Geology For Dummies
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The beginning of the Paleozoic is marked by the sudden appearance of a wide variety of animal forms in the geologic record. In fact, the fossils from this period exhibit all the animal body plans that exist even today, 540 million years later. (A body plan is how an organism’s body parts and growth patterns are organized.) This sudden appearance of complex life in the geologic record is called the Cambrian Explosion.

The Cambrian Explosion has long been defined by the abundance of creatures preserved in the fossil record at the beginning of the Cambrian period. But it is likely that this sudden appearance of life is a result of the incomplete nature of a history told in rocks. Rather than documenting the first appearance of complex life (which was already present), the Cambrian Explosion documents an important new adaption for life: the building of shells.

Toughen up! Developing shells

In the Cambrian, creatures living in the sea began to grow shells or exoskeletons. This gave them a tremendous advantage, both during their lifetimes and for being preserved in the geological record. As you can see today, this way of life has lasted for millions of years. External hard parts provide the following benefits:
  • Protection from the sun: During the Paleozoic, immense shallow seas were the primary habitat for life on Earth. Soft-bodied creatures were exposed to the sun’s harmful rays — the same rays you and I avoid with sunscreen and hats. Building an exoskeleton protects the soft tissues and internal organs of a creature from being damaged by the sun.
  • Moisture retention: Large, shallow water environments sometimes experience an occasional absence of water — similar to the beach at low tide. Animals that become stranded when the tide goes out will dry out and die unless they have a shell that retains enough moisture to help them survive until the tide comes back in.
  • Muscular support: Building an exoskeleton provides a framework for muscles to attach themselves to. This function is the same one your internal skeleton provides. Because an exoskeleton provides structure for muscle attachments, it allows an organism to grow larger than it would without such a supporting structure.
  • Protection from predators: Possibly the most important advantage a shell provides is protection from other animals that may attack and devour a soft-bodied creature.

While all of these are great reasons to build an exoskeleton, scientists are not certain which advantage started the evolutionary trend toward external hard parts. Evidence in the fossil record of creatures with damaged shells indicates that they were being hunted, attacked, and probably eaten by predators. For some scientists, this fact is enough to conclude that predation was the driving force for the evolution of exoskeletons at the beginning of the Paleozoic.

Ruling arthropods of the seafloor: trilobites

The first shelly fauna, or animals with exoskeletons, were tiny creatures; their shells were only a few millimeters in size. But it didn’t take long for other animals to follow the shelly trend. The most famous creature of the Paleozoic — possibly its mascot — is the trilobite.

Trilobites are arthropods, which today include insects, spiders, and crustaceans such as lobsters. Species of trilobites filled every nook and cranny of the ocean throughout the Paleozoic, but they did not survive the end-Permian extinction.

Trilobites had an exoskeleton that is segmented into parts, similar in appearance to roly polys (pill bugs) but with a horseshoe-shaped head segment. They ranged in size from itty bitty (just a few millimeters) to more than 50 centimeters (almost 2 feet) in length, but most of them were around 5 to 10 centimeters (about 3 to 4 inches). Some were blind, others had compound eyes (like some insects today), and certain species could roll up just like a roly poly, presumably for protection.

While trilobites continued to cover the seafloor throughout the Paleozoic, they were most diverse during the Cambrian period and begin to be overshadowed in the fossil record by the development of other types of creatures later in the Paleozoic.

A trilobite. A trilobite.

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Alecia M. Spooner has been teaching at the college level for more than 15 years. She currently teaches at Seattle Central College, where she is Professor of Earth and Environmental Sciences. Alecia teaches earth science courses that are accessible and engaging, while stressing scientific literacy and critical thinking.

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