Cone-shaped, mineralized structures that surrounded the mouths of ancient eel-like creatures and helped them grasp and mince their food weren’t teeth at all, according to a new study. The findings suggest that one theory of how teeth first appeared in creatures with backbones, a group that today includes humans, needs to be abandoned entirely.
The ancient creatures that sported these structures are known as conodonts, which in Greek means “cone-shaped teeth.” The eel-like animals that lived from 530 million to 200 million years ago in the world’s seas are some of the most primitive vertebrates in the fossil record. They had no jaws or other bones; typically, the only parts that fossilized were toothlike structures made of calcium phosphate, the same mineral in the enamel of human teeth. Only in the last few decades have researchers found fossils that also contain traces of soft tissue that reveal the creature’s size and shape.
The “teeth” are also slowly coming to light. A recent analysis of the “teeth” of one conodont species showed that they varied quite a lot in length and shape, with some of the structures likely being used to capture and restrain prey while others served to cut meals into smaller bits.
Taking a closer look, Philip Donoghue, a paleontologist at the University of Bristol in the United Kingdom, has now identified a key feature. Early conodonts had “teeth” that grew in a series of steps, with later stages of growth adding a ring of minerals only along the sides of the cone-shaped hard parts but not at the tips. Typically, the enamel coating the teeth of vertebrates forms all over the tooth all at once, late in tooth development. “Even though the end result looks like a tooth, it’s not,” Donoghue says. Despite the similarities, the new study indicates that the earliest forms of conodont hard parts evolved from a completely different set of tissues than the teeth of vertebrates do, he and his colleagues report online today in Nature.
The discovery has implications for an idea about early evolution in vertebrates. The scales covering fish, reptiles, and other creatures contain tiny, hard structures called denticles. In recent years, some researchers have proposed that denticles first appeared in the mouth as teeth and then evolved into scales. But the new findings show that denticles and teeth originated from different tissues, says Philippe Janvier, a paleontologist at the National Museum of Natural History in Paris, so it’s time to chuck that idea.