- An Evolutionary Cascade Model for Sauropod Dinosaur Gigantism - Overview, Update and Tests
- P. Martin Sander
Sauropod dinosaurs are a group of herbivorous dinosaurs which exceeded all other terrestrial vertebrates in mean and maximal body size. Sauropod dinosaurs were also the most successful and long-lived herbivorous tetrapod clade, but no abiological factors such as global environmental parameters conducive to their gigantism can be identified. These facts justify major efforts by evolutionary biologists and paleontologists to understand sauropods as living animals and to explain their evolutionary success and uniquely gigantic body size. Contributions to this research program have come from many fields and can be synthesized into a biological evolutionary cascade model of sauropod dinosaur gigantism (sauropod gigantism ECM). This review focuses on the sauropod gigantism ECM, providing an updated version based on the contributions to the PLoS ONE sauropod gigantism collection and on other very recent published evidence. The model consist of five separate evolutionary cascades (“Reproduction”, “Feeding”, “Head and neck”, “Avian-style lung”, and “Metabolism”). Each cascade starts with observed or inferred basal traits that either may be plesiomorphic or derived at the level of Sauropoda. Each trait confers hypothetical selective advantages which permit the evolution of the next trait. Feedback loops in the ECM consist of selective advantages originating from traits higher in the cascades but affecting lower traits. All cascades end in the trait “Very high body mass”. Each cascade is linked to at least one other cascade. Important plesiomorphic traits of sauropod dinosaurs that entered the model were ovipary as well as no mastication of food. Important evolutionary innovations (derived traits) were an avian-style respiratory system and an elevated basal metabolic rate. Comparison with other tetrapod lineages identifies factors limiting body size.
This review of the biology of the sauropod dinosaurs and the evolution of their gigantism, condensed into the sauropod gigantism ECM, serves to compile and synthesize the rapidly expanding literature on the subject, including this collection in PLoS ONE. It also serves as an update to an earlier review [2] in which the evidence available in late 2009 was synthesized into a unified biological scenario of sauropod gigantism, using the approach of an evolutionary cascade model. Testing the premise that it is mainly intrinsic factors rooted in the biology of the clade Sauropodomorpha that explains the historical pattern of its evolution to gigantic body size, was no the aim of this review. However, the evidence reviewed here shows at least that there is no need to invoke extrinsic, abiological factors to explain sauropod gigantism. Testing the influence of environmental change over geological time scales on the historic pattern of evolution is a valid research program, but it is not the one we pursue.
The rich new evidence accumulated in these last four years was then used to test the ECM by asking how this evidence impacted the component cascades and the entire ECM. Most of the inferred traits, selective advantages, and feedback loops in the ECM found support, sometimes strongly so, while in a few others (e.g. “High growth rate”) support weakened or relationships had to be rejected (the physiological underpinning of the feedback loop “Large gut capacity”). The ECM was also refined by splitting up traits and adding new ones. The general conclusion of Sander & Clauss [1] and Sander et al. [2] that sauropod gigantism was able to evolve because of the complex interplay of a historically contingent combination of plesiomorphic (primitive) and derived traits and characters, has emerged stronger than before. While the principle of parsimony calls for preference of simple solutions over complex ones, it is simplistic to assume that a single factor will explain sauropod gigantism. Finally, the sauropod gigantism ECM is hoped to evolve into a comprehensive framework informing us about evolutionary body size limits in herbivorous tetrapods in particular and other terrestrial tetrapods in general.
References
- 1.Sander PM, Clauss M (2008) Sauropod gigantism. Science 322: 200–201. doi: 10.1126/science.1160904
- 2.Sander PM, Christian A, Clauss M, Fechner R, Gee CT, et al. (2011) Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews 86: 117–155. doi: 10.1111/j.1469-185x.2010.00137.x
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