Perhaps the most important findings to alter our understanding of tetrapods evolution followed the discovery and analysis of Acanthostega in the late 1980s and early 1990s. A skull roof of this tetrapod was first discovered by Gunar Säve-Söderbergh and Erik Jarvik in 1933, but the significance of this animal was not realized until after additional material was recovered by Jenny Clack in 1987. Together with Michael Coates, Clack realized that this animal was clearly a tetrapod, but that it was a poor excuse for a land animal. Its legs were ill suited to support its weight and the wrists were absent. Yet, it sported well developed digits (fingers and toes). Surprisingly, the forelimbs possessed eight digits rather than the anticipated five digits, while the hindlimbs possessed seven.
Additional features from the spine, ribs, pelvis and tail corroborated the notion that Acanthostega would not be able to support itself on land. Since it was essentially contemporaneous with the apparently more terrestrial Ichthyostega, Acanthostega, like modern-day dolphins and whales, may have secondarily lost the skeletal features needed for life on land. This possibility, however, is undermined by the presence of internal fish-like gills. (It also breathed with its lungs.) Acanthostega, with its four limbs, pelvic girdle and assorted other features, was clearly a tetrapod, but it probably never left the water.
