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The New Paper That Time Forgot

In Friday's Science:

P. Martin Sander & Marcus Clauss: Sauropod Gigantism. How did sauropod dinosaurs reach body sizes that remain unsurpassed in land-living animals?, Science 322, 200f. (10 October 2008)

Being a two-page "Perspectives" paper, it has no abstract and is already so concise that I cannot summarize it unless I do it in one sentence. So, here are a few quotes:

"Extrinsic causes have repeatedly been advanced to explain the success of sauropod dinosaurs and the gigantism seen in the dinosaur era. However, physical and chemical conditions in the Mesozoic (250 to 65 million years ago) were probably less favorable for plant and animal life than they are today; for example, atmospheric O2 concentrations were much lower (4). The variation of other factors (such as land mass size, ambient temperature, and atmospheric CO2 concentrations) through time is not tracked by variations in sauropod body size (2,5). Thus, the clue to sauropod gigantism must lie in their unusual biology [...] Sauropods exhibit diverse oral, dental, and neck designs, indicating dietary niche differentiation; this variety makes reliance on any particular food source (6) as the reason for gigantism unlikely. However, one evolutionarily primitive character truly sets sauropods apart: In contrast to mammals and advanced bird-hipped dinosaurs (duck-billed and horned dinosaurs), they did not masticate their food; nor did they grind it in a gastric mill, as did some other herbivorous dinosaurs (7). Because gut capacity increases with body mass (8), the enormous gut capacity of sauropods would have guaranteed the long digestion times (6) necessary for degrading unchewed plant parts, even at a relatively high food intake."

(figure legend) "The unique gigantism of sauropod dinosaurs was made possible by a high basal metabolic rate (BMR, advanced), many small offspring (primitive), no mastication (primitive), and a highly heterogeneous lung (advanced). We hypothesize that ontogenetic flexibility of BMR was also important."

"Large body size in endothermic animals is associated with a major problem of dissipating excess body heat. A long neck also means that a large volume of air must be moved in the windpipe during ventilation before fresh air reaches the lung. These problems appear to have been solved by an evolutionary innovation shared by sauropods and theropods [...] and their descendants, the birds: a highly heterogeneous avian-style respiratory system (11) with cross-current gas exchange [uh, no, counter-current -- cross-current is what crocodiles have!] in the lung and air sacs that pneumatized the vertebrae of the neck and the trunk and filled large parts of the body cavity. Compared to mammalian or reptilian lungs, this system overcame the problem of the long windpipe of sauropods (11) and also probably helped to dissipate excess body heat via the visceral air sac surfaces (11, 12).
For selective advantages conferred by large body size to be effective (13), this large body size must be reached quickly by the individual. Uniquely among amniotes, sauropods grew through five orders of magnitude from a 10-kg hatchling to a 100,000-kg fully grown individual. [Elsewhere they say 80 t instead of 100 t... but of course their point stands.] Bone histological evidence indicates that this growth took place at rates comparable to those of large terrestrial mammals (14, 15); reproductive maturity was reached in the second decade of life and full size in the third decade of life (15), as predicted from demographic models that show higher ages at first reproduction to be incompatible with long-term population persistence (16). Such high growth rates are seen only in animals with a basal metabolic rate (BMR) of mammals and birds (17). For sauropods, rather than assuming a constant metabolic rate throughout the animals' life, an ontogenetic decrease in BMR has been suggested (12, 18). Such a decrease would reconcile rapid growth rates in juveniles with problems resulting from gigantic body size (such as overheating and high food requirements) in adults. Considering the costs of a high BMR, it may have evolved early on in sauropods, as an adaptation for the high growth rates necessary for reaching very large body size.

At the population level, egg-laying and the associated production of many small offspring (19), in contrast to the one-offspring strategy of mammalian megaherbivores, is a key characteristic of sauropod reproductive biology and of the dinosaur ecosystem (20, 21). This strategy may have guaranteed long-term survival of gigantic species (20). In mammals, large body size increases the risk of chance extinction by reducing population density and increasing population recovery time: With increasing body size, fewer offspring are produced, and these take longer to mature. The retention of the primitive feature of egg-laying might have alleviated this constraint through much higher population recovery rates than in large mammals (21)."

"Recent modeling studies found a high metabolic rate to be incompatible with gigantic body size because of the problem of heat dissipation (22, 23); however, these models assumed metabolic rate to have remained constant throughout life. [I don't know if they took pneumaticity into account, but I doubt it.] In addition, one of the studies (23) suffers from poor bone histologic constraints on sauropod growth rates, as does a study (24) arguing against fast growth in sauropods. Compared to other dinosaurs, the long bones of sauropods rarely preserve growth marks, probably because bone tissue was deposited too rapidly to record them (15). Histologic growth rate studies using skeletal elements other than long bones may provide more reliable estimates."

"Egg-laying of sauropods must have made large amounts of food available to predators in the form of many small, little-protected young sauropods. In contrast, mammalian megaherbivores withhold this food source from carnivores by rearing very few, well-protected offspring. This greatly decreased resource base may limit maximum body size of carnivores today (21).

Further progress in understanding dinosaur gigantism will come from a conservation biology approach that models the carrying capacity of Mesozoic ecosystems based on the juvenile-biased population structure of dinosaurs (19, 20). Such an approach, which goes beyond the reconstruction of an individual's metabolism (14, 15, 22, 23), will be much more informative regarding resource limitations, population growth potential, and body size evolution of dinosaurs.