Ichthyostega

Ichthyostega (Greek: "fish roof") is an early tetrapod genus that lived at the end of the Upper Devonian period (Famennian age, 374 – 359 million years ago). It was a labyrinthodont, one of the first tetrapods in the fossil record. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps. Though undoubtedly of amphibian build and habit, it is not considered a true member of the group in the narrow sense, as the first true amphibians appeared in the Carboniferous period. Until finds of other early tetrapods and closely related fishes in the late 20th century, Ichthyostega stood alone as the transitional fossil between fish and tetrapods, combining a fishlike tail and gills with an amphibian skull and limbs.

History and systematics
In 1932 Gunnar Säve-Söderbergh described four Ichthyostega species from the Upper Devonian of East Greenland and one species belonging to the genus Ichthyostegopsis, I. wimani. These species could be synonymous (in which case only I. stensioei would remain), because their morphological differences are not very pronounced. The species differ in skull proportions, skull punctuation and skull bone patterns. The comparisons were done on 14 specimens collected in 1931 by the Danish East Greenland Expedition. Additional specimens were collected between 1933 and 1955.

The genus is closely related to Acanthostega gunnari, also from East Greenland. Ichthyostega's skull seems more fish-like than that of Acanthostega, but its girdle (shoulder and hip) morphology seems stronger and better adapted to land-life. Ichthyostega also had more supportive ribs and stronger vertebrae with more developed zygapophyses. Whether these traits were independently evolved in Ichthyostega is debated. It does however show that Ichthyostega clearly ventured onto land on occasions, unlike the very first tetrapods like Elginerpeton and Obruchevichthys.

Characteristics
Ichthyostega was a fairly large animal, broadly built and about 1.5 m long. The skull was flat with dorsally placed eyes and armed with large labyrinthodont teeth. The posterior margin of the skull formed an operculum covering the gills. The spiracle was situated in an otic notch behind each eye.

The limbs were large compared to contemporary relatives, and it had seven digits on each hind limb. The exact number of digits on the forelimb is not yet known, since fossils of the manus (hand) have not been found. It had a fin containing fin rays on its tail.