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Dinosaurs were born to be big

December 20, 2012 / 11:36 AM EST / Livescience.com

Dinosaurs as a group may have been skewed more toward giant species than modern-day creatures are, researchers say.

The findings, based on comparisons of size ranges among extinct and current species, shed light on just how different the world was during the age of dinosaurs, scientists added.

Dinosaurs included the largest animals ever to walk the Earth. The giants developed early within the dinosaur lineage, with massive, long-necked, long-tailed sauropods evolving by about 200 million years ago.

Although scientists have discovered many giant dinosaurs over the years, many researchers, including, paleontologist David Hone at Queen Mary University of London, assumed dinosaurs came in the same range of small and large species as modern animals do, "and it was just my familiarity with the larger ones that distorted my perception."

"Turns out, nope, there really were tons and tons of big guys out there and not many little ones," Hone told LiveScience.

Hone and his Queen Mary colleague Eoin O'Gorman, a community ecologist, analyzed the sizes of modern birds, reptiles, amphibians, fish and terrestrial mammals. They also looked at dinosaurs, the prehistoric winged reptiles known as pterosaurs, and terrestrial mammals that vanished before the modern era. [Gallery: Stunning Illustrations of Dinosaurs]

Skew toward giant

The investigators scanned past research on the sizes of thousands of animal species, including approximately 330 dinosaur species. They found that dinosaurs were dramatically skewed toward large sizes, a pattern significantly different from all other groups of animals they studied.

The most obvious question regarding this work is whether or not large dinosaur fossils survived more successfully than smaller ones, influencing the analysis.

"Indeed, large things do tend to be more readily preserved and smaller ones harder to find, so we would expect there to be a bias in the results towards larger species," Hone said. "The question is, is that enough to affect our results?"

They don't think so.

"First off, the differences are so colossal that the amount of small species missing would be truly enormous, and it's unlikely we, as in the scientific community, have missed that many," Hone said.

Indeed, paleontologists would have had to miss 99.99 percent of dinosaur diversity to explain the huge skew toward giant sizes that O'Gorman and Hone found.

In addition, this skew is not seen with the other extinct groups of animals the scientists examined, including. This includes the pterosaurs, which wereclose relatives of the dinosaurs and lived alongside them in similar environments.

Bigger is better?

Rather, the environments and biology of dinosaurs might explain this unique skew.

"As you get bigger, you get more energetically efficient, as being big gives you a longer digestion time and returns more energy from eaten food," Hone said. "But mammals can't get that huge or they would overheat."

Mammals are endotherms, or "warm-blooded," meaning they constantly generate their own heat. In contrast, big dinosaurs probably stayed warm because their enormous mass helped retain heat, just as large pots of hot water would take longer to cool down than smaller ones.

In addition, while mammals can have bigger and bigger young as they get larger, often growing their offspring inside themselves, "you can't lay larger and larger eggs," Hone said. The embryos inside eggs need the shells of eggs to be thin enough to allow them to breathe in oxygen and get rid of carbon dioxide, and if eggs grow too massive, the shells will not be strong enough to protect the eggs.

"So large dinosaurs still laid small eggs. That meant small babies." The large number of eggs and small babies that big dinosaurs likely had suggests the roles that small species often play in environments may have been filled by the young of large species.

If these findings are true, they suggest the age of dinosaurs behaved in ways fundamentally different from the modern world.

"These must have been very harsh environments to live in, with some of the most efficient predators the planet has known," O'Gorman said. Survival amid predators that preyed on the small likely involved producing many young with very rapid growth rates. [Image Gallery: Dinosaur Daycare]

O'Gorman noted this pattern of size ranges they discovered was different between herbivorous and carnivorous dinosaurs. The sizes of herbivorous dinosaur species were biased toward giants, while carnivorous dinosaurs had nearly as many smaller species as large ones. That is likely because the herbivorous lifestyle allows a larger size limit, so that the larger they get, the more energy they can get from digested food. The same apparently did not hold true of a carnivorous lifestyle -- they could only get so big before they reached a maximum in terms of benefits.

In addition, this skew toward giant sizes generally happened toward the end of major time periods, such as the Late Triassic, Late Jurassic and Late Cretaceous. The researchers suggest the evolution of large body sizes among dinosaurs was the result of long exposures to stable environments, and was reset by the mass extinctions that occurred at the end of these periods.

A mystery that remains is why the researchers did not find this pattern in reptiles, amphibians or fish, which are like the giant dinosaurs were not endoderms. Although the vast majority of modern reptiles and amphibians are carnivores, many fish are herbivores.

"I would be very interested to explore the fish data more thoroughly, subdividing by herbivorous and carnivorous groups as we have done for the dinosaurs," O'Gorman told LiveScience. "This may help to validate the dinosaur results further if we find a consistent trend."

The scientists detailed their findings online Dec. 19 in the journal PLoS ONE.

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