Sail-backed dinos had semiaquatic lifestyle

Isotopic analyses of fossils suggest crocodile-like habits

Paleontologists may have solved the mystery of how spinosaurs and tyrannosaurs — two dinosaur groups that included many large, fierce predators — could have lived in the same regions at the same time. A new study suggests that, like many pairs of surly neighbors, they simply avoided each other.

SPLISH SPLASH Oxygen isotopes in the fossilized teeth of spinosaurs hint that the creatures (shown in an artist’s reconstruction) spent much of their time in the water, as crocodiles and hippos do today. © M. Simonetti/courtesy of CNRS

Spinosaurs, the sail-backed dinosaurs made famous by a star turn in the movie Jurassic Park III, belonged to a group of meat-eating dinosaurs called theropods. But unlike tyrannosaurs and most other theropods, spinosaurs had conical, unserrated teeth similar to those found in crocodiles, says Romain Amiot, a paleontologist at University of Lyon 1 in Villeurbanne, France, and coauthor of the new study. And although spinosaurs had long, crocodile-like snouts and fossilized stomach contents suggest their prey included fish, the creatures didn’t have any skeletal features that hint at a semiaquatic lifestyle, such as feet specialized for swimming.

But the ratios of oxygen isotopes in the creatures’ teeth tell a different story, Amiot and his colleagues report in the February Geology. Creatures that spend a lot of time in and around the water — like hippos and crocodiles — typically have lower oxygen-18 to oxygen-16 ratios in their bones and teeth than do landlubber counterparts nearby.

In spinosaur teeth from 12 sites, including southeast Asia, north Africa, England and Brazil, the researchers found that average oxygen isotope ratios were substantially lower than those of tyrannosaurs or other theropods living in those regions at the same time. The isotope ratios were, however, similar to those of contemporaneous crocs and turtles, a sign that the spinosaurs probably spent much of their time in and around lakes and rivers, the researchers speculate.

The new findings “solve the big ecological problem of how spinosaurs could live in the same areas as tyrannosaurs,” says Amiot. “They were avoiding competition for food and territory by dividing up the ecosystem,” he says.

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