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.

 History 
In 1932, Gunnar Säve-Söderbergh described four species of Ichthyostega from the Late Devonian, which he found in East Greenland. He also described one species of Ichthyostegopsis, I. wimani. These could be the same species, because their morphological differences are not very large. In this case, the only species would be Ichthyostega stensioei. The species have differences in the shape and texture of their skull bones. Ichthyostegopsis also has a shorter skull and larger eyes closer to the front of the skull. However, some species of Ichthyostega (I. watsoni and I. eigili) also have a short skull. The skulls that were compared were found in 1931 by the Danish East Greenland Expedition.

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.

A 2005 study wrote that only three species of Ichthyostega existed, and that the fossils found in both places (Gauss Halvø and Celsius Bjerg) could be grouped in the same way. Ichthyostegopsis wimani had a different skull shape because it was a juvenile of Ichthyostega eigili, and not another species. It had larger eyes compared to its skull because the eyes did not grow as fast as the rest of the skull. Ichthyostega eigili had a short and wide skull, and sparse pits on the skull roof. Ichthyostega kochii was found to be another name for Ichthyostega stensioei, which had a long and narrow skull and many pits on the skull roof. The last species, Ichthyostega watsoni, is similar to both: it had a long and narrow skull, but sparse pits. Two unnamed species from Gauss Halvø, Ichthyostega sp. A and Ichthyostega sp. B, were said to be the same as Ichthyostega stensioei and Ichthyostega wastoni from Celsius Bjerg.

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.

 Body skeleton 
The legs were large compared to other fishapods from the time. It had seven digits on each back leg. It also had a weird mass just in front of the digits that wasn't fully made of bone. It might have made the front of the leg stronger. The exact number of digits on the forelimb is not yet known, since fossils of the hand have not been found.Evolutionary developmental biology, by Brian Keith Hall, 1998, , p. 262 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. The tail of Ichthyostega had a low fin with bony fin rays, similar to those of fishes. The tail fin was not as deep as that of Acanthostega. It would have been less useful for swimming.

Ichthyostega is related to Acanthostega gunnari. They are both from what is now East Greenland. The skull of Ichthyostega was more similar to a fish skull than that of Acanthostega. However, the pelvis of Ichthyostega seems stronger and better adapted to life on land. Ichthyostega also had more supportive ribs and stronger vertebrae. The vertebrae had more developed parts to fit into each other. Researchers don't know if those evolved separately in Ichthyostega. However, these changes show that Ichthyostega could have walked on land sometimes. Other fishapods of the time, such as Elginerpeton and Obruchevichthys, could not do it.

Classification
Ichthyostega used to be grouped in the order "Ichthyostegalia", which was named after it. However, this group of fishapods does not include all of its descendants and is not used by many researchers today. Scientists have found by studying phylogeny that Ichthyostega is between other early four-limbed vertebrates. The evolutionary tree below comes from a study by Swartz in 2012.

 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 or a seal. It could not walk the same way that most quadrupeds do because its forelimbs could not turn around enough.

The ears of Ichthyostega were very different from those of other fishapods. They did not have eardrums, but were pockets full of air. Scientists think that this helped Ichthyostega hear under water, as the sound waves from the water would make the air pockets vibrate.

 Related pages 
 Tiktaalik
 Acanthostega

References

Category:Lobe-finned fish
Category:Tetrapods