Dinosaurs known as sauropods—the largest land animals that ever lived—grew huge and were an evolutionary success in part because they didn't bother to chew their food, new research suggests.
Sauropods weighed up to 88 tons (80 metric tons)—ten times more than an African elephant—and measured as high as 23 feet (7 meters).
The group of dinosaurs, which included the brachiosaurus and diplodocus, loomed over the animal kingdom for more than 140 million years until the late Cretaceous period, 65 million years ago. (See a brachiosaurus herd.)
Scientists think the animals evolved to be so large to discourage big predators, like Tyrannosaurus rex, from eating them. But how they maintained such massive body sizes has remained mysterious.
The herbivores, or plant-eaters, had hardly any teeth and are thought to have swallowed their food whole—an entire bush in one gulp, for example. They browsed large areas, barely moving and consuming vast quantities in short periods of time.
So they needed long necks to reach food high in trees and a huge gut to process and break down their unchewed meals, said Martin Sander, a palaeontologist at the University of Bonn in Germany and co-author of the study, published tomorrow in the journal Science.
"You can only have this long neck if you don't chew your food, otherwise your head would be full of teeth and too heavy to support," he said.
Paul Upchurch, an expert on sauropods from University College London, said that "most palaeontologists agree that feeding is the key to understanding sauropod gigantism."
(Related: "Bizarre Dinosaur Grazed Like a Cow, Study Says" [November 15, 2007].)
Survival Strategies
To outgrow their predators, sauropods didn't just need lots of food. They also needed to develop fast, so they could attain their full size before being eaten, experts said.
Sauropod bones show that they did indeed grow swiftly. A 22-pound (10-kilogram) hatchling could become a 220,000-pound (100,000-kilogram) grown-up in about 20 to 30 years—quick by dinosaur time.
"This tells us that they must have been warm-blooded and had a high metabolic rate compared to cold-blooded creatures," said the University of Bonn's Sander.
Like all dinosaurs, sauropods laid nestfuls of eggs. By producing so many young at a time, "a population could recover quickly, even after a big catastrophe," Sander said.
Large modern mammals, such as elephants, give birth to far fewer offspring, raising their chances of extinction should a disaster occur.
So why don't we see gigantic elephants and crocodiles roaming around today?
Experts think that reptiles, such as crocodiles, still maintain the egg-laying advantage, but their cold blood prevents them from growing fast enough to reach a great size.
Mammals have warm blood, but can't grow as big as sauropods due to their slow reproductive strategy and the need to chew their food.
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