Without a doubt, the sauropod dinosaurs were the largest animals to have ever walked the earth. Even the largest land mammal, the prehistoric rhino-relative
Despite what late-night creature features about overgrown crocodiles and gigantic spiders might have you believe, the size to which organisms can grow is not unlimited. There are constraints on how big living things can get, from matters of engineering (such as how thick legs have to be to support a heavy body) and physiology (making sure blood gets efficiently pumped around the body) to the availability of enough food, and to understand how large sauropods attained such impressive sizes, paleontologists must attempt to untangle this confluence of factors. (Sauropods did come in a variety of sizes, including dwarfed species, but the exceptionally large genera have been the most vexing.) The latest attempt to do so has been put forward in a review by paleontologists Martin Sander, Andreas Christian, Marcus Clauss, Regina Fechner, Carole Gee, Eva-Maria Giebeler, Hanns-Christian Gunga, Jurgen Hummel, Heinrich Mallison, Steven Perry, Holger Preuschoft, Oliver Rauhut, Kristian Remes, Thomas Tutken, Oliver Wings and Ulrich Witzel in a paper recently published in Biological Reviews.
As has been highlighted by recently discovered dinosaurs like Panphagia, the lineage to which the sauropods belonged started off small. The sauropodomorphs, or the diverse group of long-necked herbivores from which the first sauropods evolved, were puny when compared to their later sauropod cousins, but as soon as the first true sauropods evolved near the end of the Triassic (200 million years ago) they reached estimated body masses of 10 tons or more. In fact, extremely large body size (in excess of an estimated 40 tons) evolved multiple times in different lineages, and even though we often focus on which dinosaur was the biggest of them all the truth of the matter is that from the Late Jurassic to the end of the Cretaceous, a time spanning 85 million years, the world was populated by numerous kinds of gigantic sauropods.
The authors of the paper considered everything from changes in levels of oxygen in the atmosphere and the availability of nutritious food to the lightweight construction of sauropod bones as explanations for the prodigious growth of the sauropods. The picture that emerges from all this is that a combination of features allowed sauropods to grow large. That the sauropod lineage evolved long necks early, for example, allowed them to exploit a range of food sources to fuel their bodies, and their light bones allowed them to overcome structure constraints felt by mammals due to the heavier bones of the latter group. More than that, becoming larger would provide them some relatively safety (at least as adults) from predatory dinosaurs.
The scientists determined that changes in sauropod body size do not track changes in atmospheric oxygen content, carbon dioxide content or changes in temperature, all of which had been hypothesized as driving sauropod body size. Instead it appears that some traits sauropods inherited from their ancestors, such as swallowing large amounts of food without chewing, allowed the beginnings of increased body size, which was further allowed by the development of evolutionary novelties along the way (i.e. a bird-like breathing system that would have allowed oxygen to be supplied to their bodies more efficiently). To put it another way, there was no single cause for the observed trend in body size, but rather an intertwined mass of pressures and constraints which shaped the evolution of these dinosaurs—a constant interplay between what was evolutionarily possible and what was advantageous to local conditions at a given time. While the review does not once and for all solve the mysteries of sauropod size, its recognition of contingency and constraint in their evolution will remain important to the ongoing debate about these dinosaurs.
Sander, P., Christian, A., Clauss, M., Fechner, R., Gee, C., Griebeler, E., Gunga, H., Hummel, J., Mallison, H., Perry, S., Preuschoft, H., Rauhut, O., Remes, K., Tütken, T., Wings, O., & Witzel, U. (2010). Biology of the sauropod dinosaurs: the evolution of gigantism Biological Reviews DOI: 10.1111/j.1469-185X.2010.00137.x