Professor Per Ahlberg at the Department of Physiology and Developmental Biology, Uppsala University, together with Jennifer Clack, Cambridge University, and Viviane Callier, Duke University, have studied fossil upper arm bones from the two so-called "four-legged fishes", Ichthtyostega and Acanthostega, from Greenland. These animals, which lived during the Devonian period about 365 million years ago, were among the earliest vertebrates (backboned animals) with fore- and hindlimbs rather than paired fins. They belong to the common stem group of all living amphibians, reptiles, mammals and birds.
The researchers have identified several half-grown, as well as fully grown, upper arm bones from Ichthyostega and Acanthostega, allowing them to study how the shape of the bone changed during growth. It turns out that the two animals had different life histories.
"The upper arm bone provides a lot of information about the lifestyle of the animal, because its shape gives clues to the pattern of movement and can tell us for example whether the animal lifted the front part of its body clear of the ground," says Per Ahlberg.
Ichthyostega, which has robust limbs and only a small tail fin, appears to be the more terrestrial of the two. Its forelimb becomes better adapted to supporting weight as the animal grows up. The pattern of muscle attachments on the upper arm bone changes from a "fish-like" to a "land animal-like" configuration, and the shape of the shoulder joint changes so that it becomes possible for the animal to "lock" its forelimb into a weight-bearing position.
Acanthostega has feebler limbs and a large tail fin, and seems to have been more aquatic. In this animal, there are no corresponding changes.
"The explanation is probably that both animals laid their eggs in water just like modern amphibians, which meant that the terrestrial Ichthyostega, but not the aquatic Acanthostega, needed to undergo a lifestyle transformation as it grew from larva to adult," says Per Ahlberg.
Article: Viviane Callier, Jennifer A. Clack, Per E. Ahlberg, 'Contrasting Developmental Trajectories in the Earliest Known Tetrapod Forelimbs', Science 17 April 2009:Vol. 324. no. 5925, pp. 364 - 367 DOI: 10.1126/science.1167542