Ichthyostega

***Ichthyostega*** (meaning 'roof fish') is an extinct fishapod. It lived in Greenland during the Late Devonian. *Ichthyostega* was one of the first vertebrates with four limbs. It was also one of the first that could hold its weight on land. *Ichthyostega* had lungs and limbs that helped it live in shallow water in swamps. While *Ichthyostega* is often called a 'tetrapod' because of its limbs and fingers, it evolved long before true tetrapods, and is better called a 'stem tetrapod'. Also, while *Ichthyostega* looked like an amphibian, it is not really one, as the first modern amphibians (members of the group Lissamphibia) appeared in the Triassic Period. Until other early fishapods were found in the late 20th century, *Ichthyostega* was the only transitional fossil between fish and tetrapods, with features of both. Scientists thought it moved similar to a salamander, but newer research has shown that it moved more similar to a seal.

Researchers found more fossils between 1933 and 1955. The first body fossils were found by the expedition in 1948, just after Säve-Söderbergh died. One of the fossils (A.109) contained a tail and a back limb. Another (A.115) was the front of the body. It contained the head, shoulders, ribs and both front limbs.

## Description
*Ichthyostega* was a large animal for its time. It had a strong build and was about  long. The skull was low, with the eyes on top. The teeth were large and had many folds (labyrinthodont). The back of the skull had an operculum, which is a flap that covered the gills. There was a hole for breathing behind each eye called otic notch. *Ichthyostega* also had gill bars with grooves, like *Acanthostega*. The exact number of digits on the forelimb is not yet known, since fossils of the hand have not been found. While in water, the foot would be more similar to a fleshy paddle than a fin.

The vertebral column and ribcage of *Ichthyostega* were very different from both those of fishapods and those of later tetrapods. They were also more specialized. The thoracic vertebrae (at the front of the trunk) and those in the short neck have tall neural spines that are pointed backwards. They attach to pointed ribs which become larger and have large flat parts that overlap. Past the sixth or seventh rib, the ribs become much shorter and lose their flat parts. The lumbar vertebrae (at the back of the trunk) have strong scars from the muscle that were attached to them. Their neural spines point forwards and become smaller towards the hips. The sacral vertebrae (above the hips) have fan-shaped neural spines. The ones in front point forwards, but the ones closer to the tail point backwards. The vertebrae just behind the hips have surprisingly large ribs. They are similar to those at the front of the trunk. The tail vertebrae have thin spines that point backwards.

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## Biology 
Early vertebrates with limbs such as *Ichthyostega* and *Acanthostega* were more adapted to living on land than earlier fishapods such as *Eusthenopteron* or *Panderichthys*. Fishapods had lungs, but used gills to breathe out carbon dioxide. Fishapods swam with their bodies and tails, and steered with their fins. *Ichthyostega* may have walked on land with its front limbs and swam with its tail.

*Ichthyostega* had a large ribcage with ribs covering each other, and a stronger skeleton than earlier fishapods. Its spine was similar to that of a fish. Its front limbs were probably strong enough to help the animal get out of the water. This may have evolved so *Ichthyostega* could hold its weight on land. The back limbs were smaller than the front limbs and could not hold its weight. *Ichthyostega* could not move side-to-side because of its large ribs. The front limbs could move enough to push the body up and forward, probably allowing the animal to drag itself across flat land by "crutching" with both front limbs at the same time, similar to a mudskipper It could not walk the same way that most quadrupeds do because its forelimbs could not turn around enough.

## Related pages 
* *Tiktaalik*
* *Acanthostega*

## References