Message in a dinosaur’s teeth

Spinosaurs were large, meat-eating dinosaurs whose fossilized remains are often found in the same areas as the bones of tyrannosaurs (such as Tyrannosaurus rex). Fans of the movie Jurassic Park III may remember the spinosaur as the cranky dinosaur — the one with a sail-shaped fin on its back — that destroyed an airplane, ate a few people and took down a T. rex.

Paleontologists have wondered how such giants as spinosaurs and tyrannosaurs, both meat-eating and ferocious, could live in the same place while competing for food. In a recent study, a French researcher named Romain Amiot may have found the answer. Amiot, from the University of Lyon 1 in Villeurbanne, France, thinks that spinosaurs may have spent parts of their days in the water, thus avoiding clashes with tyrannosaurs, which lived on the land.

This study isn’t the first one in which scientists have suggested spinosaurs spent time in the water. The creatures’ fossilized skeletons show they had long snouts, the way crocodiles do, and studies of spinosaur fossilized stomachs show that the creatures ate fish. But spinosaur skeletons don’t show adaptations to living in the water or swimming — they don’t have specialized feet, for example.

In this case, however, the bones didn’t tell the whole story. For Amiot and his team, it was the dino teeth that did the talking. Spinosaur teeth are smooth and shaped liked cones — more like those of modern crocodiles than of tyrannosaurs. An analysis of the chemical makeup of the teeth turned up even more evidence.

In particular, the researchers studied oxygen. At its center, an atom contains a nucleus, and the nucleus of an oxygen atom usually contains eight protons and eight neutrons. (Protons and neutrons are the particles in the nucleus of every atom.) But some kinds of oxygen are heavier — most of its atoms may each have 10 neutrons, instead of eight, for example. When an oxygen atom has 10 neutrons, or 18 total particles in its nucleus, it is called oxygen-18. In general, when an atom has a different number of neutrons in its nucleus, it is called an isotope. Oxygen-18 is an oxygen isotope.

Animals that live in the water, such as hippos and crocodiles, have different proportions of oxygen and oxygen-18 in their bones and teeth than animals that live on land. Amiot and his team looked at the proportions of oxygen isotopes in the fossilized spinosaur teeth. Comparing these ratios to those found in fossilized teeth and bones from other animals of the spinosaurs’ day, the researchers found a closer resemblance to water animals such as crocodiles and turtles than to land animals such as tyrannosaurs.

This analysis shows that spinosaurs probably spent part of their lives in lakes and rivers. This may solve the riddle of the grumpy neighbors: If spinosaurs lived and fed in the water, then they wouldn’t be competing with tyrannosaurs on the land. (And if the spinosaur had simply stayed in the water in Jurassic Park III, the plane would have been okay, the people could have left, and the movie would have been a lot shorter.)

The study may “solve the big ecological problem of how spinosaurs could live in the same areas as tyrannosaurs,” Amiot told Science News. “They were avoiding competition for food and territory by dividing up the ecosystem.”

Going Deeper:

Perkins, Sid. 2010. “Sail-backed dinos had semiaquatic lifestyle,” Science News, February 12. http://sciencenews.org/view/generic/id/56282/title/Sailbacked_dinos_had_semiaquatic_lifestyle