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Did Triassic monster use suction to feed?

September 23, 2004 By Emma Marris This article courtesy of Nature News.

Stiff-necked sea creature stirs up palaeontologists.

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A newly discovered Triassic reptile that has a neck much longer than its body is reopening a question long thought settled: what good is a long neck?

Scientists assumed that long-necked sea monsters relied on the flexibility of their necks to hunt prey. But this new specimen has a remarkably rigid neck, forcing scientists to come up with more imaginative ideas about its purpose.

Roughly 230 million years ago, the creature Dinocephalosaurus orientalis swam in shallow seas off the coast of Pangaea, the supercontinent that dominated the Earth at that time. Some mishap entombed one of them in limestone near Xinmin, Guizhou Province, in southern China.

Its neck, which is almost twice as long as its reptilian body, supported a relatively tiny head complete with a nasty set of fangs.

You suck in, and you secure the prey between the teeth. It keeps the mouth open while you push the water out of your neck. And then comes the swallowing.
Olivier Rieppel
The Field Museum, Chicago, Illinois
But the specimen's most surprising characteristic is that, just as we have ribs protruding from the vertebrae in our backs, this creature has little riblets emerging from its 25 or so neck vertebrae. These are long and overlapping, and ran parallel to the beast's spine, so they would have stiffened the neck considerably.

This intrigues Michael LaBarbera, a biomechanist from the University of Chicago, Illinois. Prehistoric aquatic animals with extravagantly long necks were, until now, thought to use their necks like snakes, lunging and twisting as they raced after prey, or like periscopes, to breathe at the surface. "If you found such an animal that lived in the water, you'd say it had a long snaky neck," says LaBarbera.

But Dinocephalosaurus could not have used its neck in such a way. Its throat was too long to properly inflate the lungs if used periscope-style, because the pressure difference between the surface and the depth of the lungs would have been too great. What is more, Dinocephalosaurus had more vertebrae and longer neck ribs than its ancestors, indicating that the stiffness may have conferred some advantage and been selected for over time.

The creature's head and teeth gave LaBarbera a clue to what the neck might have been used for. They seem to be set up for suction feeding. This is a common tactic of aquatic predators in which prey is sucked into the gullet by a dramatic expansion in the volume of the mouth.

Hoover neck

LaBarbera thinks that Dinocephalosaurus used a variation on the suction tactic. Instead of expanding the mouth, he thinks the muscles attached to the neck ribs could have pulled them outwards, expanding the volume of the throat. Because of the neck's length, this manoeuvre would produce plenty of suction. "The neck is a long cylinder, and if you increase the diameter a little bit you increase the volume significantly," he says.

LaBarbera publishes the idea in Science1 this week, together with his colleague Olivier Rieppel, curator of fossil amphibians and reptiles at the Field Museum in Chicago, Illinois.

Rieppel explains how suction feeding would work. The animal would swim with its neck straight out and approach its prey, probably fish or squid. "You suck in, and you secure the prey between the teeth. This then keeps the mouth open while you push the water out of your neck. And then comes the swallowing," he says, with relish.

Both researchers are quick to point out that this new role for the long neck is only an inference. "Let me be the first to admit that what we have is a hypothesis that is consistent with, but not compelled by, the data we have," says LaBarbera.

Fish trap

Other possible uses for a stiff neck include faster swimming and stealth camouflage. In murky water, the innocent prey would see only the unassuming head of the beast and not its large body, far away in the gloom. This idea particularly interests LaBarbera. "The more I thought about this, the more compelling I found it," he says.

Glenn Storrs, curator of vertebrate palaeontology at the Cincinnati Museum Center in Ohio, thinks it likely that the small head and out-of-sight body were indeed a "fish trap". The idea applies to his favourite, the more recent plesiosaur, whose neck of up to 70 vertebrae might have been even stiffer: "Its head, with its needle-like teeth, would have been way out ahead of the body in dimly lit conditions." All sorts of long-necked reptiles could have been ambush predators, says Storrs.

References

  1. Li C., Rieppel O. & LaBarbera M. Science, 305. 1931 (2004).

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