LAB 5
Chapter 6: Phylum Brachiopoda
6.1 Introduction
While recent brachiopods are a rather rare and insignificant group, their long fossil history shows
that they were at times the most prominent animals in the seas. Consequently, brachiopods
receive only passing interest from zoologists, but a great deal of attention from paleontologists.
The phylum is quite important for biostratigraphy, paleoecology, and evolutionary studies
because it shows a great variety of changes in form and function through time.
Brachiopods appear near the beginning of the Cambrian, but did not become abundant until the
Early Ordovician. The remainder of the Paleozoic could be termed the Age of Brachiopodsseveral orders dominated the shallow shelf environments throughout the era, giving way only
reluctantly to the rapidly diversifying bivalved molluscs and gastropods of the Mesozoic. Only a
few groups survive today.
6.1.1 Functional Morphology
General Form
Brachiopods are solitary, entirely marine animals, each with a shell consisting of two opposing
parts (valves) that enclose most of the soft body. The animal and its shell are bilaterally
symmetrical about a plane drawn perpendicular to the line of contact of the closed valves (the
commissure). In most brachiopods, the shell is made of calcite, but a few groups have shells made
of calcium phosphate with varying amounts of organic material.
Figure 6.1 Cross section of a brachiopod showing the relationship of the internal organs to the two valves
(from Stearn, 1989).
Feeding
Inside the shell is the feeding structure characteristic of the brachiopods-the lophophore. This
consists of a pair of ciliated, twisted projections that create water currents and then filter out
microscopic food particles. Often the lophophore has a calcareous supporting brachidium. Fossil
and Recent brachiopods have a variety of accessory supports for the feeding apparatus.
In order to increase the amount of water filtered and still protect the delicate lophophore from
overly large particles, some brachiopod lineages (notably the Rhynchonellida) developed a zig-
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zag commissure. The zig-zags bring the sensitive mantle edges closer together, giving the animal
more control over the quality of incoming material (Figure 6.10c shows a zig-zag commissure).
Articulation and valve movement
The most common class of brachiopods, the Articulata, is characterized by the presence of two
opposing calcareous valves hinged along the posterior edge. They usually have a series of sockets
and teeth which allow valves to open anteriorly for feeding; they can also keep the valves firmly
closed when necessary. In some brachiopods the articulating structures have been reduced or lost
during evolution.
Two major muscle sets open and close the valves. Diductor muscles attach at one end to the floor
of the ventral valve, and at the other end to a projection (cardinal process) in the dorsal valve.
When these muscles contract, the hinge acts as a fulcrum, opening the valves anteriorly. Adductor
muscles, which are attached between the floors of both valves, contract to close the valves and
hold them shut.
Figure 6.2 Brachiopod valves are opened and closed by pairs of opposed muscles: (A) Contraction of
adductors closes the valves; (B) Contraction of diductors levers the shell around the hinge and opens the
dorsal valve (Boardman et al. 1987; from Prothero, 1998)
Relation to substrate
Most brachiopods have a fleshy stalk, termed the pedicle, that protrudes posteriorly through one
valve or between the valves and attaches permanently to the substrate. When the pedicle exits
through a valve (by definition the ventral valve), it leaves an opening that varies greatly in form
among brachiopod groups.
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Figure 6.3 The living brachiopod Magellania. (A) Dorsal view of the shell; (B) Side view showing the
pedicle and brachial valve and the position of the pedicle (from Stearn, 1989).
In many the pedicle was lost during either ontogeny or the evolution of the lineage, leaving as
evidence a hole partially or completely closed off by accessory plates or growth of the ventral
valve.
Some brachiopods had no pedicle and either lived freely on the substrate or attached their ventral
valve directly to some firm object. The free-living types developed a wide variety of devices to
protect themselves from burial in the sediment or disruption by currents (except for opening and
closing the valves and some limited movement on the pedicle, brachiopods are strictly sessile). A
few added heavy stabilizing calcite to the posteior and ventral portions of the shell; others had
spines that could attach to the substrate or function as a "snowshoe" in muddy areas. Other
brachiopods without pedicles were able to grow at a rate that kept the commissure above the
sediment surface.
Sensory structures
Recent brachiopods have series of small bristles (setae) extending from grooves at the valve and
mantle edges that serve as tactile sensory devices. Many fossil brachiopods have similar grooves,
indicating they probably had the same type of system.
