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Acanthostega Fossil range: Devonian
restoration of Acanthostega gunnari
Scientific classification
Kingdom: Animalia Phylum: Chordata Superclass: Tetrapoda Family: Acanthostegidae Genus: Acanthostega Species: A. gunnari
Binomial name
Acanthostega gunnari Jarvik, 1952

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.^[1]

It had eight digits on each hand and foot linked by webbing^{[citation needed]}, 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 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.^[1]

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.

In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish – like Eusthenopteron – exhibited a sequence of adaptations:

• Panderichthys, suited to muddy shallows;
• Tiktaalik with limb-like fins that could take it onto land;
• Early tetrapods in weed-filled swamps, such as:
  • Acanthostega which had feet with eight digits,
  • Ichthyostega with limbs.

Descendants also included pelagic lobe-finned fish such as coelacanth 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.^[1]

1. ^ ^{a b c} Jennifer A. Clack, Scientific American, Getting a Leg Up on Land Nov. 21, 2005.

• http://tolweb.org/tree?group=Acanthostega&contgroup=Terrestrial_Vertebrates
• http://www.palaeos.com/Vertebrates/Units/150Tetrapoda/150.150.html#Acanthostega
• http://faculty.uca.edu/~benw/biol4402/lecture8c/sld004.htm
• http://gatito.valdosta.edu/fossil_pages/fossils_dev/a1.html
• http://faculty.evansville.edu/de3/b39903/PDFs/12_Land_InvasionII.pdf
• http://www.wfiu.indiana.edu/amos/library/scripts/acanthostega.html
• http://theclacks.org.uk/jac/acanthostega.htm