Swimming Spinosaurs

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In 1986, paleontologists described a dinosaur unlike any that had been seen before. Named Baryonyx walkeri, it was a theropod with a long, crocodile-like snout and arms tipped in huge claws. Some preserved stomach contents confirmed that it was a fish-eater. It showed some similarities to another dinosaur that had been found decades before, Spinosaurus, and as similar dinosaurs came to light they all appeared to show adaptations for eating fish. They did not have the recurved tearing teeth of other large predatory dinosaurs, but instead had more conical teeth better-suited for grabbing prey before swallowing it, just like in living crocodiles. Despite these anatomical clues, however, it has been difficult to find more evidence of how these dinosaurs lived, but a new study published in the journal Geology suggests that they might have been spending much of their time in the water.

We are often in awe of dinosaur skeletons, but it is easy to forget that the basic materials for building their bony architecture came from their environment. Living dinosaurs took in oxygen, carbon and other elements, and isotopes of these elements became part of their bodies. An animal that primarily eats grass will have a different carbon isotope signature than one that eats leaves, for example, and an animal that spends much of its time in the water will have different oxygen isotope levels than one that spends all of its time on dry land. In some instances these isotopes can remain preserved in parts of fossil skeletons, most often teeth, and paleontologists have used these istopes to study things like what kind of plants prehistoric horses ate and how much time early whales spent in the water. The researchers behind the new Geology paper have now extended these techniques to dinosaurs in an attempt to find out how much time spinosaurids were spending in the water.

To test the semi-aquatic spinosaurid hypothesis, the researchers looked at the oxygen isotope levels in the teeth of spinosaurids, other large theropods and crocodiles (as well as some turtle shell bones). If spinosaurids were spending much of their time in the water then their oxygen isotope signatures would be closer to those of the semi-aquatic turtles and crocodiles and most different from land-dwelling theropods. This is because the oxygen isotope values of semi-aquatic animals are less prone to fluctuations as they are regularly coming into contact with oxygen in the surrounding water; an animal that has to find water to drink is more likely to have more widely-varying values.

The results of the test showed that spinosaurids did have oxygen isotope values closer to turtles and crocodiles than to other large theropods. This supports the hypothesis that they were semi-aquatic, opportunistic predators that probably specialized in hunting fish but would not turn down larger dinosaurian prey it they could get it. The precise details of their lives at the water's edge are still being discussed and debated, but if this new study is correct then spinosaurids were even stranger than previously thought.

Amiot, R., Buffetaut, E., Lecuyer, C., Wang, X., Boudad, L., Ding, Z., Fourel, F., Hutt, S., Martineau, F., Medeiros, M., Mo, J., Simon, L., Suteethorn, V., Sweetman, S., Tong, H., Zhang, F., & Zhou, Z. (2010). Oxygen isotope evidence for semi-aquatic habits among spinosaurid theropods Geology, 38 (2), 139-142 DOI: 10.1130/G30402.1

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