Cambrian “Explosion”—Or “Slow Fuse”? 177(fig. 5.6). By the Tommotian Stage (530 million years ago), a slow trickle of other groups of
larger invertebrates began to appear, including the first “lamp shells” (brachiopods), and
also members of an extinct spongelike group known as archaeocyathans. Diversity in the
Tommotian reached only about 50 genera, about the same as in the Vendian. In addition, the
sediments of the earliest Cambrian show abundant burrowing, proof that many other types
of soft-bodied worms with a true internal fluid-filled cavity (a coelom) must have lived at that
time. Thus the earliest Cambrian shows evidence of a gradual increase in diversity from the
Vendian, but no “explosion.”
Step 4: In with a Whimper, Not a Bang
Was there really a Cambrian Explosion? Some have treated the issue as semantic—
anything that plays out over tens of millions of years cannot be “explosive,” and if
the Cambrian animals didn’t “explode,” perhaps they did nothing at all out of the
ordinary. Cambrian evolution was certainly not cartoonishly fast. . . . Do we need to
posit some unique but poorly understood evolutionary process to explain the emer-
gence of modern animals? I don’t think so. The Cambrian Period contains plenty of
time to accomplish what the Proterozoic didn’t without invoking processes unknown
to population geneticists—20 million years is a long time for organisms that produce
a new generation every year or two.
—Andrew Knoll, Life on a Young PlanetThe third stage of the Early Cambrian is known as the Atdabanian Stage (515–520 million
years ago), and with this stage, we finally see a great increase in diversity: more than 600
genera are recorded (fig. 7.4). However, this number is misleading and a bit inflated. Most
of the genera are trilobites, which fossilize readily and so greatly increase the volume and
diversity of large shelly fossils. Most of the other animal phyla had already appeared by
this time (including mollusks, sponges, corals, echinoderms) or would appear later in the
Cambrian (vertebrates) or even in the Ordovician Period that followed (e.g., the “moss ani-
mals” or bryozoans).
The second misleading aspect of this apparent diversity “explosion” is that during the
Atdabanian Stage we get the first good fauna of soft-bodied fossils (the Chinese Chengjiang
fauna), so we get the apparent (but not real) first appearance of phyla only known from soft
tissues. Then in the Middle Cambrian we have the extraordinary soft-bodied preservation of
fossils from places like the Burgess Shale in Canada (fig. 7.5). As pointed out by Stephen Jay
Gould in his book Wonderful Life (1989), the soft-bodied animals preserved in these amaz-
ing deposits allow us to see what the normal fossil record is missing. We have many bizarre
wormlike and odd fossils, many of which don’t fit into any living phylum. Some, like the five-
eyed nozzle-nosed Opabinia (fig. 7.5, top left) or the soft flowerlike Dinomischus, are complete
mysteries to zoologists. Others are apparently soft-shelled arthropods. One fossil, appropri-
ately named Hallucigenia (fig. 8.17B), was a bizarre creature that seemed to have tentacles or
spikes on a wormlike body, until recent better fossils from China showed it is related to the
“velvet worms,” phylum Onychophora (discussed in chapter 8). The largest predator (about
2 feet long) was a soft-bodied swimmer known as Anomalocaris, which had a strange mouth
that looked like a pineapple slice and was originally found and misinterpreted as a sea jelly.