The Forbidden Zone was once a paradise
Large eggs must have thick shells, which make it difficult for the developing embryo to ‘breathe’ by exchanging gases with the outside world. This places an upper limit on egg size.
So, even though they grew into giants as adults, dinosaurs were forced to produce relatively tiny young. Titanosaur hatchlings, for example, were nearly 2,500 times smaller than the 4-tonne adults. By contrast, the live-born calf of an Asian elephant (Elephas maximus) is about 25 times smaller than its mother.
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.
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 favored diminutive species.
