176 Evolution? The Fossils Say YES!
Some of the little shellies look like simple cap-shaped or coiled mollusks; others look
like primitive clams (fig. 7.3). Many others are simple tubular or conical fossils whose con-
nections to any living group are hard to establish. Many of the fossils look like miniature
jacks or little spiky Christmas ornaments. These appear to have been part of the “chain-mail
armor” that studded the skins of larger organisms, such as the sponge Chancelloria, that were
soft-bodied except for these little spiky objects (much like the tiny spikes in the skin of a
shark or a sea cucumber). Most of the little shellies were made of calcium phosphate, the
same mineral that makes up the bones of vertebrates. Today, most marine invertebrate shells
are made of calcium carbonate (the minerals calcite and aragonite). To some scientists, this
suggests that some sort of environmental condition (such as low atmospheric oxygen) made
it hard to secrete calcite skeletons, but phosphate skeletons were easier to produce. They sug-
gest that the appearance of large calcified trilobites and other fossils reflects the point where
atmospheric oxygen passed a critical threshold and became abundant enough to allow this
chemical mineralization.
Whatever the reason, for almost 25 million years, the Cambrian explosion was burning
on a slow fuse. The little shellies were abundant, but larger fossils were not. The earliest
sponges had already appeared back in the late Ediacaran, but this is not surprising, consider-
ing that all lines of evidence show that sponges are the most primitive animals alive today
FIGURE 7.3. The earliest stages of the Cambrian (Nemakit-Daldynian and Tommotian) do not produce trilobites
but are dominated by tiny phosphatic fossils nicknamed the “little shellies.” Some may have been mollusk
shells (E, H, and I), while others are apparently sponge spicules or pieces of the “chain-mail armor” of larger
creatures, such as worms. (A) Cloudina hartmannae, one of the earliest known skeletal fossils, from the same
beds that produce Ediacaran fossils in China. (B) A spicule of a calcareous sponge. (C) A spicule of a possible
coral. (D) Anabarites sexalox, a tube-dwelling animal with triradial body symmetry. (E) A spicule from a
possible early mollusk. (F) Lapworthella, a cone-shaped fossil of unknown relationships. (G) A skeletal plate of
Stoibostromus crenulatus, another creature of unknown relationships. (H) Skeletal plate of Mobergella, a possible
mollusk. (I) Cap-shaped shell of Cyrtochites, another possible mollusk. All scale bars = 1 mm. (Photos courtesy
S. Bengston)
(A) (B) (C) (D) (E)(F) (G) (H) (I)