Are the late Precambrian life forms (Ediacaran biota) related to modern animals

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Are the late Precambrian life forms (Ediacaran biota) related to modern animals?

Viewpoint: Yes, some species within the Ediacaran biota of the late Precambrian are the predecessors of modern animals.

Viewpoint: No, the late Precambrian life forms (Ediacaran biota) are not related to modern animals.

In 1946, Australian geologist Reginald Sprigg discovered fossilized remains of what turned out to be creatures from 544 to 650 million years ago. Their significance lay not merely in the fact that these were some of the oldest fossils ever discovered, but in their origins from what geologists call Precambrian times—long before the rise of most animal phyla found on Earth today. Scientists dubbed the creatures whose fossils Sprigg had discovered, which were apparently jellyfish-like forms in the shape of disks, Ediacara, after the Ediacara Hills where Sprigg had found them.

Other discoveries of Ediacara followed, most notably in Namibia, in southwestern Africa. This led to a great debate concerning the meaning of the Ediacara: were they completely different life forms, unrelated to animals that existed during the present geological era? Or were they precursors of modern creatures? Either answer, of course, is scientifically interesting, and strong arguments for each are made in the essays that follow. Before diving into those arguments, however, it is worthwhile to consider a few basics as to geological time, stratigraphy, relative and absolute dating, and taxonomy.

The expression geological time refers both to the extremely long span of Earth's existence, and to the ways that this is measured. Such measurement dispenses almost entirely with the terms to which most people are accustomed to using for the measurement of time. Years, centuries, even millennia are too minuscule to have any meaning within the context of the planet's 4.5 billion -year history. Therefore, geologists speak usually in terms of millions of years ago or billions of years ago, abbreviated Mya and Gya respectively.

Divisions of geological time, however, are not usually rendered in these absolute terms, but in relative terms, according to information gathered by studying rock layers, or strata. From such information, geologists have developed a time scale that uses six basic units, which are, in order of size (from longest to shortest): eon, era, period, epoch, age, and chron. None of these has a specific length, nor is there a specific quantity of smaller units that fits into a larger one. The only rule is that at least two of a smaller unit fit into a larger one; furthermore, the longer the unit, the greater the significance of the geological and/or paleontological (i.e., relating to prehistoric life forms) events that circumscribe that period. (For example, the end of the Mesozoic era before modern Cenozoic is one marked by the extinction of the dinosaurs, probably as the result of a meteorite impact on Earth.)

The term Precambrian encompasses about 4 billion years of Earth's history, including three of the four eons (Hadean or Priscoan, Archaean, and Proterozoic) of the planet's existence. Today we are living in the fourth eon, the Phanerozoic, which began about 545 Mya ago. Paleontologists know vastly more about the life forms of this eon than about those of any preceding one, though extremely simple life forms did exist prior to the Phanerozoic. However, to leave fossilized remains, a creature must have "hard parts" such as bones or teeth, and few life forms of Precambrian time met those qualifications.

Hence the significance of the Ediacaran biota, which left fossilized remains dating back to before the Phanerozoic eon. (The term biota refers to all the flora and fauna—plants or animals—in a given region. As we shall see in the essays that follow, this term is particularly advisable in the present context, since paleontologists are not certain whether Ediacaran life forms were plant, animal, or members of some other kingdom.) The Ediacaran biota dates from the latter part of the Proterozoic eon, in the Vendian or Ediacaran era. The latter preceded the Cambrian period, which began the Paleozoic era of the present Phanerozoic eon—thus explaining the significance of the term "Precambrian."

How do we know the age of the formations in which the Ediacaran biota were found? Through a combination of absolute and relative dating. Relative dating methods assign an age relative to that of other items, whereas absolute dating involves the determination of age in actual years or millions of years, usually through the study of radioactive isotope decay.

The principles of biostratigraphy, a subdiscipline concerned with the relative dating of life forms, were first laid out by English engineer and geologist William Smith (1769-1839). While excavating land for a set of canals near London, Smith discovered that any given stratum contains the same types of fossils, and strata in two different areas can thus be correlated or matched. This led him to the law of faunal succession, which states that all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. As a result, if a geologist finds a stratum in one area that contains a particular fossil, and another in a distant area containing the same fossil, it is possible to conclude that the strata are the same in terms of time period. This principle made it possible for paleontologists studying Ediacaran forms to relate the fossils found in Namibia, Australia, and other places.

