The Evolution Of Terrestrial Vertebrates

Let's now turn our attention to the evolution of the first amphibians, the vertebrate group that first colonized the land, or partially did. The fossil record has given us a fair understanding of both the species involved in this transition and the time. A group of Devonian Period bony fish known as Rhipidistians appear to have been the ancestors of the first amphibians. These fish were dominant predators, and most or all appear to have been fresh water animals. This in itself is interesting and suggests that the bridge to land was first through freshwater. The same may have been true for the arthropods as well.

The Rhipidistians were seemingly preadapted to evolving limbs capable of providing locomotion on land by having fleshy lobes on their fins. The still-living coelacanth provides a glorious example of both a living fossil and a model for envisioning the kind of animal that did give rise to the amphibians. But another group of lobe-finned fish, the lungfish, also is useful in understanding the transition, not in terms of locomotion but in the all-important transition from gill to lung. The best limbs in the world were of no use if the amphibian-in-waiting could not breathe. There were thus two lineages of lobe-finned fishes, the crossopterygians (of which the coelacanth is a member) and the lungfish.

There is controversy about which of these groups was the real ancestor of the amphibians. Whichever it was, there is a record of the first "tetrapods," animals with four legs, in the latter part of the Devonian, meaning that the crucial transition from a fish with lobed fins and gills to an animal with four legs took place prior to that time. But when? And just how terrestrial were those first tetrapods? Could they walk on land? More importantly, could they breathe in air without the help of water-breathing gills as well? Both genetic information and the fossil record are of use here. But in some ways we are very hampered. Not until we somehow find the earliest tetrapods with fossil soft parts preserved will we be able to answer the respiration question.

Happily we have extant representatives of the crossopterygians and lung fish, and some relatively primitive amphibians. Geneticist Blair Hedges has compared their genetic codes in an effort to discover the time that fish and amphibians diverged. The "molecular clock" discov eries seem to roughly match the fossil record. The split of the amphibian stocks from their ray-finned ancestors (in this case the lobe fins) is dated at 450 million years ago, or at about the transition from the Or-dovician Period to the Silurian Period. But this may have simply been the evolution of the stock of fish from which the amphibians ultimately came, not the amphibians themselves.

Paleontologist Robert Carroll, whose specialty is the transition of fish to amphibians, considers that a fish genus known as Osteolepis is the best candidate for the last fish ancestor of the first amphibian, and this fish genus did not appear until the early to middle part of the Devonian, or, that is, this final fish ancestor did not appear before about 400 million years ago. However, the first land-dwelling amphibians may have evolved 10 million years before this time, based on tantalizing evidence from footprints recently found in Ireland. A set of footprints from Valentia, Ireland, has been interpreted as the oldest record of limbed animals leaving footprints. But were these footprint makers really on land—or were they water-breathing fish that had evolved four legs to gently pad across the muddy bottom of ponds, as suggested by amphibian expert Michel Laurin in a letter to me in 2006? There are no skeletons associated with this track way, which is composed of about 150 individual footprints of an animal walking across ancient mud dragging a thick tail. This find has set off intense debate, since it predates the first undoubted tetrapod bones by 32 million years! The footprints were found at a time interval when oxygen levels either approached or exceeded current levels, and it is at this same time that the fossil record of insects, recounted above, yielded the first specimens of terrestrial insects and arachnids. Thus, just as high oxygen aided the transition from water to land in insects, so too might it have allowed evolution of a first vertebrate land dweller.

The first tetrapod bone fossils are not known until their appearance in rocks of about 360 million years in age, so the transition from fish to amphibians was in this interval between 400 million and 360 million years ago. A rapid drop in oxygen characterizes this interval, and the first tetrapod fossils come from a time that shows oxygen minima on the Berner curve. It is likely, however, that the actual transition from fish to amphibians must have happened much earlier, nearer the time of the Devonian high-oxygen peak but still in a period of dropping oxygen. This scenario fits the proposal that the times of low, or lowering oxygen, stimulated the most consequential evolutionary changes—the formation of new body plans, which the first tetrapod most assuredly was.

