Fossils suggest earlier land-water transition of tetrapod

April 18th, 2009 - 2:59 pm ICT by ANI  

Washington, April 18 (ANI): New evidence has emerged that suggests an earlier land-water transition of a four-limbed animal with backbones, which was known to have moved from fish to landlubber.

A Duke graduate student, in his research work, came across the evidence from CT scans of fossils of Ichthyostega and Acanthostega, locked inside rocks.

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 Viviane Callier, a Duke graduate student, 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 Callier.

“If there is one take-home message, it is that the evolutionary relationship between these early tetrapods is not well resolved,” she added.

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, 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, and such shifts can help scientists deduce when in development the animal acquired the terrestrial habit.

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.

In contrast, 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.

According to Callier, “If Ichthyostega is actually more primitive than Acanthostega, then maybe animals evolved towards a terrestrial existence a lot earlier than originally believed.”

“Maybe Acanthostega was actually derived from a terrestrial ancestor, and then, went back to an aquatic lifestyle,” she said. (ANI)

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