Impressed by the spectacle of wildebeest swarming across the Serengeti? Now imagine vast herds of sauropods seasonally moving out of the dry floodplains of the western US into the highlands.
The first persuasive evidence that this happened comes from a study of dinosaur teeth from Camarasaurus, one of the most common species of sauropod. It grew to 15 metres long and lived in Wyoming and Utah in the late Jurassic. Its fossils have been found on what was once a low-lying floodplain, which would have periodically dried out.
Henry Fricke of Colorado College in Colorado Springs wondered if they migrated to nearby hills to find food and water during the dry season.
To find out, he looked at the oxygen isotopes in 32 fossil teeth. The ratio of isotopes is determined by the water the dinosaurs drank. He found the ratio in teeth was different to that in carbonate rock from the floodplain – which carries the signature of the water it formed in. This suggests that Camarasaurus sometimes left the area.
By drilling through the layers in a tooth, Fricke could track the different sources its owner drank from.
One tooth revealed that the oxygen isotopes gradually changed over five months, strengthening the idea that the migration was seasonal. The most likely interpretation, says Fricke, is that the dinosaurs moved to greener pastures at higher altitudes.
Palaeontologists have long suspected that some dinosaurs migrated, but this is the first solid evidence of it, says Paul Barrett of the Natural History Museum in London. Camarasaurus must have put a lot of pressure on food resources, so it makes sense that they moved around. Barrett suspects rarer sauropods such as Haplocanthosaurus didn’t need to migrate.
It’s likely Camarasaurus had company on the long treks. When modern herbivores migrate, they are followed by predators, so the same may have happened in the Jurassic. The most common local predator was Allosaurus, a distant cousin of T. Rex. Fricke is trying to find out if they tracked Camarasaurus. It would make sense if they did: “A mass migration,” says Barrett, “is basically a huge walking supermarket.”
Journal reference: Nature, DOI: 10.1038/nature10570