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Exercise 7 Fossils—Part 2:

Trilobites, archaeocyathids, nautiloids, graptolites

TRILOBITES:

Morphology/Terminology

The phylum Arthropoda includes an enormous variety of animals, all of which

are characterized by segmented appendages. Examples of arthropods

include insects, spiders, crustaceans (crabs, crayfish, shrimp, lobsters),

ostracodes, and trilobites. In terms of shear numbers of individuals, the

arthropods outnumber all other animal groups combined!

Trilobites were arthropods whose shells are divided longitudinally into three

lobes: the central axial lobe and two lateral or pleural lobes. The trilobite

shell also can be divided into a head region (cephalon), the segmented

thorax, and a tail region (pygidium) (Figure 1).

The shell of a trilobite is made of the organic substance chitin that has been

thickened and reinforced by CaCO3. Growth of an individual was

accomplished in a series of increments. When growth became limited by the

confines of the hard shell, the shell was discarded and a new, larger one was

secreted in its place. The process of discarding an old shell is known as

Figure 1. Trilobit anatomy and

terminology. Dorsal (top) view.

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molting (or ecdysis). A consequence of this mode of growth is that a single

individual could produce several potential fossils.

Trilobites as a group exhibit many different kinds of eyes. Some were

multifaceted as in certain insects, whereas other were a simple, single-lens

type. Many species of trilobites apparently were blind and lacked eyes

altogether.

Stratigraphic Range

Trilibites originated in early Cambrian time and they were abundant in both

the Cambrian and Ordovician periods. They decreased in abundance after

the Ordovician Period and eventually became extinct in late Permian time

coincident with the “Mother of Mass Extinctions.”

Paleoenvironmental Range

Trilobites were exclusively marine animals. Most lived in relatively shallow

shelf environments, although some apparently preferred slightly deeper

waters.

Trilobite examples

1. Cryptolithus (Ordovician) (cast). This genus has a large cephalon relative

to the rest of the body.

• Note genal spines and ornamentation (pits) along margin of cephalon

• Does this trilobite possess eyes?

2. Elrathia (Cambrian). The cephalon of this specimen is partly broken, but

otherwise this is exquisite preservation of an actual specimen.

• Make sure you can distinguish the cephalon, thorax, and pygidium

• Are segments of the pygidium fused or discrete?

3. Ditymopyge (Pennsylvanian) (cast of an assemblage of individuals). Note

that one specimen is partly enrolled. Some trilobites did this for protection

against predators, just as do modern “rolly polly” bugs (pill bugs).

• Note the well developed eyes

• Can you tell where the thorax ends and the pygidium begins?

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4. Examples of phacopid trilobites (casts and actuals specimens). The order

Phacopida includes some of the most abundant and well studied trilobites.

You may be asked to identify phacopids on the exam.

• Note that the thorax consists of a large number of segments

• Note that segments of the pygidium are not well fused (it’s difficult

to tell where the thorax ends and the pygidium begins)

• Note the well developed compound eyes (similar to the multi-faceted

eyes in certain insects)

5. Assortment of trilobites preserved in gray shale (Cambrian) (impressions,

molds, and actual shell material).

• Make sure you know what part of the trilobite you’re looking at when

examining incomplete specimens

6. Peronopsis (Cambrian). These are small, blind trilobites. Use the

microscope to see as much detail as possible.

• How many thoracic segments are present in this genus?

• Are you able to tell which end is the head and which is the tail? Are

the two ends identical?

7. Isotelus (Ordovician). This is a very distinctive trilobite from eastern

Iowa. You may be asked to identify this genus on the exam.

• Note the well developed eyes

Note the pygidium, triangular in shape and in which the segments are

completely fused

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ARCHAEOCYATHIDS:

Morphology/terminology

Archaeocyathids are enigmatic fossils that superficially resemble certain

sponges and certain corals. They differ sufficiently from both sponges and

corals so that they have been assigned their own distinct phylum,

Archaeocyatha. Archaeocyathids were very primitive, multicellular solitary

or colonial animals that secreted a calcareous skeleton. An individual

skeleton (cup) consists of a double-walled inverted cone built around a

central cavity (Figure 2). The walls are typically perforated by small pores.

The region between the wall is known as the intervallum. This region is

partitioned by numerous longitudinal septa into elongate chambers (loculi;

singular = loculum). The cup itself may have been anchored to the seafloor

or some other substrate by a root-like holdfast structure (Figure 2), or it

may have been part of a larger colony of cups (Figure 3).

Figure 2. Cut-away diagram of

a single archaeocyathid cup,

showing pores and double-

walled construction with septa

dividing the intervallum into

loculi.

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Figure 3. Line drawings of solitary (left) and colonial (right) archaeocyathid growth habits.

Stratigraphic Range

Archaeocyathids are known almost exclusively from lower Cambrian rocks.

Only a few examples are known from the middle Cambrian, just prior to the

ultimate extinction of the phylum.

