Exercise 9 Fossil Lab: Part 4 Rugose and Tabulate Corals

Exercise 9
Fossil Lab: Part 4
Rugose and Tabulate Corals; Ammonoids; Belemnites
RUGOSE AND TABULATE CORALS:
The main groups of Paleozoic corals are assigned to the orders Rugosa and
Tabulata (informally known as rugose and tabulate corals, respectively).
These Paleozoic corals differ from one another in their skeletal structure,
but the skeletal composition in both orders is calcite. In contrast, the
Mesozoic and Cenozoic corals (Order Scleractinia) secrete an aragonitic
skeleton and are believed to have originated from a sea anemone ancestor,
not from a Paleozoic coral.
The skeleton of a coral is known as a theca. Typically the theca is
partitioned by vertical elements called septa. As the animal grows and
enlarges its theca, it may also secrete horizontal partitions called tabulae
(Figure 1A).
A
B
Figure 1. (A) Basic skeletal morphology of a rugose coral. The animal itself is known as a polyp.
(B) Rugose coral with dissepiments in addition to septa and tabulae.
Certain rugose corals have less well developed septa or apparently no septa.
Instead they possess a complex skeleton consisting of tabulae and curved
elements known as dissepiments (Figure 1B).
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Rugose corals insert septa only at four locations during adult growth (hence,
the nickname “tetracorals”). They may be solitary and resemble a horn, or
they may grow in tightly packed colonies (Figure 2).
Figure 2. Rugose corals. Solitary (horn) individual (H), loosely packed colony (I), tightly packed
(prismatic) colony (J).
Tabulate corals possess weakly developed septa but very well developed
tabulae (Figure 3). They are generally colonial (Figure 4).
Figure 3. Skeletal morphology of a tabulate
coral. Note weakly developed septa.
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Figure 4. Examples of colonial tabulates. Favosites, the “honeycomb” coral (left), and Halysites, the
“chain” coral (right).
Paleoenvironmental Range:
Both rugose and tabulate corals are found most commonly in shallow water,
tropical to subtropical carbonate facies. They were capable of forming
substantial reefs in association with stromatoporoids. During the Middle
Paleozoic when they reached peak abundance, Iowa was situated in the
tropics under shallow seas. It is not surprising, therefore, that rugose and
tabulate corals are very abundant in Middle Paleozoic rocks of Iowa.
Stratigraphic Range:
Paleozoic corals, both rugosans and tabulates, originated in Ordovician time
and became extinct at the end of Permian time.
Tabulate Coral Examples:
Stations 1–5 (5 trays) are examples of the tabulate coral Favosites, also
known as the “honeycomb” coral. You will be asked to identify this genus
on the Lab Exam.
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Examine each specimen and make sure you can see the tabulae. Notice that
in some specimens the tabulae are essentially flat, whereas in others the
tabulae are curved. Can you see septa in any of the specimens?
Notice also that the thecae within a given colony are uniform in size, but
that thecal size varies among colonies.
Specimen #3 is a massive colony. This is the kind of colony that is found in
the “strome-tabulate” reefs that were so characteristic of Middle Paleozoic
shallow seas.
Stations 6–10 (6 trays) are examples of the tabulate coral Halysites, also
known as the “chain” coral. You will be asked to identify this genus on the
Lab Exam.
Thecae in Halysites are loosely arranged in a chain-like series or network.
Most of the colony was actually void space that has since been filled in with
lithified sediment.
Make sure you can see the tabulae. Notice the range in size of thecae from
specimen to specimen.
Rugose Coral Examples:
Stations 1–4 (4 trays) are examples of assorted solitary rugose corals, also
known as “horn” corals. Most, but not all of the specimens exhibit well
developed septa. A few lack septa and instead possess dissepiments.
Note, in particular, that one of the weathered specimens at Station 4
exhibits both septa and tabulae.
Stations 5–8 (5 trays) are examples of the colonial rugose coral
Hexagonaria, which is a very common fossil in Devonian rocks of eastern
Iowa. You will be asked to identify this genus on the Lab Exam.
Hexagonaria is easy to identify because the thecae in a colony are tightly
packed in a distinctive hexagonal arrangement: i.e., most thecae are sixsided and they are bordered by six neighboring thecae. Even though the
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colonies superficially resemble certain tabulate corals, notice that in
Hexagonaria the septa are well developed and prominent.
