Big may not be mighty

Why Huge Dinosaurs Had Such Tiny Babies
In the end, dinosaurs were no match for mammals and the main issue was their egg-laying.

By Jennifer Viegas
Tue Apr 17, 2012 


Larger mammals can have larger babies, but dinosaurs could not due to the physical limitations of laying eggs.
Most dinosaurs were either large or small, but mammals can fill all body size niches in the ecosystem.
When a catastrophic event wiped out larger species 65.5 million years ago, mammals were better able to recover.

dinosaur nest
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Egg-laying may have helped dinosaurs get big, but it didn't help their survival in the end. Click to enlarge this image.
Julius Csotonyi

A new study may explain many mysteries about dinosaurs, such as why enormous species had such small offspring, why non-flying dinos went extinct, and why today’s birds fly.

The paper, published in the journal Biology Letters, emphasizes how mammals and birds -- but not non-avian dinosaurs -- were able to persist beyond a major extinction 65.5 million years ago. The large body size and egg-laying ways of dinos may have helped to do them in, along with hungry mammals.

"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," lead author Daryl Codron told Discovery News.

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"It seems that with all the mammals that were now in the way, birds simply could not become bigger," added Codron, a post-doctoral researcher at the University of Zurich’s Clinic for Zoo Animals, Exotic Pets and Wildlife.

Although large flightless birds exist today, such as ostriches and emus, they are a far cry from the gigantic sauropods and other big non-avian dinosaurs.
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For the study, Codron and his team simulated both dinosaur and mammal communities comprising species from 27 size categories including the largest and smallest known species of each group. The researchers next used mathematical calculations to simulate what would happen to the adults and offspring of the animals, based on size and resource competition.

While larger mammals can have larger babies, the dinosaurs faced the physical limitations of laying eggs. This meant that even enormous dinosaurs, weighing up to 150 tons, had tiny offspring.

"One cannot have a very large egg with a very thin shell, otherwise it would simply break open," Codron explained. "The shell itself is limited in thickness. It cannot become so thick that gas can no longer diffuse through it, which would deprive the embryo of oxygen. For a very small egg-laying animal this is no problem, but it does mean that very large animals have to produce relatively small eggs."

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Since very large adult dinosaurs tended to out-compete medium-sized adults, this and the egg limitations meant that most dino species were either small or large, with a gap in the medium size range, according to the researchers. In contrast, mammals fill all body size ecosystem niches available to them.

Once the catastrophic event 65.5 million years ago wiped out larger dinosaur species, dinos then had less surviving species from which to refill empty spots in the food chain. Mammals diversified and flourished, while non-avian dinosaurs disappeared.

The theory, however, could explain why living dinosaurs -- birds -- survived and evolved flight.

Codron said that "many people assume that dinosaurs were superior competitors, but there is now evidence that mammals were already diverse in those (dino-era) times, albeit small, and mammals may have preyed upon small dinosaur individuals and or eggs."

"Thus, our results might actually provide an explanation for the evolution of flight," he continued. "These small dinosaurs simply had to find a new niche away from mammals because they suffered competition from both each other and from mammals."

Christine Janis, a professor of biology at Brown University, pointed out that a fossil of Repenomamus, the largest mammal known from the Cretaceous period, had a juvenile dinosaur in its stomach.

"Nobody thinks about the fact that all dinosaurs went through small, fairly independent stages where competition with mammals could be an issue, and that this could affect repopulation outcomes following a mass extinction," Janis said.