Acanthostega

Name: Acanthostega (Spiny roof).
Phonetic: A-can-fo-stay-gah.
Named By: Erik Jarvik - 1952.
Classification: Chordata, Tetrapoda, Amphibia, Labyrinthodontia, Ichthyostegalia.
Species: A. gunneri.
Type: Carnivore.
Size: 60 centimetres long.
Known locations: Eastern Greenland.
Time period: Famennian of the Devonian.
Fossil representation: Initially only skull fragments, further fossils have been discovered making it one of the most completely preserved species. A very well preserved specimen was discovered in 1987 by Jennifer A. Clack.

       Although it’s possible that Acanthostega could walk on land, the morphological features of the body suggest a primarily aquatic lifestyle. Neither the spine or ribs were adapted to support its weight while on land. The legs, though large, lacked the joints and degrees of motion required for efficient terrestrial locomotion. The underside of the creature had heavy scales, a feature that may have protected it from regular contact with ground, especially if it hunted in the shallows. If it were able to spend short periods out of the water the scales might also have helped protect the body as it was dragged over the land, perhaps if the creature was moving from one body of water to another.
       These and other features do point to a predator that was suited to life in the Devonian swamps. As matter is decomposed by bacteria in an aerobic process, oxygen is used up reducing the oxygen content in the water. Acanthostega lad lungs as well as internal gills, and this lung back-up would have allowed it to exploit the oxygen in the air as well, something that predatory fish would not have been able to do. The legs while crude compared to the land animals to come, would have enabled it scramble over and around submerged obstacles and aquatic plants. They would also have allowed it to free itself if it beached itself in shallow water while pursuing prey.
       Perhaps the most important feature is the teeth and jaw. Fish typically use the suction method of feeding. This is where the predator opens its mouth so fast and wide that it creates a vacuum inside its mouth that sucks water and the prey inside. In comparison Acanthostega jaw and teeth which featured two fangs, was more suited for actually biting onto prey. This terrestrial method of feeding suggests that it focused its hunting around the water edges, as opposed to exclusively hunting submerged.

Further reading
- The neurocranium of Acanthostega gunnari Jarvik and the evolution of the otic region in tetrapods. - Zoological Journal of the Linnean Society vol 122 issue 1-2 - Jennifer A. Clack - 1998.
- Terrestrial-style feeding in a very early aquatic tetrapod is supported by evidence from experimental analysis of suture morphology. - Proceedings of the National Academy of Sciences 104 (17): 7134–7138. - M. J. Markey & C. R. marshall - 2007.
- The Devonian tetrapod Acanthostega gunnari Jarvik: Postcranial anatomy, basal tetrapod interrelationships and patterns of skeletal evolution. Earth and Environmental Science Transactions of the Royal Society of Edinburgh. - Michael I. Coates - 2014.
- Descriptive Anatomy and Three-Dimensional Reconstruction of the Skull of the Early Tetrapod Acanthostega gunnari Jarvik, 1952. - PLOSone. - Laura B. Porro, Emily J. Rayfield & Jennifer A. Clack - 2015.



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