Paleozoic Life History - The Changing Earth: Exploring Geology and Evolution - page 571

Geo-inSight

Trilobites—Paleozoic Arthropods

Trilobites, an extinct class of arthropods, are probably the
favorite and most sought after of invertebrate fossils. They
lived from the Early Cambrian until the end of the Permian
and were most diverse during the Late Cambrian. More than
15,000 species of trilobites have been described,

and they are currently grouped into nine orders. Trilobites
had a worldwide distribution throughout the Paleozoic and
they lived in all marine environments, from shallow,
nearshore waters to deep oceanic settings. They occupied a
wide variety of habitats. Most were bottom-dwellers, crawling
around the sea floor and scavenging organic detritus (see left)
or feeding on microorganisms or algae. Others were free-
swimming predators or filter-feeders living throughout the
water column.

Image not available due to copyright restrictions

Axial lobe

Cephalon

1. The trilobite body is divided into three parts: a cephalon
(head), containing the eyes, mouth, and sensory organs; the
thorax (body), composed of individual segments; and the
pygidium (tail). The name trilobite does not refer to its three
main body parts, but means three-lobed , which corresponds to
the three longitudinal lobes of the thorax—the axial lobe, and
the two flanking pleural lobes. Cedaria minor , from the
Cambrian-age Weeks Formation, Utah, illustrates the major
body parts of a trilobite.

Thorax

Pygidium

Pleural lobes

3. The appendages of trilobites are rarely
preserved. However, from specimens in
which the soft part of the anatomy is
preserved as an impression, paleontolo-
gists know that beneath each thoracic
segment was a two-part appendage
consisting of a gill-bearing outer branch
used for respiration and an inner branch or
walking leg composed of articulating limb
segments. This model of the dorsal and
ventral anatomy of Triarthrus eatoni is from
the Late Ordovician-age Frankfort Shale in
New York.

2. Model of dorsal (above) and ventral
(right) anatomy of the Late Ordovician
trilobite Triarthrus eatoni .

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