99-4 Reefs, Animals and Skeletons: A Cambrian Origin Story

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part I

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The Cambrian Explosion has long been recognized as an interval of biological innovation, an aspect of which is the expansion of skeletons and new skeletal body plans among animals. Initial work on this interval noted that the appearance of skeletons must have fundamentally changed the carbon cycle, shifting the nature and locus of carbonate deposition to a skeletal factory in shallow water settings. Additional scrutiny of Cambrian and Ordovician strata determined that the true dominance of skeletal organisms occurred during the Great Ordovician Biodiversification Event, ~40 million years later. Nonetheless, certain aspects of the Cambrian skeletal carbonate factory remained significant, particularly archaeocyath reefs, that served as a clear locus of skeletal carbonate production. In recent years, a more sustained and nuanced examination of Cambrian carbonates and ecosystems globally has revealed that the Cambrian skeletal carbonate factory had an early heyday that coincided with the appearance and expansion of archaeocyath-calcimicrobial reef ecosystems. The decline of archaeocyath reefs marked a broader decline of skeletal diversity and abundance, both within and outside of reefs, and this new low was maintained until much closer to the GOBE. In this way, archaeocyath reefs acted as ecosystem engineers, creating habitat for a diversity of skeletal organisms, prior to their later early Cambrian decline. The only small biostromes that contain archaeocyaths after their initial decline in the western United States lack the diversity and abundance of their earlier counterparts, demonstrating that this ecosystem was devastated when the gregarious reefs disappeared. Although recent examination of a variety of Furongian microbial reefs has revealed siliceous sponges as framework contributors in many instances, the overall diversity and abundance of animals in these ecosystems remains low until the establishment of the sponge- and Calathium-microbial build-ups in Lower Ordovician strata. In conclusion, extensive examination of Cambrian carbonates exposes a complex mosaic of skeletal abundance across ecosystems, with reefs providing a pivotal ecological role, one that would continue throughout the Phanerozoic and into the modern ocean. Reefs are again the “canary in the coal mine” of the marine realm, and one that requires attention amidst our modern reef crisis.

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

doi: 10.1130/abs/2025AM-8429

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