The mollusks are a highly diverse group and left the most impressive fos-
sil record of all marine animals (Fig. 51). They make up the second largest of
the 21 basic animal groups, behind the arthropods. The phylum is so diverse
that paleontologists have difficulty finding common features among its mem-
bers.The three major groups are the snails, clams, and cephalopods. Snails and
slugs comprise the largest group and ranged from the Cambrian onward. An
ancient mollusk called kimberella (Fig. 52) was an odd Precambrian creature
shaped like agherkin cucumber and lived just before the Cambrian explosion.
The cephalopods, which include the squid,cuttlefish, octopus,and nau-
tilus, traveled by jet propulsion. They sucked water into a cylindrical cavity
through openings on each side of the head and expelled it under pressure
through a funnel-like appendage. Their straight, streamlined shells, up to 30
feet and more in length, made the nautiloids among the swiftest animals of the
ancient seas. The ammonoids were the most spectacular of marine predators,
with a large variety of coiled shell forms.
The arthropods are the largest phylum of living organisms, comprising
roughly 1 million species, or about 80 percent of all known animals.The giant
3-foot-long arthropods found in the middle Cambrian Burgess Shale Forma-
tion of western Canada represent one of the largest of all Cambrian inverte-
brates.Among the first and best known of the ancient arthropods were the
trilobites. The arthropod body is segmented, suggesting a relationship to the
annelid worms.Paired, jointed limbs generally present on most segments are
modified for sensing, feeding, walking, and reproduction.Figure 50Fossil casts of
brachiopods.
(Photo by E. B. Hardin,
courtesy USGS)
Historical Geology