LONDON--Life started in the water, scientists agree, and between 380 million and 350 million years ago some of the more intrepid fish crept onto land, following plants and insects. But a glimpse of how these fish made the transition to land animals has eluded researchers. Now a tiny fossil fragment appears to come from a new species, a possible missing link between fish and land animals, that could be our earliest vertebrate ancestor.
Backboned land animals, or tetrapods, share some anatomical and developmental features, including how the limbs and skull develop. Scientists therefore think that all birds, reptiles, amphibians, and mammals, including humans, can be traced back to a common fishy ancestor. But the fossil record is sketchy, and there are millions of years with no transitional fossils to link the youngest tetrapod-like fish, known as Panderichthys, and the oldest tetrapods.
Now there is a contender. Yesterday at the Natural History Museum in London, paleontologist Per Ahlberg of the museum unveiled two new fossils. Ahlberg didn't dig them up himself but found the specimens stored in museum drawers in Latvia and Estonia. Both specimens, fragments of the lower jaw several centimeters in length, had distinctive patterns of several parallel rows of teeth that were unlike anything else in the fossil record of either fishes or tetrapods. The jawbones were mostly fishlike in their arrangement. However, the new species--which will be christened in the August issue of Paleontology--is about as old as Panderichthys but is more like a tetrapod, suggesting that tetrapods may have branched off from fish earlier than paleontologists thought, Ahlberg says.
The fossils "fit in the gap [between fish and tetrapods], and it looks as if they are fairly close to the base of the [tetrapod] family tree," says paleontologist Jenny Clack of the University of Cambridge, United Kingdom. But she cautions that it's too early to say whether these creatures walked on land; to settle that, researchers would have to know whether they had fins or limbs. Ahlberg is planning to return to the Baltic Sea soon to look for more complete skeletons.