Earlier water-to-land transition found.

• Position: Fossils

New evidence gleaned from CT scans of fossils locked inside rocks may flip the order in which two types of four-limbed animals with back bones were known to have moved from fish to landlubber. Both extinct species, identified as Ichthyostega and Acanthostega, lived an estimated 370,000,000 years ago in what now is Greenland.

Acanthostega was thought to have been the most primitive tetrapod, that is, the first vertebrate animal to possess limbs with digits rather than fish tins. However, research provided by Viviane Callier, a graduate student from Duke University, Durham, N.C., indicates that Ichthyostega may have been closer to the first tetrapod. In fact, Acanthostega may have had a terrestrial ancestor and then returned to the water full time. "If there is one take-home message, it is that the evolutionary relationship between these early tetrapods is not well resolved," Callier declares.

Coauthor Jennifer Clack of the University Museum of Zoology in Cambridge, England--where she supervised Callier's work for a master's degree--found the fossils embedded in rocks collected from East Greenland. Rather than trying to remove them-an action that would have destroyed much of the evidence--the researchers studied the fossils inside the stone with computed tomography (CT) scanning. Callier "reconstructed" the animals using imaging software to analyze the CT scans, focusing on the shapes of the two species' upper arm bones, or humeri. The CT slices revealed that Clack had found the first juvenile forms of Ichthyostega. Previously known fossils of Ichthyostega had come from adults.

Anatomies can morph as animals move toward adulthood, and such shifts can help scientists deduce when in development the animal acquired the terrestrial habit. The fossils suggest that Ichthyostega juveniles were adapted aquatically, and that the terrestrial habit was acquired relatively late in development. The fossils bore evidence that the muscle arrangement in adults was...