The wave of discoveries that rewrote the story of the earliest Cambrian began when the former Soviet Union mustered sizable teams of scientists to explore geological resources in Siberia after the end of World War II. There, above thick sequences of Precambrian sedimentary rocks, lie thinner formations of early Cambrian sediments undisturbed by later mountain-building events (unlike the folded Cambrian of Wales). These rocks are beautifully exposed along the Lena and Aldan rivers, as well as in other parts of that vast and sparsely populated region. A team headed by Alexi Rozanov of the Paleontological Institute in Moscow discovered that the oldest limestones of Cambrian age contained a whole assortment of small and unfamiliar skeletons and skeletal components, few bigger than ½ in (1 cm) long. These fossils have been wrapped in strings of Latin syllables but have been more plainly baptized in English as the “small shelly fossils” (SSFs for short).
J. JOHN SEPKOSKI JR., “FOUNDATIONS: LIFE IN THE OCEANS”
THE SHELL BUILDERS
In
chapter 1, we saw that the first answer to Charles Darwin’s question about the “Cambrian explosion” was the discovery of the bacterial mats called stromatolites, which date to 3.5 billion years ago, and eventually of microfossils of cyanobacteria and other kinds of bacteria from beds of the same age. In
chapter 2, we saw how single-celled life gave rise to multicellular soft-bodied creatures of the Ediacara fauna. But what about animals with shells? When did they arise?
The problem with growing a hard shell (
biomineralization) is not as simple as you might suppose. For most animals, it is a daunting task to pull ions of calcium and carbonate, or silicon and oxygen, from the seawater and then to secrete them to construct calcite or silica shells. They need special biochemical pathways to make this kind of mineralization happen, and it is usually a very energetically expensive process.
The thick shell of a clam or a snail, for example, is built by a fleshy part of the body called the mantle, which lies just beneath the shell and surrounds the soft tissues of the mollusc. This organ has specialized structures and physiological mechanisms that allow it to pull calcium and carbonate ions from the ocean and turn them into calcium carbonate crystals. Molluscs can secrete this chemical in two kinds of minerals: calcite, the common mineral found in most limestones; and aragonite, or “mother of pearl,” which most molluscs use to line the inner part of their shells. This is why there is an iridescent “pearly” luster on the inside of most mollusc shells, such as those of abalones. This is also the mechanism that grows pearls so valued by jewelry collectors. Pearls are simply layered structures of aragonite that are secreted around a central nucleus (like a grain of sand) trapped in the mantle of certain molluscs. The coating of aragonite is secreted so that the sand grain does not continue to irritate the mantle layer.
Based on the long duration of the Ediacaran fauna (more than 100 million years), we know that large soft-bodied organisms got along just fine without hard shells for a very long time. Judging from the data from the molecular clock of the divergence times of the major animal groups, most of the major phyla (sponges, sea jellies, and anemones; worms; segmented arthropods; brachiopods, or “lamp shells”; and molluscs) existed as soft-bodied forms well back into the Ediacaran, long before they added shells to allow the further diversification of body designs.
So if shells are such a burden, why evolve them at all? In most cases, the shell serves as protection against predators. Many paleontologists have argued that when shells started to appear, they were an adaptive response to new predators on the planet that were gobbling up all the vulnerable shell-less soft-bodied creatures. For some animals, the shells also serve as reservoirs of chemicals that the body needs. And some molluscs use their shells to secrete excess waste products of various metabolic processes.
Most important, mineralized shells also allow the diversification of body plans and thus greater ecological diversity and flexibility. The handful of living shell-less molluscs (such as solenogasters) are mostly shaped like worms, but with the addition of the shell, molluscs could evolve such diverse and distinct groups as chitons, clams, oysters, scallops, tusk shells, limpets, abalones, snails, cuttlefish, squid, and the chambered nautilus. These molluscs range from the slow and simple limpets and abalones, which creep along tide-pool rocks and graze on algae; to the headless filter-feeding clams; to the extremely intelligent and fast-moving octopi, squids, and cuttlefishes, which are predators.
THE “LITTLE SHELLIES” APPEAR
The late appearance of shells after the more than 100 million years of the evolution of large soft-bodied animals suggests that the development of shells was not an easy process. Nor would we expect large shells to have appeared all at once. Indeed, that is what we see in the fossil record.
For the longest time, there was no evidence of animals any simpler than trilobites from the Early Cambrian (
chapter 4). To some, the “sudden appearance” of trilobites, with their complex segmented shells made of the protein chitin reinforced with calcite, suggested that they (and other groups of multicellular shelled animals) had arisen suddenly, without precursors, an event once called the “Cambrian explosion.”
