Ichthyostega (Greek: "fish roof") is an early tetrapod genus that lived at the end of the Late Devonian Period (Famennian age, 374 – 359 million years ago). It was a labyrinthodont, one of the first fossil record of tetrapods. 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.
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. The first tetrapods (who probably didn't walk on land) were Elginerpeton and Obruchevichthys.
Size of Ichthyostega, compared to a human.Ichthyostega was about 1.5 m long and had seven digits on each hind limb. The exact number of digits on the forelimb is not yet known, since fossils with forelimbs have not been found. It had a fin containing fin rays on its tail.
Early tetrapods like Ichthyostega and Acanthostega differed from animals like Crossopterygians (for instance Eusthenopteron or Panderichthys) in their increased adaptations for life on land. Though Crossopterygians possessed lungs, they used gills as their primary means of acquiring oxygen; Ichthyostega appears to have relied on its lungs as its primary apparatus for breathing. The skin of early tetrapods, unlike that of Crossopterygians, helped retain bodily fluids and deter desiccation. Crossopterygians used their body and tail for locomotion and their fins for balance; Ichthyostega used its limbs for locomotion and its tail for balance.
- Panderichthys, suited to muddy shallows;
- Tiktaalik with limb-like fins that could take it onto land;
- Early tetrapods in weed-filled swamps, such as:
- Acanthostega which had feet with eight digits,
- Ichthyostega with limbs.
Descendants also included pelagic lobe-finned fish such as coelacanth species.The size of an adult Ichthyostega (1.5 m) precluded completely terrestrial locomotion. Yet the massive ribcage was made up of overlapping ribs and the animal possessed a stronger skeletal structure, a more rigid spine, and forelimbs apparently powerful enough to pull the body from the water. These anatomical modifications clearly evolved to handle the lack of buoyancy experienced on land. The hindlimbs were smaller than the forelimbs and unlikely to have born full weight in an adult, while the broad, overlapping ribs would have inhibited side-to-side movements. Jennifer A. Clack suggests that Ichthyostega and its relatives spent time basking in the sun to raise their body temperatures, much as some animals do today: the Marine Iguanas on the Galapagos Island or the Gharial. They would have returned to the water to cool themselves, hunt for food and reproduce. A lifestyle that required strong forelimbs to pull at least their anterior part out of the water, and a stronger ribcage and spine to support them while sunbathing on their abdomen like modern crocodiles. New studies suggests that the juveniles were more aquatic than the adults, and the possibility that Ichthyostega came out of the water only as a fully mature adult.
Water was also still a requirement, because the gel-like eggs of the earliest terrestrial tetrapods couldn't survive out of water, so reproduction could not occur without it. Water was also needed for their larvae and external fertilization. Most land-dwelling vertebrates have since developed two methods of internal fertilization; either direct as seen in all amniotes and a few amphibians, or indirect for many salamanders by placing a spermatophore on the ground which then is picked up by the female salamander.
The Ichthyostegalians (Elginerpeton, Acanthostega, Ichthyostega, etc.) were succeeded by temnospondyls and anthracosaurs, such as Eryops, amphibians that truly developed the ability to walk on land. Until 2002, there was a gap of 20 million years between the two groups ( Romer's Gap). In 2002 a 350 million year old fossil from the lower Mississippian, Pederpes finneyae was described and helped to close the gap: it is the earliest-known tetrapod to show the beginnings of terrestrial locomotion.