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From Eldredge (1991)
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INTRODUCTION
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Arthropods are the most diverse
phylum today and probably also in the geologic past. They
are a highly specialized group which are characterized by
their bilaterally symmetrical body, paired appendages and a
chitinous (calcite in some groups) exoskeleton. As the
exoskeleton once produced remains inert, arthropods must
periodically shed their exoskeleton during molting and
re-precipitate a larger one in order to accommodate their
larger size.
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Many arthropods may be familiar to
you such as crabs, lobsters, barnacles, and insects, yet the
fossil record of the phylum is dominated by few groups,
particularly the trilobites, and to a lesser extent the
eurypterids, and ostracodes. The trilobites in particular
are unparalleled for their biostratigraphic utility for
Cambrian and Ordovician and to a lesser extent Devonian
sediments. Conversely, the eurypterids and ostracodes are
more useful in determining ancient environments at least
during mid to latter Paleozoic times.
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CLASSIFICATION
& GEOLOGIC RANGES
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Phylum
Arthropoda (Precambrian-Recent)
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Superclass
Trilobitomorpha
(Cambrian-Permian)
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Class
Trilobita (Cambrian-Permian)
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Order
Polymerida (Cambrian-Permain)
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Order
Agnostida (Cambrian-Ordovician)
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Superclass
Crustacea (?Precamb.,
Cambrian-Recent)
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Class
Ostracoda (Cambrian-Recent)
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Superclass
Chelicerata (Cambrian-Recent)
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Class
Merostomata (Cambrian-Recent)
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Order
Xiphosurida (Cambrian-Recent)
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Order
Eurypterida (Ordovician-Permain)
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TRILOBITOMORPHS
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General
Morphology
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Like other arthropods,
trilobitomorphs are characterized by numerous jointed and
paired appendages. The calcitic and/or chitinous exoskeleton
of trilobites consists of three lobes: a central axial lobe,
and two lateral pleural lobes (see Figure
1 below). As shown in
Figure
1, the exoskeleton of
trilobites can be divided lengthwise into three regions: a
fused head segment called a cephalon; a fused tail segment
called a pygidium which sometimes bears spines,
but is normally completely fused with a smooth margin;
and a mid region or thorax consisting of numerous segments.
Re-examine the previous images and be sure that you can
recognize these morphologic features on a single
specimen.
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The cephalon also contains several
unique features. The glabella is the extension of the axial
lobe into the cephalic region and is often ornamented with
furrows and ridges. Lateral to the glabella are the eyes
which can be either of compound or single lens type. Forward
of the glabella is the frontal area, a relatively flat
region which sometimes bears pits. The terminal margin of
the cephalic segment may be ornamented with genal spines.
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Figure 1 -
General Trilobite Morphology
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Modified from McRoberts (1998)
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During molting, many trilobites
split their exoskeleton along facial sutures which separates
the fixed cheek from the free cheek. Depending on their
position relative to the genal angle, these sutures can be
of four types: (see Figure
2 below) opisthoparian
terminating behind the genal angle, proparian
terminating forward of the genal angle, gonatoparian
dissecting the genal angle, and marginal along the
cephalic margin.
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Figure 2 -
Trilobite Facial Suture Types
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Modified from McRoberts (1998)
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Paleoecology
and Life Habits
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Before any attempt at a
paleoecologic or life-habit interpretation is made one must
first recognize the nature of the fossil sample. As
mentioned earlier, all arthropods periodically molt their
exoskeleton to accommodate for larger size; as such, one
individual may produce many molts or instars (or sometimes
called exuvea). To determine if a trilobite fossil is a molt
or body fossil see if the suture is split and the free
cheeks are missing. If so, the fossil may be interpreted as
a molt. If the suture is intact and there is no evidence of
splitting, the fossil may be interpreted as a body fossil.
Examine this image and see if you can identify it as a molt
or a body fossil.
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Trilobites are very common in
marine limestones and shales of the early Paleozoic,
especially from the Cambrian Period. Most trilobites were
epifaunal crawlers. Although they occupy a wide variety of
exclusively marine habitats, specific life habits are
difficult to discern by morphology alone. Nonetheless,
several aspects of trilobite morphology can indeed provide
some clue as to the life habit or activity. Examples include
the elongated cephalic shield of the example which may have
aided in ploughing through sediments.
Although most trilobites are considered to have been
benthic, the small size and non-descript morphology of
agnostid trilobites suggests that these (along with some
others) may have been nektonic or nekto-benthic. Enrolling
of trilobites may certainly have been a defensive mechanism.
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Taxonomy
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According to some texts, trilobites
are considered to have phylum status and are divided into
eight Orders. A less radical classification which is used
here (and in other texts) treats trilobites as a Superclass
or Class with two orders: the Polymerida and the
Agnostida. The Polymerida are by far the most diverse
of the two in regards to species diversity and also
morphologic and ecologic types. The Polymerids can be
identified by their larger size, a well defined cephalic
region with eyes and facial sutures, and a large number of
thoraxic segments.
An easier way to identify Polymerids is by default; if its
not a agnostid (easy to identify) then it's a Polymerid. All
of the images you have seen thus far are Polymerids, here is
another example from this diverse group.
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Agnostid trilobites are easily
recognizable by their small size, few thoraxic segments
(usually around two), and a cephalon without eyes which is
superficially similar in morphology to the pygidium.
Furthermore, agnostids lack facial sutures.
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but is normally completely fused with a smooth margin;
and a mid region or thorax consisting of numerous segments.
Re-examine the previous images and be sure that you can
recognize these morphologic features on a single
specimen.
Although most trilobites are considered to have been
benthic, the small size and non-descript morphology of
agnostid trilobites suggests that these (along with some
others) may have been nektonic or nekto-benthic. Enrolling
of trilobites may certainly have been a defensive mechanism.
An easier way to identify Polymerids is by default; if its
not a agnostid (easy to identify) then it's a Polymerid. All
of the images you have seen thus far are Polymerids, here is
another example from this diverse group. 
Furthermore, agnostids lack facial sutures.