Why Brontosaurus was huge

By Razib Khan | May 11, 2010 10:22 am

There’s a very long review out which presents a theory for how sauropod dinosaurs could scale up to such enormous sizes, Biology of the sauropod dinosaurs: the evolution of gigantism. ScienceDaily is promoting the likelihood that sauropods did not chew, and so could make do with very small heads which could be supported by long necks, as the big factor. But this is a model with many moving parts. Here’s the verbal list from the conclusion:

(1) Sauropod dinosaurs as the largest terrestrial animals ever represent a challenge to evolutionary biologists trying to understand body size evolution.

(2) The study of the upper limit of body size must address extrinsic as well as intrinsic factors, and it must be determined whether this limit is set by the bauplan of the organisms or by physical and ecological constraints imposed by the environment. Among several possible approaches, we chose the resource perspective because it has been shown that resource availability and maximal body size correlate closely (Burness et al., 2001).

(3) In the interplay of the biology of sauropod dinosaurs with their environment, a unique combination of plesiomorphic features (i.e., inherited from their ancestors) and evolutionary novelties emerge as the key for a more efficient use of resources by sauropods than by other terrestrial herbivore lineages. Plesiomorphic features of sauropods were many small offspring, the lack of mastication and the lack of a gastric mill. The evolutionary innovations were an avian-style respiratory system and a high basal metabolic rate.

(4) We posit that the long neck of sauropods was central to the energy-efficient food uptake of sauropods because it permitted food uptake over a large volume with a stationary body.

(5) In the Late Triassic and Early Jurassic (210–175 million years ago), the combination of biological properties listed above led to an evolutionary cascade in the sauropodomorph lineage characterized by selection for ever larger body size, mainly driven by predation pressure from theropod dinosaurs.

(6)From the Middle Jurassic onward, sauropod dinosaurs dominated global terrestrial ecosystems only to succumb to the catastrophic environmental change at the end of the Cretaceous 65 million years ago.

And here’s a schematic illustrating the interplay of evolutionary forces & constraints:

The paper is open access, so you should read it yourself if you’re interested.

CATEGORIZED UNDER: Biology, Evolution
MORE ABOUT: dinosaurs, sauropod
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  • MW

    I’ve not read the paper – this is just based on your post.

    We have a critter which is too big for carnivores to attack, but is very slow. (The long neck argument suggests that it spends most of its time stationary.) The problem is that there is a window of extreme vulnerability when growing up: small enough to eat, too slow to run away. One countermeasure is mentioned above: try to grow through this vulnerable window as fast as possible. This isn’t a very reliable method however.

    Logically I would expect another countermeasure: parental protection. The young stick close to the parents, and when a predator comes along they hid under mum or dad. Possibly they’d be in herds (other parents hassling predators who come near their kids also helps your kids.) One might also expect some weaponry on the adults, probably on the tail. (Perhaps such a big tail doesn’t need anything else.)

    Is there evidence (fossil footprints?) for parental protection?

  • Sandgroper

    “Thus, titanosaurid (and by extension, all other) sauropods produced numerous small eggs with very precocial young that were left to fend for themselves and suffered high mortality before reaching sexual maturity in the second or third decade of their life.”

    “Limited data from trackways and bonebeds suggest that sauropod herds were composed of a much higher proportion of juvenile animals than is observed in aggregations of mammalian herbivores. Correspondingly, trophic energy represented by large herbivore species should have been available to a predator guild to a much higher degree in the sauropod ecosystem as compared to large mammal-dominated ecosystems with reproductive output of large herbivores confined to a few well-protected young.”

    “With sauropod hatchlings being so small, there must have been strong selection pressure for high juvenile growth rates because they would have shortened the time during which the young sauropods were endangered by predators.”

    But also: “Top speeds of nearly 20 km h−1 appear possible based on preliminary computer-aided engineering (CAE) modeling. Sauropods are similar in their limb design to elephants, with sturdier sauropods having similar, or even slightly greater, strength indicators to extant proboscideans. This indicates that they were comparably athletic. Since elephants can move at speeds of up to 35 km h−1, we must assume that similarly sized sauropods achieved similar speeds, while larger animals with equal strength indicators were even faster.”

    (Citations omitted for brevity.)

    So parental protection no, herd protection maybe, fully grown adults appear immune to predation, they normally moved slowly or not at all but were capable of higher speeds, they had lots of kids who were precocious and grew fast, and a lot of the kids got eaten.

  • Toadal

    The Wiley article indicates a high atmosphere oxygen content does help to explain an increased body size, but notes there is little correlation between atmospheric oxygen content and sauropod body mass.

    Body size increases gradually from the Late Triassic to the Late Jurassic, forming a plateau in the Cretaceous. The two sharp drops in body mass in the Early and Late Cretaceous are probably due to a poor terrestrial fossil record at these times. Note the lack of correlation between atmospheric composition and sauropod body mass.

    Fig. 8 from the Wiley article provides a chart that indicates the Jurassic periods average oxygen level at ~13% and the Cretaceous average at ~17%, which is significantly less than it is today and much less that the Carboniferous periods 32.5%. Therefore we are led to believe a very low atmosphere oxygen content did not detract from body size.

    However, the article contradicts the Jurassic and Cretaceous Wikipedia entries which describes the Jurassic atmospheric oxygen content at 26% and Cretaceous at 30%; percentages that could more easily support the evolution of giant lizards.

    So whom do we believe?

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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com


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