Shortly after World War II, the Soviets began to invest great effort in the geological exploration of remote regions like Siberia, mostly to find economic resources like coal, oil, uranium, and metals. In the process, they did a lot of basic geologic mapping and fossil collection in these areas. Along the Lena and Aldan rivers, which drain north out of Siberia into the Arctic Ocean, they found much more complete sequences of the Cambrian and Ediacaran rocks than were known anywhere else on Earth at the time. Soon, they began to describe an interval in the earliest Cambrian before the trilobites appeared in the third stage of the Cambrian (which the Soviet geologists called the Atdabanian). The two earliest stages of the Cambrian, which lay beneath the earliest trilobites, were called the Nemakit-Daldynian and the Tommotian.
Although these rocks yielded no trilobites, they did contain fossils of some of the other common large shelly Cambrian groups, such as the sponges, the sponge-like extinct archaeocyathans, and the “lamp shells,” or brachiopods. But the most common finds were tiny (mostly smaller than 5 millimeters [0.2 inch] in diameter) fossils nicknamed the “little shellies” or the “small shelly fossils” (SSFs). These minute specimens were hard to find unless the fossil collector knew exactly what to look for, so it’s no wonder that they were missed for decades by geologists accustomed to discovering large, flashy trilobites. Typically, dense concentrations of these tiny creatures populated the shelly layers (
figure 3.1), and they were impossible to collect as complete specimens in the field. Instead, it was much easier to haul chunks of fossiliferous rock to the lab, and slowly dissolve the fossils out of the rock with acid. Or the chunks of fossiliferous limestone were sliced up and ground down into thin sections of rock only 30 microns thick and glued to a microscope slide. Observed through the microscope, these limestones were chock-full of a wide array of small but complex fossils (
figure 3.2).
Figure 3.1
Typical small shell fragments visible on the weathered surface of the dark band in the middle, from the Wood Canyon Formation in the White Mountains near Lida, Nevada. (Photograph by the author)
When these tiny fossils were discovered, it was not clear to what groups of familiar animals they belonged. Some were clearly shells of clam-like molluscs and snail-like molluscs. Others appeared to be pieces of “chain-link” armor for the bodies of much larger creatures. Many were the tiny needle-like or spiky elements known as spicules, which are woven together to form the only hard parts found in sponges.
Figure 3.2
Rocks from the earliest stages of the Cambrian (Nemakit-Daldynian and Tommotian) do not produce trilobites, but are dominated by tiny phosphatic fossils nicknamed the “little shellies.” Some may have been mollusc shells (E, H, and I), while others apparently were sponge spicules or pieces of the “chain-mail armor” of larger creatures, such as worms: (A) Cloudina hartmannae, one of the earliest known skeletal fossils, from the same beds that produce Ediacaran fossils in China; (B) spicule of a calcareous sponge; (C) spicule of a possible coral; (D) Anabarites sexalox, a tube-dwelling animal with triradial body symmetry; (E) spicule of a possible early mollusc; (F) Lapworthella, a cone-shaped organism of unknown relationships; (G) skeletal plate of Stoibostromus crenulatus, an organism of unknown relationships; (H) skeletal plate of Mobergella, a possible mollusc; (I) cap-shaped shell of Cyrtochites, a possible mollusc. Scale bars = 1 millimeter. (Photographs courtesy S. Bengston)
Significantly, many of them were made of calcium phosphate (the mineral apatite), not calcium carbonate, which most marine animals use to build their shells. Along with the earliest brachiopods (the lingulids) that used calcium phosphate to build their shells, this is suggestive of why it took so long for animals with large shells, such as trilobites, to evolve. It indicates that there were many hurdles and struggles to overcome before the process of mineralizing of shells got going in the Early Cambrian. First of all, none of these creatures secreted more than a few dozen tiny pieces of shell, so they were not yet ready to construct a shell as big as that of a trilobite. More important, a variety of lines of chemical evidence, along with the abundance of calcium phosphate (not calcium carbonate) shells, suggest that the atmosphere and oceans had not yet achieved the level of about 21 percent oxygen that is found on the planet today. Instead, it is estimated that the oxygen level was much lower still, which would have made it hard to run the geochemical and physiological mechanisms that allow molluscs to secrete minerals for shells.
