Fossils and the Rock Record
An amazing diversity and abundance of fossils abruptly appeared in the rock record during the beginning of the Cambrian Period. Known as the "Cambrian explosion," this time in Earth’s geologic
history represented a massive change in life from that of the Proterozoic.
Early Paleozoic Life
Organisms representing all but one of the major marine phyla showed up in the rock record of the Cambrian Period. Phyla is the plural form of the word phylum, the second-highest category in the classification
system of organisms. In particular, the development of mineralized skeletons marks the Cambrian explosion. Recall that mineralized skeletons, shells, and bones are hard parts, and all nonmineralized structures
of organisms, such as tissue, are soft parts. The development of hard parts by organisms in the Cambrian greatly increased the likelihood that an organism would become fossilized—hard parts are much more
resistant to decay than soft parts are.
Some paleontologists suggest that the Cambrian explosion was not a true explosion of new life, but rather a reflection of the development of hard parts by organisms. These paleontologists hypothesize
that many of the phyla existed in the Proterozoic, but that they are not represented as fossils because they consisted only of soft parts and therefore were not preserved. Other paleontologists suggest
that the Cambrian explosion truly was a period of rapid diversification of organisms. The final answer will likely contain elements of both hypotheses.
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Perhaps the best known Cambrian organisms that developed hard parts were trilobites. Trilobites are common in Paleozoic rocks, and particularly common in Cambrian-aged rocks. They are members of the
phylum Arthropoda, the joint-legged animals that include insects, spiders, scorpions, ticks, mites, shrimp, crabs, and lobsters. Like other arthropods, trilobites had an external skeleton. The
skeletons of most arthropods, however, are made only of organic material, whereas trilobites’ skeletons were made of both the mineral calcite and organic material. This made them easily preserved as fossils.
Trilobites had a head region known as a cephalon, a torso region called a thorax, and a tail region called a pygidium. The thorax consisted of three lobes—an axial lobe in the middle flanked on both sides
by pleural lobes. These lobes gave the organism its name—trilobite means "three-lobed." Some species of trilobites swam or floated, but most spent their lives crawling along the seafloor eating food particles.
Brachiopod fossils are also common in Cambrian rocks. A brachiopod has two clam-like shells that protect it from predators, but its anatomy is much different than that of a clam. The most striking
difference is that brachiopods feed by filtering food from the water that passes through their shells. They capture food particles on a curled, feather-like structure called a lophophore. Brachiopods
hold themselves to the seafloor or burrow with a fleshy stalk called a pedicle, which extends out near the hinge between the two shells. The brachiopods preserved in early Cambrian rocks are known as
inarticulate brachiopods. Ligaments held the shells of these brachiopods together. Articulate brachiopods became abundant later in the Paleozoic; they have tooth-like structures that fit into sockets
to help hold their shells together. Both types of brachiopods still exist today. Some Cambrian inarticulate brachiopods are indistinguishable from the living inarticulate brachiopod Lingula, a
brachiopod that has remained virtually unchanged for the entire Phanerozoic!
In addition to early arthropods and brachiopods, mollusks first appeared in the late Proterozoic and became relatively common in the Cambrian. Mollusks include such organisms as clams, snails, octopus,
and squids. The earliest mollusks had simple anatomies and tube, cap, and cone-shaped shells. By the Middle Cambrian, true snails and clams had appeared.
There were no true corals during the Cambrian. There was, however, one unusual group of organisms called archaeocyathids that did build reef-like structures such as corals do. Their shells were double-walled
and cone-shaped. Some archaeocyathids were more than a meter in length, and they grew together in clusters that formed reef-like structures. There is still some controversy regarding what type of organisms
they were. Some scientists classify them as sponges, whereas others assign them their own unique classification. Regardless, archaeocyathids likely had the same ecological niche that sponges and corals
later filled. The rein of the archaeocyathids in the shallow seas was relatively short-lived. By the early Middle Cambrian, this interesting group of organisms had become extinct.
Another major group of organisms that first appeared in the Cambrian were the echinoderms, or spiny-skinned animals. This group of organisms includes starfishes, sea urchins, sand dollars, sea cucumbers,
and crinoids. Echinoderms are unique in that they are composed of individual calcite plates, and have five-fold symmetry, as in the five arms of a starfish and the five petal-like structures on a sand
dollar. Additionally, they have internal hydraulic systems that pump water through their bodies. The earliest echinoderms did not have this five-fold symmetry, and some were quite bizarre in shape, such
as the spiral Helicoplacus. Scientists are not sure how Helicoplacus lived. This type of early experimentation in body shape, however, is characteristic of the Cambrian explosion.
Select an modern animal such as an elephant or lion, and research its anatomy. Determine which parts of the animal would likely be preserved as fossils.