by Peter Ward
PALEOECOLOGY OF THE LARGER EDIACARANS
Generally, science resolves interesting problems easily. But the nature of the Ediacarans seems to have resisted a great deal of vigorous effort. They remain mysterious. However, new work in the past few years has begun to chip away at the biggest mysteries, and some of the most important have utilized a field of paleontology that has fallen into a bit of disrespect over the past few decades—a field known as paleoecology. While a brilliant driver of paleontological research from the 1960s on, it failed to yield major new generalizations and was cogently dismissed by Stephen Jay Gould in one of his State of Paleontology addresses published in the last (twentieth) century. But this old-fashioned kind of sleuthing was used in this century by Mary Droser of the University of California, Riverside, and Jim Gehling of the South Australian Museum to arrive at perhaps the best understanding yet about the larger Ediacarans and their world.
The crux of the Gehling and Droser’s (and vice versa) work is that we need to look at the Ediacarans in the context of their association with what must have been pervasive microbial mats lining the sea bottom. The profusion of microbial mats would have been the dominating control on the ecology and especially the sedimentology of these communities. Because there were few or no burrowing organisms compared to the sea bottoms of today where burrowing is pervasive, the ecology of these communities would have been nothing like we know.
Four kinds of animal lifestyles that associate with the microbial mats could have been present: mat encrusters, forms that sat upon the mats and perhaps secreted digestive enzymes sufficient to dissolve the mats upon which they rested for food; mat scratchers, forms actually and actively grazing on the mats; mat stickers, partly emerged in the mats and growing upward as the mat level changed (because the mats would have grown upward toward the sun, as stromatolites do); and undermat miners, tunneling beneath the mat. Several of these strategies also seem to have persisted into the earliest Cambrian, but by then the world was rapidly changing because of the profusion of burrowing and larger organisms as well as active carnivores and herbivores with skeletal or hard jaw apparatuses.
This very strange and weird world of organisms can also be understood only in the context of how they were preserved. One of the interesting generalizations made by specialists who study Ediacaran organisms is that the fossils are analogous to the plaster “death masks” of previous centuries, used by dead and dying royalty and nobility of European and other civilizations. Soon after death, the face of some (then) famous person would have an impression made. The fossils we see of Ediacarans may be the same thing. Not an actual fossil from the animal, but a reproduction of the top and bottom surfaces of the creature. Making a death mask required rapid hardening of whatever material the mask was being made of, and so it is thought that the Ediacaran fossils were made of materials that hardened quickly on top of their dead bodies.
THE SPINY MICROFOSSILS OF THE EDIACARAN WORLD
In the last chapter we mentioned the work of Andy Knoll and his group at Harvard, concerning their study not of the bigger fossils of Ediacaran age, but of the microfossils. For a billion years, single-celled life dominated the world, and what fossils they left were mainly tiny, smooth-walled globes. But as the world came out of the last of the Neoproterozoic snowball Earth episodes, the fossil record becomes filled with spiny, ornamented microfossils. This somewhat short-lived episode in the fossil record may tell us important things about the nature of the overall rise of animals to complexity (these microfossils appear no more than 600 million years ago, and then are gone about 560 million years ago, and were thus survived by another twenty million years by the larger Ediacaran macrofossils). Prior to this point microfossils came exclusively from single-celled organisms, but these “spiny” microfossils may in fact be from multicellular animals. In these cases, we are seeing tiny resting stages, like cysts.
There have been several important studies of these tiny fossils, including by paleontologists/developmental biologists Nick Butterfield and Kevin Peterson,14 who suggested that the appearance of the heavily ornamented microfossils early in the Ediacaran period was in response to small early animal carnivores, such as the earliest tiny nematodes and roundworms. The spines of the microfossils were thus defensive adaptations, serving to buttress the skeletons of these fossils, interpreted to have been single celled. But the Knoll group suggests that the complexly ornamented microfossils are resting stages of early animals themselves. This suggests both a complex and early evolution for animals well before the larger Ediacaran fossils appeared at all. It also suggests that the early environments of animals were anything but like the Eden-like garden of Ediacara supposed by paleontologists in the late 1900s. Needing a resting cyst suggests a challenging environment of varying oxygen, including times when the water column had no oxygen at all, and possibly occasional doses of hydrogen sulfide. This view of life poses a world faced by early animal evolution that was challenging, extreme, and often poisonous.
The spiny microfossils disappear around 560 million years ago, and are then replaced by what was the flowering of the large, classical Ediacaran fossils, which themselves lived as the largest creatures on Earth until overthrown by a different set of animals at the base of the Cambrian period, slightly more than 540 million years ago.
