No one disputes that a huge diversity of large animals emerged with alacrity between 600 and 500 million years ago. The event itself took place in the sea, for the land areas of the time were largely barren except for lichens and perhaps a few low plants; there were no trees, no shrubs, no stemmed plants at all. Because of the lack of rooted vegetation, little soil would cling to the land surfaces.
In the shallow seas and waterways, however, life was plentiful (though clearly different from that in the seas of today, as we noted above) and was rapidly changing in composition. Stromatolites, the layered bacterial forms that had been the dominant type of life on Earth for most of the 4-billion-year Precambrian era, were by 500 million years ago nearly absent from the planet. There were literally being eaten out of existence, for a great biological revolution was creating entire suites of organisms adapted for utilizing plants as food. These newly evolved grazing animals (many looked like small worms) used the stromatolites as food. After 700 million years ago, a steep decline in stromatolite diversity took place, and newly evolved herbivores were surely its cause, although these grazing creatures left no fossil record. (They were too small and had no mineralized skeletons that could fossilize.) In most instances we simply infer their existence.
Thus the stage was set for the great evolutionary drama we call the Cambrian Explosion. It was grand theater, composed of four acts, each with its own set of characters, although some of them hung around for successive acts before exiting—by going extinct!
Act 1: The Ediacarans
The first act introduced a truly odd assemblage of creatures that looked like bizarre jellyfish, mutated worms, and quilted air mattresses somehow brought to life. This opening cast of characters is collectively known as the Ediacaran fauna.
We now know that the Ediacarans opened Act 1 about 580 million years ago and were largely gone by 550 million years ago (although a few appear in much younger rocks). Most of the Ediacaran fauna somewhat resemble members of the phyla Cnidaria and Ctenophorata—the jellyfish, sea anemones, and soft corals of our world. Two of the most common types of Ediacaran fossils resemble jellyfish and stalked, colonial sea anemone-like animals known as sea pens (still quite common in our world), and they were first interpreted as early versions of these modern forms. Other members of the fauna were more worm-like in appearance, but these were minor players.
In some cases these are large organisms—some have left fossils nearly 3 feet long, making them veritable behemoths for their time. Yet they seemed to have little organization of the sort we are so familiar with. For example, they had neither an observable mouth nor an anus. Their organization suggests a series of tube-like structures quilted together. In a 1988 essay, Stephen Jay Gould proposes that these odd animals are indeed the flowering of the
"diploblastic," or two-cell-layer body plan, a type of body plan found today only in the corals and jellyfish.
The Ediacarans were not discovered until the 1940s, when an Australian geologist named R.C. Sprigg noticed some odd-looking fossil remains on scattered slabs of sandstone mines in the Ediacaran Hills of southern Australia, a desolate and isolated locality in very arid country. The fossils were simply impressions in the sandstone, rather than preserved skeletons of any sort. Some were worm-like; others looked like giant leaves; a third group were circular in shape. Sprigg collected a few of these, noting that many of the circular impressions made in the sandstone looked like modern-day jellyfish, also known as Cnidarians. But such soft creatures as jellyfish are preserved in rock only under the most extraordinary circumstances, and because of this, many wondered whether these were fossils at all. Nevertheless, Sprigg briefly announced his find in a scientific journal, described them as "among the oldest direct records of animals in the world," and noted that "they all appear to lack hard parts and to represent animals of very varied affinities." The fossils ranged in length from less than an inch to more than 40 inches. Other fossils from this region began to turn up (see Figure 7.1), and they eventually became the passion of Australian paleontologist Martin Glaessner. He created the first biological reconstructions of the odd Ediacaran fossils and made astute observations about the nature of the environment in which these organisms lived. Intensive study of the taxonomic affinities of this varied fauna soon followed.
