Friday 16 August 2013

Why were Dinosaurs so big?

The Dinosaurs appeared in the Middle-to-Late Triassic, and rapidly became the dominant group of land animals on Earth, remaining so till the end of the Cretaceous, 65 million years ago. They evolved into a wide variety of forms filling almost every available ecological niche, but are noted in one particular trait beyond all others, the large size which so many species attained, including the largest terrestrial animals ever to live.

In a paper published in the journal PLoS One on 19 December 2012, Eoin O’Gorman of the School of Biological and Chemical Sciences, Queen Mary University of London and David Hone of the School of Biology and Environmental Science at University College Dublin, analyze the size distribution of a number of different animal groups, with the aim of estimating whether the perceived abundance of large species in Dinosaurs is real, a product of preservational bias, or a result of our obsession with the larger species of Dinosaur.

O'Gorman & Hone compared (non-avian) Theropod, Sauropod and Ornithischian Dinosaurs to modern and fossil Mammals, Birds, Reptiles, Amphibians, Fish and Pterosaurs. The inclusion of fossil Mammals and Pterosaurs being made in the hope of ruling out any bias in the fossil record due to the preferential preservation of larger species. In particular the Pterosaurs are considered to have been closely related to the Dinosaurs, and to have existed over the same span of geological time.

A mounted Tyranosaur skeleton in the Carnegie Museum in Pittsburgh, with David Hone for scale. Dave Hone's Archosaur Musings.

O'Gorman and Hone found that for Mammals, Birds, Reptiles, Amphibians, Pterosaurs and Theropod Dinosaurs, the number of smaller species significantly outnumbered the number of larger ones. This is in line with predictions; smaller species can subdivide the landscape into a larger number of small ecological niches, and have a short generation time allowing for more rapid speciation, while larger species, which take longer to reproduce and need more resources to sustain themselves until they do, are more likely to go extinct in times of environmental stress. The inclusion of non-avian Theropod Dinosaurs in this group suggests that while they may have reached larger sized than members of the other groups, they were still subject to essentially the same ecological rules.

In Sauropod and Ornithischian Dinosaurs, however, the reverse was true, and there were more large species than smaller ones, while in fish medium sized species were more abundant than either very large or very small species. Since fossil Mammals, Pterosaurs and Theropod Dinosaurs all show a bias towards smaller species, the bias for larger species in Ornithischian and Sauropod Dinosaurs was judged unlikely to be a preservational artifact, but rather a genuine response to evolutionary pressures not seen in other groups of animals. This bias for larger species appeared very early in the history of the groups, but became most extreme at the ends of the Triassic, Jurassic and Cretaceous, apparently being reset to some extinct by the end-Triassic and end-Jurassic extinctions.

O'Gorman & Hone suggest that this might be due to the unique reproductive strategy employed by Dinosaurs, which produced small young which eventually grew to vast sizes, occupying a series of ecological niches along the way, and thereby possibly excluding smaller species. They suggest that this may be an advantage for non-endothermic herbivorous animals, particularly those eating a less nutritious non-angiosperm plant diet, where a long digestive tract is likely to have been useful to slowly ferment tough food. 

There is no advantage to large size in carnivorous species, beyond being large enough to deal with the available prey, since meat is fairly easy to digest, and this preference for larger species does not appear in predominantly carnivorous groups, the Theropod Dinosaurs, Reptiles and Amphibians, nor is it a good strategy for endothermic animals (animals which generate their own body heat chemically) such as Mammals, Birds and (probably) Pterosaurs, since the generation of heat requires energy that cannot then be spent on growth.

Alone among modern vertebrate animals, Fish have both an extreme growth curve and (mostly) a non-endothermic biology, however 'Fish' is a rather poor term taxonomically, encompassing a group of creatures more diverse than all terrestrial vertebrates combined, with a range of feeding strategies not available to land-based creatures, so a different size bias is unsurprising, though it is possible that a more detailed study which broke the group into smaller units might produce similar results. 

Finally O'Gorman and Hone suggest that the tendency of the bias for larger Sauropod and Ornithischian Dinosaurs to become more pronounced towards the ends of the Triassic, Jurassic and Cretaceous imply that these were largely periods of long-term climatic stability, providing time for long lived, slow reproducing species to evolve.

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