Strophomenid brachiopods sometimes have hollow spines which may have carried continuous
strips of living mantle tissue from the shell interior to their tips. If so, then the spines would have
extended the sensory field of the animal.
6.2 Classification
The brachiopods are divided into two classes, based primarily on shell morphology. The
inarticulates have unhinged valves generally of a chitinophosphatic composition, while the
articulates are brachiopods with hinged calcareous valves.
6.2.1 *Class Inarticulata
Class Inarticulata contains five orders, only three of which are commonly encountered:
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*Order Lingulida
•
These are the most conservative of the brachiopods; one genus, Lingula, appeared in the
Ordovician and is still alive today in essentially the same form. The shells of lingulids are
made primarily of calcium phosphate, which appears dingy brown, and are always
biconvex and oval to squarish in outline. The order first appeared in the Cambrian, and it
has been found consistently throughout the record. They live burrowed in the soft
sediment, anchored by their long pedicle.
Figure 6.5 Lingula, the shell is about 3 cm long. (A) Dorsal view (B) the brachiopod at the top of its
burrow attached to the bottom by a long pedicle
Order Acrotretida
•
This group has also survived since the Cambrian with a fairly constant shell morphology.
Acrotretid valves are generally subcircular to circular, unequally biconvex, and often
have a pedicle opening. An important subgroup (the craniaceans) have no functioning
pedicle; instead, they cement the ventral valve directly to the substrate.
Order Obolellida
•
These brachiopods resemble the acrotretids and some articulates, and so are rather
difficult to identify. Their valves are circular to oval, the ventral one with a pseudointerarea and a pedicle opening of some sort. Obolellids are known only from Cambrian
rocks.
6.2.2 *Class Articulata
The articulates are a diverse and complex class. They have proven to be the most useful
brachiopods for a variety of studies. Seven orders are recognized.
*Order Orthida
•
Apparently the most ancestral of the articulates, these brachiopods are also the most
difficult to work with. They are a generalized group lacking complex morphologic
features, and they tend to resemble some of the other articulate orders. Orthids are almost
always biconvex, with a fairly wide hinge line flanked by distinct interareas on each
valve. The first known articulates to appear in the Cambrian are orthids. They change
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little in basic structure through the Paleozoic, and became extinct at the end of the
Permian.
*Order Pentamerida
•
This group usually has strongly biconvex valves that are smooth or finely costate. Their
characteristic feature is a robust spondylium, which is a curved calcareous platform for
muscle attachment in the beak region. The spondylium is seated in the ventral valve.
Pentamerids probably arose from orthids in the later Cambrian; they went extinct in the
Devonian. The pentamerids were most important in Silurian shelf seas.
*Order Strophomenida
•
No other brachiopod order shows as much morphologic diversity as the strophemenids.
Generally, they have plano-convex or concavo-convex shells with a costate surface and
no pedicle opening. This simple plan though, has led to several distinct and sometimes
exotic forms. They include:
o
Productids: These brachiopods have hollow spines and strongly concavo-convex
shells. They include several large, very spiny brachiopods and specialized reefforming groups. Productids are mostly upper Paleozoic.
o
"True Strophomenids": In this group the shell is usually wider than long and
the body cavity is small. They are important in the lower Paleozoic, but range
into the Jurassic.
o
Chonetids: These are small shells that resemble the "true strophomenids", but
they have hollow spines along the postreior edge. Chonetids are especially
prominent in upper Paleozoic rocks.
o
Oldhaminids: "A leaf in a gravy boat" is the best way to describe the most
common oldhaminid genus, Leptodus. These are confined to the Upper Paleozoic
and the Lower Mesozoic.
*Order Rhynchonellida
•
The shells of this group are strongly biconvex, many to the point of being globose,
heavily plicate, and have a very short hinge line. Rhynchonellids are simple internally
and have no complicated supports for the brachidium. Most workers believe that they
evolved from the pentamerids. The rhynchonellids appeared in the Middle Ordovician
and are still extant. Most Mesozoic brachiopods are rhynchonellids.
*Order Spiriferida
•
The most common of this group is the easiest to identify: these shells have a hinge line so
wide that they look winged. The group is defined by the possession of a spirally-coiled
brachidium that "points" toward the cardinal extremities. Most spiriferids are strongly
plicate or costate, and they usually bear a fold and sulcus. The group is important in the
middle and upper Paleozoic and parts of the lower Mesozoic.
