Finds undermine dating of early land life.

Norwegian geologists are threatening to drench established theories about when our ancestors first pulled themselves out of the water and took over the continents. A new study of rocks from east Greenland suggests that some of the earliest fossils of vertebrates with legs are not as old as paleontologists have long thought.

"These data do not fit with our current understanding of evolution," says Ebbe H. Hartz of the University of Oslo. "if we change the ages of the fossils from east Greenland, that will trigger a domino effect on many other places because a lot of evolution has been defined by this area." Hartz and his colleagues describe their work in the August Geology.

According to textbook paleontology, insects and other invertebrates retained unrivaled control of the continents until the late Devonian period, when amphibious vertebrates first hauled themselves out of the swamps (SN: 7/30/94, p. 70). These pioneers, called tetrapods, descended from fish with paired sets of fleshy fins, which at some point evolved into stout legs.

The most complete remains of primitive tetrapods hail from a sediment-filled basin in east Greenland. In 1959, geologists indirectly dated the basin as late Devonian, between 370 and 360 million years old.

This age came into question recently, when Hartz and his colleagues studied the orientation of magnetic grains embedded in rocks from the basin. The grains record the direction of Earth's magnetic field at the time the rocks solidified, indicating the site's former latitude.

According to the grains, the rocks formed when Greenland was at about 30 [degrees] S--not the expected position during the Devonian, says Hartz. Instead, the findings match Greenland's position during the subsequent, Carboniferous period.

Direct dating of the radioactive elements in the rocks confirmed this younger age. Using the argon-40/argon-39 method, Hartz and his team determined that the fossil layers of the basin were less than 336 million years old. Hartz cautions that these conclusions require verification, and he is working to date other rocks from the Greenland basin.

Paleontologists find these dates hard to accept. "I'll be dumbfounded if it's true," says Neil H. Shubin of the University of Pennsylvania in Philadelphia, who studies Devonian tetrapods in Pennsylvania.

"I'm quite convinced that this is wrong," comments Per E. Ahlberg of the Natural History Museum in London, another tetrapod investigator.

If Hartz and his colleagues are right, however, that will raise questions about the Devonian age of other early tetrapods, say paleontologists. Tetrapod fossil sites in Pennsylvania, Australia, and Russia contain fish and other animals very similar to those found in the Greenland basin, suggesting that all sites are the same age. Redating the Greenland fossils may pull many other early tetrapods into the Carboniferous as well, altering the time when vertebrates made the transition to life on land.

"If their conclusions are correct, it would suggest that the early part of the evolution of tetrapods took longer than we thought it did," says Ahlberg.