Most of our understanding about the transition from fish to amphibians comes from only a few localities, with the outcrops in Greenland being the most prolific in tetrapod remains. Although the genus Ichythostega is given pride of place in most discussions of animal evolution as being first, actually a different genus, named Ventastega, was first, at about 363 million years ago, followed in several million years by a modest radiation that included Ichythostega, Acanthostega, and Hynerpeton. Are these forms legged fish or fishy amphibians? They are certainly transitional and difficult to categorize. Of these, Ichthyostega is the most renowned. Its bones were first recovered in the 1930s, but they were fragmentary, and it was not until the 1950s that detailed examination led to a reconstruction of the entire skeleton. The animal certainly had well-developed legs, but it also had a fish-like tail. Nevertheless, the legs led to its coronation as the first four-legged land animal. It was only later that further study showed that this inhabitant from so long ago was probably incapable of walking on land. Newer studies of its foot and ankle seemed to suggest that it could not have supported its body without the flotation aid of being immersed in water.

The strata enclosing Ichthyostega and the other primitive tetra-pods from Greenland came from a time interval soon after the devastating late Devonian mass extinction, whose cause was most certainly an atmospheric oxygen drop that created widespread anoxia in the seas. The appearance of Ichthyostega and its brethren may have been instigated by this extinction, since evolutionary novelty often follows mass extinction in response to filling empty ecological niches (the traditional view)—and since it was a time of lower oxygen (the view here). And, as postulated in this book, while periods of low oxygen seem to correlate well with times of low organism diversity, just the opposite seems true of the process bringing about radical breakthroughs in body plans: while times of low oxygen may have few spe cies, they seem to show high disparity—the number of different body plans. Such was the case during the Cambrian Explosion, a time of relatively low species-level diversity but of many kinds of body plans relative to the number of species. So too with the interval of time from 365 million to perhaps 360 million years ago, with many new evolutionary experiments being tried out. Ichthyostega was one of these, and, judging from its geological record, a not too successful one. The fossil record shows that soon after its first appearance, it and the other pioneering tetrapods disappeared.

But were Ichthyostega and the two or three allied forms found with it even land-dwelling organisms? The bones of this first amphibian have been reexamined in detail by Cambridge paleontologist Jenny Clack. What she and other anatomists discovered was unexpected. Taken together, the anatomy of Ichthyostega does not seem appropriate for life on land: Ichthyostega would have been very inefficient on land, if it could walk in air at all. This creature was pretty much a fish with legs, rather than an amphibian in the sense of how we know them today. And if it were the first amphibian, we would expect this great evolutionary breakthrough to be soon followed by an adaptive radiation, the rapid proliferation of new species using the breakthrough morphology. But this did not happen. There was a long gap before more amphibians appeared. This gap has perplexed generations of paleontologists and it came to be known as Romer's Gap, after the early twentieth-century paleontologist Alfred Romer, who first brought attention to it. The expected evolutionary radiation of amphibians did not take place until about 340 million to 330 million years ago, making Romer's Gap at least 20 million years in length. This radiation took place at a time when oxygen had again risen to, or above, present-day levels, and that did not happen until later than 355 million years ago. A 2004 summary by John Long and Malcolm Gordon similarly interpreted the tetrapods living 370 million to 355 million years ago, the time of a great oxygen drop, as entirely aquatic—essentially fish with legs—even though some of them had lost gills. Respiration took place by gulping air, in the manner of many current fish, and by oxygen absorption through the skin. None were amphibians as we know them today, species that can live their entire adult lives on land. And it ap pears that none of the Devonian tetrapods had any sort of tadpole stage; they went directly from egg to land dweller without a water-breathing larval stage.

PLUGGING ROMER'S GAP?

The long interval supposedly without amphibians was "plugged" in 2003 by Jenny Clack with great media fanfare. While looking through old museum collections she came upon a fossil long thought to have been a fish. But more detailed examination showed it to be a tetrapod and, more than that, it was an animal with five toes and the skeletal architecture that would have allowed land life. More importantly, it was within the mysterious Romer's Gap. The popular press reports of this finding, which was named Perdepes, would have us believe that Romer's Gap was filled. Hardly. Perdepes may indeed have been the first true amphibian, and it did come from an interval of time within the gap: the fossil comes from the time interval between 354 million and 344 million years ago. But here is where reality sometimes escapes the news. Dating sedimentary rocks is devilishly hard. And more so for non-marine deposits. Perdepes was not an amphibian living the 10 million years from 354 to 344 million years ago. Instead, it is an admittedly (by its discoverer) short-ranging genus living sometime in that interval. Perdepes does not plug the gap—it is a small boat sailing in a vast sea of time. It does tell us that somewhere in the middle of Romer's Gap a tetrapod did evolve the legs necessary for land life. But did it breathe air? Could it live entirely emerged from the water, even for a few minutes? That we do not know. So let's demystify the gap, as alluded to earlier in this chapter.