*Order Terebratulida
•
These are the most abundant brachiopods today; they are the typical "lamp shells" known
to beachcombers. Terebratulid shells are strongly biconvex and bear a short hinge line.
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The beak is prominent and usually has a rounded pedicle opening. The surface of a
terebratulid is smooth or finely costate. This group first appears in Lower Devonian
rocks, but it only becomes abundant in Mesozoic and Cenozoic strata.
6.3 Terminology
valves
lophophore
diductor muscle
cardinal process
setae
articulates
brachial
biconvex
fold
plicate
commissure
brachidium
adductor muscle
pedicle
inarticulates
spondylium
ventral
concavo-convex
sulcus
winged
6.4 Questions
1. Live specimens: (See Appendix for helpful diagrams)
a. External examination - sketch and label the following features:
pedicle valve, brachial valve, hinge line, interarea, commissure, growth lines, plane of symmetry,
pedicle opening
b. Internal examination - identify the following features, and know their various functions:
mantle, mantle canals, diductor muscles, adductor muscles muscle scars, lophophore, brachidium,
pedicle
2. Note and describe how the shape of the brachidium and lophophore is often reflected in the
shape of the brachiopod's shell.
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3. Class Inarticulata, Order Lingulida, Lingula. The morphology of this brachiopod has persisted
relatively unchanged since the Cambrian. Note the extensive pedicle. What is Lingula's mode of
life? This animal is often referred to as a "living fossil". The hard parts are unchanged since the
Cambrian. Do you think that the soft parts are unchanged?
4. Class Articulata, Order Strophomenida, Suborder Productidina. What morphological feature do
the shells of these productids share? What function did these structures serve?
5. Sketch and label the following features:
•
zig-zag commisure
•
plications
6. Occassionally, internal structures or features may be preserved. What skeletal feature has been
preserved in these specimens?
7. Brachiopod shells are described as convex, planar or concave. The shape term for the brachial
valve precedes that for the pedicle valve. For example, a brach with a convex brachial valve and a
planar pedicle valve is called "convexi-planar". Sketch a cross-section of the following
brachiopod shell shapes, labeling brachial and pedicle valves: biconvex, plano-convex, concavoconvex. See Figure 6.8 in the Appendix for help
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8. Brachiopod classification. The phylum Brachiopoda is divided into two classes, Inarticulata
and Articulata. You need to be able to recognize these classes, the important orders within them,
and the geological ranges of the classes and orders.
Pick three orders, and make a short list or table of features that might help you distinguish one
from the other two orders (shell shape, shell outline, development of ribs, growth lines, size of
interarea, length of hinge line, prominence of fold and sulcus, shape of commisure, etc.). Sketches
are helpful also.
9. Brachiopods had a variety of modes of life. Match up the specimens with their corresponding
mode of life. Make use of shell features, if necessary:
a. Lingula
1. epifaunal: free-lying
b. a spiny productid 2. epifaunal: attached by pedicle
c. Terebratalia
3. infaunal
d. Spirifer
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Appendix
Figure 6.6 The basic anatomical orientation and symmetry of brachiopods (Neil and Tucker,1985; from
Prothero, 1998)
Figure 6.7 Terminology of the external features of the brachiopod shell (Neil and Tucker, 1985; Prothero,
1998)
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Figure 6.8 Cross sections through brachiopod shells, showing curvature. b = dorsal valve; p = pedicle
valve. (A) Biconvex, with dorsal valve less convex than ventral; (B) Biconvex, with more convex dorsal
valve; (C) Planoconvex; (D) Concavo-convex; (E) Concavo-convex, but more strongly curved; (F)
Strongly convexi-concave; (G) Gently convexi-concave; (H) Resupinate, dorsal valve convex but concave
near hinge line; (I) Convexi-planar (Moore et al, 1953; Prothero, 1998).
Figure 6.9 Internal anatomy of a brachiopod, cut along the plane of symmetry (Boardman et al., 1987;
from Prothero, 1998)
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Figure 6.10 Magellania flavescens; (a) upper surface with brachial valve (x 2 approx.); (b) lateral view (x
2 approx.); (c) anterior view (x 2 approx); (d) in life position, showing pedicle attachment; (e) internal view
of pedicle valve (x 2 approx); (f) internal view of brachial valve (x2 approx); (g) larva
((a)-(f) based on Davidson 1851; (g) based on Percival 1944; from Clarkson1979)
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