Fish-amphibian transition

During the Devonian period, the crossopterygian fishes became the forerunners of the amphibian species and, hence, of all terrestrial vertebrates. Adapting from an aquatic to a partially terrestrial existence required the development of physical and skeletal structures that could bear the weight of creatures without the buoying effect of water. Creatures living out of water had to develop the ability to breathe dry air directly. Despite these developments, amphibians remained tied to the aquatic environment to reproduce. Only when the self-contained amniote egg developed in the transition from amphibians to reptiles were vertebrate species able to exist fully outside an aquatic environment.

The Devonian Period

During the Devonian period (415 to 355 million years ago), the Earth's surface is believed to have consisted of just the single supercontinent, Pangaea, and the single world-spanning ocean, Panthalassa. As the supercontinent slowly broke apart under the influence of magmatic convection in the mantle layer of the planet, shallow freshwater and saltwater seas formed in which fish and other aquatic organisms proliferated.

The Devonian period is believed to have been a warm period in Earth's history. Reconstruction of sea surface temperatures from geological sources dated to that time indicate that the planet experienced an average temperature of 30 to 33 degrees C (86 degrees F). Water levels were correspondingly higher than during colder periods, encouraging the development of fish species in rich shallow waters. The Devonian period is accordingly known as the age of fishes and is marked by the dominance of the placoderms, the first appearances of ray-finned and lobe-finned fishes, and the proliferation of primitive shark species. Several important developments in the evolutionary history of fish took place during the Devonian period, the most important being the transition from fish to amphibian. In this evolutionary process, fish became the common ancestral line of all terrestrial vertebrates.

Transitions and TIKTAALIK ROSEAE

The direct ancestors of the amphibians, according to fossil records, were the crossopterygian lobe-finned fishes, of which Coelacanthus is a representative member. The dental and cranial structures of the Crossopterygii are similar to those of the earliest known amphibians. These structures are characterized by teeth with a highly folded structure and by a cranium covered by bony plates with two nasal openings, two eye sockets, and one pineal orifice. The first amphibian species appear in the fossil record of the Devonian period. Transitional fossils for these creatures have been notably absent, however, and while species such as Coelacanthus traditionally have been regarded as likely candidates for this role, the revelations provided by the study of the present-day coelacanth have made this much less likely.

The modern coelacanth possesses many of the features that would characterize amphibian physiology, such as thickened forefins adapted to moving the creature's body along a solid surface and lungs that would enable the creature to breathe air directly. Research has revealed, though, that the modern-day coelacanth dwells at a depth of about 300 meters (984 feet) and does not attend the surface. Its lung structure has long since lost the ability to utilize air and functions exclusively as a swim bladder to control the buoyancy of the creature at depth, and its forefins serve to guide movement in the water and not at the surface.

Careful research and theorization of the time period in which a true transitional creature should have arisen led paleontologists to the Canadian Arctic in 2004, where they discovered the 375 million-year-old remains of Tiktaalik roseae, a creature colloquially referred to as the fishapod. The creature was a fish, but it also had unusual physical characteristics, such as robust pectoral fins designed to raise its body in the shallows at water's edge but not sufficiently articulated to promote a walking movement. Tiktaalik exhibited many of the characteristics of fish (fins, scales, gills) and several characteristics normally associated with tetrapods (flattened head with top-mounted eyes, functional neck, and supportive ribs). In the absence of evidence to the contrary, it appears that T. roseae possessed the requisite characteristics to represent an intermediary phase of evolution from fish to tetrapod amphibian. These tetrapod-like features appear in Tiktaalik some 12 million years before the appearance of the first known tetrapods in the fossil record.

Life and the Ecological Niche

An ecological niche is a specific environmental situation in which a species can obtain food and other survival needs by adapting its behavior and physiology to suit. In the Devonian period, the plant and invertebrate life that had colonized the dryland eolian environment represented the food supply, while the land and air constituted the environments in which aquatic species could adapt.

Separated by this distance in time, one can only speculate about the absolute reasons for water-dwelling creatures to have evolved from fish to amphibians. The fossil record and geological evidence indicate that the transformation took place primarily in freshwater habitats, which in the Devonian period would have been warm and presumably rich feeding grounds for both predators and prey. At the same time, the vast areas of dry land had been colonized by plants and invertebrate species; however, no other advanced creatures were in existence to exploit that territory. Vertebrates as such existed only in the seas. These two factors provide perhaps the best depiction of the evolutionary forces working toward the development of amphibians.