PRESTON CLOUD’S PREDICTIONS
The field of Precambrian geology and paleontology was virtually nonexistent until 1954, when Stanley Tyler and Elso Barghoorn discovered and published the first evidence of Precambrian microscopic fossils. One man in particular became the pioneer and dominant figure of Precambrian biology and geology starting in the 1950s and 1960s, and remained so until his death: Preston H. Cloud. I met Pres several times in my career, and as both J. William Schopf, in Cradle of Life, and I recall, he was a towering figure in the field—even though he was only a slim 5 feet, 6 inches tall and had a shiny bald head and a bristly beard. But he was (in Schopf’s words) “a giant, a wiry wonder, full of energy, ideas, opinions, and good hard work. And he was probably the greatest biogeosynthesist the United States ever produced…. Cloud was not given to idle chatter and struck some colleagues as a bit imperious (one of them referred to him as ‘the little general,’ though never to his face). Yet Cloud had an overriding saving grace. He was brilliant.”
Cloud had a long career both in academia (especially at the University of California, Santa Barbara), and at the U.S. Geological Survey, where he built the paleontological branch into a powerhouse. Cloud’s innovative and wide-ranging thinking made him an expert in many areas, from brachiopods to bauxite mining to oceanography to coral reefs to carbonate petrology. In 1974, he began writing books that warn about the future of the planet, about limited resources and peak oil, and about the ecological and environmental disasters that humans are creating on Earth. His two major books on this topic (Cosmos, Earth, and Man: A Short History of the Universe [1978] and Oasis in Space: Earth History from the Beginning [1988]) were the first to connect his broad understanding of 4.5 billion years of Earth history with predictions about how humans are likely to destroy the planet.
Long before anyone else was working on the evidence for early life, Cloud pushed for more and more studies of Precambrian microfossils and stromatolites, as well as for the search for more Ediacaran fossils. Even more important, he created the framework of our understanding of Precambrian Earth—the period of 3 billion years of low oxygen levels, the slow evolution of single-celled life, and the explosion of eukaryotic cells during the “oxygen holocaust” between 2 and 1.8 billion years ago—and he came up with many innovative ideas for how Precambrian geochemistry, atmospheres, and oceans had worked. His famous paper “A Working Model of the Primitive Earth” (1972) has been the foundation of nearly every study on the Precambrian in the past forty-plus years.
CLOUDINA
Like many other geologists, Cloud was frustrated with the big difference between the large but unshelled Ediacaran creatures and the shelled trilobites. Late in his life, he was overjoyed with the discovery and description of the Early Cambrian “little shellies,” closing most of that gap. Still, why were there no shelled fossils before the Cambrian? Why did there appear to have been this evolutionary break between Edicarans and SSFs?
Then, in 1972, Gerard J. B. Germs described fossils from the Nama Group in Namibia (at that time, the South African colony of South-West Africa), which dates to the Late Precambrian. He reported a strange calcareous fossil about 6 millimeters (0.2 inch) across and about 150 millimeters (6 inches) long. It was constructed of a set of nesting conical shells, with a hollow tubular cavity inside (
figure 3.3). There is still no agreement as to which modern group of animals it belongs to (such as a worm group that secretes a tubular skeleton), or even if it belongs to a modern group at all. The organisms are usually found associated with stromatolites, so they preferred shallow-water microbial-mat habitats. And there is some evidence of other creatures nibbling on them, so true predation had begun.
Whatever these mysterious creatures were, they were the first shelled animals on the planet (along with a Chinese tubular fossil called Sinotubulites), and they occurred around the world in the latest Precambrian: not only in Namibia, but also in Antarctica, Argentina, Brazil, California, Canada, China, Mexico, Nevada, Oman, Spain, Uruguay, and especially Russia. Appropriately, in 1972, Germs named it Cloudina, in honor of Preston Cloud and his huge number of contributions to Precambrian biogeology. Although subsequent years brought waves of argumentation about and reinterpretation of these frustratingly simple and incomplete fossil, it seems very appropriate that the oldest shelled animal on Earth was named after Preston Cloud.
Figure 3.3
Reconstruction of Cloudina, showing the cone-in-cone outer structure and the cylindrical internal chamber, which was occupied by the soft-bodied shell maker. (Drawing by Mary P. Williams, based on several sources)
The “Cambrian explosion” was not an explosion at all, but a “slow fuse” (
figure 3.4). From about 600 to about 545 million years ago, the only multicellular life on the planet was the large soft-bodied, shell-less Ediacarans. Apparently, the geochemical conditions (especially low oxygen level) did not allow for the evolution of large shelled animals. Along with the mysterious Ediacarans, the precursors of the “little shellies,” especially
Cloudina and
Sinotubulites, lived among the stromatolitic mats.
