Summary:
1. Sauropod dinosaurs were the largest terrestrial animals ever, and the combination of selective pressures that might have lead to such extraordinary sizes has long been discussed.
2. Here, we argue that a previous suggestion that large size may be a response to unusually high C/N ratios in available plant foods has been prematurely discarded. C/N ratios were likely to be high during much of the Mesozoic, and C/N ratio is entirely different from gross energy density as a measure of the value of a plant as food. In addition, we use recently published allometric equations for herbivore nitrogen and carbon use to make tentative calculations which suggest that if Mesozoic C/N ratios were greater than extant ones, this would have selected for one of two strategies: gigantism in ectothermic herbivores or endothermy (and selective foraging on high N material) in very small herbivores.
3. We speculate that smaller-bodied juvenile sauropods might have had a broader omnivorous diet and/or had higher mass-specific metabolic rates than adults. The former is potentially testable by changes in dentition; the latter matches evidence of high growth rates of juvenile sauropods. press release
