How does an undulating stingray, without a bone in its body, chomp its hard-shelled prey? Adam Summers, a graduate student at the University of Massachusetts, Amherst, observed the cownose ray eating mollusks, unusual food for the soft-bodied, boneless creature.

Curious about evolution, engineering and form, Summers examined the internal structure of the ray in greater depth than anyone else up to now. Though the cownose ray is common to the Atlantic coast, Summers said, "it is not a typical model animal for biologists."

The research team, publishing their results in this week's journal Nature, says the cownose jaw has not one but two traits that make the cartilage rigid, normally only found in animals with bone skeletons.

The walls of the cartilage in the jaw are hardened with layers of tiny tiles called "tesserae". Also, the cartilage has internal struts to distribute the strength of the bite, similar to the way a building distributes weight by a system of girders.

These strengthening traits have only been seen before in ancient fossils of sharks. That the cownose has retained these traits reflects an evolutionary freeze-frame, since only about five percent of cartilaginous animals eat hard-shelled prey, according to Summers.

Dr. Scott McRobert, director of the biodiversity lab at Saint Joseph's University in Philadelphia, says its a mystery why few stingrays have these traits. "The evolution of new traits involves a bit of luck," he says. "If traits convey an advantage, then genes for those traits take hold in a population." He notes that the traits must already exist if they are to develop. For instance, if another Ice Age hit, rabbits wouldn't suddenly grow thick fur. But those rabbits already with thick fur would endure, and that trait would become dominant.

Summers is hesitant to speculate why few stingrays retain the hard jaw trait. Perhaps it relates to the availability of types of food during their evolutionary growth. Their wide-winged relative, the manta rays, also have strut-enforced jaws even though they eat tiny plankton. Apparently in spite of inertia or non-use, a trait is harder to lose than gain.