Then, between 545 and 520 million years ago (Nemakit-Daldynian and Tommotian stages), the largest creatures on the planet were soft-bodied animals with tiny bits of mineralized armor in their skins, or sponges woven of small spicules, as well as little shelled molluscs and brachiopods. At 520 million years ago, at least 80 million years after larger multicellular animals first appeared, we finally get animals with large calcified shells: the trilobites. Thus there was no “Cambrian explosion,” unless you count 80 million years (beginning of the Ediacaran to the Atdabanian) or 25 million years (duration of the first two stages of the Early Cambrian) as an “explosion.”
Creationists and others are determined to ignore this evidence and distort the fossil record for their own purposes by promoting a false version of the “Cambrian explosion.” As Harvard paleontologist Andrew Knoll put it:
Figure 3.4
A detailed examination of the stratigraphic record of fossils through the late Precambrian and the Cambrian shows that life did not “explode” in the Cambrian, but appeared in a number of steps spanning about 100 million years. The large soft-bodied Ediacaran fossils first appeared 600 million years ago, in the Vendian stage of the Late Precambrian (see
figure 2.2). Toward the end of their reign, we see the first tiny shelly fossils, including the simple conical
Cloudina and
Sinotubulites. The Nemakit-Daldynian and Tommotian stages of the Cambrian are dominated by the “little shellies” (see
figure 3.2), plus the earliest brachiopods, the conical sponge-like archaeocyathans, and many burrows showing that worm-like animals without hard skeletons were also common. Finally, in the Atdabanian stage, around 520 million years ago, we see the radiation of trilobites and a big diversification in the total number of genera, thanks to the mineralized shells of trilobites, which preserve particularly well (histograms on the right side of the diagram). Thus the “Cambrian explosion” took place over more than 80 million years and thus was not a “sudden” event, even by geological standards. (Redrawn from Donald R. Prothero and Robert H. Dott Jr.,
Evolution of the Earth, 7th ed. [Dubuque, Iowa: McGraw-Hill, 2004], fig., 9.14)
Was there really a Cambrian Explosion? Some have treated the issue as semantic—anything that plays out over tens of millions of years cannot be “explosive,” and if the Cambrian animals didn’t “explode,” perhaps they did nothing at all out of the ordinary. Cambrian evolution was certainly not cartoonishly fast…. Do we need to posit some unique but poorly understood evolutionary process to explain the emergence of modern animals? I don’t think so. The Cambrian Period contains plenty of time to accomplish what the Proterozoic didn’t without invoking processes unknown to population geneticists—20 million years is a long time for organisms that produce a new generation every year or two.
FOR FURTHER READING
Attenborough, David, with Matt Kaplan. David Attenborough’s First Life: A Journey Back in Time. New York: HarperCollins, 2010.
Conway Morris, Simon. “The Cambrian ‘Explosion’: Slow-fuse or Megatonnage?” Proceedings of the National Academy of Sciences 97 (2000): 4426–4429.
——. The Crucible of Creation: The Burgess Shale and the Rise of Animals. Oxford: Oxford University Press, 1998.
Erwin, Douglas H., and James W. Valentine. The Cambrian Explosion: The Construction of Animal Biodiversity. Greenwood Village, Colo.: Roberts, 2013.
Foster, John H. Cambrian Ocean World: Ancient Sea Life of North America. Bloomington: Indiana University Press, 2014.
Grotzinger, John P., Samuel A. Bowring, Beverly Z. Saylor, and Alan J. Kaufman. “Biostratigraphic and Geochronologic Constraints on Early Animal Evolution.” Science, October 27, 1995, 598–604.
Knoll, Andrew H. Life on a Young Planet: The First Three Billion Years of Evolution on Earth. Princeton, N.J.: Princeton University Press, 2003.
Knoll, Andrew H., and Sean B. Carroll. “Early Animal Evolution: Emerging Views from Comparative Biology and Geology.” Science, June 25, 1999, 2129–2137.
Runnegar, Bruce. “Evolution of the Earliest Animals.” In Major Events in the History of Life, edited by J. William Schopf, 65–93. Boston: Jones and Bartlett, 1992.
Schopf, J. William. Cradle of Life: The Discovery of Earth’s Earliest Fossils. Princeton, N.J.: Princeton University Press, 1999.
Schopf, J. William, and Cornelis Klein, eds. The Proterozoic Biosphere; A Multidisciplinary Study. Cambridge: Cambridge University Press, 1992.
Valentine, James W. On the Origin of Phyla. Chicago: University of Chicago Press, 2004.
