Ichthyostega

Ichthyostega (Greek: "fish roof") is an early tetrapod genus that lived in the Upper Devonian (Famennian) period, 367-362.5 million years ago. It was a labyrinthodont that represents an intermediate form between fish and amphibians. 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.

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. The first tetrapods (who probably didn't walk on land) were Elginerpeton and Obruchevichthys.

Characteristics
Ichthyostega was about 5 feet (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.

Adaptations for land life
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.



The size of an adult Ichthyostega (1.5 m or 4 ft) precluded much completely terrestrial locomotion. Juveniles, though, could have moved about on land much more easily. The massive ribcage, though. was made up of overlapping ribs and the creatures 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. 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. In that case, they would need 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. The greater mobility of juveniles on land would have helped them avoid aquatic predators.

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.

Ichthyostegoids (Elginerpeton, Acanthostega, Ichthyostega, etc.) were "succeeded" by temnospondyls and anthracosaurs, such as Eryops, an amphibian that truly developed the ability to walk on land. There is a gap of 20-30 million years between both groups. This gap, a classic in vertebrate paleontology, is known as Romer's Gap, after the American paleontologist Alfred Sherwood Romer. In 2002 a 350 million year old fossil named Pederpes finneyae was found.