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A cladogram of the evolution of tetrapods showing the best-known transitional fossils. From bottom to top: Eusthenopteron, Panderichthys, Tiktaalik, Acanthostega, Ichthyostega, Pederpes.

Ichthyostega was a Paleozoic stem-tetrapod, about 4 to 5 feet long, that lived during the Devonian epoch and had adaptations for living in water as well as on land. Ichthyostega had seven digits on its hind limb instead of five, and its limb bones included a humerus and a femur as with later tetrapods. Ichtyostega had a strong rib cage that could support the animal's lungs, thereby enabling it to breathe on land but probably restricting the animal's side to side movement. Its ear also was apparently adapted to hear both on land and in the water. Ichthyostega also had gill arches as well as lungs.[1][2]


The fused rib cage almost certainly restricted the movements of Ichthyostega on land and it is impossible to reconstruct what the animal did there.

  1. It may be that Ichthyostega lumbered safely across land feasting on large invertebrates that had evolved no defenses against vertebrate types of predation. Further invertebrate predators may not have been triggered to attack a vertebrate moving differently from their typical invertebrate prey.
  2. It may be that Ichthyostega was in danger from invertebrate predators whenever it ventured far from water. Ichthyostega may have stayed near water except when desperately hungry or desperately in need to escape problems in the water where it had lived. Such problems may have included a pool or waterway drying up or perhaps a watery area becoming overpopulated, thus when Ichtyostega needed to move it could cross land and find a new waterway in which to live.
  3. Ichthyostega may have left the water mainly to bask, staying near water and able to return fast if needed.

See also[edit]