DURHAM, N.C. -- New evidence gleaned from CT scans of fossils locked inside rocks may flip the order in which two kinds of four-limbed animals with backbones were known to have moved from fish to landlubber.
Both extinct species, known as Ichthyostega and Acanthostega, lived an estimated 360-370 million years ago in what is now 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 fins.
But the latest evidence from a Duke graduate student's research indicates that Ichthyostega may have been closer to the first tetrapod. In fact, Acanthostega may have had a terrestrial ancestor and then returned full time to the water, said Viviane Callier, who is the first author of a report on the findings to be published in today's issue of the journal Science.
"If there is one take-home message, it is that the evolutionary relationship between these early tetrapods is not well resolved," Callier said.
Co-author 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 (Amira and Mimics) 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 towards adulthood, Callier said. And such shifts can help scientists deduce when in development the animal acquired the terrestrial habit. The fossils
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