18 April 2012

Eggs failed the dinosaurs

Agence France Presse
Their reproductive strategy spelled the beginning of the end: The fact that land-bound dinosaurs laid eggs is what sealed their fate of mass extinction while live birthing mammals went on to thrive, scientists said.
dinosaur, egg size, ecological niche, extinction

While mammals occupied the various ecological niches with different species (left), the egg-laying dinosaurs occupied the same niches with few large species - in their respective different growth stages (right). Consequently, there was no room in the niche for smaller and medium-sized species (far right). Credit: University of Zurich; Jeanne Peter

PARIS: Their reproductive strategy spelled the beginning of the end: The fact that land-bound dinosaurs laid eggs is what sealed their fate of mass extinction while live birthing mammals went on to thrive, scientists said.

The fact that dinosaurs laid eggs put them at a considerable disadvantage compared to viviparous mammals. Together with colleagues from the Zoological Society of London, Daryl Codron and Marcus Clauss from the University of Zurich investigated why and how this ultimately led to the extinction of the dinosaurs and published their findings in the journal Biology Letters today.

In a new explanation for mammals’ evolutionary victory over dinosaurs, researchers said a mathematical model has shown that infant size was the clincher.

Tiny babies

In comparison to the adults, dinosaurs had very small young. Some came out of the egg weighing as little as two to 10 kg yet had to bulk up to a hefty 30 or 50 tonnes. The staggering difference in size between newly hatched dinosaurs and their parents was down to the fact that there are limits to the size eggs can become. After all, larger eggs require a thicker shell and as the embryo also needs to be supplied with oxygen through this shell, eventually neither the shell nor the egg can grow any more. Consequently, newly hatched dinosaur babies cannot be larger in the same way as in larger species of mammal.

Growing up, the youngsters had to compete in several size categories with adults of other animal groups for food, Clauss said.

This meant that all the small and medium animal size categories supported by the natural environment were ‘occupied’, leaving no room for smaller dinosaur species in which to thrive. Young mammals are fed with milk directly by the mother, so they don’t take any niche away from smaller species.

Only small species survived

“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,” Clauss explained. “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.”

The catastrophic event that wiped out all larger life forms some 65 million years ago meant the end for terrestrial dinosaurs.

Mammals did not have the same limitations in size spread, said Clauss. This meant there were smaller mammal species able to cope with the new post-catastrophe environment and evolve into new species.

“The question that haunted some people including me is … why did the mammals survive and why did the dinosaurs not. I think we have a very good answer for that,” Clauss said. The researchers said egg size is constricted by upper limits to the thickness of shells, which have to allow oxygen through to the embryo.

The average four-tonne titanosaur, the largest type of vertebrate that ever lived, was 2,500 times heavier than its newborn. A modern-day elephant mother weighs 22 times more than her calf. Scientists say all animals with a bodyweight of more than about 10 to 25 kg died in the mass extinction event.

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