Its ridge of paddle-shaped plates and its spike-tipped tail make Stegosaurus an instantly recognizable icon, even to preschoolers. Now, new analysis suggests these bizarre features served the same purpose for stegosaurs, allowing the dinos to recognize members of their own species.
Dinosaurs evolved some strange body parts, from the skull crests of duck-billed hadrosaurs to the horns and frills of Triceratops. So the row of plates on Stegosaurus were in good company. A favored explanation for these so-called scutes is that they radiated and absorbed heat. Earlier analysis bolstered this hypothesis by identifying large "pipes" inside the scutes that were thought to carry blood to the plates.
But a new look at the fossilized tissue and bone, reported in an upcoming issue of Paleobiology, refutes this idea. A team of researchers led by paleontologist Kevin Padian of the University of California, Berkeley, compared slices of fossil scutes from Stegosaurus and several of its relatives, including two of its smaller ancestors. The scutes of all species examined shared similar structures--thin outer layers of dense bone over a porous interior--suggesting a common evolutionary origin. But the first scutes most likely did not function in temperature control, since those in the oldest species, the 1.2-meter-long Scutellosaurus, were only nickel-sized--too small to exchange much heat. Moreover, the large pipes in the stegosaurs' scute interior did not connect to blood vessels on the scute exterior and appear to be soft-tissue-filled cavities, not blood vessels. The presumed radiator plumbing was not there, the team concludes.
Other explanations are also unlikely, says Russell Main, a biologist at Harvard University and a co-author on the study. The porous plates appear too lightly built to provide much defense, and males and females look the same, making sexual display an unlikely scute purpose. But, he adds, these alternatives, including a passive role in heat regulation, may be secondary functions of the plates.
"Everything seems to be pointing towards species recognition" to explain many of dinosaurs' fanciful features, says study author and paleontologist John Horner of Montana State University in Bozeman. "We think there may have been more than one Stegosaurus species" living in the same area, Horner says. "You may not need to defend yourself every year," he says, "but you do have to mate."
"It's the great dinosaur detective story," says paleontologist Kristi Curry Rogers, curator at the Science Museum of Minnesota in Minneapolis. "They're looking at it from an evolutionary perspective." And, she adds, "Rather than searching for some bizarre function, they're searching for something more realistic."