transition from forward-leaning to backward-leaning as they approach the tail. The vertebrae right behind the hips have unusually large ribs similar to the thoracic region. The caudal (tail) vertebrae have slender spines which lean backwards. The tail of Ichthyostega retained a low fin supported by bony lepidotrichia (fin rays). The tail fin was not as deep as in Acanthostega, and would have been less useful for swimming. Ichthyostega is 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 life on land. Ichthyostega also had more supportive ribs and stronger vertebrae with more developed zygapophyses. Whether these traits were independently evolved in Ichthyostega is debated. It does, however, show that Ichthyostega may have ventured onto land on occasions, unlike contemporaneous limbed vertebrates such as Elginerpeton and Obruchevichthys. History In 1932 Gunnar Säve-Söderbergh described four Ichthyostega species from the Late 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. Classification Traditionally, Ichthyostega was considered part of an order named for it, the "Ichthyostegalia". however, this group represents a paraphyletic grade of primitive stem-tetrapods and is not used by many modern researchers. Phylogenetic analysis has shown Ichthyostega is intermediate between other primitive stegocephalian stem-tetrapods. The evolutionary tree of early stegocephalians below follows the results of one such analysis performed by Swartz in 2012. Paleobiology Early limbed vertebrates like Ichthyostega and Acanthostega differed from earlier tetrapodomorphs such as Eusthenopteron or Panderichthys in their increased adaptations for life on land. Though tetrapodomorphs possessed lungs, they used gills as their primary means of discharging carbon dioxide. Tetrapodomorphs used their bodies and tails for locomotion and their fins for steering and braking; Ichthyostega may have used its forelimbs for locomotion on land and its tail for swimming. Its massive ribcage was made up of overlapping ribs and the animal possessed a stronger skeletal structure, a largely fishlike spine, and forelimbs apparently powerful enough to pull the body from the water. These anatomical modifications may