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| Left to right: Two examples of archaeocyathids preserved in limestone from the lower Cambrian Poleta Formation, roughly 520 million years old. At left is a branching specimen assigned to the Class Irregularia. At right is a more common, typical archaeocyathid preservational aspect--a natural cross-section of the extinct calcareous sponge (member of the living phylum Porifera) showing the hollow interior surrounded by a distinctive double wall separated by many partitions. |
| A chunk of limestone from the lower Cambrian Poleta Formation that reveals several natural cross-sections of archaeocyathids--an extinct calcareous sponge (phylum Porifera)--in their classic preservational aspect--a hollow interior surrounded by a double-wall exterior separated by many partitions. The specimens are approximately 520 million years old. |
| This is an Olenellid trilobite cephalon (head shield) from the lower Cambrian Poleta Formation. It is certainly of the genus Nevadia, and most likely can be assigned with a fair degree of accuracy to Nevadia parvoconica. The specimen is roughly 518 million years old. |
| A slab of quartzite (heat and pressure-altered sandstone) from the lower Cambrian Poleta Formation bearing numerous annelid (worm) trails--the sinewy ridges that crisscross the rock surface. Such ichnofossils are typical of Poleta Formation exposures that lie directly above (hence, they are younger) the archaeocyathid-rich limestone accumulations at the base of the Poleta stratigraphic section. The specimens are roughly 518 million years old. |
| An Olenellid trilobite cephalon (head shield) from the lower Cambrian Montenegro Member of the Campito Formation. Angle and length of genal spines, in combination with thickened area at top of cephalon and general shape of glabella all help to place this specimen in the genus Nevadella sp., which is about 521 million years old. |
| An Olenellid trilobite cephalon (head shield) from the lower Cambrian Montenegro Member of the Campito Formation. As far I can determine, it matches no known previously described Campito Formation trilobite. I found this specimen in virtually the same spot that I located the Campito Formation Nevadella specimen, directly above, so presumably it came from an identical early Cambrian biozone--usually referred to by paleontologists as the Fallotaspis Zone, in honor of the most characteristic trilobite present at that specific stratigraphic horizon. The extinct arthropod is around 521 million years old. |
| Here's the best trilobite I found in the Lower Cambrian Montenegro Member of the Campito Formation during a visit to Esmeralda County, Nevada. 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 Fallotaspis cf. bondoni. 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 Fallotaspis, 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. Estimated geologic age of the specimen is approximately 521 million years old. |
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| Left to right: Inarticulate brachiopods from the lower Cambrian Poleta Formation, roughly 518 million years old, from shales that occur immediately above the archaeocyathid-bearing limestones lowest in the Poleta Formation. Specimen at left is genus Lingulella sp.; brachiopod at right is called scientifically Mickwitzia muralensis. |
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| Here are two well-preserved, virtually complete specimens of the extinct Helicoplacus guthi--the oldest fossil echinoderm yet discovered in the geologic record; they're from the lower Cambrian Poleta Formation of approximately 518 million years ago. Note the partial US penny for perspective at upper right and lower left in photographs (left to right), respectively. Photographs courtesy of Taphonomy and Environmental Distribution of Helicoplacoid Echinoderms by Stephen Q. Dornbos and David Bottjer, PALAIOS, 2001, V. 16, p. 197-204. |