A New History of Life (28 page)

Arthropods are not the only colonists making a new life on land of course. Gastropod mollusks also made the evolutionary leap onto land, but did not make this transition until the Pennsylvanian (thus they were part of the second wave), when oxygen levels were even higher than at any time during the first wave. Another group that made shore were horseshoe crabs, at about the same time that the mollusks landed. But these are minor colonists compared to the group that most concerns this history of life—our group, the vertebrates. But amphibians did not just burst out of the sea. They were the culmination of a long evolutionary history, and before they emerge onto land in our narrative, let us look at the Devonian period, a time long called the Age of Fish. To do this we want to feature one of our favorite field areas, the Devonian-aged Canning Basin of Western Australia, where we two coauthors spent multiple field seasons in one of the most extraordinarily beautiful (if hot!) places on the planet. The Canning Basin preserves the world’s best fossilized barrier reef system. It is as if the Great Barrier Reef were suddenly turned to stone and the water removed. While much of the work to date has been in studying that giant Devonian reef, in fact the rocks deposited in deeper water nearby during the Devonian Period have yielded some of the most
extraordinary of all fossil studies that certainly need to be featured in any self-styled “new” history of life.

While common in salt water to freshwater and all salinities in between, in fact fish fossilize all too rarely. It usually takes a low oxygen sea bottom where a dead fish is rapidly buried for an entire fish to be preserved. Scavengers are all too efficient at tearing fish corpses apart. But here and there beautiful fish fossils can be preserved. Sometimes they appear in two-dimensional form, as from the Eocene-aged Green River shale of Colorado, perhaps the place where more fish fossils have been found than in any other locality. But other fish parts, especially big fish skulls, are sometimes preserved in large round balls of rock called concretions. These cannonball-like objects are often found in sedimentary rocks, and they can contain the most beautifully preserved of fossils. Such preservation is found in strata from northern Ohio of Devonian age, where gigantic fish skulls have been found for a century, including the skull of one of the iconic monsters, an ancient fish called Dunkleosteas, lately featured in the usually cheesy Discovery Channel programs about ancient predators. But such preservation is also found in a curiously named rock formation called the Gogo Formation, the same age rocks (but deeper water equivalents) of our own Devonian Age research. Among these cannonball concretions are some of the most important fossils ever found. They give us a window of the platform from which our amphibian ancestors ultimately emerged. To understand the conquest of land we first have to know the Devonian world of fish in all its diversity and complexity. In recent years Australian paleontologist John Long, a professor at Flinders University in Adelaide, Australia (but also with a long professional stint at the Los Angeles County Museum of Natural History), has taken new high-resolution-scanning technology to make breakthrough discoveries about the ancestry of all modern fish, as well as the lineages in deep time that are in our own DNA.

Long is a rarity in Australian academics in having a successful and thriving career in science outreach, and is the author of numerous
books. But Long’s “day job” has shown us that the evolution, morphology, diversity, and ecology of Devonian age fish was far more complex than is now portrayed in textbooks. By pioneering the use of imaging technology such as CT scanning, which bombards fossils with energy sufficient to produce 3-D slices of the fossils, Long has literally looked into the heads of the various fish groups.

The four “traditional” fish groups—today represented by lampreys and hagfish; sharks; the most diverse, the “bony” fish; and an entirely extinct group, the placoderms (the first jawed fish)—are far more complicated in all aspects than they have been long portrayed. Long’s major discoveries from his field expeditions to the Gogo fossil sites included the first complete skull of one of the first bony fish, named
Gogonasus
, which showed that this species had large spiracles, or holes, previously unknown in fish on top of its head. But the most surprising discovery—beyond demonstrating a hitherto unknown diversity of other kinds of early fish, including new types of lungfish (closely related to the fish that ultimately crawled onto land) as well as strange fish called arthrodires—was the discovery of the first Devonian fishes showing embryos inside them. This latter discovery was the first time that reproduction by internal fertilization was demonstrated, as well as the oldest evidence for vertebrate viviparity yet discovered. One of his specimens was the only known fossil to show a mineralized umbilical structure linked to the unborn embryo. Long used his new high-tech methods to remarkably preserve 3-D muscle tissues, nerve cells, and microcapillaries, all new kinds of detail from fossil fish. But most important for understanding the move onto land, his soft tissue discoveries gave entirely new insight into how a fish could evolve ancestors that could walk—even upright on two legs.

The transition of our own group from purely aquatic organisms to true terrestrial inhabitants began with the evolution of the first amphibians. The fossil record has given us a fair understanding of both the species
involved in this transition and the time it happened. A group of Devonian period bony fish known as rhipidistians appears to have been the ancestors of the first amphibians. These fish were dominant predators, and most or all appear to have been freshwater 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 are 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 are 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.

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 Ordovician 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 in this transition, considers a fish genus known as
Osteolepis
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 before about 400 million years ago.

