Exercise 7 Fossils—Part 2: Trilobites, archaeocyathids ...
Transcript of Exercise 7 Fossils—Part 2: Trilobites, archaeocyathids ...
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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?