Large dinosaurs migrated huge distances, say scientists

An analysis of dinosaur teeth suggests the creatures may have migrated from flood plains to uplands during the dry season

The largest dinosaurs to walk the Earth may have embarked on seasonal migrations that covered hundreds of kilometres when local watering holes dried up and food became scarce.

Evidence that giant sauropods set off on epic journeys came to light when scientists examined fossilised teeth recovered from the remains of beasts unearthed in Wyoming and Utah in the US.

The analysis of 32 teeth belonging to two species of Camarasaurus, among the most common sauropods found in North America, suggests the creatures migrated during hot, dry summers, from their usual habitats on flood plains in search of food and water in surrounding uplands.

Some return journeys required the dinosaurs to cover distances of around 300 kilometres (190 miles) in each direction. The long-necked herbivores measured 20 metres from nose to tail in adulthood and weighed around 18 tonnes.

The arduous treks pushed the lumbering animals to their limit, and some appear to have died soon after returning to their lowland homes, before the rainy season brought fresh water to parched soils and vegetation flourished once more.

Understanding the ranges and seasonal movements of the animals will help scientists piece together the role of migrations on Jurassic ecology and any bearing this had on the evolution of gigantism among dinosaurs.

"The question of how sauropods got to be so big is one that is still being actively studied. There's evidence that some of the reason is that they didn't have the dental morphology to chew their food, so in order to get enough energy their guts got bigger, and they did more processing in their stomachs," said Henry Fricke, head of geology at Colorado College in Colorado Springs, who led the study which is published in Nature.

"Migration could come into the story of gigantism as a feedback process. Once they started to get big, it would be easier for them to migrate and get more food more consistently, which would help them to grow even more," he added. Moving long distances gets more energetically efficient the bigger strides a creature can take, so it would be highly inefficient for a mouse, for example, but much more efficient for a large dinosaur.

Fricke's team attempted to reconstruct camarasaur migrations by measuring oxygen isotopes (variants of particular elements that have different numbers of neutrons in their nucleus) in their teeth. The work relied on the fact that ratios of two oxygen isotopes differ markedly in the waters of streams and lakes, depending on local environmental conditions, such as how high and arid the landscape was at the time.

The dinosaurs kept an unwitting record of these oxygen isotopes as they roamed the land, because the oxygen in the water they drank became incorporated into successive layers of enamel as their teeth developed.

Most of the teeth, from remains collected at Thermopolis in Wyoming and Dinosaur National Monument in Utah, were worn and retained only a month or two of enamel growth, but others were in far better condition with up to four or five months of enamel still intact.

The scientists analysed oxygen isotopes in the dinosaurs' teeth and compared them with ancient soil samples from their lowland habitats and bordering uplands. From this, they pieced together the dinosaurs' movements over several months of their lives, concluding that the beasts made seasonal migrations to the uplands. Studies of one tooth suggest the dinosaur left its lowland habitat to find food and water in the highlands and returned home within five to six months.

"What was up in the highlands food-wise we don't know, the land is weathered away, but the conditions may not have been as hot and dry, and it may even have rained more continuously at the higher elevations," Fricke said.

"This is a neat example of how we can bring geochemical methods to bear on an issue, how we can learn something about dinosaur behaviour that we can't learn from looking at the morphology of the fossils themselves," he added.


Ian Sample, science correspondent

The GuardianTramp

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