Acanthostega
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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 helped to disprove 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.
Fish-Like Features
Acanthostega was still very much fishy in morphology. For example, it had a flat head, internal gills, paddle-like hindlimbs, tail fin, and lateral lines on the skull.
Evolved Features
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 8 fingers and 8 toes but always more than typical five seen in many later vertebrae species, and wrist and elbow joints.
Morphology of Land-Walkers
Those who conquered land needed to overcome many obstacles. The two opposite environments require several different characteristics and different morphology. 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.
Environment
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