Glaessner ultimately placed the entire Ediacaran fauna, as he called them, into known phyla, such as the Cnidaria, a phylum thought to be among the most primitive of all animals. To him, the Ediacarans thus represented the first flowering of the animals and belonged to taxonomic groups still present on Earth today. Using the tree analogy, Glaessner viewed his Ediacarans as "missing links" between the small, presumably simple ancestors of all animals, and the jellyfish and anemones still alive today. The Ediacaran world seemed to resemble a Cnidarian world, and this agreed nicely with most biologists' view of how the metazoan radiation may have unfolded, beginning with the most "primitive" of phyla, the sponges and Cnidarians, followed only later by more complex fauna such as arthropods (and the trilobites, which are members
of the phylum Arthropoda). According to this view, our modern-day animals are descendants of the Ediacarans. This opinion is still held today by many specialists, including Simon Conway Morris of Cambridge University.
In the nearly four decades since Glaessner's interpretations were first published, the Ediacaran fauna has achieved new importance. First, the strange fossils making up this assemblage have been found beyond Australia. In the White Sea region of Russia, Arctic Siberia, Newfoundland in Canada, and Namibia in southern Africa, other fossilized examples of these strange creatures are preserved, showing that the Ediacaran fauna was essentially worldwide in distribution at the end of the Precambrian. (Many of these localities have now been dated via radiometric geochronometry; some of the oldest known on Earth are those at Mistaken Point, in Newfoundland, dated as 565 million years old.) Second, the Ediacarans seem to have a longer strati-
graphic range than previously supposed, and in a few places on Earth, they may actually have coexisted briefly with the undoubted animal faunas. Finally, some workers believe that the Ediacaran fauna do not represent animals at all but are large plants, fungi, or even lichens. Others class them as animals, but animals belonging to taxonomic groups now extinct. The Ediacarans have thus gone from logical precursors of the Cambrian Explosion to much more controversial players in the evolutionary drama.
This latter view, that the Ediacarans are not a main branch of the tree leading to animals, but rather represent a side branch now extinct (and thus have nothing to do with the ancestry of all current animal life), has been most effectively championed by paleontologist Adolf Seilacher of Yale and Tübingen University. He suggests that the resemblance between the Ediacarans and living creatures such as jellyfish and sea pens is coincidental. In his view, the Ediacarans represent an extinct assemblage of organisms—a separate biological "experiment" involving creatures with tough outer walls and fluid-filled interiors. Seilacher has speculated that during the time of the Ediacarans, a thick mat of bacteria covered the ocean bottoms, and this could answer the very perplexing question of how organisms without hard parts became fossilized.
This bacterial mat may explain why the soft-bodied Ediacaran fossils were so commonly preserved as fossils. As sand settled over the Ediacarans, they were pushed downward into the bacterial mat. Their tough outer walls could not be crushed readily, and the impressions they made in the mats were preserved in three dimensions by the overlying sand. Seilacher suggests that the evolution of new, efficient grazing animals such as mollusks at the start of the Cambrian rapidly brought an end to such mats and changed the way sediment accumulated in the earliest Cambrian.
Perhaps the most intriguing aspect of the Ediacarans is that there is no evidence of predation on them,- we have no records of Ediacarans preserved with bite marks or missing pieces. Did these creatures live in a time without predators—in a "Garden of Ediacara," as paleontologist Mark McMenamin has dubbed the time?
What actually happened to the Ediacarans? Conway Morris asks whether they were diluted out of existence. That is, did the great radiation of animals at the base of the Cambrian simply overwhelm them in the fossil record? (In other words, they existed but were too few to fossilize.) Was the disappearance of the Ediacaran body form a result of the first mass extinction on planet Earth? Were they ecologically replaced or even preyed on by the new animals—driven to extinction by efficient new predators against which they had little defense? The fossil record is still enigmatic. In some places on Earth, the Ediacarans are gone before the first "Cambrian" animals appear, which suggests that the new animals were simply filling the niches of the extinct Ediacarans. But as we have said, in other places there is a clear overlap between the two, suggesting that a competitive interaction ensued.
