Acanthostega, from Ancient Greek ἄκανθα (ákantha), meaning "spine", and στέγη (stégē), meaning "roof", is an extinct genus of stem-tetrapod, among the first vertebrate animals to have recognizable limbs. It appeared in the late Devonian period (Famennian age) about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and those that were able to come onto land.

Discovery

thumb|left|240px|Holotype skull, [[Zoological Museum of Copenhagen]]

The fossilized remains are generally well preserved, with the famous fossil by which the significance of this species was discovered being found by Jennifer A. Clack in East Greenland in 1987, though fragments of the skull had been discovered in 1933 by Gunnar Säve-Söderbergh and Erik Jarvik.

Description

thumb|[[Life restoration|left]]

thumb|192x192px|Simplified phylogeny of the fish–tetrapod transition.

The Acanthostega had eight digits on each hand (the number of digits on the feet is unclear) linked by webbing. It lacked wrists, and was generally poorly adapted for walking on land. It also had a remarkably fish-like shoulder and forelimb. The front limbs of Acanthostega could not bend forward at the elbow, and therefore could not be brought into a weight bearing position, appearing to be more suitable for paddling or for holding on to aquatic plants. Acanthostega is the earliest stem-tetrapod to show the shift in locomotory dominance from the pectoral girdle to the pelvic girdle. It had internal gills that were covered like those of fish. It also had lungs, but its ribs were too short to support its chest cavity out of water.

Lifestyle

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 many times independently from another. Muscle scars on the forelimbs of Acanthostega were similar to those of crown-tetrapods, suggesting that it evolved from an ancestor which had more terrestrial adaptations than itself.

Development

A histological study of Acanthostega humeri, assisted by synchrotron 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.

See also

  • Parmastega

References

Sources

  • Laurin M. 2010. How Vertebrates Left the Water. Berkeley: University of California Press.
  • Steyer J-Sb. 2012. Earth Before the Dinosaurs. Bloomington: Indiana University Press.
  • Tolweb.org