Lampreys and other living relatives of primitive vertebrates don't have jaws. But that doesn't mean they're mild-mannered vegetarians; some attack their prey of fish with the fury of a wolverine. Equally nasty were the eel-shaped conodonts, an extinct jawless fish. These and other observations have led some researchers to propose that jaws evolved in response to increasingly aggressive hunting habits by jawless ancestors. But a new study takes a bite out of this widespread idea.
Mark Purnell of the University of Leicester, United Kingdom, tested the notion by looking at the heterostracans, a group of extinct jawless fishes that were a dominant slice of vertebrate life for 140 million years until they went extinct at the end of the Devonian. They were relatives of the conodonts, and they shared a more recent common ancestor with the first jawed fish. Some researchers argue that the heterostracans were predators.
In the 7 January issue of the Proceedings of the Royal Society, however, Purnell argues that they were better suited for filter feeding. Using an electron microscope, Purnell took a close look at the oral plates that line the mouth. The plates were not worn on their tips, as one would expect if they had bit into prey. And they were covered with denticles not much more than 100 micrometers long and delicately shaped like triangles and maple leafs. "If this animal tried to grab hold of prey, the denticles would get broken," he says. Moreover, because the denticles pointed toward the front of the mouth, they would have prevented anything other than microscopic food from entering.
Philippe Janvier of the National Natural History Museum in Paris says the study presents "the first good evidence of filter feeding in these early jawless fossil vertebrates."