The Ediacarans do make good theater—enigmatic, mysterious, and the first on the stage. They were a hard act to follow, but a great wave of diversification was occurring at the end of their time in the limelight, a wave that continues on planet Earth still. With the second act of the Cambrian Explosion, undoubted animals appear on the scene.
Acts 2 and 3: Trace Fossils and Small Shellys
We can combine the next two acts, because the cast of characters is both incomplete and poorly characterized. In Act 2, a new group of players, seemingly wearing masks to disguise their true identity, replace most of our opening troupe. We detect them only by the footprints they left on the stage itself, for we have no true "body" fossils (usually the remains of skeletal hard parts). The second assemblage of life making up the Cambrian Explosion has left only squiggles and tracks in the ancient sediment. Such fossilized remains are known as trace fossils; they are not the remains of animals, but evidence of their behavior, and thus record the trackways or feeding patterns of ancient organisms. Yet they are of enormous significance. Whereas the Ediacarans simply sat in place their entire lives, these first trace fossils tell us that large animals capable of locomotion had appeared on Earth. Perhaps they were large worms or flatworms, or perhaps they belonged to phyla now extinct. The first and most primitive trace fossils appear in rocks as old as the Ediacarans, but they diversify and take center stage in younger rocks. Trace fos sils are still being formed today and have been common in the rock record since the Cambrian. But they are clearly formed by many different organisms, and it is doubtful that the organisms that formed the first trace fossils survived into much more recent times than the Cambrian period itself.
Our Act 3 introduces an assortment of tiny calcareous tubes, knobs, and twisted spines, none larger than about '/i inch, all coming from animals that it is still impossible to reconstruct completely. Some are the remains of larger skeletons that have been fragmented into pieces, but most are single elements of some sort of a multielement skeleton, like individual spines coming from a porcupine. Collectively, they are known as small shelly fossils, or SSFs. The small shellys are first found in rocks dated to around 545 million years ago. These extremely significant fossils tell us that another great biological breakthrough had been achieved: The SSFs are the first large animals with mineralized skeletons.
Act 4: The Trilobite Faunas
Act 4 of our play is a grand finale featuring fossil icons much more familiar to us than the previous actors. They include the first trilobites, brachiopods, and a host of newly evolved mollusks and echinoderms. The characters are now far larger—and greater in number—than in any of the three previous acts, and ironically, these actors were long thought to mark the start of the Cambrian Explosion, rather than its end. This last group didn't appear until about 530 million years ago. Its diversification proceeded for another 30 million years. By about 500 million years ago, the Cambrian Explosion was finished.
The trilobites are by far the most diverse and obvious part of this assemblage. The oldest trilobites, of which the genus Olenellus is diagnostic, were spiny, somewhat resembled annelid worms, and had large crescent-shaped eyes. They all had walking legs and gills, and all appear to have fed by ingesting sediment or particulate material on the sea floor. They showed little adaptation for defense against predation.
Another curious group to appear contemporaneously with the trilobites were immobile, coral-like animals called archeocyathids. This group had conical skeletons made of lime and lived gregariously. They appear to have been the world's first reef-forming organisms and seem to have lived in the same environments favored by corals today. In addition to being the first of a long line of reef formers, the archeocyathids have another somewhat dubious claim to fame: They were perhaps the first animal phylum to go extinct. The basic body plan of the archeocyathid skeleton is unlike anything alive today. Taxonomists place these creatures in the same phylum as the living sponges, but this is as much for convenience as anything else. They appear to have constituted a separate phylum—and one of the few phyla we know of to have suffered utter extinction.
The remarkable Burgess Shale fauna of British Columbia has yielded extraordinary insights into animals living among the trilobites. Because of the lack of oxygen in this ancient environment, even soft parts were preserved, and these remains offer us an unparalleled window into the past. The Burgess Shale reveals how diverse the marine ecosystems were by the time trilobites evolved. Yet by the time of the Burgess fauna, some 505 million years ago, the majority of animal phyla appear to have been present.
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