Paleoenvironmental Range

Archaeocyathids lived in carbonate shelf environments that probably were

similar to those of the modern tropics. They formed massive reef

structures in the early Cambrian seas, and as such, they are Earth’s earliest

reef-forming organisms!

Archaeocyathid Examples (all early Cambrian)

1. This is a cross section of a solitary individual.

• Note the well defined inner and outer walls of the cup, and the

intervallum

• Note the septal partitions and the loculi

• This specimen has a relatively large central cavity by comparison with

the thickness of the walls and intervallum

2. Polished slab containing several individuals in a colonial group. Be sure to

examine both sides of this slab.

• Note that these archaeocyathids possess a relatively small central

cavity by comparison with the walls and intervallum

• Note that the intervallum and septa exhibit a complex, “spongy”

network

3. This sample contains several randomly oriented solitary individuals.

• Note that the walls, septa, intervallum, and loculi are very well defined

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4. This is a thin section of an archaeocyathid from the same rock sample as

no. 3, above. Use the microscope to observe the crystalline details of the

walls and septa.

• What is the material filling the central cavity and loculi?

• What is the mineralogic composition of the skeletal material itself

(walls, septa)?

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NAUTILOIDS:

Morphology/terminology

Nautiloids are a special group of cephalopod mollusks, which include among

their extant members the octopus, squid, cuttlefish, and Nautilus (Figure 4).

Extinct cephalopods include the ammonoids and belemnoids (more on those

groups in another lab).

Nautiloids are characterized by an external, multi-chambered shell (or

conch) of the mineral aragonite (CaCO3, but different from calcite – also

CaCO3). The conch may be coiled or straight. The chambers of the conch

are separated from one another by septa. A thin, delicate, calcareous tube,

the siphuncle, extends through the septa along the entire length of the

shell. The siphuncle is permeable and allows the exchange of gas between

the living animal (which resides in the final, living chamber) and previously

occupied chambers. The addition and removal of gas to chambers enables

the animal to control the bouyancy of its conch, much as the bouyancy of a

submarine is governed.

Nautiloids are further characterized by simple, unfluted septa. The

intersection of the margin of a septum with the outer surface of the conch

Figure 4. Sectioned conch of the modern

nautiloid, Nautilus. Living chamber is the

large, final chamber at the bottom of the

conch. Previously occupied chambers are

separated by simple, slightly curved septa.

Siphuncle is not preserved, but its trace is

evidenced by perforations in the center of

each septum.

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is known as a suture. Nautiloid sutures are always smooth and never complex

(Figure 5). (Complex sutures are characteristic of ammonoid cephalopods.)

Figure 5. Comparison of suture morphology between a

coiled nautiloid and the three kinds of ammonoids.

Stratigraphic Range

Nautiloids appeared in Cambrian time and they are still extant, although

represented today by just a single genus, Nautilus. They are fairly abundant

in rocks of early and middle Paleozoic age (Cambrian-Devonian), less

abundant in rocks of late Paleozoic age (Mississippian-Permian), and rare in

rocks of Mesozoic and Cenozoic age. The Ordovician rocks of northeastern

Iowa contains large numbers of well preserved specimens.

Paleoenvironmental Range

Nautiloids are exclusively marine, nektonic (swimming) organisms. In the

modern oceans Nautilus has been observed in neritic, bathyal and even

abyssal depths. Empty shells, however, commonly wash ashore on islands.

Ancient nautiloids are inferred to have occupied environmental niches similar

to that of Nautilus. The occurrence of fossil nautiloids in a sedimentary

rock, therefore, conveys little paleoenvironmental information other than

that the rock was deposited in a marine setting.

Ammonoids (variably

complex sutures)

Nautiloid (simple sutures)

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Nautiloid Examples

1. Modern Nautilus and a Cretaceous fossil nautiloid. These are examples of

coiled nautiloids.

In the modern Nautilus, make sure you note the final living chamber and

earlier (unoccupied) chambers, the septa, and perforations in the septa

where the siphuncle once extended. Unfortunately, the siphuncle is not

preserved in this specimen because it was very thin and delicate.

• Note that you cannot observe the sutures in the modern Nautilus

because the original outer shell material is still intact. The color

bands and fine textures you see are simply surface ornamentations.

• Note that the mineralogy of the conch is aragonite, a relatively

unstable form of CaCO3. When aragonite exhibits an irridescent,

pearly luster it is called “mother of pearl.”

• What might be the adaptive value of the color bands?

In the Cretaceous fossil the outer shell material has been removed. It was

probably dissolved during or after burial.

• Note that because the outer shell is no longer present, you can

observe the sutures in this specimen

• Is the living chamber still preserved?

2. This is a polished section of a straight nautiloid.

• Note that the siphuncle is still intact as a thin, calcareous tube in the

center of the conch

• Note the simple, gently curved septa

3. Another example of a straight nautiloid exhibiting simple sutures.

4. Dawsonoceras (a 3-dimensional cast and a 2-dimensional impression).

This straight nautiloid has slightly inflated chambers, imparting a ribbed

appearance to the conch.

