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PHYLUM MOLLUSCA
The mollusks are a large group of animals that are
still abundant. These invertebrates
are mostly shelled organisms. Their
hard parts often formed fossils. Most
are made of the minerals calcite or aragonite.
Mollusks are classified into smaller classes that include the gastropods,
pelecypods/bivalves, and cephalopods.
CLASS CEPHALOPODA
This is a class of mollusks that includes octopus,
squid, and nautilius. Ammonites, bacculites,
belemnites and orthoceratoids are representatives of this fossil class.
Bacculites are actually a straightened form of nautilus.
CLASS PELECYPODA
This class consists of any mollusks with two half-shells
or valves. The valves are mirror images
of each other but are asymmetrical (uneven). (Note: only one side or half the shell may
be available) One of the most commons fossils, this class includes the clams,
mussels, scallops, and oysters.
CLASS GASTROPODA
Gastropods are “stomach-footed”
mollusks. They have one-piece snail-like
shells that are usually twisted into a spiral with the largest diameter at the
open end. The coiling of the spiral
may be left or right-handed, regular or irregular.
PHYLUM BRACHIOPODA
This group of fossils is easily confused with the bivalve-mollusks,
but they are not mollusks! A brachiopod
shell is symmetrical (even), but one half is always larger,
Brachiopods’ shells were attached to stalks or pedicles.
The pedicle helped the animal attach itself to a hard surface. (Brachiopoda are also known as lamp shells
because they look like ancient Roman lamps)
PHYLUM ARTHROPODA
This is a group of organisms that includes insects,
spiders, crabs, millipedes, and barnacles.
Arthropod literally translates as “jointed foot.” Very few are found as fossils, except for an
extinct group called the trilobites. Trilobites
are found world-wide and are representative of a period of geologic time.
They became extinct after the Permian period, 250 million years ago. Because of this trilobites are on of the more familiar index fossils.
There were more than 10,000 species of trilobites, and they all lived
in a marine environment. Some were bottom dwellers, while others floated
or swam. Most trilobites had two eyes.
Trilobite means “three-lobed” and corresponds to the animal’s three parts
or lobes of the body. The legs, antennae, and other soft parts of
thse organism were very seldom preserved.
PHYLUM CHORDATA
This is a group of organisms that has a large number
of species (including humans) still in existence, those with nerve cords. One of the more common fossils remains is shark
teeth. These fossils can be traced back
to the Devonian Period. Because the
shark’s skeleton was cartilage it is rarely found in fossilized form.
Sharks continue to grow new teeth to replace old ones that drop out. Therefore, many shark teeth have been found, with lengths up to
4 inches across.
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