Where the dinosaurs roamed

The largest animals to walk on Earth were the sauropods, dinosaurs such as Diplodocus and Brachiosaurus, some of which might have been longer than 100 feet and weighed around 100 tons.

A herd of sauropods could not have remained in one place for long before it had consumed all available fodder. Paleontologists reasoned that they probably migrated, perhaps seasonally. From large accumulations of bones of single species and from their tracks, we’ve known for some time that sauropods traveled in herds.

Although biologically plausible, such behavior doesn’t fossilize. Or does it?

Oxygen, like most other elements, occurs in different varieties called isotopes, which differ slightly in their mass. Oxygen has two common isotopes — oxygen 16 and oxygen 18. Different geographic areas have drinking water with different ratios that are determined, at least in part, by elevation.

Sauropods such as Diplodocus likely migrated
Sauropods such as Diplodocus likely migrated

The hard component of bones and teeth is the mineral apatite, a compound made of the elements calcium, phosphorus and oxygen. Apatite in tooth enamel is relatively resistant to chemical change, so the oxygen isotope ratios in fossil teeth can show us the ratios of those areas where the animals lived when the teeth were being formed.

Research recently published in the journal Nature looked at the oxygen isotope ratios of 32 teeth of Camarasaurus, a sauropod that was about 60 feet long and weighed about 25 tons. The teeth were found in the famous Morrison Formation of Wyoming and Utah.

Some parts of the teeth had ratios that matched the areas in which the bones were found. But other parts had ratios that did not. Those ratios indicated that those parts of the teeth were formed when the animal lived at a higher elevation.

That higher area was probably a region called the Mogollon Highlands, a volcanic area in Arizona that was at least 180 miles away. From known growth rates of the teeth, the animals completed the 370-mile round trip in about five months.

Continuing research will look at smaller dinosaurs, ones presumably too small to have made such long journeys, and at carnivorous dinosaurs, which supposedly stayed in one area and waited for their dinners to come to them.

– by Dale Gnidovec, curator of the Orton Geological Museum at Ohio State University