April 18, 2009

Fossils Suggest Tetrapod Adapted To Land Earlier Than Estimated

New research from Duke University suggests a reversal in the order in which two four-limbed creatures transitioned from water to land.

The Ichthyostega and Acanthostega, lived some 360-370 million years ago in modern-day Greenland.

Scientists had previously considered Acanthostega to be the first vertebrate animal to possess limbs with digits rather than fish fins, but Viviane Callier, a Duke University graduate student, found fossilized evidence to suggest that Acanthostega may have had a terrestrial ancestor and then returned full time to the water.

"If there is one take-home message, it is that the evolutionary relationship between these early tetrapods is not well resolved," Callier said.
Jennifer Clack of the University Museum of Zoology in Cambridge, England, discovered the fossils in rocks from East Greenland. Clack was responsible for supervising Callier's work.

Researchers analyzed the fossils while they were still fixed inside the stone using computed tomography (CT) scans. The images were used to reproduce what the animal would have looked like.

The images marked the first discovery of juvenile Ichthyostega fossils. Previously discovered fossils had been from adult Ichthyostegas.
Callier said scientists can use the shifts toward adulthood to determine "when in development the animal acquired the terrestrial habit."

They found that the fossils suggest Ichthyostega juveniles were aquatically adapted, and they adapted to terrestrial environments later 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.

In Acanthostega "there is less change from the juvenile to the adult. 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.

"If Ichthyostega is actually more primitive than Acanthostega, then maybe animals evolved towards a terrestrial existence a lot earlier than originally believed," she said. "Maybe Acanthostega was actually derived from a terrestrial ancestor, and then, went back to an aquatic lifestyle."

"It seems like there were different species evolving the same or similar traits independently -- evidence of parallel evolution," Callier said.

"The terrestrial environment posed new challenges like feeding and moving on land and breathing air, to which the first tetrapods had to evolve solutions. Sometimes different lineages stumbled upon similar solutions."


Image Credit: Wikipedia


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