Stegosaur Plates Used for ID, Not Defense, Study Says

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Form Over Function?

Stegosaurs lived primarily during the Jurassic period, roughly 210 million to 144 million years ago.

Researchers behind the new study examined what may be the most well-known stegosaur species, Stegosaurus stenops.

Weighing two tons, the plant-eater measured about 20 feet (6 meters) in length. It had a powerful tail with two pairs of three-foot-long (one-meter-long) spikes and sported a double row of finlike plates along its back.

Examining fossilized S. stenops plates and spikes, the researchers found that the features were bony structures laced with blood vessels. The structures had probably been covered with keratin, the same protein found in fingernails, horns, and claws.

Various theories have been proposed to explain the function of stegosaur plates, or scutes. One of the most common explanation suggests that stegosaur scutes may have been used for defense.

But Main, the Harvard University biologist, said the plates would not have been very effective as armor. He noted that the scutes, which grew only on the stegosaur's back, left the animal's flanks completely exposed and that the structures themselves were "relatively fragile."

Another theory holds that stegosaur scutes evolved as a kind of sexual display mechanism. Main believes that explanation is unlikely given the fact that both male and female stegosaurs had the spinal plates.

"If you had male-male combat over females, just the males would have large or showy structures," the biologist said. "If there were male-female displays where the male was trying to impress the female in some way, as some birds do, you would expect the males to have much more elaborate structures than the females."

Heat Exchange

A third theory proposes that stegosaur scutes served as a kind of heat exchange, functioning in a manner similar to the way elephant's ears work: radiating heat on hot days and absorbing warmth on cool days.

The numerous blood vessels found in stegosaur scutes lend support to this theory, but the researchers discount it.

"The cooling device theory has always been a problem, because the closest relative to Stegosaurus stenops has spikes instead of plates," said Horner, the Montana paleontologist. "If cooling was the most important thing for that dinosaur, then obviously its closest relative would also need the same thing."

The researchers believe their findings—that stegosaur scutes served as species ID—can be applied more broadly to other families of dinosaurs.

Within dinosaur families that had relatively similar body types, such as the hadrosaurs (duck-billed dinosaurs) or ceratopsians (such as Triceratops), there is an incredible range of diversity. The number of horns, plates, frills, or spikes; the ornaments' size, placement, even possibly their color—all these help to distinguish one species from another.

"In order to gain a fuller picture, especially in extinct organisms, you have to look at the structures within an evolutionary context—where these structures came from ancestrally and what they've become or evolved into down the line," Main said.

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