It is understandable that the pressure to survive in shallow waters in the face of numerous predators could give an advantage to creatures that could purposely retreat to the shallowest margins of a body of water where predators were less able to pursue. Additionally, of those creatures that could retreat to the shallows, the ones that would be able to survive would be those that could capitalize on the availability of insect and arthropod life in that area. Thus, creatures like T. roseae would naturally develop physiological features that would favor survival. In Tiktaalik's case, this meant the development of more robust pectoral fins that would enable it to raise its forebody somewhat out of the water, a skeletal structure that was stronger in the vertical plane to better support its weight in the absence of water buoyancy, and eye placement that enabled it to see upward and straight ahead.

The survival advantages of these adaptations are easy to realize when considered in the context of the relative locations of prey creatures and the essential differences of the environments in and out of the water. The air-based environment that existed above the waterline would require some special adaptations for creatures that had to that point existed only in the water-filled environment. Eyes that had developed in the constant wetness of an aqueous environment would be subject to damage by the dryness of air. Similarly, gills developed for extracting dissolved oxygen from liquid water would not function in air. It is readily imagined that creatures with fleshy tissue structures around their eyes would perhaps fare better in air than those that did not have such features and that in time this would develop into protective membranes and actual eyelids that could be purposefully closed to replenish the moisture of the eyes.

Adapting to Air

Breathing the air presents a problem of an entirely different nature in regard to adaptation; because gills are unable to extract oxygen from air, a new organ was required. This organ is the internal lung, which functions to enable the exchange of oxygen from moisture-laden air to the blood. The moisturizing of air as it is breathed in ensures a constant moisture level and effectively prevents the lung tissue from becoming dried out by the air that it breathes. In some cases, creatures developed adapted gills that could function, to an extent, in air.

The prime example of this adaptation is a creature known as Gerrothorax, which had an elongated and flattened body, hind legs extending directly backward, short pectoral “arms” rather than fins, a wide mouth, upward-peering eyes, and feathery external gills on both sides at a point that would correspond to its neck. Gerrothorax was apparently designed to be an ambush hunter, to lie in wait in shallow water for prey to approach. A similar structure survives in the present day in the axolotl and the mudpuppy. However, the external gill structure is exceedingly vulnerable, and it did not become the air-breathing replacement that was required for adaptation to an air-based environment. The modern-day coelacanth and a great many other fishes possess an internal organ that is undoubtedly more suitable for this adaptation. This organ is the swim bladder, which can be inflated or deflated as needed to adjust the mass of the fish to neutral buoyancy, a mechanism that greatly reduces the amount of energy and, hence, food required for the fish to maintain a certain depth.

In the transitional creatures between fish and amphibian, such as Tiktaalik and Ichthyostega, fossil evidence indicates the presence of air-breathing lungs in the same anatomical location as would have been occupied by the swim bladder of crossopterygian lobe-finned fishes, such as the ancient Coelacanthus. It is therefore most probable that, like present-day lungfish, the swim bladder became the means by which fish and fish-like creatures could leave the water for limited periods of time and survive by breathing air directly.

Amphibians Still Tied to the Water

The ability to breathe air allows amphibians to occupy dry land, but it does not remove their complete dependence upon the water for survival. The reason for this is the reproductive process. Ancient fishes are known to have been both oviparous and viviparous. It is more accurate to refer to the species that give live birth to their young as ovoviviparous rather than strictly viviparous.

In these species, the fertilized eggs are incubated within the female's body until they hatch. Upon hatching, there may be some further incubation in which the dominant offspring survive, as in some modern-day sharks, and the surviving young are then expelled. Other fish are known to have deposited roe for external fertilization. In both cases, the entire process occurs within a water environment, not in an air environment. The ova, in all known cases, are in many ways like an overly large animalian cell in that they are enclosed in a soft oxygen-permeable membrane rather than in a hard shell that contains the amniote in a discrete isolated package. This development characterizes the transition from amphibians to reptiles. Fish and amphibians, however, require the surrounding environment of water for the survival of their eggs and subsequent offspring.

