Tetrapods—From Water to Land

Tetrapods—including the modern forms of amphibians, reptiles, birds, and mammals—are loosely defined as vertebrates with four feet, or limbs. Many species we see today, like the snakes or whales, may not appear to be tetrapods, but their lack of well-developed limbs is a secondary adaptation to their habitat. This means that they originally had four limbs, but lost them as they adapted to a certain style of living. In the fossil record scientists often see intermediate forms which has reduced limbs. In modern skeletons of these animals you can often see vestiges of appendages that indicate that they are, indeed, tetrapods.

Life on Land

The appearance of tetrapods on land signaled one of the most hazardous and important evolutionary events in the history of animals. Life began in water. The body systems of early organisms were adapted to a mode of life in which water provided buoyancy against gravity. Desiccation, or drying out, was not a problem for animals whose bodies were constantly bathed in fluid. Movement from place to place usually required little energy and often involved simply floating along with the current. Reproduction was easier when sperm and eggs could be released into the water for fertilization . So the transition from living in the ocean to living on land required that ancestral vertebrates (who gave rise to the tetrapods) have physical traits that would helped them make this shift.

Tetrapods were not the first animals to make the move to land. Around 400 million years ago, primitive arthropods quickly followed the invasion of the first land plants, such as the mosses and liverworts, the first organisms to establish a foothold in the drier, but still moist, habitats, such as shorelines streams, and marshes. Both plants and insects continued to evolve and invade increasingly arid and varied habitats. They provided an important and diverse potential food base for the future land vertebrates.

Fins and Legs

Paleontologists believe that only 50 million years after the first plants left their aquatic environments, two conditions existed that paved the way for the first tetrapods. First, competition for food in the oceans was extremely fierce. Second, a group of bony fishes called the lobe-finned (or sarcopterygian) fishes had developed the physical characteristics necessary for the transition from water to land.

In particular, one group of lobed-fin fishes, the Rhipidistians, had a general body plan that was very similar to and can be traced back to early amphibians. Rhipidistians had pairs of front and back fins with sturdy bones, instead of cartilaginous rods, for resting their bulky bodies on the sea floor. These special fins were strengthened by a particular arrangement of the bones that resembled the structure of tetrapods in many ways. Large single, heavy bones in the fins were located nearest to the body and attached to a central girdle, like the pelvis.

When the land animals began to use the fins for standing and walking, the girdle bones had to become stronger to support the body on the legs. These bones eventually became the femur (upper leg bone) and humerus (upper arm bone) of the tetrapods. Pairs of bones (the radius and ulna of the arm, and the tibia and fibula of the leg) were attached to the femur and humerus, followed by a series of smaller bones that correlate to the fingers of tetrapods.

However, the Rhipidistian fishes had many more fingers and finger bones. The loss and reduction of the numbers of bones of these animals is one of the way scientists can trace the more modern animals from the older ones. Modern living members of this group of extraordinary fishes include the coelacanths (considered to be living fossils) and the lungfishes. Lung-fishes do not have gills, but have primitive lungs that enable them to remain out of water for extended periods of time. The mouth and skull structures of lungfishes are very similar to those of ancestral amphibians.

Tetrapods in the Fossil Record

The fossil record of tetrapods is not complete. There are many gaps that prevent scientists from clearly understanding the relationship between ancestral amphibians and modern ones. In addition, the transitional form of animals representing the shift from amphibian to reptiles is still poorly understood. However, places like China, Europe, Mongolia, North America, and South America constantly provide new fossil information about the history of tetrapods, and tetrapod fossils have even been discovered in Greenland and Antarctica.

What is well-known about the history of tetrapods starts about 400 million years ago when the first terrestrial (no longer dependent on water for a complete life cycle) vertebrates appeared. By the beginning of the Triassic period many unusual amphibians ruled the land. A group of large and slow-moving creatures, the labyrinthodonts, and the smaller newtlike and salamander-like lepospondyls, dominated the swamps and humid environments of Earth.

Labyrinthodonts were named because the structure of their teeth whose outer and inner surfaces reminded researchers of a labyrinth, or maze. The teeth of lepospondyls were very simple and cone-shaped as compared to the labrinthodonts. The labyrinthodonts were some of the largest amphibians to have ever lived. Some genera, like the Euryops, were about 2 meters (six feet) long.

Euryops is considered one of many possible ancestors of modern amphibians The great diversity of these types of tetrapods has been vastly reduced. Only the salamanders and frogs are still living, but are found over most of the world. Amphibians still have the primitive fishlike trait of laying eggs in water and have never lost their dependency on water-rich environments. As their skin does not retain moisture, they must live near a wet habitat to keep from drying out. They are good examples of the tetrapods' link to watery beginnings.

On the Land and in the Sea

The development of the amniotic egg and the growth of scales that prevented water loss allowed tetrapods to move into newer, more arid environments. An evolutionary explosion then occurred that produced the early ancestors of the turtles, crocodiles, lizards, snakes, dinosaurs, and even mammals. Many other tetrapods, like the pterosaurs, also emerged. In the marine environment, several tetrapods returned to the sea. Fierce marine reptiles like the mosasaurs and plesiosaurs found an abundant food source in the huge stocks of fishes in the oceans. These animals, although aquatic, were structurally tetrapods. They retained the tetrapod body plan of a thick, upper-arm bone connected to a girdle (hip and chest bones), two smaller bones, and a series of small bones for fingers. However, instead of appearing like an arm or a leg the bones were covered by tissue that formed a flipper.

Around 65 million years ago at the close of the Cretaceous period, a massive extinction killed many life forms including the large tetrapods (dinosaurs) and other animal and plant species. However, many life forms did not die out. Mammals, reptiles, and birds survived, and eventually became the most abundant tetrapods on Earth.

Modern Tetrapods

The most familiar modern group of tetrapods is the mammals, which includes humans. These furred animals began their evolutionary history back with the beginnings of the dinosaurs. Early mammals were small, about the size of a rat. After the large Cretaceous extinction, mammals survived to become some of the largest tetrapods on Earth, including elephants and whales. Just as the marine reptiles had done, whales returned to the sea, developing fins as a secondary adaptation.

Tetrapods today can be found in nearly every environment. Unfortunately, as the world continues to change, the numbers of many types of tetrapods are declining. With increasing habitat loss and pollution many tetrapods are in danger of extinction. The loss of so many species of birds, reptiles, and mammals, such as the panda, is a serious threat to the continued survival of the tetrapods.

see also Living Fossils; Phylogenetic Relationships of Major Groups.

Brook Ellen Hall

Bibliography

Carroll, R. "Vertebrate Paleontology and Evolution." New York: W. H. Freeman and Company, 1988.

Lillegraven, J. A., Z. Kielan-Jaworowska, and W. Clemens. "Mesozoic Mammals; The First Two-Thirds of Mammal History." Berkeley: University of California Press, 1979.

Macdonald, D. "The Encyclopedia of Mammals," New York: Facts on File Publications, 1987.