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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).

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).

A B

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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 six-

sided 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).

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

saddles

lobes

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advanced ammonoids are the ammonites, in which both the saddles and lobes

are serrated (Figure 6).

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

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

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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.