Accordingly, the life cycle of amphibians includes both a wholly aquatic period and a period in which the creatures live primarily on land. Present-day toads are the classic example of this existence. Among early amphibians, Protobatrachus appears to closely resemble toads and frogs as an early predecessor with its squat, shortened body and its frog-like head. Unlike its more recent descendants, though, Protobatrachus's hind legs were not adapted to the motion of jumping and it retained a rudimentary tail. Typically, amphibian eggs are deposited in quantity in some part of an aquatic environment, where they will be protected by camouflage while the embryos inside have developed sufficiently for hatching. The young emerge with an attached yolk sac that sustains their nutritional needs as they grow to a size at which they are capable of securing their own nutrition and develop into their adult or terrestrial forms. By comparison, these stages proceed within the confines of the hard-shelled amniote eggs of reptiles, not requiring the presence of an aquatic environment. Upon attaining their terrestrial form, the amphibian is free to leave the water and occupy its corresponding ecological niche on land.

It is a crucial piece of fossil evidence that markings characteristic of the attachment of a yolk sac have been identified in some recovered fossil remains, indicating that the amphibian life cycle operated in the past in much the same way that it does in the present. Fossilized remains of egg masses that can be attributed to the amphibian life cycle have also been identified. The source material is presumed to have become entrapped by fine sediment in the sudden collapse of a silt bed, which would have buried the egg mass almost instantly.

Continuation of the Amphibian Transition

Given that evolution is a slow, continuous process of living systems adapting to environmental conditions, it should be asked whether the process of transitional evolution functions in the present as it did in the past. Adaptation is a dynamic process that requires long periods of time for a discernible change in the overall characteristics of a species to appear. This makes it unlikely to be observed in any normal human lifespan. Also, the fossil record cannot be considered a complete record of life on Earth despite the millions of fossil remains that have been recovered. Billions more have gone unrecorded, and the likelihood that any intermediate forms would be discovered is exceedingly small. This makes the intentional search for and discovery of T. roseae all the more important.

Biologists observe creatures, such as lungfish, that are capable of leaving the water and moving across dry land for a distance. These scientists are looking for physical and physiological changes through time that would indicate evolutionary progression from fish to amphibian. Results of such observation are inconclusive at best. It is as likely that some fish species are adapting to environmental conditions that favor their survival in a manner that steers them to someday becoming amphibians rather than fish, as it was in the past. The presence of terrestrial organisms renders that ecological niche much less available, though, and would act against such an evolutionary development.

That the process continues through time is a notion supported by the fossil record itself, as new transitional species appear at different times in the past. Ichthyostega and Tiktaalik, both considered transitional species between fish and amphibians, were separated by several million years. Further paleontological research is likely to turn up new species that also will be identified as transitional species. Still, it is valuable to recall that evolution also works in reverse, as creatures that had adapted from one environment to another may subsequently return to that original environment. The fossil record contains many known examples of such reversion, and scientists now believe that the lung developed from the swim bladder for breathing dry air by ancient fish, such as Coelacanthus, has since lost that ability in the coelacanth and functions once again only as a swim bladder.

Principal Terms

amnion: the sac-like structure that lines the inside of a shelled egg and contains the embryo, the yolk sac, and the amniotic fluid medium

amniote: descriptive of eggs consisting of a tough, mineralized, air-permeable, outer shell enclosing a fluid-filled sac called the amnion

amphibian: meaning “of double lives” and referring to creatures whose life cycle includes both aquatic and terrestrial existences

lacustrine: descriptive of sedimentary deposits formed from lake-bottom residues

nictitation: the movement, such as blinking, of a specialized protective membrane in creatures without eyelids to cover and uncover the surface of the eye; the movement of a nictitating membrane

oviparous: of creatures that reproduce by fertilized eggs that incubate and hatch externally; typical of aquatic species, amphibians, reptiles, and birds

ovoviviparous: of creatures whose fertilized eggs are incubated and hatched internally before the expulsion of live offspring

transitional: having the characteristics of both preceding and successive classes of organism without clearly belonging to either

vertebrate: possessing a bony, internal-skeletal structure based on a central vertebral column

viviparous: of creatures who reproduce by giving birth to live offspring without the intermediate, defined structure of an egg

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