In addition to the principles of geological and paleontological time classification at work in the study of Ediacara, there are the concepts of biological classification embodied in the discipline of taxonomy. As with the geological time-scale, there are a number of levels of taxonomic groups or taxa, from kingdom down to species, though we are concerned here only with the two largest significant groupings, kingdom and phylum. The number of smaller groupings within a larger group is, as with geological time periods, not fixed: for example, it is conceivable that a phylum could have just one species, if that species were so significantly different from all others as to constitute a distinct category. (This can easily be understood by reference to the taxonomy of political entities: though New York City is much larger than the Vatican City, the latter happens to be its own country, whereas New York is a city within a state within a country.)

Ediacara may or may not represent a distinct phylum, depending on the answer to the question of whether they were precursors to modern life forms. Even more fundamental, however, is the question of whether they were plants or animals. Although plant and animal are common terms, there are no universally accepted definitions. One of the most important characteristics of plants is the fact that they are capable of generating their own nutrition through the process of photosynthesis, or the conversion of electromagnetic energy from the Sun into chemical energy in the plant's body. Animals, by contrast, must obtain nourishment by consuming other organisms. This is a much more significant distinction than some of the most obvious-seeming ones, such as the ability or inability of the organism to move its body: thus sponges, which might seem like plants, are actually animals.

—JUDSON KNIGHT

Viewpoint: Yes, some species within the Ediacaran biota of the late Precambrian are the predecessors of modern animals.

Some species within the Ediacaran biota of the late Precambrian are the predecessors of modern animals. Although these soft-bodied organisms have occasionally been described as a "failed experiment" that died off before the Cambrian explosion, new fossil finds and scientific analyses indicate otherwise. Recent studies now show that the Ediacaran fauna not only persisted far longer in geologic time than originally thought, but also may well have given rise to many of the animals that currently live on Earth.

The notion that the Ediacaran animals are ancestors of modern life is not new. Discovered in Australia a century ago, the Ediacaran biota remained an enigma for about 50 years. Then paleontologist Martin Glaessner of the University of Adelaide asserted that they may be the missing link in the tree of life. Scientists had long pondered the evolutionary origins of the organisms associated with the Cambrian explosion, a period of spectacular diversification. Nearly all basic body plans appear during the Cambrian period which began about 540 million years ago (540 Mya), but where did they come from? Glaessner answered that the Ediacaran biota was the logical place to look. They represented the first definite multicellular animals and were prevalent in the late Precambrian era (also known as the Vendian Period). The scientific community embraced the idea and declared the Ediacaran biota as the forebear of modern animal life. A few vocal dissenters, including such outspoken scientists as Adolf Seilacher and Stephen Jay Gould, have challenged Glaessner's ideas, and his hypothesis has lost favor over the past two decades.

Now, however, paleontologists are finding more and more fossils that lived further and further into the Cambrian Period, and the view is shifting again toward the notion that at least some of the Ediacaran organisms were indeed the ancestors of modern life.

Overlap in Time

One of the biggest arguments against the Ediacaran-ancestor hypothesis was that these odd-looking organisms became extinct several tens of millions of years before the Cambrian era, and therefore could not have given rise to modern phyla. Although many scientists suspected that at least some Ediacaran animals survived into the Cambrian, fossil proof was lacking. MIT geologist John Grotzinger is quoted in the Discover magazine as saying, "In sections around the world, you could walk up through a succession of layers that contained Ediacaran fossils, and then you wouldn't see any Ediacaran fossils for a long, long time—I should say a great, great thick-ness—and then you would see the early Cambrian fossils." The gap suggested a lengthy separation in time between the occurrence of the two groups of organisms. The tide turned suddenly in 1994 when Grotzinger's research group went to a site in Namibia, which is located in southwestern Africa, and discovered a swath of Ediacaran fossils mingling with early Cambrian fossils, indicating that the two coexisted. Precise dating techniques verified the extension of Ediacaran animals into the next geologic period. Scientists now had solid evidence that the Ediacara lived during the time frame spanning 600-540 Mya, pushing their survival at least into the beginnings of the Cambrian explosion.

