Ichthyostega: A Fish with a Roof

Ichthyostega lived at the end of the Late Devonian Period and was was one of the first four-limbed vertebrates in the fossil record. It is an early genus of tetrapodmorph, and although its possession of both limbs and fingers has lead to it often being labeled as a tetrapod, it was more primitive than crown tetrapods and could therefore be more accurately described as a stem tetrapod, or stegocephalian. Whereas crown tetrapods are the group identified as the most recent common ancestor of all living amphibians, the term “stem tetrapods” (tetrapodmorphs) is used for any animal that is more closely related to living amphibians, reptiles, birds, and mammals than to living lungfishes.  Until other finds of early stegocephalians,  Ichthyostega was alone as a transitional fossil between fish and tetrapods. Ichthyostega may have used its tail for swimming, and its forelimbs to move on land. Its forelimbs seem to have been strong enough to pull its body out of the water, and were larger than its hindlimbs. It had a large ribcage with overlapping ribs, which would have limited its ability to make side-to-side motions, and probably moved by dragging itself as its forelimbs did not have the range of rotational motion needed to move in typical quadrupedal gaits.