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Science Olympiad
Phylum Brachiopoda

Members 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 radial symmetry and are sessile or free-floating as adults.

Bilateral Symmetry & Coelomates

The 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. This 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 sediments 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 protection for these soft-bodied animals. Coelomates diversified into segmented worms, mollusks, echinoderms, and lophophorates. Bryozoans and brachiopods are both lophophorates.

Brachiopods

Brachiopods 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 is through both shells instead of between shells as in mollusks. In some species the pedicle valve has a fleshy 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 ventral valve. The brachiopods soft body is sandwiched between the two valves or shells.

The 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 for filter-feeding. 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 brachidium can be important in helping to identify brachiopods.

Class Inarticulata

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 an anus. The shells are made of calcium phosphate and chitin (chitinophosphatic shells), although some species have shells made of calcium carbonate (calcite). Inarticulate brachiopods were common in the Cambrian. During the Ordovician articulate forms displaced inarticulate brachiopods. Lingula is the best-known inarticulate brachiopod alive today.

Class Articulata

Articulates (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 no 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 their shells. The adductors close the 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.

Brachiopods 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 intertidal zones. 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 shed sperm and eggs into the water during the breeding season. Shell-less larvae swim in the water 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 to decline thereafter. 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 marine environments.

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.

 


Brachiopod
Rafinesquina ponderosa
Waynesville Formation Ordovician
St. Leon, IN
Note: This Rafinesquina, an articulate brachiopod, is covered with inarticulate brachiopods


Brachiopod
Cyrtospirifer rudkinensis
Brachial Valve
Devonian
Voronezh, Russia
4.5 cm long x 2 cm wide

Mucrospirifer
Brachial Valve
Devonian
New York

3.5 cm wide x 1.8 cm tall

Mucrospirifer
Pedicle Valve
Devonian
New York
3.5 cm wide x 1.8 cm tall

Atrypa
Pedicle Valve
Devonian
Iowa
1.9 cm wide x 1.7 cm tall

Atrypa
Brachial Valve
Devonian
Iowa
1.9 cm wide x 1.7 cm tall


Composita
Pennsylvanian
Missouri
1.8 cm long x 1.3 cm wide

 


Composita
Pennsylvanian
Missouri
1.8 cm long x 1.3 cm wide

Side View Showing Aladdin Oil Lamp Shape


Juresania sp.
Side View of Ventral Valve
Pennsylvanian
Graham Formation
Texas
3.7 cm across




Juresania sp.
Dorsal Valve
Pennsylvanian
Graham Formation
Texas
3.7 cm across


Platystrophia sp.
Ordovician
MacMillan Formation
Kentucky
3.5 cm wide


Bibliography

Hickman, 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.

Rich 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.


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