National Geographic Daily News
Dinosaur picture: Camarasaurus.

A Camarasaurus dinosaur (illustrated above) could grow longer than a school bus.

Illustration by Dorling Kindersley, Getty Images

Brian Handwerk

for National Geographic News

Published October 27, 2011

Sauropod dinosaurs may well have been the largest animals to ever walk—and walk and walk and walk—the Earth.

A new fossil-teeth analysis has uncovered the best evidence yet that dinosaurs migrated seasonally like modern-day birds or elephants, according to a new study.

(See pictures of great migrations.)

Chemical signals in prehistoric tooth enamel reveal that roughly bus-length Camarasaurus dinosaurs walked hundreds of miles on marathon migrations in late Jurassic North America.

Responding to shifts in food and water availability, the long-necked plant-eaters likely trudged from floodplain lowlands to distant uplands and back again as the seasons changed across parts of what are now Utah and Wyoming, researchers say. (See a sauropod picture.)

"On the African Serengeti the large mammals do a wet season-dry season migration and that's sort of what we're envisioning here," said Colorado College geochemist Henry Fricke, who led the study, published today in the journal Nature.

Fricke and colleagues theorize that the dinosaurs left a basin floodplain area at the onset of the summer dry season, when droughts may have been common and plant supplies limited.

The sauropods moved nearly 200 miles (300 kilometers) to highlands that were presumably cooler and wetter in that season, Fricke said. That's tough to prove, though, because erosion of the areas' ancient rock has deprived scientists of crucial evidence needed to "reconstruct" the long-gone highlands, he said.

The animals likely returned to the lowlands during the wet winters, when food and drink were again plentiful, he added.

(Related: "Huge New Dinosaur Trackway Found in U.S.")

The Telltale Teeth

Scientists have long suggested that some dinosaurs might have migrated, but it's been extremely hard to find any evidence for the behavior.

Fricke and colleagues compared ratios of oxygen isotopes—varying versions of the element—in the fossil Camarasaurus teeth with oxygen isotopes found in prehistoric layers of lowland soil.

(Also see "Biggest Dinosaurs Grew Huge By Not Chewing Their Food.")

Because the dinosaurs' teeth were replaced roughly every five months, each tooth offers a unique record of what the animal drank during the tooth's life span.

The soil and teeth turned out to have distinctly different oxygen-isotope ratios, suggesting the teeth had formed elsewhere, for the most part. The ratios in the teeth, tellingly, were akin to what you'd expect had the teeth grown at high elevations.

"We concluded that the sauropod had to be leaving the basin areas and going somewhere else," Fricke said.

And they may not have been alone. Fricke wonders whether the huge plant-eaters were followed in their migrations by predators like Allosaurus (picture), like wagon trains trailed by fleeter-footed marauders—the subject of a future study, he said.

Quiz: Test your dinosaur IQ >>

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