of the kingdom Animalia are multicellular, eukaryotic organisms.
They are typically heterotrophs that lack cell walls. As
we have seen, the phylum Porifera represents the most primitive
animal, which lacks tissues; filter feeds and is sessile
as adults. The phylum Cnidaria is a little more complex
in that they have tissues, but no organs. Cnidarians have
symmetry and are sessile or free-floating as adults.
Bilateral Symmetry & Coelomates
next step in animal evolution is represented by the worms,
which have an elongated body with a definite head (anterior)
end and tail (posterior) end. A one-way digestive tract
runs from mouth to anus, which leads to bilateral symmetry.
organization produces an organism that actively seeks
its food. Most worms (except for flatworms) have a coelom
or true body cavity. Coelomates have a "tube-within-a-tube"
body arrangement. The outer body wall surrounds an inner
tube, the digestive tract. The space between these two
tubes is the coelom. The coelom is of great evolutionary
significance. The fluid-filled coelom provides a space
for organs and organ systems and serves as a hydrostatic
fluid skeleton in primitive forms. The hydrostatic fluid
skeleton aids in movement and allows the organism
to burrow (Hickman, 1970, p. 109). Deep burrows in marine
first appear in the fossil record during the late Precambrian
(Prothero, 1998, p. 227). Burrowing into the sediments
allowed organisms to exploit a new niche, which provided
for these soft-bodied
animals. Coelomates diversified into segmented
worms, mollusks, echinoderms, and lophophorates. Bryozoans
and brachiopods are both lophophorates.
are shelled, solitary marine organisms that range from
the Cambrian to recent times. They resemble bivalve mollusks
in having two valves (shells). Most bivalve mollusk shells,
like clams, are mirror images of each other. Brachiopod
shells are both unlike and unequal. Brachiopods have a
larger ventral shell (pedicle valve) and a smaller dorsal
shell (brachial valve). The symmetry
of the brachiopod
both shells instead of between shells as in mollusks.
In some species the pedicle valve
stalk called the pedicle with which the brachiopod attaches
itself to a substrate. Muscles that open and close the
valves attach to the inside of the pedicle valve. Muscle
scars on the pedicle valve can help in identification.
The pedicle exits a hole (foramen) near the beak of the
The brachiopods soft body is sandwiched between the two valves or shells.
basic body plan of brachiopods consists of a visceral mass
with organs, a mantle that secretes the shell, the
pedicle, and the lophophore. Brachiopods
and bryozoans have a horseshoe-shaped structure with
ciliated tentacles called the lophophore, which is used
The lophophore moves food towards the mouth. The dorsal
or brachial valve bears the feeding organ (lophophore
or brachium). In some brachiopods the lophophore is supported
by a calcareous structure called the brachidium. The
can be important in helping to identify brachiopods.
are traditionally grouped into two classes. Members
of the class Inarticulata have shells that lack tooth-and-socket
articulation or hinges. Since the shells are not enclosed
at the base the one-way digestive tract can end in
shells are made of calcium phosphate and chitin (chitinophosphatic
shells), although some species have shells made of calcium carbonate
Inarticulate brachiopods were common in the Cambrian. During
the Ordovician articulate forms displaced inarticulate brachiopods.
Lingula is the best-known inarticulate brachiopod
(class Articulata) make up 95% of the known brachiopod
genera. Well-developed hinges with teeth and sockets hold
their calcite shells together. Articulates have a U-shaped
digestive tract that ends blindly, with
anus. Waste is regurgitated as small pellets and expelled by
snapping of the valves. Articulate brachiopods have two
sets of muscles used to open and close
shell, while the diductors open the valves. Clams have
only adductors to close their shells and when they die
their shells become disarticulated. Brachiopod shells remain
together when they die and are often found fully articulated
as fossils. Articulate brachiopods also have adjuster muscles
to move the pedicle.
are benthic marine invertebrates that inhabit both deep
water and shallow environments. However, the majority
of brachiopods lived and still live in shallow water and
Most brachiopods are sessile filter feeders and are incapable
of burrowing. Thus, they must live in areas were currents
provide food, carry away waste and allow for gas exchange.
Brachiopods do not fair well in turbid waters as their
lophophore becomes clogged with sediment. Living brachiopods
sperm and eggs into the water during the
and eventually attach themselves to a surface. A shell
is then secreted and grows through accretion. Much is still
unknown about the life of fossil brachiopods. The brachiopod
is the state fossil for Kentucky.
Success in the Paleozoic
Brachiopods are the most abundant and diverse shelled invertebrates
of the Paleozoic. Their great diversity and abundance make
them useful tools for paleoecology, biostratigraphy, biogeography
and evolutionary studies. Brachiopods representing different
species can have very similar looking shells, but have
very different internal structures. Paleotogists often
make thin sections through fossil brachiopods to study
the hinge and internal calcarious supports
of the lophophore (brachidium) to aid in identification.
Brachiopods evolved and diversified for more than 300 million
years. Brachiopods suffered greatly
from the Permian extinction and continued
Today, students may learn about brachiopods in biology
class by studying one of living representatives Lingula.
In a paleontology class brachiopods are obviously a major
topic of study because of their great success in ancient
Science Olympiad Fossil Event
The 2016 Science Olympiad Fossil List inludes the class Inarticulata
and the class Articulata. The genus Lingula represents
the inarticulates. The articulates are represented by the following
genera: Platystrophia, Atrypa, Mucrospirifer, Rafinesquina, Leptaena,
Composita, and Juresania. The order Rhynchonelida is also included.
C.P. (1970). Integrated Principles of Zoology.
St. Louis: The C.V. Mosby Company.
Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology.
New York: McGraw-Hill.
P.V., Rich T. H., Fenton, M.A., & Fenton, C.L. (1996). The
Fossil Book: A Record of Prehistoric Life. Mineola,
NY: Dover Publications, Inc.