Stegosaur Plates Used for ID, Not Defense, Study Says

Hillary Mayell
for National Geographic News

May 25, 2005

Scientists have long puzzled over the sometimes bizarre ornamentation found on dinosaurs. Such features range from feathers, bills, and horns to spikes and plates, among others.

A new study of the finlike plates that lined the backs of the Jurassic-period plant-eaters known as stegosaurs suggests that the plates served a rather simple purpose: dinosaur I.D.

Researchers behind the study think stegosaur plates, as well as many structures in other dinosaur species, evolved to help the ancient animals recognize their own kind.

Russell Main, a biologist at Harvard University in Cambridge, Massachusetts, co-authored the study, which is published in latest issue of the research journal Paleobiology.

Referring to many dinosaur families, the biological classification above genus and species, he said, "If you look at [them] from the neck down, their body form is relatively generalized, they're all relatively similar."

"Dinosaurs have come up with a number of weird structures, and people have always wanted to give them interesting and sort of strange functions," he said. "But perhaps the simplest explanation is recognition between groups."

Jack Horner, a paleontologist at the Museum of the Rockies in Bozeman, Montana, joined Main in the study, which was led by Kevin Padian at the University of California, Berkeley.

"The skeletons of these dinosaurs [stegosaurs] below the neck are identical," Horner said. He points to deer as a modern-day analog: While the skeletons of mule deer and white-tailed deer look very similar, the animals themselves have different colors, and their ears and tails are also different.

Horner says these deer attributes serve a similar function to that of the elaborate frills, crests, and back spikes found in dinosaurs. They allow mule and white-tailed deer to identify members of their own species.

Referring to the dinosaurs, Horner noted that "all of these big features on dinosaurs are very expensive" in terms of the energy required to grow them. He added that such features must therefore be extremely important.

Some scientists have theorized that stegosaur plates might have evolved as defense mechanisms, a notion Horner disputes.

"When you think about defense, it's something you don't have to do all the time. Not every animal gets attacked," he said. "But when it comes to species recognition, [that] is something you need all the time. Species recognition is the most important thing, if you're planning to have more stegosaurs."

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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