The first shark: to bite or not to bite?

Fossil fish scales discovered in Colorado reveal that sharks have been haunting the seas since the middle of the Ordovician period 455 million years ago, a time far more ancient than paleontologists previously thought. Yet these early sharks may not have brandished the fearsome maws of their modern descendants. In fact, the ancient fish may have lacked jaws altogether, says Ivan J. Sansom of the University of Birmingham in England. Sansom and his colleagues describe the new finds in the Feb. 15 Nature.

"We've got enough evidence to say that these are sharks. But we don't have enough evidence to say that they were sharks with jaws," says Sansom.

The scales measure about 1 millimeter in length and have multiple, overlapping cusps that look like a miniature mountain range. The distinctive shape matches scales from later sharks, leading Sansom and his colleagues to identify the Colorado species as a shark or shark ancestor. Until now, the oldest shark remains had hailed from the Silurian period, some 25 million years younger. The Colorado fossils fall into a peculiar taxonomic position, comments paleontologist Philippe Janvier of the National Center for Scientific Research in Paris. Because they resemble later sharks so closely, the Ordovician animals qualify as the closest relatives of the gnathostomes, or jawed vertebrates. "There is no known jawless vertebrate with similar scales," Janvier says. Yet without more complete fossils, scientists cannot tell whether these early fish actually possessed teeth or jaws. The appearance of jaws marked a major leap in the evolution of vertebrates, in some ways almost as dramatic as the jump from invertebrates to the first vertebrates (SN: 2/3/96, p. 74). Whatever their dental details, the Ordovician sharks apparently shared the ancient oceans with true gnathostomes. "Almost certainly, there must have been jawed vertebrates at the time," says Janvier.

He notes that recent fossil finds from China indicate the existence of several different jawed fish early in the Silurian, which began 438 million years ago.

This diversity implies that the first jaws must have appeared well before then, presumably by the time of the Colorado shark.

Other paleontologists, however, argue that gnathostomes may not have evolved that early. "It is debatable whether these scales came from animals that we would identify as gnathostomes. I'm not fully convinced," says Mark Wilson of the University of Alberta in Edmonton.

Wilson argues that proof would come only when scientists find complete fossils. He asks why, if gnathostomes existed in the Ordovician, scientists haven't found complete fossils of these animals.