Acanthostega

Skull cast, Musee De L'Histoire Naturelle Brussels

It has been inferred that Acanthostega probably lived in shallow, weed-choked swamps, its legs apparently being adapted for these specific ecosystems. Apart from the presence of limbs, it was not adapted in any way for walking on land. Jennifer A. Clack interprets this as showing that Acanthostega was primarily an aquatic animal descended from fish that never left the sea, and that the specializations of the tetrapod lineage were exaptations: features which would later be useful for terrestrial life, even if they originated for a different purpose. At that period, deciduous plants were flourishing and annually shedding leaves into the water, attracting small prey into warm oxygen-poor shallows that were difficult for larger fish to swim in; Clack remarks on how the lower jaw of Acanthostega shows a change from those of fish that have two rows of teeth, with a large number of small teeth in the outer row, and two large fangs and some smaller teeth in the inner row. This difference likely corresponds to a shift in stem-tetrapods from feeding exclusively in the water to feeding with the head above water or on land.[2]

Research based on analysis of the suture morphology in the skull of Acanthostega indicates that the species was able to bite prey at or near the water's edge. Markey and Marshall compared the skull with the skulls of fish, which use suction feeding as the primary method of prey capture, and creatures known to have used the direct biting on prey typical of terrestrial animals. Their results indicate that Acanthostega was adapted for what they call terrestrial-style feeding, strongly supporting the hypothesis that the terrestrial mode of feeding first emerged in aquatic animals. If correct, this shows an animal specialized for hunting and living in shallow waters in the line between land and water.[5]

Model reconstruction at the State Museum of Natural History in Stuttgart, Germany

While normally considered more basal than Ichthyostega, it is possible that Acanthostega was actually more derived. Since Acanthostega resembles juvenile Ichthyostega and shows a lot less differences from juveniles to adults than the latter, it has been suggested that Acanthostega might be descended from a neotenic lineage. Although it appears to have spent its whole life in water, its humerus also exhibits traits that resemble those of later, fully terrestrial stem-tetrapods (the humerus in Ichthyostega being somewhat derived from, and homologous with the pectoral and pelvic fin bones of earlier fishes). This could indicate that vertebrates evolved terrestrial traits earlier than previously assumed, and numerous times independently from another.[6] Muscle scars on the forelimbs of Acanthostega were similar to those of crown-tetrapods, suggesting that it evolved from an ancestor that had more terrestrial adaptations than itself.[7]

A histological study of Acanthostega humeri, assisted by synchotron scans, indicates that the animal matured slowly. Some individuals reached sexual maturity (based on a fully ossified humerus) at more than six years of age, and adult fossils are much rarer than juveniles. Late ossification of the humerus supports a fully aquatic lifestyle for Acanthostega. There is barely any correlation between humerus size and maturity, suggesting that there was significant size variation among individuals of the same age. This may be due to competitive pressures, differing adaptive strategies, or even sexual dimorphism. However, the small sample size prevents recognition of a bimodal distribution which could confirm the latter hypothesis.[8]