Dinosaurs were doomed because laying eggs made them too hugeS

Sure, an impact event is what started wiping out the dinosaurs 65 million years ago. But why did some species manage to survive while most of the dinosaurs perished? A fascinating new theory suggests it's all about laying eggs.

To understand what's going on here, we first have to consider just why so many dinosaurs got so gigantic, with even mid-sized dinosaurs outpacing the largest land mammals alive today. Earlier explanations have suggested changes in atmospheric conditions, such as heightened oxygen levels or higher temperatures, might have provided dinosaur species with the energy required to evolve into much larger creatures. But recent research at Germany's Ludwig Maximilian University suggests there's no such link between changing atmospheric conditions and when dinosaurs began to get big. So what else could be going on?

The next most likely explanation is that the dinosaurs had biological incentives to expand in size. As Nature reports, University of Zurich zoologist Daryl Codron suggests the answer may lie in the eggs dinosaurs laid. There are strict limits in how big an egg can get before its shell would be to thick to take in enough gases to sustain the embryo inside, which means even the biggest dinosaurs would produce very small young, some 2,500 times smaller than the adult versions. By comparison, large land mammals are only about 25 times bigger than their young. And Nature explains why such tiny young would have paradoxically made dinosaurs grow huge:

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.

Such large creatures would have enjoyed huge advantages over other dinosaurs, but their giant stature also meant they stood no chance of surviving the cataclysms brought on by the giant impact 65 million years ago. By this point, there were so few small dinosaur species left that the odds were tiny that any of them would endure, with only the bird-like species able to live on and ultimately evolve into, well, birds. You can check out Nature for the whole story.

Image by Linda Bucklin, via Shutterstock.