The first land-dwelling amphibians may have evolved at this time, based on tantalizing evidence from footprints found in Ireland. A set of footprints from Valentia has been interpreted as being the oldest record of limbed animals leaving footprints, dated at about 400 million years in age. But there are no skeletons associated with this trackway, which is composed of about 150 individual footprints of an animal walking across ancient mud dragging a thick tail. This find has set off
debate, since it predates the first undoubted tetrapod bones by 32 million years. Interestingly, however, the trackway dates to 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 the 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 uncertainty about the age of the first vertebrate footprints on land was slightly alleviated by a discovery made in 2010, of a second set of tracks that was discovered to be 395 million years in age. They were preserved in marine sediments of the southern coast of (now) Poland. They were made during the Middle Devonian period. The tracks, some of which show digits, are thus 18 million years older than the oldest-known tetrapod body fossils. Additionally, the tracks show that the animal was capable of a type of arm and leg motion that would have been impossible in the more fish-like tetrapods and near tetrapods, such as the aforementioned
Tiktaalik
and its probable descendant,
Acanthostega
.

The animal that produced the tracks was large for the time: some estimates peg it at more than eight feet long. Perhaps this creature and its ilk were scavengers on the tidal flats, feeding on washed-up marine animals stranded by the tide, or the numerous land arthropods, including scorpions and spiders.

The first tetrapod bone fossils are not known until rocks about 360 million years in age, so the transition was in this interval between 400 and 360 MA. A rapid drop in oxygen characterizes this interval, and the first tetrapod fossils come from a time that shows minimal oxygen on the Berner curve. It is likely, however, that the actual transition from fish to amphibian must have happened much earlier, nearer the time of the Devonian high-oxygen peak but still in a period of dropping oxygen.

Most of our understanding about these crucial events comes from only a few localities, with the outcrops in Greenland being the most prolific in tetrapod remains. Although the genus
Ichthyostega
is given
pride of place in most texts 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
Ichthyostega, Acanthostega
, and
Hynerpeton
.

Of these,
Ichthyostega
was the most renowned—until
Tiktaalik
, that is. Yet the new notoriety of
Tiktaalik
is a bit misplaced. It was a fish.
Ichthyostega
was something else. An amphibian, 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 had well-developed legs, but it also had a fish-like tail. Later that further study showed that this inhabitant from 363 million years 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 tetrapods from Greenland came from a time interval that was soon after the devastating Late Devonian mass extinction, whose cause was most certainly a drop in atmospheric oxygen 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. But the success of
Ichthyostega
and its brethren was short-lived: the fossil record shows that within a few million years after its first appearance, it and the other pioneering tetrapods disappeared.

The appearance of
Ichthyostega
and its late Devonian brethren poses crucial questions. If these were indeed the first terrestrial vertebrates, why wasn’t there a succeeding “adaptive radiation” of their descendants? But this did not happen. Instead there is a long gap before more amphibians appear. This gap has perplexed generations of paleontologists. In fact it came to be known as Romer’s gap, after the early twentieth-century paleontologist Alfred Romer, who first brought attention to the mysterious gap between the first wave of vertebrates invading the land and the second. In fact, the expected evolutionary radiation of amphibians did not take place until about 340
to 330 million years ago, making Romer’s gap at least 30 million years in length.

A 2004 summary by John Long and Malcolm Gordon similarly interpreted the tetrapods living in the 370- to 355-million-year-old interval, the time of a great oxygen drop, as entirely aquatic, essentially fish with legs, even though some of them had lost gills. Respiration was by gulping air in the manner of many current fish, and oxygen absorption through the skin. They were not amphibians as we know them today—species that can live for their entire adult lives on land. And it appears that none of the Devonian tetrapods had any sort of tadpole stage.

The long interval supposedly without amphibians was “plugged” in 2003 by Jenny Clack. While looking through old museum collections she came upon a fossil misinterpreted as a fully aquatic fish, but which she showed to be a tetrapod with five toes and the skeletal architecture that would have allowed land life. This fossil was given a new name,
Pederpes
, and it lived long after
Tiktaalik
. It indeed may have been the first true amphibian, and it did come from the time interval between 354 and 344 million years ago known as Romer’s gap. But like so much about the past, sometimes fossils raise more questions than answers. It does tell us that somewhere in the middle of Romer’s gap, a tetrapod did evolve the legs necessary for land life. However, it is still not known if it could breathe air or whether it could even emerge from the water for even for a few minutes.

Alfred Romer thought that the evolution of the first amphibians came about because of the effect of oxygen. Romer considered that lungfish or their Devonian equivalents were trapped in small pools that would seasonally desiccate. He thought that the lack of oxygen brought about by natural processes in these pools, as well as the drying, was the evolutionary impetus for the evolution of lungs—the amphibians-in-waiting were forced out of the pools and into the 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. It may be that the transitional forms had both gills and primitive lungs.