| 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. 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 Cambroinyoella wallacei. This is just about the oldest articulated trilobite one can be expected to find in the geologic record. A species of Eofallotaspis occurs in a rather narrow 30 centimeter bed directly below where I found my Cambroinyoella, but they're all fragmentary, disarticulated remains of head shields--except, that is, for a few very rare, poorly preserved complete specimens; and quite rare, stray occurrences of trilobites have been spotted a few tens of feet below the Eofallotaspis bed in the Andrews Mountain Member of the Campito Formation. 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. Geologist and Cambrian trilobite specialist J. S. Hollingsworth has had Nevada's Esmeralda County under serious study for over a decade now; he is the leading expert on the trilobite fauna of the Lower Cambrian Campito Formation. |