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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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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.

Figure 4. Scleractinian corals (all colonial

examples).

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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.