THE SEARCH FOR “BILATERIANS”
If the spiny microfossils are small animal resting stages, rather than large protists (single-celled organisms) of some kind, what kind of animals were they? About the same time that the ornamented microfossils appear in the geological record, it is supposed that another great evolutionary event took place: the first animals with bilateral symmetry, something that greatly improved locomotion. The advent of a bilaterally symmetrical body plan was another great evolutionary milestone. A bilaterally symmetrical animal has a distinct “front” and “back,” with internal organs roughly symmetrical on either side of this front to back, tubelike body. It was the kind of ancestor we would expect the diversifying animal phyla to have sprung from. But the age of these enigmatic fossils was long debated.
Genetic work suggests that this ancestor should have been alive well between ~570 million and ~660 million years ago.15 But the fossil record has been opaque to what must surely have been a tiny (perhaps a millimeter in length), wormlike creature without a skeleton. While this is a case deserving not a little of the scorn heaped on it ever since Darwin, the fossil record should be cut some slack: the chance of a tiny, soft, wormlike creature without hard parts leaving any fossil record of itself is low indeed.
Fossils from China came to the rescue.16 Rocks of an age considered to be the best guess of when the first bilaterian may have lived were found in China early in the twenty-first century. These rocks were then slowly and laboriously dated with higher precision, so that a very specific time interval, when it is thought that bilaterians must have first appeared, was identified. When this was completed, the search for the theorized fossils began. None of it was easy.
It took three years and the completion of more than ten thousand individual “thin sections” (in which a hunk of rock is sliced to a very narrow width and then polished, so that light can be transmitted through it while on a microscope stage), and just such an animal was found. And it was much smaller than an eighth of an inch: tiny fossils that were as long as a human hair is wide were found, examined, and studied. The age of these tiny wonders, named Vernanimalcula, was nearly 600 million years old.
Here again is a missing link no longer missing. Small, unassuming, and true revolutionaries, these early bilaterians paved the way for what was to come. And there was more from these strata. In addition to the bilaterian fossils, the Doushantuo Formation of southwest China yielded both eggs and embryos of earliest animals. It has also given us a new window into the world of 600 million years ago, and how animals changed the very nature of the sedimentological record.
Prior to animals, there was no “bioturbation,” the disruption of newly accumulated sediment
ary layers by the action of organisms. This is so pervasive today that it is hard to envision a time before it was the rule rather than the exception. Only strange environments today have this preanimal mode of sedimentological preservation, such as the bottom of the Black Sea. There the bottom is firm, and the sediments for the first meter below this surface show both lamination (layering) and a very low water content. Contrast this with any modern oxygenated sea bottom: the few centimeters above the bottom’s substrate is filled with organic goo—mucus, feces, pseudofeces, dissolved organic material, etc. Going deeper you find a lack of lamination; all has been burrowed and consumed, over and over. The slow-moving invertebrates are either feeding while moving (sediment in, sediment-rich feces out) or escaping and leaving locomotion burrows. A significant thickness of the bottom sediment has a high water content because of all the business of all the locomotor animals.
As changes go, this one was huge. In the late twentieth century it was dubbed the “agronomic revolution,” and it is the main characteristic feature of the Proterozoic and Phanerozoic sea bottoms—and the stratigraphic records they left behind.17 The new bilaterians were moving, and not just atop the sediment-water interface they were increasingly colonizing. A vertical component to burrowing also began. Our own take on this is that could not have taken place without high levels of oxygen in the sea: oxygenation is difficult at best when burrowing through sediment, and surely would have been impossible at global oxygen levels less than, let’s say, 10 percent. The old view is that the newly evolved animals increasingly ate the stromatolites and microbial mats out of existence near the Proterozoic-Cambrian boundary. The new view is that the tiny bilaterians were not just eating the nutrient-rich microbial mats: they were also making the firm substrates required for the mats to change from ubiquitous to virtually nonexistent.
By the latest Proterozoic time, the world was primed for animals. The genetic “toolboxes” necessary for the evolution of larger sizes, skeletons, and the many kinds of tissues necessary for activity were in place. Only one thing was lacking: oxygen. After the last snowball 635 million years ago, animals were poised, but oxygen levels were too low. Yet by (approximately) 550 million years ago, that had changed: oxygen levels had risen.
Making oxygen levels rise permanently requires increasing the fraction of organic carbon buried in the sediments, rather than buried as limestone. Most organic carbon is sequestered by clays eroding from the continents, so any factors that increase the clay flux— particularly in the tropical oceans where productivity is highest—will notch up the atmospheric oxygen level. One suggestion is that the rise of a terrestrial biosphere of some sort may have increased the production of clays through weathering,18 which is certainly true after vascular land plants evolved the ability to make deeply penetrating roots. However, shifts in the position of continents relative to the equator also have a big effect, as physiochemical weathering is much higher in the warm tropics than at the cold poles. Near the beginning of Cryogenian time (but before the snowballs, at about 800 million years ago) there was a stepwise change in the carbon cycle that lasted about 15 million years, during which time the fraction of organic carbon being buried dropped precipitously. This Bitter Springs event was first discovered in Central Australia, and has since been found at many other sites around the globe. It presumably caused a transient drop in surface oxygen. The cause of this event was a mystery until a group led by Adam Maloof at Princeton University discovered that the onset and termination of this event coincided with a pair of very rapid ~60° oscillatory shifts in Earth’s rotation axis (from an unpronounceable package of rock in Svalbard called the Akademikerbreen Group).19 This type of shift is termed a “true polar wander event” (discussed at length below), and involves the geologically rapid motion of the entire solid Earth, right down to the liquid metal at the core-mantle boundary. These particular shifts, however, moved a large chunk of the supercontinent of Rodinia off of the equator into the mid-latitudes, and then back again, fluctuating carbon burial and oxygen production in sync. When paleomagnetic and geochemical data from vastly different parts of the globe show the same shifts, at the same time, and in sync, we might learn something about how the planet works. In this case it is oxygen. We now think that there might be as many as thirty of these TPW events during the past 3 billion years,20 many of which coincide with interesting events like the Cambrian explosion.
