October 27, 2011
Dinosaur Teeth Reveal Large Migrations
An analysis of fossilized teeth from dinosaurs in the western United States has provided the first concrete evidence that sauropods undertook seasonal migrations in search of food.
Scientists have often assumed that dinosaurs did, in fact, migrate. However, it is difficult to determine from fossils even what dinosaurs looked like, let alone infer their behavior.
But Henry Fricke, head of geology at Colorado College in Colorado Springs, and his colleagues seem to have found solid evidence that sauropod dinosaurs moved hundreds of miles each year.
"In a theoretical sense, it's not hugely surprising. They are huge – they would probably have eaten themselves out of house and home if they stayed in one place," Fricke told Nature News.
"Now we have evidence that demonstrates that, and a method to move forward and study other dinosaurs."
Fricke and his colleagues analyzed fossil enamel chipped from 32 teeth that belonged to sauropods of the genus Camarasaurus. The teeth were collected at Thermopolis in Wyoming and Dinosaur National Monument in Utah, and date from the late Jurassic period (160 million to 145 million years ago).
Some samples 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 researchers measured the ratios of two isotopes -- oxygen-16 (16O) and oxygen-18 (18O)-- in the enamel, and compared them with the ratios in the sedimentary rocks found in the surrounding area.
Isotopes are variants of particular elements that have different numbers of neutrons in their nucleus.
In vertebrates, the oxygen ratio relates to the ratio of oxygen isotopes in the water they were drinking as their teeth were growing.
Since the sedimentary rocks provide a record of local ratios, if there is a mismatch between the oxygen ratios in a tooth and those of the surrounding rocks, the creature must have been somewhere else when growing that tooth -- which is precisely what the researchers found.
Some teeth matched the basin, however others had a much lower proportion of 18O, indicating that the sauropods had likely spent time at higher elevations, where oxygen levels are low because the heavy isotope is rained out of clouds first as they rise and cool.
Fricke hypothesized that the Morrison basin Camarasaurus must have migrated at least 185 miles between the basin and highlands to the west.
The researchers then analyzed the teeth from one animal, beginning at the bottom of the tooth and working up, and found that the youngest enamel from the base of the teeth indicated a highlands location, while the older enamel nearer the tip was formed in the basin environment.
This implied that the dinosaur had moved out of the basin into the highlands when the teeth were forming.
However, because the sauropod was found in the basin, it must have moved back there at some time -- likely in a seasonal migration.
"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 behavior that we can't learn from looking at the morphology of the fossils themselves.”
Understanding the movements of the dinosaurs will help scientists determine the role these migrations play on Jurassic ecology, and any impact this may have 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," Fricke said.
"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.
Indeed, moving long distances becomes more energetically efficient the bigger the strides a creature can take. For instance, it would be highly inefficient for a mouse to travel long distances, but much more efficient for a large dinosaur.
Fricke said he intends to conduct similar studies using teeth from other dinosaurs found in the region.
"The ones we're really interested in right now are the associated carnivores," he said.
"The question is whether the carnivores stayed in on place and waited for the sauropods, [or whether] they followed them on their migrations."
While results from the current study are only preliminary, Fricke said he is confident he will be able to determine whether other dinosaurs remained at home or joined the sauropods on their migration.
The study was published online in the journal Nature.
Image 1: Artist's impression of a C. supremus herd. Credit: Dmitry Bogdanov/Wikipedia (CC BY 3.0)
Image 2: Justin Hencecroth at Dinosaur National Monument. Credit: Colorado College
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