An artist's depiction of Camarasaurus - an ancient sauropod that may have undertaken seasonal migrations to find food at higher altitudes.
Credit: Wikimedia
Scientists compared dinosaur teeth to soil samples from a stretch of Jurassic rocks, and determined from isotopes that the dinosaurs would have been getting their water from higher altitudes.
Credit: Henry Fricke/ Colorado College
ABERYSTWYTH: Large, herbivorous dinosaurs undertook seasonal migrations of thousands of kilometres, travelling to high ground in search of food and water, according to new research.
Whether dinosaurs migrated has been difficult to prove, but by analysing the composition of fossils, researchers have been able to reveal the migratory movements patterns of sauropods - the largest animals ever to have lived on land.
"To date there hasn't been any direct evidence for migration of sauropod dinosaurs. The existence of trackways has been used to suggest that sauropods moved together in herds, but that is indirect evidence," said lead author Henry Fricke, from Colorado College in the U.S. of the paper published in Nature today.
"Similarly it has been suggested that these animals must have migrated because they needed to eat a lot and they would soon strip a landscape of all vegetation if they didn't move around. But again, this is a supposition, not direct evidence. This lack of solid evidence is one of the reasons the result of this study is so interesting."
Ancient giants
Sauropods were notable for their large size - with even dwarf sauropods like Europasaurus (around 5 to 6 m long) counted among the largest animals in their ecosystem. Complete fossil finds for these animals are rare, and still in these rare cases it can be difficult to determine behavioural information from bare bones.
Fricke and colleagues compared the chemical signature in teeth from a Camarasaurus, one of the most numerous sauropods discovered, to ancient soil samples taken from two sites in the Morrison basin, which is a stretch of Jurassic rocks in the western United States.
As air rises, it cools and condenses. Water molecules start to rain out, with those that contain heavier oxygen-18 atoms going first. As a result, mountain water is low in oxygen-18 and rich in oxygen-16. When compared to these sediments, Camarasaurus teeth were found to have far less of the heavier oxygen-18 isotope, relative to the lighter oxygen-16. This result implies that the animals must have obtained water from higher ground.
"Because mean isotope ratios and variance in isotopic ratios of the tooth enamel carbonates are different than those in the surrounding sediments, we concluded that the sauropods must have been drinking 'non-basin' water, and thus must have left the basin at some point during the year," said Fricke.
Further analysis of the fossilised teeth has also suggested that this migrational behaviour was seasonal. When slices were taken through individual teeth, going from the oldest outer enamel to the young root; repeated variances were found in the ratios of oxygen-18 to oxygen-16.
These patterns suggest the giant dinosaurs showed a seasonal migratory pattern as they moved from the basin into the highland regions, probably over the course of four to five months, during the dry season in search of food and water to survive.
An exciting new technique
"The exciting thing about palaeontology is that it is becoming more and more interdisciplinary. People are bringing new ideas and new techniques to the table that help answer questions we could only dream about previously," commented Phil Bell, Australian palaeontologist based at the Philip J Currie Dinosaur Museum at Alberta, Canada.
"Isotopes are a wonderful example of how palaeontology is changing and how these new techniques are influencing our way of thinking. This study finally gives good evidence for where and how far certain animals could migrate and that's a big step forward in this migration debate. Certainly, it doesn't pertain to every species but it gives researchers a method to test for behaviour, and that's something that dead things don't give up easily, no matter how perfectly a skeleton is preserved."
The researchers have also suggested these findings may reveal new clues on how these sauropods evolved to their gigantic size.
"My thinking is that if large dinosaur such as Camarasaurus - and perhaps other large sauropods - migrate, but smaller coexisting dinosaurs do not, then it could be possible to argue that migration and the evolution of super-large body sizes go hand in hand," said Fricke.
"Although Sauropods have no modern ancestor, I do think it is interesting, however, that migration appears to be a common behaviour regardless of the type of animal - as in many large mammals and birds today."