Faced with this new evidence, scientists realized that they might have been too hasty in their characterizations of the geological strata. Perhaps the Ediacaran animals coexisted with early skeletonized (shelled) animals in other areas, too, but simply left no trace in the rock. After all, the creatures were soft-bodied with no hard parts, and were therefore ill-suited to preservation. Most Ediacaran remains are merely impressions in sandstone beds. In addition, the Cambrian period appears to have ushered in the first predators, which would have greatly decreased the chances that an Ediacaran animal would have died intact. A digested Ediacaran creature leaves no trace in the fossil record. Simon Conway Morris of the University of Cambridge has gone so far as to say that skeletons arose in the Cambrian in response to pressure from predators.

Other scientists note that the disappearance of Ediacaran animals from the fossil record was likely caused by the disappearance of microbial mats. Explained James Gehling of the University of California—Los Angeles, "A form of 'death mask' resulted from bacterial precipitation of iron minerals in the sand that smothered decaying microbial mats and megascopic benthic organisms." He suggested that the mats provided a defense against erosion, while triggering mineral encrusting and sand cementation. As grazing organisms evolved and attacked the benthic mat communities, they began to disappear. Without the mats, the Ediacaran animals subsequently biodegraded without preservation.

Despite the poor odds, however, some Ediacaran animals were fortuitously preserved in the Cambrian strata. Preservation was typically a combination of the animals' habitat on the muddy sea bottom and the sudden deposition of sandy debris atop them. At Mistaken Point, Newfoundland, where a massive fossil bed is located, the Ediacaran animals were covered by volcanic ash. One scientist has even dubbed the Canadian site an "Ediacaran Pompeii" in reference to the volcanic ash from the eruption of Mount Vesuvius that buried and preserved the people of Pompeii back in a.d. 79.

Grotzinger's Namibia discovery was the first evidence of Ediacaran persistence past the Precambrian-Cambrian border, but it was not the last. Among a string of findings, James W. Hagadorn of the California Institute of Technology, and Ben Waggoner of the University of Central Arkansas, reported in 2000 that the Ediacaran fauna from the southwestern United States also survived to near the base of the Cambrian. They found numerous Ediacaran animals, including a variety of tubular Ediacaran organisms and a frond-like form known as Swartpuntia. Specifically, they noted that Swartpuntia persisted "through several hundred meters of section, spanning at least two trilobite zones." Scientists have also reported Cambrian Ediacaran fossils from numerous sites, including the famed Burgess Shale, a fossil bed located in the Canadian Rockies.

Odd-looking Ancestors

The second prominent argument against the Ediacaran-ancestor hypothesis is simply that the Ediacaran animals are just too different from modern metazoan (multicellular animal) groups to be related. In the past decade, however, many research teams have pointed out enough similarities between specific pre-Cambrian and Cambrian forms to pronounce at least a few Ediacaran animals to have likely given rise to modern metazoans.

Classification of organisms is a tricky business, even among the extant species. When scientists are dealing with fossils, taxonomy is more difficult. When the fossils are as rare and incompletely preserved as the Ediacaran animals, it can be excruciating. However, the Ediacaran specialist Waggoner cautions against taking the easy route and placing unusual or incomplete organisms in new taxa that are unrelated to anything else. "[P]roblematic fossils must be incorporated into phylogenetic analyses. By definition, their synapomorphies (features that are shared) with other taxa will not be obvious; these synapomorphies must be searched for and documented," he recommended in a 1996 paper. For example, he cited the argument that Edicarans could not have given rise to modern phyla because many Ediacaran animals appear to exhibit glide reflectional symmetry and extant animals do not. In this type of symmetry, the organisms have mirror-image right and left sides, with one side slightly shifted so the two sides are off-kilter. Most modern animals have bilateral symmetry, with directly opposite right and left halves. Waggoner argues that the glide reflectional symmetry seen in some Ediacaran fossils may actually be just a result of shrinkage or distortion, and even if it isn't, that type of symmetry is not unknown among more evolutionary advanced animals, including Cambrian specimens from the Burgess Shale. In addition, he points out that while modern animals may not exhibit glide reflectional symmetry in their overall body plan, some do have such symmetry in their gross anatomy. In sum, the mere presence of glide reflectional symmetry does not preclude an organism from being a metazoan.

