Acanthostega is a prehistoric tetrapod of the Late Devonian Period and among the first tetrapods to have recognizable limbs. Acanthostega evolved from Lobe-Finned Fish, and was the ancestor to the majority of species that followed in later periods, including land species. The findings of Acanthostega has forced scientists to reassess the theory that the evolution of limbs and digits came only after animals had started walking on land, probably in response to the food resources on land. Since it is evident that the limbs of Acanthostega were more adapt to swimming through water than walking on land, it is now believed that the evolution of limbs and digits happened in water. Acanthostega probably lived a life similar to that of present day crocodiles and alligators, preying on those animals smaller than them who come close to the shores or in the shallow waters.
Acanthostega was still very much fishy in morphology. For example, it relied on internal gills in order to breathe, an indication that it depended on and spent a great deal of its life in the water. So even if the rest of its body were adapted for land walking, Acanthostega still would be tied to a water environment, unable to venture far from its shores. Another fishy characteristic was its tail fin. The tail fin only would have been able to sweep from side to side, thrusting the animal forward. The tail also had little bones, called finrays, protruding from the top and bottom which would have allowed a fin to grow on both sides of the tail bone.
Even though the animal had evolved limbs and digits, its limbs were still weak and more than likely Acanthostega could not walk on them. The limbs were sideways and backwards facing rather than front facing and comprised of only three different bones, the humerus, radius, and ulna, along with a distinct bony wrist. The limbs, especially the hind limbs, would have been used more like paddles to help the animal get through the water plants. The skull also had fish-like features, such as lateral lines. Lateral lines are part of the system of sense, used to detect the vibration of the water surrounding the animal. Evidence of this is found in several fossilized skulls of Acanthostega.
Even though these prehistoric tetrapods were still mostly aquatic living animals, they had several evolved features that would later become detrimental for land walking. First and foremost Acanthostega had developed Zygapophyses, which are interlocking vertebral pegs that stick out from the ends of vertebrae and allow the spine to stiffen and carry more weight. Tetrapods used these vertebral pegs in order to harden their spines so that they can carry their bodies off of the ground. Also characteristic of Acanthostega and other tetrapods were the limbs that evolved, probably from the fins of their ancestors the lob-finned fish. In each limb were multiple digits, usually around eight fingers and seven or eight toes but always more than typical five seen in many later vertebrae species, and wrist and elbow joints.
Water Versus Land
The two opposite environments require several different characteristics and different morphology, and as such those who finally conquered the land needed to overcome many obstables. For example, if a fish is taken out of the water for an extended period of time they become blind because their eyes dry up, they are unable to move any distance, unable to breathe the oxygen, they die soon after coming out of their watery environment. So in order to survive in the new oxygen rich environment our vertebrae ancestors would have needed to evolve numerous features that would allow them to adapt to life out of the water. First would have been to evolve a wider and deeper set of lungs more capable of breathing oxygen. A stronger set of limbs would be needed in order to walk on land, which would have been necessary to find food and shelter or to escape predators. The eyes, ears, and other sensory organs would have also needed to adapt to life on land. It seems as though Acanthostega was the next evolutionary step towards life on land.
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