Prologue - Fish Out of Water
Fossils of Carboniferous amphibians were relatively well known by the turn of the century, as were their probable piscine (fish-like) ancestors, the Devonian lobe-finned fishes. But there was a sizable anatomical gap between the two, and the transition between fish and tetrapod was frustratingly obscure.
The Drying Pond
If transitional forms were absent from the fossil record, then perhaps evidence could be collected elsewhere. One conspicuous feature of Late Devonian geology was the prevalence of red sediments in Europe and North America (the "Old Red Sandstone Continent"). In 1916, Joseph Barrell argued that these oxidized sediments were evidence of a harsh landscape subject to severe droughts. He also argued that this severe climate was a major driving force in the evolution of air-breathing vertebrates, including tetrapods.
Elaborations on the idea that Devonian droughts were the driving force for the evolution of tetrapods culminated in 1950s with a "Drying Pond" scenario proposed by Alfred Sherwood Romer. In this scenario, tetrapods evolved from lobe-finned fishes driven onto the land by drought. As one pool or stream dried out, the fishes ventured onto the parched earth in search of other bodies of water. Over time, natural selection would favor those fishes with more efficient terrestrial locomotion (i.e., with more limb-like fins). In other words, tetrapods evolved from fish out of water.
Preliminary reports on the anatomy of Ichthyostega during the 1950s reinforced Romer's scenario. This Late Devonian tetrapod apparently had well-developed limbs similar to those of some Carboniferous amphibians, but it also had a fish-like tail. A picture emerged of Ichthyostega as the evolutionary consequence of Romer's drought refugees. It was seen as an essentially terrestrial tetrapod, albiet one with a fishy tail and almost certainly dependent on water for its aquatic young.
An Alternative, and Frustration
Romer's Drying Pond was embraced by the public at large and by many in the scientific community. However, it was challenged by a number of his peers. The geologic underpinning of Romer's scenario, that red sediments were evidence of persistently severe drought, was effectively refuted by Paul Dimitri Krynine as early as 1949. It was also challenged by contemporary evolutionary biologists and paleontologists, including Gordon Gunter, Robert Inger, and Raymond B. Cowles, who preferred scenarios in which marginal aquatic environments (e.g., lagoons, lake margins or wetlands) provided the crucial evolutionary setting.
Scientists investigating the vertebrate transistion from aquatic to terrestrial during the 1950s through to the 1980s were frustrated by a paucity of materials. The fossils of Middle Devonian lobe-fin fishes on one end of the transition and Carboniferous tetrapods on the other end were relatively common, but aside from Ichthyostega and a couple of isolated fragments, there was precious little to study despite intensive efforts to discover new material. Specimens of the best known Devonian tetrapod, Ichthyostega, were monopolized by Erik Jarvik.
Given these obstacles, some of them focused attention on the tetrapods' lobe-fin predecessors. Discoveries of new and better lobe-fin fossils greatly enhanced our knowledge of these fishes and helped to resolve their relationships both among themselves and with the tetrapods. These relationships were the subject of protracted debate, but by the 1980s most authorities settled on the idea that tetrapods evolved from osteolepiform ancestors sometime during the Late Devonian.
Many of these same scientists also explored ecological, anatomical, and physical adaptations (and pre-adaptations) that would have facilitated the transition to land. For example, possible ecological "incentives" were escaping large aquatic predators, exploiting terrestrial food resources, and finding safe places to breed. The ability to breath air would come in useful when shallow waters became stagnant, while changes to their muscular lobe-fins might prove useful in navigating shallow or marshy habitats. One particularly interesting speculation was proposed by both Jerome Rackoff and Keith Thomson. They argued that the forward bending of our forelimbs at the elbow and the backward bending the hindlimbs at the knee is an evolutionary consequence of lobe-fin locomotion in shallow water. The forelimbs would prop up the front of the body (and prevent crushing the lungs), while the hindlimbs would propel the animal forward by pushing backward.
It was not until the late 1980s and the 1990s that a series of recent findings upset the old order.
- Fenton, C.L. and M.A. Fenton. 1958. The Fossil Book: A Record of Prehistoric Life. Doubleday & Company, Inc.: Garden City, New York.
- Scientific Papers
- Barrell, J. 1916. "Influence of Silurian-Devonian climates on the rise of air-breathing vertebrates." Geological Society of America. Bulletin. v. 27: 371-379.
- Romer, A.S. 1958. "Tetrapod limbs and early tetrapod life." Evolution 12: 361-369.
- Rackoff, J.S. 1980. "The origin of the tetrapod limb and the ancestry of the vertebrates." pp. 255-292. In: A.L. Panchen (ed.). The Terrestrial Environment and the Origin of Land Vertebrates. Academic Press. London.
- Thomson, K.S. 1969. "The biology of lobe-finned fishes." Biological Reviews 44: 91-154.
- Thomson, K.S. 1980. "The ecology of Devonian lobe-finned fishes." pp. 187-222. In: A.L. Panchen (ed.). The Terrestrial Environment and the Origin of Land Vertebrates. Academic Press. London.
- Thomson, K.S. 1993. "The origin of the tetrapods." American Journal of Science 293-A 33-62.