29-2 Paleoenvironmental Analysis of the Earliest Cambrian Trilobites in Laurentia

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts (Posters)

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Abstract:

As current literature is surprisingly quite sparse on the subject, this research aims to elucidate the particular paleoenvironmental conditions of the habitat of the very first trilobites of Laurentia during the Cambrian Explosion. Current literature hypothesizes that trilobites evolved in the early Cambrian (Terreneuvian) in shallow marine environments. However, the finer details of the conditions of these paleoenvironments are unknown and thus testing this longstanding hypothesis constitutes the basis of this research. Current literature has only considered the paleoenvironment that the first trilobite body fossils occur in but fails to account for the arthropod trace fossils that occur much earlier than the first mineralized trilobite fossils.

This project involves studying the paleontology and sedimentology of the lower Cambrian Campito Formation (most notably the basal Gold Coin Member) and the upper strata of the underlying Deep Springs Member in Western Nevada and Eastern California. The Gold Coin Member contains specimens of some the of the oldest Cambrian trilobites in North America. This member is characterized by thinly bedded siliciclastic siltstones and sandstones. Arthropod trace fossils occur throughout the member, mainly Rusophycus and Monomorphicnus. The upper part of this member occurs within the Fritzaspis Assemblage Zone. This assemblage zone marks the start of the very first occurrence of trilobite fossils (genus Fritzaspis). Abundant disarticulated fragments, as well as whole articulated trilobites occur in this unit, as well as arthropod traces. This member is thought to represent a lowstand systems tract.

This study utilizes the Natural History Museum of Los Angeles County Invertebrate Paleontology collection of Campito Formation trilobites and Deep Springs Member arthropod trace fossils along with their associated sedimentary strata. Preliminary results of a size comparison study of Campito body fossils versus pre-trilobitic Deep Springs trace fossil size indicates that the trace fossils exceed the average trilobite body size in the Campito Formation. This suggests a need to pay greater attention to the arthropods and their environment in the Deep Springs Member. Additional studies of thin sections of the rocks associated with museum specimens are being used to further interpret depositional environment.

Overall, this research will provide a deeper understanding of the habitats and evolution of the earliest trilobites, and by proxy, the evolution of complex life on Earth during the Cambrian Explosion.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6761

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