Cambrian “Explosion”—Or “Slow Fuse”? 173Step 2: The Garden of Ediacara
Aspiring paleontologists are typically attracted to the large, flashy specimens such as
carnivorous dinosaurs and Pleistocene mammals. But to find the real monsters, the
weird wonders of lost worlds, one must turn to invertebrate paleontology. Without
question the strangest of all fossilized bodies are to be found among the Ediacarans.
—Mark McMenamin, The Garden of EdiacaraThe next step in our progression from single-celled life to Cambrian trilobites is the appear-
ance of fossils of multicellular life. Contrary to the creationists’ myths, we have abundant
fossils in rocks older than the Early Cambrian (prior to 545 million years ago). Some of
these date to 600 million years ago, and they are known as the Ediacara fauna (pronounced
“Ee-dee-AK-ara”). This period of time from 600 million to the beginning of the Cambrian
545 million years ago is known as the Ediacaran Period of the Proterozoic Era. First discov-
ered in the Rawnsley Quartzite in the Ediacara Hills of Australia by Reg Sprigg in 1946, the
Ediacara fauna is now known from a wide variety of localities around the world, includ-
ing many spectacular localities in China, Russia, Siberia, Namibia, England, Scandinavia,
the Yukon, and Newfoundland. Most of these fossils (fig. 7.2A) are the impressions of soft-
bodied organisms without skeletons, so there are no hard parts that make up the bulk of
the later fossil record. Instead, these impressions have reminded some paleontologists (such
as Martin Glaessner, who studied the classic Australian Ediacara fauna) of the impressions
made by sea jellies, worms, soft corals, and other simple nonskeletonized organisms. Over
2,000 specimens are known, usually placed in about 30–40 genera and about 50–70 species,
so they were relatively diverse.
Although the Ediacara fauna clearly represents fossils of multicellular organisms
(some reach almost a meter in length), paleontologists have a wide spectrum of opinions
about what made these impressions. The more conventional interpretation (fig. 7.2D) is
that they are like fossils of groups we know today: sea jellies, sea pens, and worms of vari-
ous sorts. Some do look like sea jellies, but if so, they have symmetry unlike any living sea
jellies. Others vaguely resemble some of the known marine worms, although their sym-
metry and segmentation do not match any groups of worms alive in the ocean today. Nor
do the “worms” have evidence of eyes, mouth, anus, locomotory appendages, or even a
digestive tract.
For this reason, other paleontologists have suggested that the Ediacara fauna was com-
posed of organisms unlike any that are alive today. They point to the lack of modern patterns
of symmetry and the apparent large size of many of the fossils and argue that they are an
early failed experiment in multicellularity. Adolf Seilacher (1989), for example, calls them
the “Vendozoa” and suggests that they were constructed in a quilted or “water-filled air
mattress” fashion that maximizes surface area. Instead of using internal digestive and cir-
culatory systems to solve the problem of large multicellular bodies, Seilacher suggests that
these simple organisms had no internal organs but instead received all their nutrients and
oxygen and got rid of waste through the huge surface area of their outer membranes. Mark
McMenamin (1998) suggested that they housed symbiotic algae (as do many living large
invertebrates, such as reef corals and giant clams). In his “garden of Ediacara” hypothesis,
McMenamin suggests that the large surface area of the Ediacarans maximizes the area of
exposure of sunlight for these internal algae, which then help such large organisms metabolize.