Paleontologists have long debated whether an ancient group of armored fish called placoderms, the earliest known vertebrates with jaws, actually had teeth or whether the structures on their jaws were simply bumps of bone. Because fossils of these fearsome creatures (sculptor's reconstruction of Dunkleosteus, top) are relatively rare and researchers are loath to slice them up to look at their internal structure, most previous studies have focused on the shape and surface appearance of the remains. But now, researchers have used powerful x-rays to create CT-like scans of placoderm fossils with resolution high enough to identify layers of bone representing growth lines and to distinguish among several types of cells. The new analyses reveal that placoderms, which lived from about 420 million years ago to about 360 million years ago, had true teeth with dentine and pulp cavities, the researchers report online today in Nature. Unlike the style of tooth growth seen in modern-day bony fish and their ancient kin, in placoderms teeth grew on the surface of the jawbone and had no roots (cross section of jaw from placoderm Compagopiscis croucheri, bottom). This style of tooth growth suggests that placoderm teeth couldn't be replaced when they wore down or broke—a potentially devastating prognosis for these very active predators. The evolutionary origin of teeth and jaws has long been shrouded in mystery, but the new findings reveal that even the earliest jawed vertebrates had choppers.
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