Dinosaur Eggs Reveal Potential Answers

The Cretaceous – Tertiary impact, 65 million years ago, was one of the great cataclysms in the history of Earth. After this huge meteorite impact, all of the animals on earth with a body weight greater than 20 or 55 pounds were wiped out. In this mass extinction, the entire group of dinosaur species disappeared off of the face of the earth. This much has been well known for years, but there remained some questions that scientists could not answer.

Why was it that, after the impact, it was mammals rather than dinosaurs that adapted to the new conditions? Why did mammals thrive and dominate the earth, while dinosaurs vanished? Now, thanks to mathematical modeling, researchers at the University of Zurich say they have an answer.

The downfall of the dinosaurs came for a simple reason: they were egg layers. And, as such, they faced challenges unknown to mammals.

The Size of Dinosaur Eggs was not Relative to their Adult Size

Because dinosaur eggs shells need to be thin enough to let oxygen through to the embryo, there is an upper limit on how large dinosaur eggs can be. So there is also an upper limit on the size of newborn dinosaurs. As a result, large dinosaurs grew through an astonishing range of sizes. A dinosaur might hatch from its dinosaur egg weighing only two to ten kilograms, and yet it would grow to reach an adult size of thirty to fifty tons. The adult Titanosaur – the largest vertebrae ever to walk the earth – weighed 2500 times as much as its newborn. In comparison, an African elephant fully grown is only twenty two times the size of its newborn.

Smaller Sized Young From Dinosaur Eggs Meant More Competition

Because of the small size of dinosaur eggs and this demand for incredible growth, a young dinosaur would have to compete in the full range of size categories as it grew up. With the help of their powerful parents, these young dinosaurs were so successful that they crowded out any smaller species from the evolutionary chain. And so when we picture the age of dinosaurs, it is huge lumbering reptiles that we picture. And correctly so, because there were few smaller species. Any smaller dinosaurs were crowded out of the ecosystem by the larger ones. Unable to compete, they could not survive in this world of giants.

Monstrous Size Led to Ultimate Downfall in a Nuclear Winter

For 150 milllion years, massive dinosaurs thrived. But when the meteorite hit, their strength became a weakness. Upon impact, millions of tons of ash and dust were lifted into the atmosphere, blocking out the sun and destroying vegetation. These harsh conditions created a “nuclear winter” where larger animals could not survive. The Titanosaur, the Tyrannosaurus Rex, all of the great creatures of the Cretaceous Period could not adapt to the harsh new environment. And so we know them today only by the bones they left behind.

Perhaps, if there had been a greater diversity of smaller dinosaurs, they would have been able to survive in the nuclear winter as small mammals did. But for millions of years the larger species had dominated the land, with their small but powerful offspring taking up as many resources as the environment could provide. Smaller species had no chance and could not evolve. Then when the meteorite wiped away the large dinosaurs, there were no small ones to take their place. The entire dinosaur group was eliminated, and it was mammals – up to that point a minor category of life – that survived and multiplied and eventually evolved into the species we know today.

The Co-existance of Diverse Mammals Led to Improved Survival

Because young mammals suckle from their mothers, they do not need to compete for food until they are closer to fully grown. So each species competes only within a narrow size range. This has allowed a diverse array of mammals to co-exist, from tiny rodents to enormous elephants. When the mass extinction occurred, the diversity of mammals meant that there were small species capable of survival. So the tiny mouse could live on, scavenging for what little vegetation remained in this cold, dusty world. But the enormous Tyrannosaurus required more resources than the earth could provide. The great dinosaurs were far too large to survive in this harsh climate, and soon they had all vanished. Dinosaurs had to be born small, and born small meant that they had to compete with other small species for survival. In this competition it was impossible for small dinosaurs to survive and evolve when pitted against larger mammals. There was only room for large species and mammals beat the dinosaurs to the punch. So only a rare few types of small dinosaurs existed – enough to evolve into modern birds, but not enough to maintain the group’s dominance…all because of the design of dinosaur eggs.