Fossil enigma bares teeth, tells its tale.

Once regarded as the greatest unsolved puzzle in paleontology, tiny fossils known as conodonts are finally revealing their true identity as some of our most ancient relatives, according to two new studies reported this week. The new evidence threatens to overturn theories of the evolution of early vertebrates more than half a billion years ago.

Extremely common in rocks from 520 to 205 million years old, the pointy, millimeter-sized conodont fossils have mystified paleontologists for 150 years. Whereas some thought the comblike pieces belonged to ancient mollusks, others saw them as fossilized algae or plants, and many more suggested that conodonts represent teeth from some type of unknown, extinct animal.

In the early 1980s, paleontologists discovered conodonts arranged near the front of an eel-like body, proving that they were parts of a larger creature -- perhaps the earliest vertebrate. But critics contended that the animal lacked many typical vertebrate features.

"For many years, we as vertebrate paleontologists did not want anything to do with conodonts," says Peter Forey of the Natural History Museum in London.

Now, studies of a giant conodont from South Africa reveal that these animals had distinctive eye muscles seen only in other vertebrates, report Sarah E. Gabbott and her colleagues from Leicester University in England. The muscles sit outside the actual eye, permitting the animal to shift its gaze up and down and from side to side. In the April 27 Nature, Gabbott and her coauthors argue that this discovery places conodonts securely within the vertebrate line.

The new evidence has swayed at least one prominent skeptic. In an accompanying comment, paleontologist Philippe Janvier of the Centre National de la Recherche Scientifique in Paris admits his conversion. "Considering this and other evidence for the vertebrate affinity of the conodonts that has accumulated over the past 10 years, I think it is time for me to stop playing Devil's advocate against this theory," he says.

In the same issue of Nature, Mark A. Purnell of Leicester University puts to rest arguments about the function of the conodont elements. Although the fossils look like teeth, some paleontologists discounted the resemblance because they could find no signs of wear. Instead, researchers argued that the comblike structures served as sieves for filtering out fine plants and animals from seawater. This picture fits with standard images of the earliest vertebrates as slow, filter-feeding fish that lived on the ocean floor.

Purnell, however, finds evidence of microscopic scratches and pits etched into the conodont fossils. "They must have been used to crush and shear food," he says.

Together with the evidence of large, movable eyes, Purnell's findings suggest that conodonts were predatory animals that devoured fairly large prey. If so, this challenges the textbook picture of vertebrates starting out as lethargic filter-feeders.

The emerging portrait suggests that the earliest vertebrates may have distinguished themselves by an active, predatory mode of life, says Purnell.