What killed the dinosaurs? Laying eggs, scientists say

Land-based dinosaurs died out because they laid eggs in a world in which size mattered, scientists claim.

In a new explanation for mammals' evolutionary victory over dinosaurs, University of Zurich scientists said that eggs limited the size of offspring — to as little as two kilograms. 

Growing up, the youngsters had to make it through a broad size range, competing with several size categories of adults from other animal groups for food, researcher Marcus Clauss, told Agence France-Presse.

Nature puts this in perspective, using the example of Titanosaur hatchlings, which were nearly 2,500 times smaller than the 4-tonne adults, while the live-born calf of an Asian elephant is only 25 times smaller than its mother.

As those categories being supported by the natural environment were already occupied, there was no room in which smaller dinosaurs could thrive, according to the researchers' findings, published in Biology Letters.

Further, the asteroid that hit the earth 65 millions years ago wiped out most large-bodied animals, and almost almost obliterated small dinosaur species, leaving only the birds, Nature wrote.

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By contrast, the new-look planet favored the many small mammals alive at the time, the journal added.

"There is a lot of room in the ecosystem for small species, but [in such a scenario] that room is taken up by the young ones of the large species," AFP quoted Clauss as saying. 

"That was not a problem for 150 million years but as soon as something happens that takes away all the large species so that only small species remain, if there are no small species to remain you are gone as a whole group."

Lead author Daryl Codron, a post-doctoral researcher at the University of Zurich’s Clinic for Zoo Animals, Exotic Pets and Wildlife, told Discovery News that: "The most successful [dinosaurs] were the very large ones that were able to escape the competition trap and replenish their numbers. After the mass extinction, they again tried to evolve large size, but to escape the competition trap they had to become multi-ton animals." 

There is contention among scientists over whether dinosaurs died out before or after a meteorite smashed into Earth and ash and dust filtered out light from the Sun, triggering a "nuclear winter" that decimated food sources.

However, Clauss made the point that mammals not only did not have the same limitations in size spread, but also did not need to compete with other species for food, instead suckling on their mothers. 

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