Acanthostega and Ichthyostega, the first limbed tetrapods

Acanthostega and Ichthyostega are the two best-known early four-legged animals, found in Late Devonian rocks about 365 million years old in East Greenland. Ichthyostega was first collected in 1931 by Swedish expeditions led by Gunnar Save-Soderbergh, and studied for decades by Erik Jarvik. Acanthostega was first collected in the 1950s but came to be understood through Jenny Clack's Cambridge expeditions from 1987, and with Michael Coates she published its limb structure in 1990. Both animals were about a metre long, with a long body, a tail fin, ribs and limbs ending in digits.¹²

The surprise was the digits. Acanthostega had eight fingers on each hand, and Ichthyostega seven toes on the foot, rather than the five in later animals. Acanthostega also had internal gills, a fish-like tail and limbs too weak to carry its body on land. That means limbs with fingers first evolved for life in shallow, plant-clogged water, to push through vegetation and hold position, not for walking on land. Ichthyostega had a sturdier chest and may have hauled itself on shore. These animals build on the lobe-finned fish like Tiktaalik, which lived about 10 million years earlier.¹²

Their anatomy shows the order of change: limbs and digits came first and air-breathing and walking later. Later work, notably by Per Ahlberg, has shown that tetrapods may be even older, as trackways from Poland dated to about 395 million years ago are claimed to show, though that dating is debated.³