Useful Decorations on Dinosaurs
Maybe because they're not around to tie up traffic, dinosaurs seem to be everybody's favorite animal. It's as true for scientists as schoolchildren---and for scientists who work on them, there's the added career advantage that no live dinosaur will show up to dispute an appealing theory about the big reptiles.
However, deciphering the scanty evidence dinosaurs have left behind in the rocks is iffy enough to keep argument alive among human interpreters of the fossil record. Consider, for example, the debate over some dinosaur features.
It started with stegosaurs. A distinctive feature of this dinosaur family is a row of bony plates jutting upright from their backs. The plates looked like defensive armor, useful if odd-looking protection against taller predatory saurians that might bite down on a stegosaur's spinal column. That was pretty much the assumption until about ten years ago---after all, dinosaurs tried lots of defensive structures that look odd to us marmmals. Then James Farlow of lndiana University proposed that the plates were actually Tadiators. If the animals circulated blood through those plates, they could cool down. It would be a useful adaptation in hot climates.
This isn't a wild idea. Biologists interested in more modern animals are familiar with the value of radiator structures that suit climate. Alaskans have a local example: arctic hares are close kin to southern jackrabbits, but have much smaller ears than their desert-dwelling cousins do. Ears can bring a lot of blood close to the air. If that air is likely to be cold, little ears are better, since they mean less heat loss. Hotter air gives the advantage to bigger ears; they radiate excess heat away from an animal's body.
The fossil record supports the notion that dinosaurs, like their bird descendants and lizard cousins, didn't have much in the way of external ears. Fancy projections like those emerging from stegosaur backs would have done even better as radiators, so the theory seems logical.
Recently it received more support, from another scientist working with another group of dinosaurs. J. Keith Rigby of Notre Dame thinks the frill over the necks of ceratopsid dinosaurs was another such radiator.
The best-known ceratopsid, Triceratops, looks like a rough draft of a rhinoceros. It had three horns, as its name implies, and a huge flaring bony plate---the frill---over its neck. That frill left an unusual fossil for paleontologists to find in what is now Montana, and it made Rigby think he'd found evidence of another saurian radiator.
Usually, the rock matrix is removed from a fossilized bone. In this case, the rock was unusually hard, and it stuck to the fossil. Rigby ended up removing the fossil from the rock. The frill fossil left a clear impression in the rock, and the impression revealed thick grooves in the top of the frill. The grooves branched into smaller-diameter tubes lower in the frill. Rigby read the grooves as arterial pathways. He believes they carried blood to the top surface of the frill, where the animal dissipated excess heat before the blood returned to the body interior.
It' s a nice theory, but hardly the only theory. Other specialists point out that the frill served as an anchor site for powerful muscles attached to this herbivorous dinosaur's lower jaw. Some speculate that since the frill shape and size varied for each species, the big structures were useful chiefly as signs by which ceratopsids could recognize their own kind.
My favorite view of the frill comes from Robert Bakker of the University of Colorado. He doesn't see any of the theories as excluding any of the others. Rather, he describes the frill as a multipurpose appendage: "the Swiss army knife of dinosaur haberdashery."
