Like many birds and mammals today, ancient plant-eating dinosaurs migrated hundreds of miles each year as seasons changed, according to a study published online Wednesday by the journal Nature.
Scientists have long suspected that camarasaurus – a 50-foot-long, 20-ton dinosaur that lived 145 million years ago during the late Jurassic Period - migrated.
But what really surprised scientists was how far these big lizards walked: a six-month, 186-mile trek from lowlands to the mountains and then back again.
"That's a lot of walking to do over the course of a year," said the study's lead scientist, Henry Fricke of Colorado College.
The research touches on key questions among dinosaur experts: How did these giant beasts behave, and why were they so big?
Fossilized teeth and chemicals called oxygen isotopes may have unlocked a few clues.
Fricke and his team spent four years analyzing oxygen isotopes in fossilized camarasaurus teeth found in Wyoming and Utah.
Here's a basic idea of how it worked. Water across the ancient landscape contained specific ratios of two isotopes: oxygen 18, which has eight protons and 10 neutrons in its nucleus, and oxygen 16, which has eight protons and eight neutron in its nucleus. Researchers were able to track locations where the dinosaurs drank their water by examining the isotopes built up in the fossilized tooth enamel, like a "tiny tape recorder of what animals were drinking," Fricke said. From this data, scientists tracked the dinosaurs from lowlands in what is now Wyoming and Utah to mountainous regions to the west.
They also used the teeth to build a timeline to track approximately when the dinosaurs drank their water by examining the layers of oxygen isotopes in their tooth enamel, sort of like rings in tree trunks.
So, why did these giant beasts migrate? Their motivation was probably to find the food and water they needed to survive, Fricke said.
"They left the lowland basin in the dry season, presumably summer, when plant growth was limited and drought may have been common. They returned in the wet season, presumably winter." A wetter annual climate would have supported more plant growth, Fricke speculates, and may have resulted in less movement by the camarasauruses. Fricke believes analyzing tooth fossils of other large dinosaurs might bring similar results.
This general approach to studying animal migration has been around for decades, Fricke said, "but this is the first time it's been applied to dinosaurs."
Renowned Montana State University paleontologist Jack Horner, who served as technical adviser for the 1993 film "Jurassic Park," says behavior is among the most difficult things to learn about dinosaurs, because so much depends on what evidence is found.
"Unfortunately, it has to do with serendipity more than anything else," he said. "It's frustrating, but on the other hand, we make a lot of discoveries we may not expect."
"Jurassic Park" really drives home how hard it is to infer behavior based on fossil evidence, Fricke said. "All the questions that the paleontologists had in that movie were about behavior."
One question that paleontologists still don't have an answer for is, why did camarasaurus evolve into such gigantic animals? Fricke suspects there is a relationship between dinosaur size and migration.
"The ability to migrate gave camarasaurus an option to obtain food, which could allow them to get big," Fricke said. "Once you get big, it's easier to migrate, because you've got longer limbs to walk with. So it becomes a sort of feedback cycle."