The New Paper That Time Forgot

[David Marjanovic <david.marjanovic@gmx.at>]

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.