The fact that teeth are always organized--into sturdy rows of molars, for instance--has led researchers to assume that teeth evolved just once, in the common ancestor of jawed vertebrates. Now there's evidence that they also evolved at another time, independently, in a group of extinct jawed fishes called the placoderms. If they evolved twice or more, that could help shake up a significant portion of the vertebrate family tree.
Some 408 million years ago, armored fish known as placoderms ruled the seas. The first jawed vertebrates caught their prey with bumpy gums or bony cutting blades made of so-called semidentine. Teeth were believed to have developed as a better way to capture prey in a relative of placoderms, one that gave rise to all of the other major groups of jawed vertebrates, such as sharks and bony fishes.
But in the 21 February issue of Science, Moya Meredith Smith of King's College London and Zerina Johanson of the Australian Museum in Sydney argue that a group of advanced forms of placoderms, the Arthrodira, had true teeth. Instead of a random assortment of tiny spikes called denticles, they sported conical structures arranged in rows. In studying this tooth wear on museum specimens, the scientists realized that two members of the Arthrodira added new teeth to the end of a row, a pattern seen today in certain fish. Meredith Smith was able to slice through a few teeth and show that they are made of regular dentine, not semidentine. However, Philippe Janvier of the National Museum of Natural History in Paris cautions that the independent origin of placoderm teeth depends on the placoderms being a separate branch on the family tree, which he says has not yet been rigorously established.
Meredith Smith and Johanson speculate that teeth might have originated even three or more times among jawed vertebrates. That wouldn't surprise evolutionary biologist Jukka Jernvall of the University of Helsinki, Finland. "Multiple origin of all the things that have something to do with teeth seems to be an emerging theme in evolutionary biology," he says. And that could mean that the vertebrate family tree might end up needing some revising. "The relationships of the main groups of jawed vertebrates are entering a state of flux," explains Michael Coates of the University of Chicago.
Introduction to the Placodermi
More on the jawed vertebrates