Exercise 11
Fossil Lab — Part 6:
Gastropods, bivalves, scleractinian corals
GASTROPODS:
Gastropods (“snails” and their allies) are mollusks characterized by
distinctive torsional (“corkscrew”) coiling of their soft anatomy. Most
possess an aragonitc shell which also is coiled in a corkscrew or helical
fashion, but some lack a shell altogether (slugs, for example). Typical
gastropod shells are illustrated in Figure 1.
Figure 1. Marine gastropod shells exhibiting characteristic “corkscrew” coiling.
Paleoenvironmental Range:
Gastropods are a varied group in terms of their environmental range. Some
live in marine environments, some live in freshwater environments, and some
actually possess lungs so that they can live on land (slugs, land snails). Most
marine gastropods are mobile predators.
Stratigraphic Range:
Gastropods originated in the Cambrian Period and they are still around
today. They were fairly abundant in Paleozoic time, but they were greatly
outnumbered by sessile, filter-feeding Paleozoic taxa such as brachiopods,
bryozoans and crinoids. The end-Permian mass extinction caused a great
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reduction in the diversity of sessile, filter-feeding organisms and apparently
facilitated an adaptive radiation of mobile predators such as the gastropods.
Gastropods became much more abundant in Mesozoic and Cenozoic times.
Gastropod Examples:
1. Modern land snail in leucite. This specimen exhibits the
characteristic gastropod helical coil as well as soft anatomy. The
fleshy “foot” that protrudes from the shell has sensory organs and
also is used for locomotion.
2. Assorted fossil snails (all internal molds). Note that this collection
includes examples of both high- and low-spired coiling types.
3. Floydia (Devonian of Iowa). You will be asked to identify this genus
on the Lab Exam. Note that the coil expands rapidly, but the overall
shape in neither very high-spired nor very low-spired.
4. These two specimens from the Eocene Epoch are good examples of
low-spired gastropods. What is the mode of preservation?
5. Example of a Pennsylvanian high-spired gastropod. Why do think most
gastropods are preserved as internal molds?
6. An Ordovician low-spired gastropod. How would you distinguish
between a gastropod like this and a coiled cephalopod such as a
nutiloid or an ammonoid?
7. Eocene Turritella agate (silicified). These rock samples are composed
almost exclusively of silicified Turritella shells in the absence of
other invertebrates. This is known as an “impoverished” or
“depauperate” fauna. What inferences can you draw about the
environment of deposition?
8. Small Ordovician high-spired gastropods. Note the striking similarity
between these Ordovician snails and Turritella from the Eocene!! A
beautiful example of evolutionary convergence!!
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BIVALVES:
Bivalves are the group of mollusks that includes clams, mussels and oysters.
Like gastropods, most bilvalves secrete an aragonitic shell and most are
mobile predators.
Most bivalves exhibit bilateral symmetry on either side of the plane
separating the two valves of the shell. In other words, each valve is a mirror
image of the other. Brachiopods also exhibit bilateral symmetry, but the
plane of symmetry bisects each valve into identical right and left sides. The
two valves of a brachiopod shell usually are quite different in size and shape.
Oysters and mussels are irregularly shaped bivalves that do not exhibit
bilateral symmetry. In fact the two valves of an oyster shell are radically
different in size and shape. Some representative bilvalves are illustrated in
Figure 2.
Figure 2. Bivalve shells. Specimens at right and in center are clams; specimen at left is an oyster.
Rudists are bizarre Cretaceous bivalves that superficially resemble horn
corals (Figure 3). In rudist shells, one valve has been elongated to form a
deep cup-like structure and the other valve serves as a small cap or lid.
Some rudists grew very large, approaching 6 feet in length! In middle and
late Cretaceous time rudists formed large reefs that temporarily displaced
scleractinian coral reefs. Rudists became extinct at the end of the
Cretaceous Period or early in the Tertiary Period, after which time
scleractinians again were the dominant reef-forming organisms.
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Figure 3. Rudists. Specimens at left mimic corals in possessing deep, cup-like lower valves. Specimen at
right possesses two grotesquely contorted valves.
Paleoenvironmental Range:
Like gastropods, bivalves occur in a wide range of environments. Clams live in
both marine and freshwater environments. Oysters and mussels are mostly
marine, but they can tolerate fairly low salinity such as in bays and estuaries.
Mussels and oysters are abundant live in brackish water or along rocky
coasts where few other invertebrates thrive.
Stratigraphic Range:
Again, like gastropods, bivalves originated in the Cambrian Period and are
still alive today. They were abundant during the Paleozoic Era, but their
diversity increased significantly after the end-Permian mass extinction when
they were able to exploit niches left vacant by brachiopods and other sessile
filter-feeders.
Bivalve Examples:
1. Modern bivalve with soft anatomy. In this specimen only a single valve
is preserved. Note that it is asymmetrical. The bilateral symmetry
observed in most bilvalves arises because the two valves are identical.
2. Assorted Anadara shells. Examine the inside surface of these shells
and notice that each possess a pair of muscle scars where muscles
formerly were attached to the shell. In bivalves, muscles must be
contracted to close the shell. When muscles are relaxed (or upon
death of the individual), the shell opens. This is just the opposite of
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the case in brachiopods, and it explains why bivalve shells usually
occur as single valves whereas brachiopod shells are usually preserved
intact.
