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Macroevolution through evograms → The origin of tetrapods
The origin of tetrapods
The word “
tetrapod” means “four feet” and includes all species alive today that have four feet — but this group also includes many animals that don’t have four feet. That’s because the group includes all the
organisms (living and
extinct) that descended from the last common ancestor of amphibians, reptiles, and mammals. So, for example, the ichthyosaur, an extinct swimming reptile, is a tetrapod even though it did not use its limbs to walk on land. So is the snake, even though it has no limbs. And birds and humans are tetrapods even though they only walk on two legs. All these animals are tetrapods because they descend from the tetrapod ancestor described above, even if they have secondarily lost their “four feet.”
Tetrapod phylogeny from
The Tangled Bank, used with permission of the author, Carl Zimmer, and publisher, Roberts & Company, Greenwood Village, Colorado.
Tetrapods
evolved from a finned organism that lived in the water. However, this ancestor was not like most of the fish we are familiar with today. Most animals we call fishes today are ray-finned fishes, the group nearest the root of this evogram. Ray-finned fishes comprise some 25,000 living species, far more than all the other vertebrates combined. They have fin rays — that is, a system of often branching bony rays (called lepidotrichia) that emanate from the base of the fin.
In contrast, the other animals in the evogram — coelacanths, lungfishes, all the other extinct animals, plus tetrapods (represented by Charles Darwin) — have what we call “fleshy fins” or “lobe fins.” That is, their limbs are covered by muscle and skin. Some, such as coelacanths, retain lepidotrichia at the ends of these fleshy limbs, but in most fleshy-finned animals these have been lost.
The
common ancestor of all those different organisms (ray-fins, coelacanths, lungfishes, tetrapods, etc.) was neither a lobe-fin nor a ray-fin. This ancient vertebrate
lineage had fins (with lepidotrichia), scales, gills, and lived in the water. Yet they also had air bladders (air-filled sacs) connected to the back of their throats that could be used for breathing air (i.e., as lungs) or for buoyancy control. The air bladders of many ray-fins no longer connect to their throats, and so they are not able to breathe air. In these ray-fins, the air bladder is used mainly for buoyancy control and is known as a swim bladder. By contrast, tetrapods have taken an alternative route: they have lost the buoyancy control function of their air bladders, and instead this organ been elaborated to form the lungs that we all use to get around on land.
When we get past coelacanths and lungfishes on the evogram, we find a series of fossil forms that lived between about 390 and 360 million years ago during the Devonian Period. During this interval, this lineage of fleshy-finned organisms moved from the water to the land. Many parts of the skeleton changed as new innovations that permitted life on land evolved.
For example, the ancestors at the base of this evogram lived fully in the water and had skulls that were tall and narrow, with eyes facing sideways and forwards. This allowed them to look around in their watery environments for
predators and
prey. However, as ancestors of the first tetrapods began to live in shallower waters, their skulls evolved to be flatter, with eyes on the tops of their heads. This probably allowed them to look up to spot food. Then, as tetrapods finally moved fully onto land and away from the water, many lineages once again evolved skulls that were tall and narrow, with eyes facing sideways and forwards, allowing them to look around their terrestrial environments for predators and prey.
As lineages moved into shallower water and onto land, the vertebral column gradually evolved as well. You may have noticed that fishes have no necks. Their heads are simply connected to their shoulders, and their individual vertebrae look quite similar to one another, all the way down the body. Mobile necks allow land animals to look down to see the things on the ground that they might want to eat. In shallow water dwellers and land dwellers, the first neck vertebra evolved different shapes, which allowed the animals to move their heads up and down. Eventually, the second neck vertebra evolved as well, allowing them to move their heads left and right. Later tetrapods evolved necks with seven or more vertebrae, some long and some short, permitting even more mobility.
Read on, but I doubt you will . . .