Cambrian Fossils in the Bright Angel Shale
of Northern Arizona:
"SW"
Locality
Site
and Key
Trilobite
Fossils Section
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The dominant trilobite at
this site - Zacanthoides.
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Updated 6/7/17
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This site, located
south of the Grand Canyon is one of two BAS localities in which
we found locally abundant fossils. They are preserved as highly
detailed molds, occuring in dense layers 1 - 2 inches thick.
Finding the correct shale horizon is the trick here, then that
layer is worked horizontally. Underlying the trilobite layer
is a mass mortality layer of an unidentified colonial animal,
and a nearby hill is a higher section in the stratographic column
which contained only trace fossils at the base, and the Muav
Limestone at the top which contained no fossils.
Note: All images
are small thumbnails, click for a full size image!
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Site Images:
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Paleontologist Rose Fowers splitting shales, and taking precise measurements.
The shales are green and slightly micaceous in nature, and the fossiliferous horizons
are usually a few inches thick separated by non fossiliferous zones. Fossils are found
as external molds of dorsal and ventral impressions, and also as calcareous replacement
of small delicate specimens. Whole trilobites are rare, and as usual - most are molts.
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Key Fossils:
Trilobites - Complete or nearly complete specimens Type 1 - Zacanthoides walapai
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Two types of trilobites are found here. This is "Type 1", and we have found
several complete specimens. This one is missing the huge free cheeks and
stubby genial spines.
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Another nearly complete specimen minus one free cheek and both genial spines.
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Highly flattened specimen, with both cheeks but missing parts of the
cranidium and genial spines.
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This slightly distorted specimen only missing one side of the cephalon shows relatively short
genial spines and how oversize the cephalon is compared to the highly tapering thorax. The
eye lenses are apparent on this one!
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A nice thorax and pygidium I found, the red dot is 1/4" diameter.
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Thorax and pygidium of another type 1 that we collected.
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Thorax and pygidium in a partially enrolled state.
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Trilobites - Reconstruction of individual Exuvia - Type 1
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I took some of the better parts we found and layed them out so you can see
the taphonomic preservational variation in a single species. Top we have
cranidiums, center thorax, and bottom pygidiums.
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Free cheeks attached to cranidium and rescaled.
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Trilobites - A few select key specimens of Type 1 (Zacanthoides walapai)
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Cranidiums are flat and contain substantial palpebral lobes and a stout
cephalic border. A small hypostome appears to its right.
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Free cheek (Librigena) with genial spine. Note the sharp edge to where the
eye would be attached at the bottom of the cheek. Zacanthoides had very
narrow eyes that were literally slits that could see 180 degrees. It would be
able to see the horizon but not above the animal.
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Click for a full size image!
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Another free cheek with genial spine. A very tiny (3mm) pygidium of a
juvinile is on its right. Smallest pygidums we found were 1mm.
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In this dime sized field, this pygidium shows a rounded axial lobe termination,
and fan-like furrows. This one has a rounded profile on the bottom.
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Another pygidium, very well preserved about 1 cm. A lip like border can be
seen here.
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Thorax, the top forward end is complete with a nice articulating half ring. Note
the crisp axial and pleural furrows here!
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Hypostomes (mouth cover plate)
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is a nice external mold of a Zacanthoides hypostome. A large
number of these were found amongst the crandiums on the green
shale slabs. The size ranged from less than .1" to about.35"
in length. |
Click for a full size image!
| Postitive
cast from a hollow mold preserving the external appearance in
three dimensions. This is a small specimen, the arrow is about
.2" wide. |
Trilobites - Type 2 - Amecephalus Althea
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A major find was the type 2 trilobite which just over 50% of the body was
seen. This allows a digital reconstruction seen below to aid identification. The
cephlon of a much larger trilobite is overprinting it. The size is approximately
2 cm.
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By digitally folding over the visible half, we can get a more complete picture
of what a compete specimen would have looked like. Note that this one even has
facets on its eyes!
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