The earliest teeth were not individual structures, but rather tough, bumpy plates that ancient fish used like sandpaper to crush and shred their food. Now, a new study reveals that for at least one species those so-called tooth plates didn’t form all at once: They expanded gradually with the accumulation of toothlike tissue as the fish grew in size. That’s the conclusion of the first detailed analysis of the tooth plates of a 400-million-year-old creature known as Romundina stellina, an armored fish that may have been among the first animals to sport teeth. A better understanding of how teeth evolved may provide clues about how other tissues—such as hearts, kidneys, and other major organs—might have developed.

Researchers have long looked to sharks to figure out how teeth evolved. That’s because sharks have been around for more than 420 million years, yet their teeth—and presumably the biological processes by which teeth develop as they grow—are very similar to those of creatures that evolved later. But there are even more ancient creatures than sharks that also had toothlike features, says Philip Donoghue, a paleobiologist at the University of Bristol in the United Kingdom. That includes R. stellina, a species of small, armored fish that lived about 400 million years ago. These fish, whose skin included armor to protect against predators, are among the earliest known jawed vertebrates, Donoghue says.

He and Martin Rücklin, a vertebrate paleontologist at the Naturalis Biodiversity Center in Leiden, the Netherlands, decided to take a look at the tooth plate fossils of R. stellina to tease out their secrets. The researchers performed 3D x-ray scans of a fossil tooth plate recovered along with other remains of the creature from Canada’s Prince of Wales Island. Those scans were so detailed that the researchers could spot features as small as 10 micrometers across.

The images clearly showed that the tooth plates didn’t form all at once. In cross section, Donoghue says, he and Rücklin could readily identify the solid lines that marked the surfaces of individual structures that, like modern teeth, had a core of dentine and a tough enamel-like surface. The images also suggest that the tooth plate formed around what the pair calls a “pioneer tooth.” As the ancient fish grew, new toothlike structures appeared in nearby tissue and then merged onto the edge of the expanding tooth plate, the researchers report online today in Biology Letters.

Even though teeth seem to be simple tissues, they form through a complex process, says Zerina Johanson, a vertebrate paleontologist at the Natural History Museum in London. “By understanding the evolution of teeth in primitive fish, we can get new perspectives on how these tissues develop in other creatures as well,” she notes.

The same general set of biological processes that creatures use to form teeth is also used to generate other tissues, Donoghue says. So having better insights into the evolution of teeth can provide scientists with clues about how major organs such as hearts, kidneys, and digestive tracts developed as well, he notes.