General Range Chart
Phylum Arthropoda
In terms of species, Arthropoda is the largest phylum in the animal kingdom. Arthropoda consists of about
700,000 species or about three fourths of all known kinds of animals. It consists of spiders, crustaceans (crabs,
lobsters, shrimp, ostracods, etc.), insects, centipedes, and the extinct trilobites. Trilobites are the focus of this
laboratory.
Arthropod means “jointed foot”. Jointed and paired appendages are important features of the arthropods.
Another important feature is the chitinous exoskeleton. Arthropods have been around for approximately 550
million years.
Arthropods have a segmented body that is bilaterally symmetrical. In other words, if an imaginary plane passes
through the length of the trilobite along its axis, then the two halves produced are mirror images of each other.
The body of an arthropod has a hard external covering composed of chitin. This hard external covering serves
as protection for the arthropod, as support for the soft tissues, and as an attachment for its muscles.
Arthropods develop from an egg and then pass through a larval stage. Most reproduction is sexual and the male
fertilizes eggs in the female body.
Class Trilobita
As mentioned, trilobites are extinct arthropods. Trilobites form a very important part of the early fossil record.
They first appear at the base of the Cambrian approximately 530-525 m.y. ago, and reach their maximum in the
mid-Ordovician. Trilobites declined during the Silurian and Devonian, were rare in the Pennsylvanian, and
disappeared in the middle Permian (about 270 m.y ago).
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Trilobites are exclusively marine and are common in shallow sea deposits of the Cambrian and Ordovician.
Trilobites found at the base of the Cambrian are structurally complex, suggesting that they evolved from an
earlier ancestor. Trilobites have not been found in Precambrian rocks. It is possible that the acquisition of hard
parts led to fossilizable parts. Species lacking hard parts have a very poor chance of being preserved.
Trilobites are composed of chitin impregnated with varying proportion of calcium phosphate or calcium
carbonate. The trilobite body is divided into three areas: the Cephalon, the Thorax, and the Pygidium.
Trilobite means “three lobed” and is named for the axial lobe and the two pleural lobes. They range in size from
6 mm (0.25 in) to a few giant ones near 75 cm (30 in) long. Most trilobites range from 2 to 7 cm (0.8 to 2.7
inches) in length and about 1 to 3 cm (.04 to 1.2 in) in width. During growth trilobites molts its exoskeleton and
sheds its hard parts, which may then become fossilized. These molts help with our studies of trilobite growth
stages
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©1999 - 2007 by S. M. Gon http://www.trilobites.info/trilobite.htm
Dorsal View.
Ventral View
http://www.trilobites.info/trilomorph.htm
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LAB
Use the diagrams above to help you complete the lab. GENTLY HANDLE ALL FOSSILS!!!!!!!!
I. Sketch the following sample (PUT A SCALE ON ALL OF YOUR DRAWINGS):
 No. 3 ; Parodoxides bohemicus, Age : Mid Cambrian, Locality : Bohemia, Ginetz
Identify and label the following features on your sketch:
1. Cephalon
head shield
eyes
glabellar lobes and furrows
2. Thorax
axis
axial furrows
pleura
axial lobe
pleural lobes
3. Pygidium
4. Anterior
5. Posterior
6. Ventral and dorsal side
7. Bilateral symmetry by sketching a plane through the axis to show the symmetry.
II. Observe the following trilobites. Make sure that you can identify the features you labeled above:
1. No. 12 ; Ollenellus Burtoni
Age : Cambrian
Locality : British Columbia, Canada
4. No. 174 ; Phacops rana Green
Age : Devonian - Erian
Locality : Ohio, half a mile north of Silica ;
Hamilton Gp.,
Silica shale
2. No. 20 ; Acadagnostus Sp.
Age : Mid Cambrian
Locality : Utah, Millard Co., House Range;
Swasey Fm. ; Condor shale member
5. No. 185. Tranthurus becki
Age: Devonian
Cold County , Oklahoma
3. No. 92 ; Isotelus gigas
Age : Ordovician
Locality : Carter County, Oklahoma ;
III. Answer the following questions:
1. When did triolobites first appear? _______________
2. When did trilobites become extinct? _______________
3. Trilobites are exclusively marine T/F
4. Why do you think that most of the trilobites that you are observing are found in very fine-grained rocks,
such as shales and other mudrocks, and not in sandstones?
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IV.
Plot the following on the Range Diagram and use the trilobite lineage diagram above to complete the
following.
You find rock layer B below rock layer C. The trilobites shown below are found in the units.
A. Mudstone
A
B. Well sorted, well rounded, medium grained quartz arenite.
`B
1. What is the age of unit B? _______________________
2. What is the age of unit A? _______________________
3. Is there a difference in ages between Units A & B?_____________
If so, how many years? _______________________
How would you account for this gap? _______________________
4.
Which rock was deposited in a higher energy environment? _______________________
5.
Which rock was deposited in a lake or river ?__________________
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Cenozoic
Quaternary
Tertiary
Mesozoic
Cretaceous
Jurassic
Triassic
Paleozoic
Per
Car
Dev
Sil
Ord
Cam
Fossil/T PreCam
axa
Unit
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Cenozoic
Mesozoic
Quaternary
Tertiary
Cretaceous
Jurassic
Triassic
Per
Paleozoic
Car
Dev
Sil
Ordo
Cam
PreCam
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