Trilobites were the most successful species of the Cambrian period
The Cambrian period spanned the time between 543 to 490 million years ago. During the four billion years prior to the start of this period, the only life forms that had evolved were plankton, bacteria and multi-celled algae. The Cambrian period marked an era of increasingly favorable planetary conditions, which encouraged an "explosion" of life. Many species of animals and plants evolved during the Cambrian period.
Trilobites
Trilobites (so-called because of the three lobes that formed their body) were arthropods. Arthropods have an external skeleton, a segmented body and jointed appendages. There were many different trilobites during the Cambrian period and most were predators or scavengers living on the seabed. Some were able to swim and fed on plankton. Trilobites are thought to be the first life forms to develop complex eye structures. Trilobites were the most successful and diverse creatures at the time, but by the end of the Cambrian period the advancing glaciers and subsequent changes in the chemistry of the sea caused many to become extinct.
Brachiopods
Brachiopods were diverse sea-floor dwellers during the Cambrian period. Brachiopods were invertebrates that had clam-like shells, although they are not related to the modern clam. Some brachiopods had a long fleshy appendage which they used to anchor themselves to the sea bed. All brachiopods are filter-feeders. More brachiopods than trilobites survived after the end of the Cambrian period and brachiopods have a greater number of living descendants.
Sponges and Coral
Descendants of ancient coral can still be found today.
Stromatolites are coral-like structures created during the Cambrian period by colonies of bacteria. True corals and sponges also developed during this period. Most corals and sponges fed on bacteria and other food particles in the water. Descendants of Cambrian sponges and corals exist today.
Other Animal Life Forms
A number of other life forms thrived during the Cambrian period, such as various mollusks, nautiloids and echinoderms. The ancestors of modern echinoderms like starfish, sea urchins and sand dollars evolved during the Cambrian period. Some nauliloids looked like long, thin ice-cream cones. Types of nautiloid were numerous during this period and a few species exist today. They are often called the "living fossil" as their structure has changed little since they first evolved.
Algae
Plant life was limited to single and multi-celled green algae during the Cambrian period. Land was still a hostile environment so all plant species evolved in the sea. Single-celled algae grew together in colonies, giving them the appearance of being one plant. Some Cambrian multi-celled algae looked like modern kelp, or seaweed.
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