Other litmus tests have been proposed and later discounted. Adolf Seilacher, who challenged Glaessner's original Ediacaran-ancestor hypothesis, proposed that Ediacaran animals did not have mouths. Waggoner countered that at least two Ediacaran species, Marywadea and Praecambridium, clearly show digestive structures akin to those in arthropods. A third, Metaspriggina, also shows indications of a digestive tract.

Even Seilacher, who has disputed many Ediacaran-Cambrian species links, has proposed the existence of late-Precambrian organisms he calls Psammocorallia, which are sand-filled, cnidarian coelenterates. The fossil record also shows a diversification of worm-like Edicaran animals, such as Helminthopsis, Cloudina, and Dickinsonia, at the end of the Precambrian period. In a review of Vendian fossils found in the former Soviet Union and Mongolia, paleontologists Alexei Rozanov and Andrey Zhuravlev pointed out that organic "skeletons" appeared in Precambrian organisms, including Redkinia and Sabellidites. Cloudina is of particular interest since its fossil tubes show evidence of calcification. With the ability to precipitate calcium carbonate, this worm exhibited perhaps an early step toward the development of skeletal elements. Another species, the sedentary Tribrachidium, also shows signs of a mineralized skeleton.

Other late-Precambrian, or Vendian, organisms show marked similarities with animals of the Cambrian period. Charnia, a large and flat Ediacaran animal, had synapomorphies with the pennatulaceans, or sea pens, which are colonial coral-like organisms. Eoporpita is an animal with radially oriented tentacles that may demonstrate a phylogenetic relationship to Coelenterates, which also show radial symmetry. The disk-shaped Arkarua adami exhibits some primitive traits akin to echinoderms, which encompass the spiny, marine invertebrates. Several geologists and biologists believe the small, disk-shaped Beltanelliformis has ties to the sea anemones, because it is "strikingly similar to the base of some anemones and especially to the bases of Paleozoic burrows generally attributed to anemones," said geologist Guy Narbonne of Queen's University in Ontario, Canada. Likewise, other Ediacaran animals show some of the characteristics of trilobites or of sponges.

Several species show indications of relationships to arthropods. Spriggina had the head shield and segmented body that make it a likely arthropod ancestor. Parvancorina exhibited segmentation, a shield-like, ridged carapace, and at least 10 paired appendages as well as more than a dozen thinner appendages toward the rear. Noted Narbonne, "The similarity of Parvancorina to the Paleozoic arthropods of the Marellomorpha group may indicate that it is close to the ancestors of the Crustacea."

One of the most compelling Ediacaran fossils is Kimberella. Originally thought to be a box jellyfish, researchers now describe it as "a mollusc-like bilaterian organism" complete with a shell, a foot like the one that molluscs use for locomotion, and possibly a mouth. The authors of the research assert, "We conclude that Kimberella is a bilaterian metazoan … plausibly bearing molluscan synapomorphies such as a shell and a foot…. This interpretation counters asser tions that the Ediacaran biota represents an extinct grade of non-metazoan life."

Failed Experiment?

Did the Ediacaran animals give rise to modern metazoans? The fossil evidence and scientific comparisons say that some did. Many species likely died off before or near the beginning of the Cambrian, some perhaps succumbing to predation or competition with the new species associated with the Cambrian explosion. Many others, however, survived and became the rootstock for modern animals. The research has clearly shown that the Ediacaran animals were in the right place at the right time to spark the Cambrian animal diversification, and exhibit striking similarities to some important features of Cambrian metazoans.

Although the geologic record is less than perfect and in some cases downright scant in the case of Ediacaran organisms, fossil finds and subsequent analyses have provided ample fodder against discounting the role of these soft-bodied Vendian animals in the Cambrian diversification. The link between the Ediacaran and Cambrian biota implies that the animal "explosion" so long described as a strictly Cambrian event may have started long before with the Ediacaran biota. Without a doubt, the debate over the role of the Ediacaran fauna in the evolutionary tree of life will persist for many years, and scientists will continue their quest for new fossils, additional insights through scientific investigations, and ultimately a better view into the past.

