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The order Ichthyostegalia was erected for Ichthyostega, and contained until the 1980s only three genera (Ichthyostega, Acanthostega and Tulerpeton).While Ichthyostegalia in principle contain the most basal of animals with toes rather than fins, Clack and Ahlberg uses it for all finds more advanced than Tiktaalik (the closest relative of tetrapods known to have retained paired fins rather than ...
Tiktaalik also lacked a characteristic most fishes have—bony plates in the gill area that restrict lateral head movement. This makes Tiktaalik the earliest-known fish to have a neck, with the pectoral (shoulder) girdle separate from the skull. This would give the creature more freedom in hunting prey on land or in the shallows.
Ichthyostega (from Greek: ἰχθῦς ikhthûs, 'fish' and Greek: στέγη stégē, 'roof') is an extinct genus of limbed tetrapodomorphs from the Late Devonian of what is now Greenland. It was among the earliest four-limbed vertebrates ever in the fossil record and was one of the first with weight-bearing adaptations for terrestrial locomotion.
A cladogram of the evolution of tetrapods showing some of the best-known transitional fossils. It starts with Eusthenopteron at the bottom, indisputably still a fish, through Panderichthys, Tiktaalik, Acanthostega and Ichthyostega to Pederpes at the top, indisputably a tetrapod
In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish such as Eusthenopteron exhibited a sequence of adaptations: Panderichthys, suited to muddy shallows; Tiktaalik with limb-like fins that could take it onto land; early tetrapods in weed-filled swamps, such as Acanthostega, which had feet with eight digits, and Ichthyostega, which had limbs.
This suggests tetrapod evolution is older than the dated fossils of Panderichthys through to Ichthyostega. 375-350 Ma Tiktaalik. Tiktaalik is a genus of sarcopterygian (lobe-finned) fishes from the late Devonian with many tetrapod-like features. It shows a clear link between Panderichthys and Acanthostega. Acanthostega Ichthyostega
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In the tetrapod and higher clades from the lower-middle Famennian there are several defining changes on the basis of anatomy of Ichthyostega, Tulerpeton, and Acanthostega. In the cranium, there is a stapes derived from the hyomandibular of fishes; a single bilateral pair of nasal bones, and a fenestra ovalis in the otic capsule of the braincase ...