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.
[Illustration: 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