CHAPTER VIII
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The Cambrian Explosion: 600–500 MA
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Photographs of Charles Darwin when he had reached seventy years old seem to show a man weathered well beyond this chronological age. This seems a man perhaps eighty or older. Yet at seventy, Darwin was in his last years, and perhaps this physical antiquity was a product of stress as well as the diseases he may have contracted in the tropics when, as a young man on HMS Beagle, he slowly circumnavigated the globe. Perhaps it was being so vexed by his many critics, as well as his own distress at his inability to understand how organisms inherited traits—genetics was not accepted until the early twentieth century when the work of Gregor Mendel was “rediscovered”—and especially the nature of the Cambrian explosion must have taken their emotional as well as physical toll. Darwin hated the fossil record in general and the Cambrian in particular. A vexation with the Cambrian fossil record followed him to his grave. It was this, and his inability to know how genetics worked, that surely were among his greatest regrets.
Since well before the time of Darwin it was known that fossils of animal life seemed to appear suddenly in the fossil record: the great English geologist Adam Sedgwick, the author and definer of the Cambrian period itself, mapped its base as the strata containing the first trilobites. While we now first think of the various geological periods as time, in fact they came into existence as a succession of strata, with a bottom bed defined by some fossil first appearing, and its top defined either by the extinction of some fossil, or better yet, a different species first appearing. In this case this was the Cambrian System, based on piles of strata in Wales. The Cambrian period is the time during which these Cambrian strata accumulated—no more, no less.
Sedgwick found that over short stratal intervals, sedimentary rocks seemingly bereft of fossils were found to be overlain by rocks with a profusion of highly visible fossils, the most common being trilobites. Trilobites are fossil arthropods, and thus their fossils are the remains of highly evolved and complex animals. This observation was vexing to Darwin (and hugely comforting to his critics), as it seemed to fly in the face of the then newly proposed theory of evolution.1
Charles Darwin thus went to his grave cursing the fossil record. His genius was such that he knew he was right, yet till the end of his life, he was bedeviled by critics who pointed out that the “first” life on Earth was of such complexity that it was inconceivable that the evolutionary processes so eloquently argued by Darwin in the many editions of his great work On the Origin of Species could have produced such complications as—a trilobite. Yet the great irony is that trilobites did not appear until the Cambrian was at least half over.2
One of the iconic fossils, trilobites were arthropods that dominated oceanic habitats relatively early in the history of animals on Earth. But how early? In Darwin’s time, trilobites were thought to be the earliest of all animals. Yet they are undoubtedly complicated, with three body sections, complex eyes and limbs—and large size. Some of the earliest could be up to two feet long. This was not what the earliest animals ought to have looked like—small and generalized, not large, complicated kinds of animals. We now know that trilobites were not—not even close, in fact, to being—the first animals.3
The history of the origin of animals on Earth is one of life’s most fascinating chapters, and also one of the most controversial. There is also a great deal of new information that has been gleaned even in the last ten years. There are two distinct lines of evidence giving quite different views on the timing of the first diversification of animal phyla. One o
f these lines comes from the pattern of appearance of animal fossils in rocks, the second from molecular clock studies on extant animals. They give important clues to one of the greatest of all paleontological mysteries: the rapid diversification of animals.
Illustration of trilobites from the nineteenth century. At that time, these were thought to be the oldest fossils on Earth. Trilobites were used to “mark” the start of the Cambrian Period.
The first major line of evidence about the Cambrian explosion comes from fossils. The appearance of animals leaving evidence of themselves in the rock record came in four successive waves. The first began around 575 million years ago and has been called the Avalon explosion, a name coming from the part of eastern Canada where the oldest of this group were found. The second wave is coincident with the almost complete disappearance of the Ediacarans, and is characterized not by actual fossils but with accurate traces of their locomotion. These numerous “trace fossils” could only have been formed by active locomotion of multicellular organisms—animals. These are as old as 560 million years, but most are about 550 million years in age. The sea bottoms would have been alive with actively moving, small wormlike forms.4