LAB 4
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
Brachiopods-several 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-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.
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:
*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
pseudo-interarea 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 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:
- Productids: These brachiopods
have hollow spines and strongly concavo-convex shells. They include several
large, very spiny brachiopods and specialized reef-forming groups.
Productids are mostly upper Paleozoic.
- "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.
- 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.
- 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. 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.
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6.3
Terminology |
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valves |
commissure |
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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, didcutor muscles, adductor muscles muscle scars, lophophore,
brachidium, mouth, pedicle
2. Note and describe how the
shape of the brachidium and lophophore is often reflected in the shape of the
brachiopod's shell.
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 the following brachiopod shell shapes,
labeling brachial and pedicle valves: biconvex, plano-convex,
concavo-convex. See Figure 6.8 in the
Appendix for help
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:
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a.
Lingula |
1.
epifaunal: free-lying |
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b.
a spiny productid |
2.
epifaunal: attached by pedicle |
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c.
Terebratalia |
3.
infaunal |
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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)
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)
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)