—LESLIE MERTZ

Viewpoint: No, the late Precambrian life forms (Ediacaran biota) are not related to modern animals.

Before 1946, a huge gap existed in the fossil record. There seemed to be no complex animal fossils at all until the start of the Cambrian period, about 544 Mya, when nearly all animal phyla now on Earth appeared in a relatively short time span. Because of this gap, many people thought either that no animals evolved before the Cambrian, or that Precambrian animals couldn't leave fossils because they had no shells or hard skeletons.

Ancient Life Revised

Then, in 1946, while exploring an abandoned mine in the Flinders Range of mountains near Adelaide, Australia, in the Ediacara (an Aboriginal expression meaning "vein-like spring of water") Hills, mining geologist Reginald Sprigg found fossilized imprints in quartzite and sandstone of what looked to have been a collection of soft-bodied organisms. Some were disk-shaped and looked like jellyfish, others resembled segmented worms, and some had odd, unrecognizable forms.

At first Sprigg thought the Ediacara fossils might be from the Cambrian (named for Cambria, the Roman name for Wales, where rocks of this age were first studied). But later work showed them to predate the Cambrian by about 20 million years or more. The fossils dated from the late Precambrian, between 650 to 544 Mya—a period variously known as the latest Proterozoic, the Vendian, or the Ediacaran.

The Proterozoic (meaning "early life") was the last era of the Precambrian, between 2.5 Bya and 544 Mya. Fossils of primitive single-celled and more advanced multicellular organisms appeared abundantly in rocks from this era. The Vendian (sometimes called the Ediacaran) was the latest period of the Proterozoic era, also between 650 and 544 Mya.

The first Ediacaran grouping was recorded, although not by that name, in Namibia (near Aus) in 1933, and reports of other Precambrian soft-bodied fossils had appeared in the scientific literature from time to time beginning in the mid-nineteenth century. But the fossils Sprigg found in the Ediacara Hills were the first diverse, well-preserved assemblage studied in detail. Since his discovery, paleontologists have found Ediacaran-age fossils at more than 30 places around the world, on every continent except (so far) Antarctica.

The best known Ediacara locations are Namibia, the Ediacara Hills in southern Australia, Mistaken Point in southeast Newfoundland, and Zimnie Gory on the White Sea coast of northern Russia. Fossils have also been found in Mexico, central England, Scandinavia, Ukraine, the Ural Mountains, Brazil, and western United States.

Some Ediacaran fossils are simple blobs that could be almost anything. Some are like jellyfish, worms, or perhaps soft-bodied relatives of arthropods. Others are harder to interpret and could belong to extinct phyla. Vendian rocks also hold trace fossils, possibly made by segmented worm-like animals slithering over sea mud. Trace fossils record past behavior and can include tracks, trails, burrows, and any other mark made by an animal or plant.

Analyzing trace fossils (a discipline called ichnology) has been called the Sherlock Holmes approach to paleontology. Even worms, whose bodies have no hard parts to become fossilized, can inscribe into sediments many kinds of trace, track, trail, and burrow patterns. Such patterns have been important in interpreting the Ediacaran fossils, because they tell about the nature of original surfaces and environments, and help paleontologists reconstruct the behavior and evolutionary transformations of soft-bodied life forms.

Today most paleontologists divide Ediacara fossils into four general groups, but there is no consensus on whether the fossils are animals, modern or otherwise. The groups are i) jellyfish or sea anemones, ii) frond-like organisms, iii) trace fossils of possible shallow burrows in mud, and iv) unusual forms.

After decades of research and investigation, paleontologists, paleobotanists, and paleogeologists still do not agree about what this group of fossils represents. Over time the fossils have been theorized to be ancestors of modern animals, algae, lichens, giant protozoans, and a separate kingdom of life unrelated to anything alive today.

Default Assumption

From the first discovery of Ediacaran fossils in Australia, many investigators assumed that because the Ediacarans were preserved together and had certain similar characteristics, they must be the same sort of life form—that all the Ediacara fossils are members of the same high-level taxon and must fit the same plan of construction. But this default assumption is starting to crumble. According to Ben Waggoner, assistant professor of biology at Central Arkansas University in Conway, the long-standing debate about the nature of Ediacaran biota no longer has two sides—it has several.