Alfred Romer thought that evolution of the first amphibians came about because of the effect of oxygen. But the pathway may not have been that supposed by Romer, who considered that lungfish or their Devonian equivalents were trapped in small pools that would seasonally dry up. In his view the lack of oxygen brought about by natural processes in these pools, and the drying, was the evolutionary impetus for the evolution of lungs. His idea was that seasonally drying swamps spurred the jump to land or smaller freshwater ponds or lakes. Accord ing to this idea, then, the amphibians-in-waiting were forced out of these pools and into air. Gradually, those animals that could survive the times of emersion from water had an advantage. These fish still had gills, but the gills themselves allowed some adsorption of oxygen. The problem was that the gills quickly dried out. By evolving ever-tighter and water-resistant pockets around the gills, the transition from gill to lung was under way. But a gill is still an evagination, even if in a pocket. There had to be a complete inversion of this system, for a lung is a series of sacs, whereas a gill is a series of protuberances. It may be that the transitional forms had both gills and primitive lungs.

The transition from aquatic tetrapods such as Ichthyostega or, more probably, Perdepes, involved changes in the wrists, ankles, backbone, and other portions of the axial skeleton that facilitate breathing and locomotion. Rib cages are important to house lungs, while the demands of supporting a heavy body in air, as compared to the near flotation of the same body in water, required extensive changes to the shoulder girdle, pelvic region, and the soft tissues that integrated them. The first forms that had made all of these changes can be thought of as the first terrestrial amphibians and the Perdepes, found in rocks younger than 355 million years in age, may indeed have been the first of all, according to Long and Gordon. But there may be a continuation of the gap after Perdepes. The great radiation of new amphibian species did not occur until 340 million to 330 million years ago. But when it finally took off, it did so in spectacular fashion, and by the end of the Mississippian Period (some 318 million years ago) there were numerous amphibians from localities all over the world.

Let's reexamine the radiation of amphibian species in the context of atmospheric oxygen levels. A gill is very inefficient when it must act as a lung, and a primitive lung must evolve through many steps before the complex and high surface area surface of internal pockets, all vas-cularized, with concomitant changes to the circulatory system, is effected. While all these respiratory and circulatory changes are happening, the respiratory system in the stock leading to amphibians would have been less efficient at delivering oxygen than either a gill in water or the complex lung in air that would later be evolved. The high-oxygen peak in the early Devonian would have provided the extra oxy-

Reconstructions of the earliest known tetrapods, Tiktaalik (left) and Acanthostega (right), shows how the transition from fish to amphibians took place. In spite of their limbs, both of these were probably fully aquatic and unable to climb onto land because of inadequate (for land life) respiratory and locomotory systems.

gen necessary to make this system work, as would have the high oxygen of the latter parts of the Mississippian and Pennsylvanian of 330 million to 300 million years ago.

The Berner curve starting this chapter suggests that there was a great drop in oxygen near the end of the Devonian and coincident with this is the Devonian mass extinction, one of the five most severe mass extinctions in Earth's history. While investigators have been searching for clues to this extinction for decades and have invoked causes ranging from an asteroid impact to climate change, it is not known for sure what the causes of the Devonian mass extinction were. Ammonite workers have long known that at this time the oceans showed a marked change to low-oxygen conditions. The extinction took place over about 2 million years, from 370 to 368 million years ago, at a time when the Berner curve shows a very low level of atmospheric oxygen of about 12 to 14 percent.

Here is where the new terrestrial arthropod data from Conrad Labandeira and the new land vertebrate range data from Michel Laurin can help solve the mystery of "Romer's Gap," and support the hypothesis presented below that the animal conquest of land happened in two initial phases separated by a time of low oxygen. The figure from our paper is shown here:

Land

Colonizations:

First O Second

Silürian

Devonian

Mississippian

Pennsylv.

Permian

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Responses

  • Kaari
    How did bony fish evolve cloac in transition to amphibians?
    3 years ago
  • Lotho
    When did vertebrates evolve in the ordovician period?
    2 months ago

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