• Note that the cast allows you to determine that the sutures were

simple

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• Note that the impression preserves the fine surface ornamentation of

the conch. Approximately 4 or 5 thin, wiggly lines cover each

chamber.

5. Straight nautiloids from Ordovician rocks at Graf, Iowa (impressions,

sectioned specimen, and many 3-D specimens).

• Have a close look at these specimens and note that the siphuncle

pores are observable in some.

• Note also the septa and simple sutures in every specimen

• Some of the specimens still possess the outer shell material, albeit it

has been altered from the original aragonite to more stable calcite (no

“mother of pearl” still present)

• At Graf, many of the nautiloids are preserved in a bizarre “cone-in-

cone” fashion, just like so many ice cream cones. Nobody has been

able to come up with a satisfactory explanation for this type of

preservation, but suggestions range from group sex to hydrodynamic

sorting to some kind of pressure-related phenomenon. Whaddya

think?

6. More straight nautiloids exhibiting simple septa and sutures (internal

molds).

7. An assortment of large, straight nautiloids (all internal molds). These

are big specimens, but they are by no means the largest known. Some

Paleozoic nautiloids reached lengths of several feet, reminiscent of

“characters” in giant squid horror films.

• Note that one specimen possesses a large hole where the siphuncle

once was

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GRAPTOLITES:

Morphology/terminology

Graptolites are a group of extinct, marine organisms that constructed fairly

simple to highly complex colonies. Both planktonic and sessile forms are

known. Although their taxonomic affinity has been much debated, recent

specialists assign them to the rather obscure phylum Hemichordata (not

closely related to true chordates). The graptolite skeleton is composed of

the proteinaceous substance chitin. A colony, or rhabdosome, consists of a

pointed terminal structure (nema) and one or more stipes along which large

numbers of thecae (tiny living chambers) are arranged (Figures 6 and 7).

Figure 6. Sketches of simple graptoloid graptolite rhabdosomes showing arrangement of thecae

along stipes.

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Two main categories of graptolites are recognized. Dendroid graptolites

constructed highly elaborate, branching, fan-shaped rhabdosomes that may

have been anchored to the seafloor (like certain seaweeds) or suspended

from a floating bulb (again, like certain seaweeds) (Figure 8).

Figure 7. Enlarged view of a graptolite stipe

showing individual thecae and thecal apertures.

A tiny animal resided in each theca.

Figure 8. Fossil rhabdosome of a dendroid graptolite.

Note overall fan-shaped morphology. Nema (not

shown) is located just out of view at bottom, left of

colony. Graptolites are usually preserved as two

dimensional, carbonized impressions in fine grained

matrix.

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Graptoloid graptolites were exclusively planktonic and their rhabdosomes

are generally simpler, mostly consisting of four or fewer stipes (Figure 6).

Both graptoloid and dendroid graptolites are normally preserved as

carbonized, two-dimensional impressions in fine grained rocks. In some

instances, though, exquisitely preserved three-dimensional specimens have

been recovered intact by gently etching them from enclosing limestone

matrix.

Stratigraphic Range

Graptolites as a group appeared in Cambrian time and persisted into

Pennsylvanian time. Graptoloid graptolites, which are extremely useful

biostratigraphically, are found only in Ordovician, Silurian, and early

Devonian rocks, with specimens being most abundant in Ordovician rocks. As

with nautiloids, certains Ordovician rocks of northeastern Iowa contain very

good graptolite faunas.

Paleoenvironmental Range

As they were mostly planktonic marine organisms, graptolites are not

especially useful as paleoenvironmental indicators. Their presence in a rock

indicates only that the rock was deposited in a marine setting. Because of

their small size, delicate construction, and non-mineralized composition, they

are best preserved in very fine grained rocks. Such rocks formed under

quiet water conditions, probably below fair weather wave base in neritic and

deeper bathymetric zones.

Graptolite Examples

1. This is a latex model, greatly enlarged, showing several thecae along a

stipe and a detailed view of the ultrastructure of an individual theca.

2. Assorted graptoloid graptolites. Because graptoloids evolved very

rapidly they are considered index (or guide) fossils. Use the microscope to

observe details of the thecae.

• Note that rhabdosomes consist of 1, 2, or 4 stipes. The number of

stipes and the arrangement of thecae are key taxonomic characters

that differentiate genera and species

• Note that graptolites are almost always preserved as 2-dimensional,

carbonized impressions

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3. Assortment of dendroid graptolites. Curiously, although dendroid

graptolites are more complex than graptoloids, they apparently evolved at

slower rates and are not as useful biostratigraphically.

• Note the complex, fan-shaped rhabdosomes, each of which possesses

considerably more than 4 stipes

4. Graptoloid graptolites from Ordovician rocks of eastern Iowa.

• Note how abundant and delicate these specimens are. They come

from the same rock unit as the straight nautiloids (no. 5 on the

nautiloid table).

• Under what kind of conditions might these graptolites have been

preserved?