Mistaken Point

In 1967 geologist S.B. Misra discovered great numbers of unusual fossils of late Precambrian age on exposed rock surfaces along the southern coast of Newfoundland's Avalon Peninsula. The most famous area is Mistaken Point, a crag at the southern tip of the Peninsula where more than 50 ships have wrecked. The rock slabs Misra saw held many imprints of soft-bodied organisms. Some resembled Ediacaran fossils found elsewhere, especially Charnwood Forest in central England, but most were unique to Mistaken Point and didn't look like any known life form.

The Mistaken Point fossils are also unique because they were preserved in large numbers in layers of fine volcanic ash, which created snapshots of the sea floor at the time they were preserved. And unlike most other Ediacaran biota, the Mistaken Point life forms seem to have lived in deep water, far below sunlight or surface waves.

Typical Mistaken Point biota were large frond-like leafy forms: some had stalks; some were bushy or cabbage-like; others had branching, tree-like or network-like shapes; and some looked like spindles and were long and pointed at both ends. There were also lots of large, lumpy disk-shaped fossils. Thanks to dating techniques for volcanic ash, the Mistaken Point fossils were shown to be 565 million years old, so far the oldest complex Ediacara fossils to be accurately dated.

There is at least one argument for the non-animal nature of the Mistaken Point fossils. Some Ediacara sites have trace fossils, which represent unambiguous evidence that animals were present at the time, although in most cases pale-ontologists can't link the trace fossils to specific animal body fossils. One exception is Newfoundland—not a single trace fossil has been found at the Ediacara sites there. This could mean that none of the fossil forms discovered there were living animals.

Quilted Organisms and Metacellularity

By the 1980s paleontologists had described more than two dozen species from around the world, along with tracks on the sea floor, and more detailed studies and reconstructions of the life forms showed that some were not similar to jellyfish, corals, or worms.

At that time at least two researchers—German paleontologist Adolf Seilacher, a professor at Yale University and the Geological Institute at Tübingen University in Germany, and Mark McMenamin, a geology professor at Mount Holyoke College in Massachusetts—argued that the Ediacarans were unrelated to any living organisms and represented a new kingdom that fell victim to a mass extinction at the Vendian-Cambrian boundary.

Seilacher hypothesized that they were a failed experiment of evolution unrelated to modern animals. He called them vendobiota—a name some still use for the Ediacaran biota—and described them as large, thin, quilted air-mattress-like life forms.

Some experts, including Simon Conway Morris, professor of evolutionary paleobiology at the University of Cambridge, find the quilted Ediacaran structure to be unlike any animal form in existence today, and say it is "genuinely difficult to map the characters of Ediacaran fossils onto the body plans of living invertebrates; certainly there are similarities, but they are worryingly imprecise."

Because the fossil forms had no mouth, teeth, or jaws for biting or chewing, and no gut or way to eliminate waste, Seilacher suggested they took nutrients and handled wastes by diffusion from and into the sea water they lived in, or from internal organisms that lived by photosynthesis (drawing energy from sunlight) or chemosynthesis (drawing energy from chemical reactions). Mark McMenamin created the phrase "garden of Ediacara" to embrace this concept.

In a 1998 book of the same name, McMenamin theorized that the Ediacaran organisms were multicellular and related to animals, but not to those in existence today. According to his hypothesis, their body organization—a simple symmetry that divided their bodies into two, three, four, or five parts—was established early in development by individual cells that gave rise to separate populations of cells, which then grouped together to form the organisms.

Ancient Lichens

But not everyone thinks the Ediacaran fossils necessarily belong to animals, extinct or otherwise. In 1994, University of Oregon paleobotanist Greg Retallack published an article in Paleobiology proposing that the Ediacarans were huge ancient lichens—symbiotic groups of algae and fungi—and that at the time there were "great sponges of vegetation all over, draping the landscape."

Retallack compared their thickness to that of much younger tree trunk fossils, and said the Ediacaran fossils seemed to have resisted compaction after they were buried, much like sturdy logs. Several features suggested that they were lichens—their size (up to 3.3 ft [1 m] across) and sturdiness; the lack of a mouth, digestive cavity, or muscle structure; and evidence from microscopic examination.