AMMONOIDS:
Remember from an earlier lab that cephalopod mollusks include two
geologically significant groups, (1) the nautiloids and (2) the ammonoids.
We’ve already learned that nautiloids are characterized by simple septa and
smooth sutures. The ammonoids, in contrast, possess variably undulating or
folded septa and correspondingly complex sutures.
The curves of ammonoid sutures are called saddles and lobes. Saddles are
the curves that are convex in the direction of the living chamber or aperture
(i.e., they point forward), whereas lobes are the curves that are convex in
the direction away from the living chamber or aperture (i.e., they point
backward) (Figure 5).
saddles
lobes
Figure 5. Goniatite ammonoid exhibiting well developed saddles and lobes.
Ammonoids are classified into three main categories on the basis of
serrations in the sutures. The simplest ammonoids—the goniatites—have
sutures that lack serrations. The ceratites are the next more advanced
group. Ceratitic sutures have smooth saddles and serrated lobes. The most
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advanced ammonoids are the ammonites, in which both the saddles and lobes
are serrated (Figure 6).
Figure 6. Stages of sutural
complexity. Nautiloids possess simple
sutures. Ammonoids possess folded
sutures with saddles and lobes.
Goniatite, ceratite and ammonite
ammonoids are distinguished on
serrations of the saddles and lobes
Paleoenvironmental Range:
Ammonoids were exclusively marine organisms that are preserved in a
variety of marine sedimentary environments. They are most common in
deeper water shales and limestones, but empty shells sometimes were
washed into shallower water where they were preserved in nearshore
environments.
Stratigraphic Range:
Ammonoids are extinct. Goniatite ammonoids evolved from nautiloids in late
Silurian or Devonian time, flourished in the late Paleozoic, and then became
extinct at the end of the Permian Period. Ceratite ammonoids originated in
the late Pennsylvanian or Permian and became extinct at the end of the
Triassic Period. Ammonite ammonoids originated in the late Triassic or
Jurassic and became extinct at the end of the Cretaceous Period.
Ammonoid Examples:
1. Assorted ammonoids (1 tray). Carefully examine these examples and make
sure that you can recognize goniatitic, ceratitic and ammonitic types of
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sutures. Do you know which parts of the suture are the saddles and which
are the lobes? Notice also the various kinds of coiling represented.
2. Study those sutures (1 tray). This tray contains two specimens, a
gonitaite and a ceratite. Can you determine which is which?
3. Goniatite examples (2 trays). Examine the sutures in these specimens and
verify that neither the saddles nor the lobes are serrated.
4. Jurassic ammonoids (probably ceratites) from England (5 trays, all plastic
casts). These specimens illustrate various types of coiling and surface
ornamentation. No sutures are visible because the outermost layer of the
shell is intact.
5. Large ceratite (1 tray). Examine the sutures in this specimen and notice
that the saddles are smooth and the lobes are serrated.
6. Large Jurassic ammonoid (probably a ceratite) (1 tray). This specimen is
a good example of “evolute” coiling (i.e., every volution is visible). Specimens
in which the outermost volution conceals previous ones are said to be
“involute.”
7. Straight ammonites (2 trays). These trays contain examples of the
straight ammonite Baculites. You will be asked to identify this genus on
the Lab Exam. Notice the complex ammonitic suture in which both saddles
and lobes are strongly serrated. Notice also that some of the original
aragonitic shell material is preserved, the characteristic molluskan “mother
of pearl.”
8. Large ammonite (1 tray). This partially crushed specimen exhibits good
ammonitic sutures. Note the complexity of serrations on both saddles and
lobes.
BELEMNITES:
Belemnites are extinct cephalopods (Jurassic-Cretaceous) that lacked an
external shell, but possessed an internal chambered skeleton (Figure 7).
The posterior portion of the internal skeleton was unchambered and heavily
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calcified. It is this feature, known as the guard, that is most commonly
preserved. In terms of their overall appearance, belemnites closely
resembled the modern squid. Belemnite guards are sometimes called “fossil
cigars” because of their tapered cylindrical shape and brownish color. A
piece of a belemnite guard from the Jurassic PeeDee Formation in South
Carolina originally served as the global standard for oxygen isotope analyses,
but the δ 18O standard now is “standard mean ocean water” (or “SMOW”).
Figure 7. Reconstruction of a belemnite.
9. Belemnite guards (1 tray). You will be asked to identify belemnite
guards on the Lab Exam.
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