Sauropods (such as the iconic Apatosaurus) are among the very biggest creatures that ever lived. Their fossils have been found in the American West, preserved in the famous Jurassic-aged Morrison Formation and uncovered at places like Dinosaur National Monument. The geologic record tells us that this region was a seasonally dry floodplain at the time when sauropods roamed across it.
Many paleontologists have suggested that they would have had to migrate elsewhere to find food and water during the dry summer months. This week, a paper published in Nature presents geochemical evidence indicating that this migration did, indeed, take place.
Their method measures stable isotopes in water (16O and 18O, specifically), which vary geographically due primarily to fractionation during evaporation and condensation. Since 18O (which has two more neutrons than 16O) has slightly more mass, it’s a little more difficult to evaporate, and a little easier to condense. One consequence of that is that the ratio of 18O to 16O in rain water changes along storm tracks, as the 18O preferentially rains out more quickly.
The oxygen in that rain water is incorporated into carbonate that precipitates in lake sediment and soils, leaving a geologic record of this process. Analysis of oxygen isotopes in carbonate from the floodplain region of the Morrison Formation shows that it differs from the volcanic highlands that were once to the west (this is prior to the formation of the Rocky Mountains).
The oxygen isotopes in vertebrate tooth enamel also reflect the precipitation of the region in which the organisms live (for the same reason that the carbonates do). Researchers took advantage of this to analyze teeth from a genus of sauropods called Camarasaurus for evidence of migration between the seasonally dry floodplain and the nearby volcanic highlands—a journey of about 300 kilometers.
They analyzed samples taken at intervals along the length of 10 individual teeth from Dinosaur National Monument to look for changes in oxygen isotopes that were locked in place during the formation of the tooth. This technique has been successfully applied to modern animals, where seasonal trends in the data can be clearly seen. The sauropod teeth appear to have formed over 4-5 months, and do show a clear change from floodplain isotope ratios to highland ratios.
Finally, paleontologists have some evidence to support the idea that (at least some) sauropods were migratory animals. And this must have been some migration—many of these Camarasaurs were about 50 feet long.
The odd thing is that none of the teeth record the return to the floodplain, even though that’s where all the teeth were found. The authors suggest that these individuals may have died soon after returning from the highlands, as it takes roughly 2-4 weeks for enamel to start to reflect environmental conditions.
The researchers are already subjecting teeth collected from other locations to this analysis in order to paint a more detailed picture of migration patterns. They hope to find out whether migration was universal among sauropods, or populations differed from place to place.