Cambrian “Explosion”—Or “Slow Fuse”? 175Still other hypotheses (such as the ideas that they are lichens, proposed by Greg Retallack)
have been put forth. Unfortunately, because the Ediacarans are known entirely from the
impressions on the soft sea bottom and not from any body fossils with internal organs or
other important features, it is very difficult to resolve this controversy. Whatever the bio-
logical affinities of the Ediacara fauna, it is very clear that they are multicellular organisms,
whether animals, plants, fungi, or some early experimental kingdom not in any living group.
Even more intriguing is the fact that some molecular clock estimates of the diver-
gence times of the major invertebrate groups (fig. 5.7) place the branching points of the
major invertebrates as old as 800 to 900 million years ago (Runnegar 1992; Wray et al.
1996; Ayala et al. 1998). We still do not have any fossils nor even any undisputed bur-
rows or other evidence to support this prediction, but either way, it is clear that advanced
multicellular life (but still soft-bodied without any trace of fossilizable skeletons) was on
earth 600 million years ago (more than 50 million years before the Cambrian), and pos-
sibly as early as 900 million years ago.
Step 3: The Little Shellies
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 geologi-
cal resources in Siberia after the end of World War II. There, above thick sequences
of Precambrian sedimentary rocks, lie thinner formations of early Cambrian sedi-
ments 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 1/2 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).
—Jack Sepkoski, The Book of LifeIf the soft-bodied multicellular (but nonskeletonized) Ediacara fauna represents the next
logical step up from single-celled life, then the next step beyond that would be the appear-
ance of mineralized, fossilizable skeletons. But if life took almost 3 billion years to develop
the ability to mineralize shells, we expect that it would be a difficult process and would not
arise fully fledged. Sure enough, the earliest stages of the Cambrian (known as the Nemakit-
Daldynian and the Tommotian stages, from 520 to 545 million years ago) are dominated by
tiny (only a few millimeters) fossils nicknamed the “little shellies” or the “small shelly fos-
sils (SSFs)” in the trade (fig. 7.3). For decades, these little fossils were overlooked as people
hunted the beds above them for the more spectacular trilobite fossils. But as the quotation
from Sepkoski points out, the Soviets were the first to study their long and detailed sequences
of Proterozoic and Cambrian sediments in detail and named the stages of the Cambrian. And
when they looked closer at the beds below the trilobites and took samples back to the lab to
dissolve in acid or slice into thin sections, it became apparent that these long-neglected beds
were chock-full of tiny fossils.