3. This is a fairly rare specimen in which both valves are preserved.
Note the bilateral symmetry.
4. This Eocene specimen exhibits more typical preservation in which the
shell has become disarticulated upon death and the two valves were
separated from one another.
5. Check out this slab containing small Permian bivalves. This is another
example of an “impoverished” or “depauperate” assemblage, just as in
the Turritella agate (gastropods, #7). What inferences can you draw
with respect to the environment of deposition?
6. Inoceramus (internal mold). You will be asked to identify this genus
on the Lab Exam. Inoceramus was a giant Cretaceous clam. Some
specimens exceed a foot in length. Note the overall elongate shape of
the shell and coarse, concentric ornamentation.
7. More examples of Inoceramus. You will be asked to identify this
genus on the Lab Exam. What is the mode of preservation?
8. These specimens are examples of the bizarre Cretaceous coiled
oyster Exogyra. Note that the inside of the shell has only one muscle
scar (compare with Anadara at station #2). Examine specimen Mp-29
very carefully. How would you distinguish this oyster from a similarly
coiled gastropod?
9. These are examples of the Jurassic oyster Gryphaea. You will be
asked to identify this genus on the Lab Exam. Recall from Chapter
7 in your textbook that evolution in the genus Gryphaea is cited as a
good example of phyletic gradualism. Gryphaea shells are sometimes
called “devil’s toenails” because of their claw-like shape.
10. Here are two examples of the bizarre Cretaceous bilvalves known as
rudists. How would you distinguish rudists from horn corals?
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SCLERACTINIAN CORALS:
Scleractinians are “modern” corals that originated in middle Triassic time
somewhat after the “Mother of Mass Extinctions.” They are classified as
cnidarians along with the Paleozoic coral orders Rugosa and Tabulata, but
most specialists agree that the scleractinians and Paleozoic corals are not
closely related. In fact, the scleractinians are believed to have evolved
from a sea anemone ancestor. One line of evidence pointing to a unique
origin of the scleractinians is the complete absence of corals during early
Triassic time. In other words, there cannot be an evolutionary connection
between Paleozoic corals and scleractinians because the Paleozoic corals
became extinct some 10-15 million years before the scleractinians
originated. A second distinction between the Paleozoic corals and
scleractinians is skeletal mineralogy, with the skeleton in Paleozoic forms
being calcite and that in scleractinians being aragonite.
Scleractinian corals provide the rigid framework for all of the coral reefs
of the modern oceans. Corals grow rapidly under suitable environmental
conditions that include warm, shallow, clear, well agitated water of normal
marine salinity (~35‰). Rapid secretion of skeletal aragonite is facilitated
by photosynthetic algal symbionts known as zooxanthellae. Deep-water
corals lack these symbionts, and therefore do not grow as rapidly or form
reef structures.
Scleractinians are sometimes known as “hexacorals” because the earliest
formed part of the skeleton possesses six septa and later-formed parts of
the skeleton possess septa in multiples of six (e.g., 12, 18, 24, 30, etc).
Scleractinians may be either solitary or colonial and therefore they may be
difficult to distinguish from certain rugosans. The distinction requires
determination of skeletal mineralogy (impossible in hand specimens!),
recognition of septal insertion patterns (nearly impossible for non-experts!)
or knowledge of the age of the specimen. Some scleractinians are
illustrated in Figure 4.
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Figure 4. Scleractinian corals (all colonial
examples).
Paleoenvironmental Range:
Scleractinians require normal marine salinity. Reef-forming scleractinians
further require shallow (well lit), warm and well agitated water. For these
reasons, coral reefs occur almost exclusively in the tropics. Also for these
reasons, the health of coral reefs is a key environmental indicator (i.e.,
demise of coral reefs may be associated with global climate change or water
pollution). Non-reef-forming scleractinians may live in relatively deep and/or
cold waters in temperate latitudes.
Stratigraphic Range:
Scleractinians originated in middle Triassic time and they are still around
today.
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Scleractinian Coral Examples:
1. Assorted modern corals in leucite. This block includes both
scleractinian corals and octocorals, with the latter being the more
delicate, “lacy” ones.
2. Modern, solitary scleractinian coral. In life, the basal part of the
polyp (or soft tissue of the animal) would have been complexly folded
within the septal framework of the aragonitic skeleton, and the upper
part of the polyp would have been perched on top of the skeleton.
3. Assorted scleractinian corals. Examine these carefully. How would
you distinguish these scleractinians from superficially similar rugose
and tabulate corals from Paleozoic rocks?
4. Massive colonial coral head (modern). Large numbers of corals like
this are capable of forming the skeletal framework of reefs.
5. Modern Diploria, the “brain coral” (4 trays). You will be asked to
identify this genus on the Lab Exam. This is a massive colonial coral
that exhibits a distinctive “brain-like” folding of its skeletal
structure. Examine the upper and lower surfaces of these specimens
and notice the variety of other invertebrates that have encrusted the
corals. These so-called “epibionts” include bivalves, calcareous worm
tubes and calcareous algae.
6. Jurassic, stick-like colonial scleractinian. How would you distinguish
this from a similarly shaped bryozoan?
7. Miocene colonial scleractinian.
8. Eocene stick-like scleractinian.
9. Pleistocene colonial scleractinian.
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