Ichthyostega
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Ichthyostega Temporal range: Late Devonian, 365–360 Ma |
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| Skeleton of Ichthyostega in Moscow Paleontological Museum | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Sarcopterygii |
| Clade: | Tetrapodomorpha |
| Clade: | Stegocephalia |
| Genus: |
Ichthyostega Säve-Söderbergh, 1932 |
| Type species | |
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†Ichthyostega stensioei Säve-Söderbergh, 1932 |
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| Species | |
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| Synonyms | |
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Genus synonymy
Species synonymy
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Ichthyostega (Greek: "fish roof") is an early tetrapod genus that lived at the end of the Upper Devonian period. 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.
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. Although the forelimbs have never been recovered, they are generally thought to have been larger than the rear limbs, perhaps as an aid to carrying its body out of the water. It had a fin containing fin rays on its tail.
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.
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