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Temporal range: Late Devonian, 365 Ma
|Acanthostega gunnari skeleton reconstruction|
Acanthostega (meaning spiny roof) is an extinct genus of stem-tetrapod, among the first vertebrate animals to have recognizable limbs. It appeared in the late Devonian period (Famennian age) about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and those that were fully capable of coming onto land.
Acanthostega had eight digits on each hand (the number of digits on the feet is unclear) 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 could not bend forward at the elbow, and thus could not be brought into a weight bearing position, appearing to be more suitable for paddling or for holding on to aquatic plants. It had lungs, but its ribs were too short to give support to its chest cavity out of water, and it also had gills which were internal and covered like those of fish, not external and naked like those of some modern amphibians which are almost wholly aquatic. Acanthostega is the earliest stem-tetrapod to show the shift in locomotory dominance from the pectoral to pelvic girdle. There are many morphological changes that allowed the pelvic girdle of Acanthostega to become a weight-bearing structure. In more ancestral states the two sides of the girdle were not attached. In Acanthostega there is contact between the two sides and fusion of the girdle with the sacral rib of the vertebral column. These fusions would have made the pelvic region more powerful and equipped to counter the force of gravity when not supported by the buoyancy of an aquatic environment.
Therefore, paleontologists surmise that it probably lived in shallow, weed-choked swamps, the legs having evolved for some purpose other than walking on land. Jennifer A. Clack interprets this as showing that this was primarily an aquatic creature descended from fish that had never left the sea, and that the tetrapod lineage 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. Clack 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 stem-tetrapods from feeding exclusively in water to feeding with the head above water or on land.
While normally considered more primitive than Ichthyostega, it is possible it was the other way around. Because Acanthostega looks more like juvenile Ichthyostega and shows a lot less differences from juveniles to adults than the latter, it has been suggested the ancestors might have evolved by neoteny. Even if it appeared to spend the whole life in water, its humerus exhibits traits that resembles those of later fully terrestrial stem-tetrapods, while the humerus in Ichthyostega is more similar to earlier fishes. This could indicate that vertebrates evolved terrestrial traits earlier than previously assumed, and numerous times independently from another.
Research based on analysis of the suture morphology in its skull indicates that the species was able to bite prey at or near the water's edge. Markey and Marshall compared the skull with the skulls of fish, which use suction feeding as the primary method of prey capture, and creatures known to have used the direct biting on prey typical of terrestrial animals. Their results indicate that Acanthostega was adapted for what they call terrestrial-style feeding, strongly supporting the hypothesis that the terrestrial mode of feeding first emerged in aquatic animals. If correct, this shows an animal specialized for hunting and living in shallow waters in the line between land and water. Newer research also indicates that it is possible Acanthostega evolved from an ancestor that had more terrestrial adaptations than itself.
Acanthostega was 2 feet (60 centimeters) long.
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.
Acanthostega is seen as part of widespread evolutionary radiation in the late Devonian period, starting with purely aquatic finned tetrapodomorphs, 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.
-  Archived February 25, 2007, at the Wayback Machine.
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- Fossils suggest earlier land-water transition of tetrapod
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