PhysOrg.com has a story on new research on Ichthyostega and Acanthostega. The new research was prompted by the discovery of new fossils:
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.
The find of juvenile material is incredibly important for the insight it can provide from a developmental perspective – in this case habitat use:
The fossils suggest that Ichthyostega juveniles were aquatically adapted, and that the terrestrial habit was acquired relatively late in development. The fossils bore evidence that the muscle arrangement in adults was better suited to weight-bearing, terrestrial locomotion than the juvenile morphology. It is possible that Ichthyostega came out of the water only as a fully mature adult.
Acanthostega shows something a little different:
Although Acanthostega appears to be aquatically adapted throughout the recorded developmental span, its humerus exhibits subtle traits that make it more similar to the later, fully terrestrial tetrapods,” Callier said [one of the authors of the study]
One of the implications is that Acanthostega is secondarily aquatic…
The research is being published in Science
(if any one has access could you email me a copy?)