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Friday, January 4, 2013

Dinosaurs really were gigantic

Non-avian dinosaurs (more commonly simply ‘dinosaurs’) are best known for being enormous. As a group, they include the largest land animals ever known. But were these gigantic specimens merely outliers? After all, among extant mammals, the largest ones (elephants, lions, horses, etc) are better known than shrews or voles, yet the majority of mammals are in fact rather small. Was this the case for dinosaurs as well, or did they truly have a unique pattern of size distribution?

To find out, Eoin O’Gorman and David Hone of University College London and Queen Mary University of London collected body mass information on five groups of living vertebrates: modern birds, reptiles, amphibians, fish and terrestrial mammals, and on three types of extinct animals: dinosaurs, pterosaurs and Cenozoic terrestrial mammals.

As you can see from the diagrams below, the distribution of dinosaur sizes (panel a) is significantly different from that of the other seven groups. There is far higher percentage of extremely large dinosaur species than there is of large species of any other vertebrate group. 


Frequency distribution of species body size for eight different animal groups: (a) extinct dinosaurs; (b) extant birds; (c) extant reptiles; (d) extant amphibians; (e) extant fish; (f) extant mammals; (g) extinct pterosaurs; and (h) Cenozoic mammals.
Silhouettes of the largest and smallest animal in each group are also shown (provided by Matt van Rooijen).

Gigantism gives an organism a number of advantages. They can conserve body heat in cold climates, have greater digestive efficiency and are less susceptible to predators. On the other hand, larger animals can overheat more easily and require far more food and space than small animals do. It’s not surprising that each vertebrate group comprises both large and small species, the bigger surprise is why dinosaurs don’t fall into the typical pattern.

The fact that extinct mammals and pterosaurs don’t have the same skewed size distribution as dinosaurs indicates that it’s not simply that prehistoric times favored larger animals. It’s also not that the largest modern animals have been hunted to extinction. In either of these cases, all the extinct animals would show the same pattern. And in fact, it isn’t even all dinosaurs that exhibit this trend. When the authors subdivided dinosaurs into three groups, Ornithischia, Sauropodomorpha and Theropoda, only the first two were skewed to contain mostly large specimens. Theropoda species fell into a bell-shaped distribution with most species being middle-sized.

I’ve discussed one possible reason for this unique size distribution before, namely that a single species of dinosaur can cover many different niches in the environment as it grows. You may not have sufficient habitat to support both small and large species if the offspring of the large species already fill all the niches. Of course, this same adaptation may have ultimately led to the dinosaurs’ demise.

Eoin J. O’Gorman, & David W. E. Hone (2012). Body Size Distribution of the Dinosaurs PloS ONE 7(12): e51925. doi:10.1371/journal.pone.0051925.