March’s Fossil of the Month - Acanthostega (Acanthostega gunnari)
Family: Acanthostega Family (Acanthostegidae)
Time Period: 365 Million Years Ago (Late Devonian)
In life, Acanthostega gunnari would have likely resembled a cross between a lungfish and a salamander, and this isn’t dramatically different to what it really was; Acanthostega was a stem-tetrapod, an early member of the lineage of animals that now contains all reptiles, birds, mammals and amphibians, and is believed to be an example of a key stage in the transition between fleshy-finned fishes and the earliest terrestrial vertebrates. Although it possessed 4 short limbs ending in wide, 8-toed feet, the lack of any clear wrist or ankle joints suggests that Acanthostega likely couldn’t support its weight on land, and this combined with its well-developed pelvic bones implies that it was likely a fully-aquatic animal that primarily relied on a paddle-like fin on its tail to propel it forwards while its limbs were used to steer or possibly to grasp aquatic vegetation. During the late Devonian much of the world (including the area of what is now Greenland where Acanthostega fossils were first discovered) was covered in humid, swampy deciduous forests, and this combined with Acanthostega’s anatomy suggests that it likely inhabited warm, oxygen-poor forest pools, which would also explain one of its more unusual characteristics; in addition to possessing fish-like internal gills (as suggested by the presence of gill arch like structures at the base of its skull), a rudimentary rib cage implies that Acanthostega likely had lungs, allowing it to extract oxygen from water as well as air and thereby survive in shallow, oxygen-starved pools that fishes and larger stem-tetrapods would have struggled to breathe in. The teeth of Acanthostega (which were arranged in two rows and were short and sharp, with two larger fangs on the lower jaw) implies that it was likely carnivorous (possibly feeding on terrestrial arthropods caught from above-water beds of vegetation or the banks of its home pools), and comparisons of the anatomy and mineral makeup of fossils of smaller individuals (believed to be juveniles) with those of larger individuals (which are generally believed to be adults) implies that it grew slowly, possibly taking up to 6 years to reach full maturity (at which point most individuals were around 60cm/23.6 inches long, although the difference in the length of seemingly mature individuals suggests that, as with many fish, adverse environmental conditions could considerably limit Acanthostega’s growth.) Although it is unlikely that Acanthostega or its descendants ever succeeded in colonizing land, it is generally accepted that (having become so well-suited to life in the oxygen-poor pools they inhabited) they had little need to, and as several of Acanthostega’s fellow stem-tetrapods (such as the significantly larger Ichthyostega, which had jointed, six-toed limbs and a more developed rib-cage that likely allowed it to haul itself onto land for prolonged periods like modern mudskippers or seals) are known to have done so, the study of the anatomy and lifestyle of this strange little swamp-dweller can still help to shed light on how the variety of land-dwelling vertebrates seen today came to be.
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Image Sources
Fossil: https://commons.wikimedia.org/wiki/File:Acanthostega_gunnari.jpg#/media/File:Acanthostega_gunnari.jpg
Restoration: https://www.10tons.dk/acanthostega-gunnari