Until about 20 years ago, a scarcity of tetrapod fossils led paleontologists to make several basic assumptions about the evolution of the first land animals. Central to an interpretation of tetrapod origins was the idea that legs developed as a means for walking on land. This idea made sense based on the timing of known fossil specimens and the best information available about the specimens' ancient habitats. These basic assumptions have changed dramatically in recent years due to a renaissance of tetrapod fossil discoveries. These recent discoveries have broadened the time span of the tetrapods' earliest development and have provided more data about the kinds of environments in which the animals thrived.
To understand the evolution of ideas surrounding the transition of fishes to tetrapods, it is instructional to explore the theories of paleontologist Alfred S. Romer (1894-1973), a pioneering thinker in this field. Romer was an innovative researcher who combined knowledge from vertebrate paleontology, comparative anatomy, and embryology to form a more broadly informed understanding of extinct vertebrates. One of his special areas of interest was the evolution of the first land vertebrates from fishes. In exploring this field, Romer questioned many of the once-accepted reasons for the migration of backboned animals from sea to land.
During the 1960s, Romer was at the center of the debate about the origin of terrestrial animals. At that time, several theories about the development of the first land vertebrates were being debated. One suggestion was that an increasing abundance of atmospheric oxygen lured animals from the sea to the land. Romer objected to this theory because some lungfishes of the Paleozoic had already developed air-breathing lungs and "could obtain all the atmospheric oxygen they wished by merely sticking their heads out of water." Hence, venturing onto land was unnecessary for them.
Another theory about the movement of vertebrates to the land was that some species were escaping the ocean to avoid predators. Romer also discounted this idea; he reminded his colleagues that the closest relatives of the first tetrapods—the lobe-finned fishes—were generally considered to be aggressive predators in their own right and not likely to be forced out of many habitats for this reason.
Other paleontologists of Romer's time believed that the key to the mystery of migration to land was linked to food sources. In the opinion of these scientists, the mere opportunity to acquire new food sources on land was enough to effect the steady but slow adaptation of some fishes to onshore habitats. Once again, this theory did not satisfy Romer. He argued that because the first land vertebrates were predators, they were unlikely to have found suitable prey in large enough numbers during the Late Devonian Period. Romer's objection made perfect sense in the 1960s because it was thought at that time that insects and other small invertebrates were not widely available during the Late Devonian.
Underlying all of Romer's objections to the then-prevalent theories about the migration of vertebrates to land was that such explanations were not based on the anatomical and physiological realities of the animals in question; rather, they were based on "urges or directives" that caused the animals to seek life out of the oceans. In response to the prevailing theories, in 1968, Romer presented a well-conceived theory about taking to land that was based on asking a biological question about these land-seeking creatures: "Can such an explanation be found for the development in fishes of such terrestrially useful structures as lungs and legs?"
Romer answered his own question with an emphatic, "Yes." He found reasons why some fishes could have developed adaptations such as lungs and legs to improve their chances for survival, even if they were purely water dwellers. His explanation? Lungs and legs would have been important to the survival of fishes caught in shallow waters during times of drought. This would have been particularly important to freshwater species that lived in ponds, lakes, and streams that were subject to periodic dry spells. Lungs and primitive limbs in the form of pectoral fins would have enabled fishes to survive temporarily in shallow, stagnant water, or even out of the water, by allowing the fishes to breathe and to move across a muddy, drying streambed or pond to reach the water again. Such fishes certainly were doomed if water did not return in a reasonable time to reinstate their watery habitat, but these adaptations nonetheless made sense even in the absence of a migration to dry land on a full-time basis. This explanation was the key to the way in which Romer believed that amphibians evolved from lobe-finned fishes. Over time, in those cases in which both lobe-finned fishes and the earliest amphibians were victimized by drought, it would be the amphibians' ability to get up and walk away to a nearby stream or pond that would gradually select them as the progenitors of all land creatures that followed. The survival of early tetrapods in those circumstances led to the continuance of their anatomical traits in vertebrates that followed.
Although lungs and legs made life on land possible, Romer concluded that both of those adaptations had not evolved "in some mystic drive or urge toward a career on land" but had arisen "as structures useful in enabling a water dweller to survive as a water dweller." Romer suggested this premise more than 40 years ago; however, as intriguing as the idea was, it was not supported by much fossil evidence. In fact, the fossil record has a significant gap at the very place where Romer's idea might best be proved. The first known tetrapods—Ichthyostega and Acanthostega—date from about 365 million years ago, in the Late Devonian Period. Following the Late Devonian and extending into the Carboniferous is a gap of about 30 million years in the fossil record between those earliest known tetrapods and the tetrapods that are known from later in the Carboniferous. This span came to be known as "Romer's Gap," in part because of Romer's own tireless efforts to fill the gap with new fossil evidence. This 30 million-year void in the fossil evidence of early land vertebrates was significant because it was during that span that tetrapods fully developed many of the key anatomical traits that made life on land possible: legs, lungs, supporting skeletal structures, and the eyes and ears necessary for interpreting the world outside of the water.
A spurt of fossil discoveries since about 1990 has begun to fill Romer's Gap, shedding light on the critical transition of fish to tet-rapod. The transition from fish to tetrapod did not proceed along a simple straight-line of adaptation; it now appears that some extinct lobe-finned fishes had limblike fins with skeletal features similar to those seen in tetrapods. One such fish specimen is Sauripterus, described by Ted Daeschler of the Academy of Natural Sciences of Philadelphia and Neil Shubin of the University of Chicago and which was found in sedimentary rocks of Late Devonian age in Pennsylvania. The limb elements of this specimen contain many fish and tetrapod features, including paddlelike fin rays and a tetra-podlike internal fin skeleton.
In 1987, Jennifer Clack and her associates discovered the best specimen yet of the early tetrapod Acanthostega. Their discovery revealed that this animal, considered to be one of the first tetra-pods, was in reality poorly equipped for life on land. Its limbs were punier than originally surmised and lacked the kind of wrist and ankle joints necessary to maneuver effectively on land. Acanthostega most certainly had lungs, but its short rib cage suggests that it relied on buccal ventilation rather than on breathing powered by abdominal muscles. Acanthostega also had gills; this suggests that it still had not completely made the transition from water to land. Its tail was decidedly long, vertically flattened, and rayed like that of a lobe-finned fish. These features paint a picture of Acanthostega different from what had been envisioned originally. Rather than being primarily a land-dwelling creature, Acanthostega most likely was a water dweller with paddlelike legs. It spent much of its time lying in shallow waters, where it could view the outside world with its dor-sally positioned eyes and occasionally take a breath of air.
An understanding of the kinds of environments that once hosted Devonian tetrapods also has grown with the discovery of more specimens around the world. Key fossil localities are still somewhat scarce, but they are spread across the globe; they include Greenland, Russia, Quebec, Pennsylvania, Estonia, Latvia, New South Wales, and northwestern China. Tetrapods appear to have originated in environments that were not strictly marine but instead featured a combination of freshwater and saltwater. The climates associated with early tetrapods include extreme tropics subject to monsoonal winds and also more temperate subtropics.
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