During the early Cambrian, organisms with robust skeletons began to integrate with microbialite reef structures. Specifically, archaeocyathan sponges were among the first metazoan reef-builders. Here we investigate the transition from microbial-dominated reef environments to metazoan-based reefs from strata in the western Basin and Range of California and Nevada. This study integrates point count data from petrographic thin sections with stable carbon isotopic and elemental composition of carbonates. From the earliest reef bearing formations to the latest, metazoan framework contribution increases from zero to 29.7%. This increase is linked to the addition of new framework-building organisms, namely coralomorphs, as well as an increase in archaeocyath body size. Correspondingly, Shannon's diversity increases from 0.652 to 1.492. However, skeletal contributions from additional organisms within the reefs (e.g., trilobites, echinoderms) appear unchanged and their diversity is not correlated with framework-builder diversity. A positive carbon isotopic excursion within the Lower Poleta Formation correlates with decreases in the abundances of uranium and molybdenum that suggest a global change in organic carbon burial as opposed to localized or diagenetic factors. This allows for chemostratigraphic correlation to published carbon isotopic data and provides a proposed regional age constraint of roughly 517 million years. Overall, early Cambrian reefs at this location exhibit a pattern of increasing metazoan contribution during the transition from microbial- to metazoan-based reef support, however, diversity remained low until additional organisms evolved to inhabit these ecosystems.
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Vol. 34 • No. 3