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Weird wonders at the dawn of animal life

By Hans-Dieter Sues

Shortly after World War II, an Australian geologist named Reginald Sprigg discovered peculiar impressions on slabs of ancient quartzite in the Ediacara (pronounced “Ee-dee-acra”) Hills of South Australia.

These finds came from a level well below the rocks containing the oldest Cambrian fossils in the region. They did not fit the traditional notion that there were no large and complex life forms during the Precambrian, that vast interval of Earth’s history prior to the appearance of abundant multicellular life.

A report submitted by Sprigg to the leading scientific weekly Nature was quickly rejected, and initially nobody paid much attention to Sprigg’s discovery.

Starting in the 1950s, very similar fossils were found in other regions of the globe–England, Russia, Ukraine, Newfoundland, and North Carolina. (An earlier find of such remains had already been reported from what is now Namibia in the 1930s.)

500 million-year-old rocks

“Ediacarans” (as these organisms are collectively referred to) have now been recorded from some 40 localities worldwide and sedimentary rocks that range in age from 575 to 541 million years. A few rare finds are known from the Cambrian, but most ediacarans apparently vanished at the end the Precambrian.

Once researchers had accepted the Precambrian age of ediacarans, they compared these enigmatic fossils to various groups of present-day marine invertebrate animals, especially jellyfish, octocorals (sea pens), polychaete worms, and even arthropods and echinoderms.

Individual ediacarans were either “shoehorned” into one of these major groups or interpreted as their precursors. However, all ediacarans lack hard parts and any apparent structures for feeding and locomotion. Any similarities to later animals appear, for the most part, to be superficial.

Although only about 100 species have been described, ediacarans show a surprising range of unusual bodyplans.

Many are disk-shaped, often with a particular pattern of radial symmetry. Others are bilaterally symmetrical, with distinct front and back ends, and may have been mobile.


Photo by Smith609/Wikimedia Commons

However, some ediacaran bodyplans are unlike anything seen among present-day animals: The rangeomorphs typically have spindle-, leaf- or comb-shaped, often large structures composed of repetitive patterns of frondlets attached to a stem, which often has a holdfast at its base for attachment to the sea floor.

Superficially, rangeomorphs resembled present-day sea pens but their detailed structure and growth pattern are very different.

Finally, the ernettiomorphs have two rows of quilted tubes alternately arranged along a midline. They are typically ribbon- or bag-shaped.

No trace of mouth or gut

None of the ediacarans shows any trace of a mouth and gut. This has led to much speculation about their way of life and biological relationships.

Impressed by the unusual structure of ediacarans, some researchers argued that they were not even animals, interpreting them instead as lichens, relatives of fungi, giant protists, or even a unique kind of early multicellular life. However, all of these interpretations turn out to be inconsistent with what is known about the structure and development of ediacarans.

Other paleontologists suggested that at least certain ediacarans housed symbiotic algae and derived nutrients from the photosynthetic activity of their guests. However, the presence of these ediacarans in deeper-water settings (where light levels would be too low to permit photosynthesis) rules out that hypothesis.

Recent studies by a team of Canadian and American researchers have made a compelling case that ernettiomorph and rangeomorph ediacarans could have directly absorbed organic carbon from the seawater through their body surfaces.

This mode of nutrient acquisition is called osmotrophy and would explain the absence of a mouth and gut.

It is also consistent with the large surface area characteristic of these ediacaran bodyplans.

Ediacarans provide an important object lesson that life is far more diverse than even scientists can imagine.

Ediacarans provide an important object lesson that life is far more diverse than even scientists can imagine.

Ediacaran communities as dominated by a variety of sessile organisms that lived close to the interface between ocean water and sediment where the presence of microbial mats generated an abundance of dissolved nutrients.

In addition, some forms may have been mobile and grazed on the microbial mats, and trace fossils indicated the presence of others that burrowed into the sediment.

Many modes of feeding found in later marine animals had apparently not yet evolved.

Ediacarans represent a range of probably only distantly related organisms just before the “Cambrian explosion” 542 million years ago when multicellular animals suddenly appeared in great diversity and abundance. They may have included the stem-forms of at least some of the later groups of marine animals.

Despite much recent progress in research, much remains to be learned both about these intriguing life forms and the environmental conditions that led to the “Cambrian explosion.”

Hans-Dieter-Sues.jpgHans-Dieter (Hans) Sues is a vertebrate paleontologist based at the National Museum of Natural History in Washington, D.C. He is interested in the evolutionary history and paleobiology of vertebrates, especially dinosaurs and their relatives, and the history of ecosystems through time.

A former member of the National Geographic Committee for Research and Exploration, Hans has traveled widely in his quest for fossils and loves to share his passion for ancient life through lectures, writings, and blogging.

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