Welcome to Acanthostega
Name Definition
Spiny roof
Name Given By
Jarvik, 1952
Location
Eastern Greenland
Classification
Animalia, Chordata, Elpistostegalia, Stegocephalia
Size
60 cm long (23.6 in)
Temporal Range
Fammenian age of the late Devonian, 365 million years ago
Ecological niche
Small aquatic piscivore
Species/Sub Species
A. gunnari
Diet
Acanthostega lived off of a diet of small fish
Introduction
Acanthostega is a genus of stem-tetrapod vertebrates that lived in Greenland during the late Devonian period. Acanthostega means "spiny roof", while the specific epithet honors Gunnar Säve-Söderbergh who, along with Erik Jarvik, discovered the original fragments of its skull.
Acanthostega had eight manual digits (fingers) on each hand, although the number of digits present on the feet are currently unknown. They were linked together by webs, similar to extant waterfowl, which is one adaptation for an aquatic lifestyle. Unlike its successors, Acanthostega lacked wrists in their forelimbs, meaning that they could bend forward from the elbow, so a weight-bearing stance in Acanthostega is rather unlikely, however, despite lacking wrists, these forelimbs do seem to be more suited to paddling and holding onto underwater vegetation. Acanthostega is the first known stem-tetrapod to show the transition between locomotion which has been observed in its skeleton. Even if it doesn't mean that Acanthostega itself could stand in a weight-bearing position, its pelvic girdle started to show adaptations for a weight-bearing stance, however the two sides of the girdle are not attached. The two sides of the girdle did come in contact however, and they were also fused to the sacral ribs protruding from the vertebral column. The fusion of the pelvic girdle and the sacral ribs would have made it more powerful in countering gravity when not supported by buoyancy in water. Acanthostega had internal gills like fish. Acanthostega also had a pair of lungs which are another sign of transition, allowing it to breathe air, though the only times Acanthostega would need to breathe air would be above the water when its aquatic habitat was oxygen-poor, because not only were its ribs too short to support its chest cavity out of water, but also because oxygen levels are always smaller in the water than in the air, giving it an advantage over the contemporaneous fish it lived with.
Acanthostega is thought to have lived in shallow, weeded swamps, since its limbs show adaptations for this kind of habitat. Jennifer A. Clack interprets these adaptations as showing that Acanthostega was a primarily aquatic animal that never left the water, and that the true tetrapods would later evolve specializations which would be useful for terrestrial locomotion. She also adds the contrast between the mandibles of fish that have two rows of teeth and Acanthostega, which shows a big amount of teeth on the outer row, while the inner row consists of two fangs and some other smaller teeth. This likely is another indicator of the transition from tetrapods feeding exclusively in the water and then to feeding above water or on land.
Usually considered more primitive than to the other stem-tetrapod Ichthyostega, Acanthostega may have been more derived instead. Acanthostega resembles juvenile Ichthyostega much more than the adult form of Ichthyostega, suggesting that Acanthostega may have descended from a neotenic lineage of stem-tetrapods.
Analysis of the suture morphology in Acanthostega suggests that it could bite prey near the water, yet another transitional trait. Markey and Marshall compared the skull of Acanthostega to that of fish, which are known to use suction feeding to eat prey, and those of predatory tetrapods that are known to directly bite their prey. They concluded that Acanthostega was capable of biting into prey like predatory tetrapods, and this strongly supports the hypothesis that terrestrial-style feeding first evolved in aquatic animals. This indicates that Acanthostega was specialized for hunting in shallow waters and near the water's edge. Some more recent research suggests that Acanthostega may have evolved from an ancestor with more terrestrial adaptations than itself.
The remains of Acanthostega are relatively well-preserved, and the famous fossil that made Acanthostega gunnari significant was discovered by Jennifer A. Clack in eastern Greenland, 1987, albeit fragments of its skull had been discovered a few decades earlier in 1933 by Gunnar Säve-Söderbergh and Erik Jarvik, the man who the specific epithet is named after and the man who described Acanthostega respectively.
Acanthostega is important in understanding the evolutionary radiation of tetrapods and their relatives during the late Devonian, which began with purely aquatic and finned tetrapodomorphs, and their descendants being capable of breathing air and related adaptations of the jaws, moving their head around more because of their muscular necks, and limbs homologous with the fins of their ancestors that were used to support body weight. These features were present in the earlier and more primitive Tiktaalik, which like Ichthyostega and Acanthostega showed adaptations that allowed for increased terrestrial locomotion, but is also thought to have been primarily aquatic animal.
In the Late Devonian speciation of sarcopterygians, there are many iconic genera that exhibited a series of adaptations that started to move towards a fully terrestrial lifestyle. Descendants of open-ocean sarcopterygians like Eusthenopteron started to evolve structures of skull roofing bones similar to those of the earliest known tetrapods. Panderichthys evolved the capacity to breathe air and also capable of using its pectoral fins to haul itself on land, being adapted to shallow-water environments. In Tiktaalik, these muscular fins became even more exaggerated and would have allowed Tiktaalik to move overland in order to get to other bodies of water. Then Acanthostega makes its debut, being one of the first stem-tetrapods to evolve true limbs, though these limbs were rudimentary compared to the later tetrapods. Finally, Ichthyostega evolves with strongly developed limbs and girdle morphology is stronger and more adapted to life on land. From then on, the 367 million-year legacy of the tetrapods would start, giving rise to some of the most diverse and successful animal groups such as dinosaurs and eventually humans. However, not all descendants of the Devonian sarcopterygians moved to land. Today, coelocanths and lungfish still live in the water, with coelocanths returning back to the open sea, and the lungfish staying in freshwater swamps and rivers.
Sources:
- https://en.wikipedia.org/wiki/Acanthostega