Lower Cambrian Campito Formation

Esmeralda County, Nevada

Here's the best trilobite I found in the Lower Cambrian Montenegro Member of the Campito Formation during a visit to Esmeralda County, Nevada; in actual size, the specimen is 25 millimeters long (approximately an inch). Must have been beginner's luck. I wasn't at the main trilobite quarry more than five minutes, splitting chunks of shale others had neglected to split thoroughly, when a nondescript piece of shale literally cleaved in my hands to reveal a mostly complete specimen of Eofallotasps. This is just about the oldest articulated trilobite one can be expected to find in the geologic record. Rare specimens of Repinaella--the oldest trilobite yet discovered in the geologic record--occur in the underlying (and hence, older) Andrews Mountain Member of the Campito Formation several feet below where I found my Eofallotaspis, but they're all fragmentary, disarticulated remains of cephalons, thoraxes, and pygidia. The Eofallotaspis bed exposed throughout Esmeralda County, Nevada, marks the base of the proposed Montezuman Stage of the Early Cambrian Waucoban Series, which is that point in the geologic record where the first common trilobites begin to appear.

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