The fact that land-bound dinosaurs laid eggs is what sealed their fate of mass extinction millions of years ago while live birthing mammals went on to thrive, scientists said Wednesday.
In a new explanation for mammals' evolutionary victory over dinosaurs, researchers said a mathematical model has shown that infant size was the clincher.
Given physical limitations to egg size, dinosaurs had comparatively small young. Some came out of the egg weighing as little as two to 4.4 to 22 pounds, yet had to bulk up to a hefty 30 or 50 tons.
Growing up, the youngsters had to compete in several size categories with adults of other animal groups for food, University of Zurich scientist Marcus Clauss told AFP.
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, according to the findings published in Biology Letters, a journal of Britain's Royal Society.
"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.
Scientists disagree on whether the scaly reptiles died out before or after a meteorite smashed into Earth in what is known as the Cretaceous-Tertiary impact, causing billions of tonnes of wind-borne ash and dust to filter out light from the Sun and triggering a "nuclear winter" that cooled the planet and withered vegetation.
Mammals did not have the same limitations in size spread, said Clauss, because their young were not born as comparatively small and did not need to compete with other species for food, instead suckling on their mothers.
This meant there were smaller mammal species able to cope with the new post-catastrophe environment and evolve into new species alongside birds, which are also dinosaurs.
"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 22 to 55 pounds died in the mass extinction event.