We’ve talked here about prehistoric fish and about prehistoric land dwellers, but never about the evolutionary crossover between these two. Tetrapods (meaning four feet, or “four-legged fish”) are considered to be the link between fish and land dwellers. They have many fish-like attributes such as gills and fins but are capable of walking and breathing on land for short periods of time due to their lungs and boned legs, and are the predecessors to modern-day amphibians. Of the tetrapods, Ichthyostega isn’t the most popular or well-known, but they were the first tetrapod discovery and the species which made scientists aware of how the transition from aquatic to terrestrial animals may have occurred. They lived in the Devonian period, 365 million years ago, before any dinosaurs ever existed. The Devonian Period is also known as the “Age of Fishes”.
Ichthyostega was discovered in the early 1930s in Greenland by Gunnar Säve-Söderbergh and the Danish East Greenland Expedition. The name “Ichthyostega” means “fish roof”. The tetrapod was about five feet long and weighed close to 50 pounds. While they could get around on land, they were too heavy to do it for any long period of time, and so it is believed that this animal spent most of its time underwater. While on land, they probably would have pulled themselves along slowly. Their legs didn’t have the range of motion needed for walking raised up off of the ground. Scientists have likened the probable movement of this tetrapod to that of a seal or a sea lion.
The diet of Ichthyostega likely consisted of fish that it caught while moving around with ease in the water, or of small lizards that they hunted on land. They had both lungs and gills but relied mostly on their gills, and so fish probably made up the largest portion of their diets. Ichthyostega kept to the shallows, swamps, and rivers and streams while in the water, keeping close to land at all times.
So why did some fish evolve to start making the trek onto land? During the Devonian period, the seas were the center of life and the environment was fiercely competitive. Many fish were huge or armored and smaller or more vulnerable fish might not have survived if they didn’t branch out. By moving onto land, a whole new landscape of both prey and plant life, which had just begin to spread out across the Northern hemisphere, were available to them.
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