When the young of large animals start out small, they must grow through a large size range before reaching adulthood, and compete with species of many different sizes as they do so.
Codron and his colleagues developed a model that suggests that there was intense competition among small and medium-sized dinosaurs, so that it was difficult for medium-sized adults to make a living, and adults had to keep growing until they reached very large sizes to gain a competitive edge. In comparison to mammals, relatively few dinosaurs are known with adult sizes between 1 kilogram and 1,000 kilograms.
But being big also has drawbacks. When an asteroid impact at the end of the Cretaceous period (65 million years ago) wiped out most large-bodied animals, there were so few small dinosaur species that the group was almost obliterated, with only the birds surviving. However, the many small mammals alive at the time were well suited to a world that favoured diminutive species.
“I love the idea that egg-laying led dinosaurs to engage in such different niche occupation from mammals and that this played a part in their eventual extinction,” says John Hutchinson, an evolutionary biologist at the Royal Veterinary College in Hatfield, UK. “But we are going to see a lot of debate over how accurate these models really are,” he adds.
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