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Living Arthropods include insects, spiders,
scorpions, ticks, crabs, lobsters, barnacles, and centipedes. Trilobites
belong to an extinct class of marine organisms called Trilobita. This name refers to
the three-part (tri-lobes) latitudinal and longitudinal shape of a
trilobite's exoskeleton. The latitudinal lobes consist of the cephalon
(head), segmented thorax (body), and pygidium (tail); the longitudinal
lobes consist of two lateral lobes (on each side of the body) and an axial
lobe (central back area of the exoskeleton).
When did they live?
More than 500 different trilobite species have been
found across Utah, in a broken band of Cambrian Period (570 to 500 million
years old) limestones, siltstones, and shales that trends
northeast-southwest across the western part of the state. During the Early
Cambrian (about 570 to 540 million years ago), western Utah was covered by
a shallow sea. Slow-moving rivers flowed across the sandy lowlands of
eastern Utah deposited sediment into the sea. The heavier sediment (mostly
sand) was deposited near the shoreline which metamorphosed through time
into quartzite. The lighter sediments (mostly silt) were deposited farther
out into the sea, and through time lithified into siltstone and shale. The
deepest part of the sea was an ideal environment for the precipitation of
calcium carbonate, which lithified to limestone.
Regional subsidence during the Middle and Late
Cambrian (about 540 to 500 million years ago), caused the sea's shoreline
to migrate eastward across Utah, allowing the deposition of a fairly
complete sequence of Cambrian sediment in western Utah. Utah was located
near the equator during the Cambrian, so the water temperature was warm.
The combination of warm, shallow water and nutrient-rich silt allowed
several marine genera to thrive. The most common and diverse of these were
trilobites, which occupied several different marine environments.
Where did they live in the sea?
Most trilobite species were bottom dwellers that
crawled over sand and mud. Some of them could curl up like modern pill
bugs. Other trilobites burrowed into bottom sand and mud using their
shovel-shaped cephalons. These crawling and burrowing trilobites were
either scavengers, or they ingested mud and silt, digesting the organic
material contained in it like modern day worms (annelids). Some trilobites
lived in shallow burrows where they could keep their heads near the
surface of the sand or mud, and grab passing prey. Fossil evidence
suggests some trilobites were capable of swimming. The bodies of swimming
trilobites are narrower and the eyes are closer to the sides of the
cephalon, than those of bottom-dwelling trilobites. Swimming trilobites
may have been predators, or they may have been "filter-feeders"
using special appendages to remove nutrients from the surrounding water.
The smallest trilobites were plankton-like and lived close to the water
surface.
Where are trilobites found in Utah?
Trilobites are probably the most common fossils
collected in Utah, many world-class specimens from this state reside in
museums throughout the world. In Utah, trilobites can be found at several
localities.
House Range
The Wheeler Amphitheater in the House Range, Millard County is one of the
more well-known collecting areas. Most of the trilobites in this area come
form the Middle Cambrian formation called the Wheeler Shale. The Wheeler
Shale contains interbeds of shaley limestone, mudstone, and thin platy
limestone. Another trilobite-bearing unit that directly overlies the
Wheeler Shale in the central part of the House Range is the Marjum
Formation. This formation consists of thin-bedded, fine-grained, silty
limestone with interbeds of shale and mudstone. Also located in the central part of the House Range
is a fossiliferous limestone called the Weeks Formation, that crops out in
North Canyon near Notch Peak. The Weeks Formation overlies (is younger
than) the Marjum Formation and also contains trilobites.
Wellsville Mountains
Another trilobite-bearing unit is the Spence Shale Member of the Langston
Formation in the Wellsville Mountains, Box Elder County. Here, trilobites
can be found in Miner's Hollow, Cataract Canyon, Dry Canyon, and the area
between Antimony and Hanson Canyons.
Photo by Raquel Halladay
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