I. stensioei
I. watsoni
I. eigili
I. kochi
Ichthyostega Säve-Söderbergh, 1932 is an early tetrapod genus living in the Upper Devonian (Famennian ) period, 367-362.5 million years ago, and the first to be discovered. Being one of the first animals with legs, arms, and finger bones, Ichthyostega is seen as a hybrid between a fish and an amphibian. Ichthyostega had legs but its limbs probably weren't used for walking as once believed, but were used instead to negotiate its way through the swamps of the time.

History and Systematics

Säve-Söderbergh, 1932a described four Ichthyostega species from the Upper Devonian of East Greenland and one species belonging to the genus Ichthyostegopsis, I. wimani. These species can be synonymous (in which case only I. stensioei would remain), because their morphological differences are not very pronounced. They are based on differences in skull proportions, skull punctuation and skull bone patterns. This work was based on 14 specimens collected in 1931 by the Danish East Greenland Expedition. Additional specimens were collected between 1933 and 1955.

The species is/are 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 zygaphophyses . The first tetrapods (who probably didn't walk on land) were Elginerpeton and Obruchevichthys.

Characteristics

Ichthyostega was about one meter long and had seven digits on foot and hand.

Adaptations for land-life

Primitive amphibians like Ichthyostega and Acanthostega differed from animals like Crossopterygians (for instance Eusthenopteron or Panderichthys ) in that although Crossopterygians had lungs, they used their gills as the primary means of acquiring oxygen. Ichthyostega probably used lungs as its primary means of breathing. Primitive amphibians had a special type of skin that helped them retain bodily fluids and deter desiccation whereas Crossopterygians did not, and a stronger skeletal structure allowed Ichthyostega to live more comfortably with the increased burden of weight on land. Moreover, Crossopterygians used their bodies and tails to move about while using their fins for balance while Ichthyostega instead used its limbs for locomotion and its tail for balance.

Amphibians gained a tremendous advantage by moving on land as there was no competition for food (the first tetrapods had to be herbivores) and they could avoid large predatory fish that ruled the rivers and lakes. The move also came with disadvantages such as the new requirement of a moist, gas-permeable skin in order to aid the inefficient lungs. Water was still a requirement too because the jelly-like eggs of an amphibian cannot survive out of water, so reproduction could not occur without it. Water was also needed for the external fertilization that is characteristic of amphibians. Most land-dwelling animals have since developed various methods of internal fertilization.

Ichthyostegoids (Elginerpeton, Acanthostega, Ichthyostega,...) 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.

External links

• Excellent site on early tetrapods
• Course site
• Course site

References

Blom, H. (2005) — Taxonomic Revision Of The Late Devonian Tetrapod Ichthyostega from East Greenland. Palaeontology, 48, Part 1:111–134

Westenberg, K. (1999) — From Fins to Feet. National Geographic, 195, 5:114–127