CAMBRIAN LIFE
There can be no mistake as to the prolific development of life in Cambrian seas, for fossils of this age are to be found in many parts of the world, where ancient sea bottoms now form part of the land surface. Invertebrate animals appear to have made much progress, but plants were either scarce or too small and delicate to be productive of fossils. It is probable, however, that seaweeds and other algae were flourishing along with the invertebrates, because animal life is directly or indirectly dependent on the existence of plants. The latter sustain themselves by taking carbon and nitrogen from air, water, and soil, but animals must obtain their requirements by eating plants or eating each other. They cannot obtain what they need from the inorganic world without this help from the vegetable kingdom.
One group of animals stands out prominently above all its contemporaries. Known as the trilobites they were by far the most distinguished and most characteristic of Cambrian invertebrates. Trilobites inhabited the warmer seas of this period and several later ones, but were extinct by the end of the Paleozoic era. Hundreds of species have been described, most of them under four inches in length. Well-known distant relatives now living are the shrimps, and other crustaceans. The name Trilobite has reference to the three lobes which are apparent in the form of the upper surface, the central lobe forming a broad ridge extending along the back. Beneath the outer lobes on each side there was, during life, a row of short, jointed legs used for swimming and walking, but these delicate appendages are seldom preserved in the fossils.
Second in importance among the animals of the period were the brachiopods or lamp-shells, not true mollusks although they were provided with similar shells composed of calcium phosphate or calcium carbonate. Shells are of two parts (bivalved) as in the case of clams, but the valves are above and beneath the body instead of on the right and left sides, which is the arrangement among mollusks. Although abundant as individuals, there were only a few species during the earlier part of the period; the number of species increased, however, and the race became very persistent. About seven thousand species have been described, and the race is not yet extinct although the number of living species is relatively small.
Cambrian life evidently included representatives of all the great divisions of invertebrates; sponges, jelly-fishes, worms, and primitive corals have been reported. At the end of the period there was an elaborate molluscan fauna. The closing of the period in North America was apparently a gentle elevation of continental areas and a consequent withdrawal of the sea.
Invertebrate Fossils
Only a few prominent types have been selected from thousands of invertebrates known to zoologists. The forms illustrated are of frequent occurrence as fossils.
CRINOIDS CEPHALOPODS Coiled types Ammonite Scaphite Straight-shell type Baculite TRILOBITE BRACHIOPODS BIVALVES Inoceramus Oyster GASTROPODS Snail-like Univalves PROTOZOA UNICELLULAR FORMS Radiolaria (Microscopic) Fusulina limestone Foraminifera (Enlarged) MULTICELLULAR FORMS Cup coral Reef coral Sponge Bryozoa