An evolutionary cascade model for sauropod dinosaur gigantism--overview, update and tests.

P Martin Sander,Andrew A Farke

doi:10.1371/journal.pone.0078573

Abstract

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.

Highlights

Dinosaurs of the clade Sauropoda were the largest terrestrial animals that ever lived [1,2,3]
Revised ECM for Sauropod Gigantism The remarkable amount of evidence that has accumulated over the last few years, and that is the focus of this collection, considerably refines the evolutionary cascade model of sauropod gigantism proposed by Sander et al in 2010 [2] by testing many of its components
The question of why no multi-tonne ground birds evolved in the early Tertiary after the demise of the non-avian dinosaurs remains prominent [184], considering that birds seem to show all of the traits in the revised sauropod gigantism ECM in which a gastric mill, obligatory in herbivorous birds, is not necessarily seen as limiting food intake rate

Summary

Introduction

Dinosaurs of the clade Sauropoda were the largest terrestrial animals that ever lived [1,2,3]. Extrinsic controls, such as changing global environmental parameters, were largely excluded from consideration in the ECM because those environmental parameters that are known or can be reasonably well inferred show no correlation with sauropod body size evolution [2] This hypothesis of no correlation was tested by Sookias et al [13] using maximum-likelihood analyses of Late Paleozoic to Jurassic terrestrial vertebrate evolution, and they showed that biological factors alone are sufficient to explain patterns of size evolution in dinosaurs [13]. Traits of sauropod reproductive biology, i.e., the lack of parental care and the large number of small offspring, must have resulted in increased predation pressure which in turn would have led to strong selection for larger body size.

Discussion

Conclusions