Acanthostega
Encyclopedia : A : AC : ACA : Acanthostega
Acanthostega is an extinct tetrapod genus, among the first vertebrate animals to have recognizable limbs. It appeared in the Upper Devonian (Famennian) about 360 million years ago, and was anatomically intermediate between lobe-finned fishes and the first tetrapods fully capable of coming onto land.Jennifer A. Clack, Scientific American, [Getting a Leg Up on Land] Nov. 21, 2005.
Description
It had eight digits on each hand and foot linked by webbing, it lacked wrists, and was generally poorly adapted to come onto land. Acanthostega also had a remarkably fish-like shoulder and forelimb. The front foot of Acanthostega couldn't bend forward at the elbow, and thus couldn't be brought into a weight-bearing position, appearing to be more suitable for paddling. It had lungs, but its ribs were too short to give support to its chest cavity out of water, and it also had gills.Therefore, paleontologists surmise that it probably lived in shallow, weed-choked swamps, the legs having evolved for some other purpose than walking on land. Jenifer Clack interprets this as showing that this was primarily an aquatic creature descended from fish that had never left the sea, and that tetrapods had evolved features which later proved useful for terrestrial life, rather than crawling onto land and then gaining legs and feet as had previously been surmised. At that period, for the first time, deciduous plants were flourishing and annually shedding leaves into the water, attracting small prey into warm oxygen-poor shallows that were difficult for larger fish to swim in. She remarks on how the lower jaw of Acanthostega shows a change from the jaws of fish which have two rows of teeth, with a large number of small teeth in the outer row, and two large fangs and some small teeth in the inner row. It differs, having a small number of larger teeth in the outer row and smaller teeth in the inner row, and she suggests that this change probably went with a shift in early tetrapods from feeding exclusively in water to feeding with the head above water or on land.
Related species
Acanthostega is seen as part of widespread speciation in the late Devonian period, starting with purely aquatic lobe-finned fish, with their successors showing increased air breathing capability and related adaptions to the jaws and gills, as well as more muscular neck allowing freer movement of the head than fish have, and use of the fins to raise the body of the fish. These features are displayed by the earlier Tiktaalik, which like the Ichthyostega living around the same time as Acanthostega showed signs of greater abilities to move around on land, but is thought to have been primarily aquatic.
From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.Discovery
The fossilized remains are generally well preserved, with the famous fossil by which the significance of this species was discovered being found by Jennifer A. Clack in East Greenland in 1987, though fragments of the skull had been discovered in 1933 by Gunnar Säve-Söderbergh and Erik Jarvik. Sources
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.