by predator/prey ratios and using different sources of data. (A) Bone data (based on a Late Creteceous deposit). (B) Track data, normalized for biomass (based on a Late Jurassic tracksite). Data from Lockley (1990).

such ecological reasoning that very large, 20+ metric-ton herbivores, represented by some sauropods, such as Late Jurassic diplodocids and Cretaceous titanosaurids (Chapter 10), must have ingested large amounts of low-quality (woody) plants. This would have happened regardless of whether they were endothermic or ectother-mic. In short, difficult-to-find food of high quality (those with high kJ/g) was sacrificed for the sake of immediately accessible quantity. Accordingly, low-quality plant foods would have needed extensive residence time in the gastrointestinal tract for fermentation by symbiotic anaerobic bacteria. Thus, digestion required a longer and larger gut, which in turn necessitated a larger animal. Gastroliths presumably aided in this digestion, whether they were present in a muscular crop, gizzard, or both (Chapters 5 and 14). Unfortunately, these pieces of evidence, along with the small amount of coprolite data linked to herbivorous dinosaurs' dietary choices (Chapter 14), do not provide adequate answers to questions about their thermoregulation. Consequently, other sources of information from a wide range of choices must be examined.

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