On the head size of sauropodomorph dinosaurs: Implications for ecology and physiology

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Abstract
The heads of prosauropod and sauropod dinosaurs appear to be small, compared with those of extant endothermic mammals. This has been considered inconsistent with the hypothesis, supported by several other anatomical characteristics, that these animals had cellular metabolic rates significantly above the usual reptilian level. However, prosauropods and sauropods had only moderately developed cranial musculature, comparable with that of more typical reptiles. This musculature is plesio‐morphic, in contrast with the specialized and powerful muscles of the mammalian cranium. In feeding, these dinosaurs used their heads exclusively in cropping vegetation, a modest amount of oral processing occurring only in certain sauropods. Thus, the heads of prosauropods and sauropods did not have to be as long, at any given body mass, as those of mammals, for the posterior part of the mammal cranium is expanded to accommodate a food processing apparatus as well as the large brain. Statistical comparisons show that sauropodomorph skulls are significantly shorter, relative to body mass or predicted metabolic rate, than those of mammals of comparable mass. In contrast, the width of the muzzle is subequal to that of mammals at any given body mass and predicted metabolic rate. This suggests that the cropping area was not inadequate in mass to enable these animals to ingest sufficient food to maintain an elevated metabolic rate.