Phylogeny of Acanthostega
Acanthostega is related to modern day tetrapods and is one of the most important stem tetrapods that we have fossils of. Acathostega is placed on phylogenies at the origin of digits, enlargement of hind limb and pelvic girdle, and interarticulations between vertebrae (Clack, 2009).
In depth phylogeny of Acanthostega showing its relations to other early Tetrapods, Elpistostegalia, and Sarcopterygians. From Clack et al., 2009.
Simplified phylogeny of Acanthostega. Created by Justin Andell.
Sarcopterygian
Sarcopterygii are the lobed finned fish. This group includes Lungfish, Coelocanths and Eusthenopteron, and are characterized by their muscular fleshy paired fins. The fins are attached to the pelvic and pectoral girdle by a single bone. Fossil records and genetic analysis show us that terrestrial vertebrates evolved from these lobed finned fish.
Simple reconstruction of Eusthenopteron
How is Acanthostega related to modern Tetrapods?
Acanthostega is classified as stem tetrapoda, meaning Acanthostega is more closely related to four limbed vertebrates than it is to lungfish and their other fishy ancestors. Their closest relatives are the living tetrapods such as frogs, lizards, humans, etc.