Incorporation, Morphology, and Extinction of Framework-Building Metazoans in Early Cambrian Reef Ecosystems from the Western Usa and Mongolia and Their Effects on Reef Diversity

The early Cambrian represents an important transition in the evolution of life, perhaps most vividly exemplified by reef ecosystems as they changed from microbial-supported to metazoan-supported framework reefs. Microbial reefs were initially composed of Renalcis- and Epiphyton-group calcifying microbes. Subsequent reefs began to incorporate archaeocyathan sponges within this framework. This represents a shift in the source of carbonate production, which can be quantified using thin section point counts. In archaeocyathan reefs from the western USA, carbonate contribution from metazoan framework builders increased from zero to 29.7%. Similar reefs from Mongolia increased from zero to 5.0%. Increases in Laurentian archaeocyath contributions are not associated with shifts in carbon isotopic composition or changes in global redox conditions, while Gondwana examples might be associated with a negative carbon isotopic excursion and increase in redox sensitive elements. The incorporation of metazoan framework builders is not associated with an increase in reef dwellers, as one might expect based on the niche supporting roles that framework builders play in modern reefs. To further explore the timing of reef dweller biodiversity, a literature survey was conducted that shows an increase in reef-dweller abundance (17.9% in the Cambrian to 28.8% considered “frequent” in the Ordovician), functional richness (3.8 to 5.9 functional groups), and skeletonization. Furthermore, archaeocyath gross morphologies are also highly constrained to a few (3-6 categories) simple morphologies and smaller body sizes compared to lithistid and modern demosponges. It therefore may not be unusual for early Cambrian reefs to have reduced reef-dweller diversity, potentially due to a combination of low ocean productivity and restricted morphological diversity. Archaeocyaths went extinct at the end of Stage 4 of the Cambrian. This ushered in a period of low reef carbonate contribution from metazoans. This post-archaeocyath interval is preserved in western Nevada, but this locality does not contain substantial evidence of either metazoan or microbial framework building. As archaeocyaths were an important framework builder, their extinction may have resulted in a local reef eclipse. This work highlights the early Cambrian as a transitional period between the minimal diversity of the Proterozoic and high diversity reefs of the later Paleozoic