Monday, April 20, 2009

Getting a Leg Up On the Tetrapod Land-Water Transition

Contrasting Developmental Trajectories in the Earliest Known Tetrapod Forelimbs. 2009. V. Callier et al. Science 324: 364 – 367.

Click HERE for an animation of the humerus from MGUH 29017a. The bone rotates 360° around its long axis, which is oriented vertically.

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.

New evidence 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.

The researchers studied the fossilized bones inside the rock with computed tomography (CT) scanning rather than trying to prepare them out of the matrix. The CT slices revealed that Clack had found the first juvenile forms of Ichthyostega.

The humeri (upper arm bones) 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.

Because the shapes of its adult limbs seemed the most fin-like, scientists had previously concluded that Acanthostega was "more primitive," Callier said. "But now, if we look at the details of the humeri, Ichthyostega's are actually more similar to earlier fishes."

Ironically, the shape of Acanthostega's limbs, in both adult and the newly-discovered juvenile forms, is more "paddle-like" than Ichthyostega's, Callier said. "They would have been really good swimmers. So, although Acanthostega had limbs with digits, we don't think it was really terrestrial. We think even the adults were aquatic."

"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." link