Rock lichens do live in the sea (shallow-water lichens) and on land, but some paleontologists who otherwise find Retallack's notion plausible for some of the Ediacaran life forms question whether lichens could have survived in the deep-sea Vendian deposits, like those found at Mistaken Point, where the deep sea environment was dark, rich in methane or sulfur, and low in oxygen.

Continuing Debate

Paleontologists have debated the nature of the Ediacara fossils over the years since the fossils were discovered for two reasons: No one knows how or whether they are related to the Cambrian explosion of life forms, and no one knows how this group of apparently soft-bodied organisms came to be preserved in the first place. These questions have yet to be answered to everyone's satisfaction.

Even today, some paleontologists think at least some of the Ediacaran fossils could be related to Early Cambrian life forms and so to modern animals, and some agree with Seilacher and others that the fossils represent a failed experiment of evolution.

The question about preservation holds a potential argument for the failed-experiment view. If the Ediacaran life forms are unrelated to modern animals, their bodies may have been made of cells and tissues that differed substantially, and were more enduring, than those of soft-bodied creatures alive today.

Some taxa (named groups of organisms) are now known to have lived into the Cambrian period—since 1990 there has been a growing number of Cambrian-age discoveries of Ediacaran fossils from Cambrian deposits—and others may have evolved into different forms.

But most Ediacarans vanished from the fossil record near the beginning of the Cambrian. Some experts think this is evidence of mass extinction; others propose that conditions in the environment, especially the appearance of hard-shelled predatory animals with claws that could crawl and burrow, may have contributed over time to their decline.

—CHERYL PELLERIN

KEY TERMS

ARTHROPOD:

The largest group of animals in the animal kingdom, characterized by a outer skeleton (exoskeleton) and jointed body parts (appendages). These include spiders, scorpions, horseshoe crabs, crustaceans, millipedes, centipedes, and insects.

CAMBRIAN:

The earliest period of the Paleozoic era, between 544 and 505 Mya. It is named after Cambria, the Roman name for Wales, where rocks of this age were first studied.

CNIDARIA:

Jellyfish, corals, and other stingers. The name comes from the Greek word cnidos, for stinging nettle. Cnidarian forms are very diverse, but all are armed with stinging cells called nematocysts. Thousands of species live in the oceans, from tropics to poles and from surface to bottom; a smaller number live in rivers and fresh-water lakes.

EDIACARAN FAUNA (ALSO VENDIAN BIOTA):

Fossils first found in Australia's Ediacara Hills and later worldwide that represent the first true multicellular animals at the end of the Precambrian era. They have been variously considered algae, lichens, giant protozoans, and a separate kingdom of life unrelated to anything now living.

PHANEROZOIC:

The time between the end of the Precambrian and today. The Phanerozoic (also called an eon) starts with the Cambrian period, 544 Mya, and encompasses the period of abundant, complex life on Earth.

PRECAMBRIAN:

All geologic time before the beginning of the Paleozoic era. This includes 90% of all geologic time—from the formation of Earth, about 4.5 billion years ago, to 544 Mya. Its name means "before Cambrian."

PROTEROZOIC ("EARLY LIFE"):

The last era of the Precambrian, between 2.5 billion and 544 Mya. Fossils of both primitive single-celled and more advanced multicellular organisms begin to appear in abundance in rocks from this era.

STRATIGRAPHY:

The branch of geology concerned with the formation, composition, ordering in time, and arrangement in space of sedimentary rocks.

SYNAPOMORPHY:

A shared (derived) trait, which suggests that two organisms are related evolutionarily.

TRACE FOSSILS:

Record of the past behavior of organisms and include tracks, trails, burrows, and any other mark made by an animal walking, running, crawling, resting, or feeding on or within soft sediment such as sand or mud. Plants also leave traces.

VENDIAN:

The latest period of the Proterozoic era, between 650 and 544 Mya, and sometimes called the Ediacaran period. The Vendian is distinguished by fossils representing a characteristic collection of complex soft-bodied organisms found at several places around the world.

Science in Dispute