Depauperate skeletonized reef-dwelling fauna of the early Cambrian: Insights from archaeocyathan reef ecosystems of western Mongolia
Abstract Reef-dwelling organisms in the early Cambrian include typical members of the Cambrian Fauna such as trilobites, chancelloriids, and hyoliths. However, in comparison to later Ordovician reefs, Cambrian reefs often do not appear as diverse. Here, we investigate early Cambrian microbial-archaeocyathan reefs of the Salaagol Formation in the Zavkhan Basin of Mongolia as well as perform a literature survey of 40 early Paleozoic reefs to understand the timing of reef biodiversity. By using point count analysis, we can quantify the carbonate contribution of framework builders and reef-dwelling organisms in these ecosystems. We find that Mongolian reefs contained several genera of framework building archaeocyaths (5.0% by volume) and associated microbial forms (5.4%), but are unevenly distributed through the formation. Archaeocyathan contribution increases are potentially concurrent with negative shifts in carbon isotopic composition, similar to other localities in Gondwana. However, reef-dwelling biota was minimal (0.3%) throughout the Salaagol Formation and did not correlate to changes in framework builders or geochemical proxies such as U, V, and carbon isotopes. A literature survey of Ediacaran through Ordovician reefs suggests that there is a trend of increasing reef-dweller abundance, functional richness, and skeletonization through time. The changes in abundance (17.9% to 28.8% considered “frequent”) and functional richness (3.7 to 6.0 functional groups) could be due to an increase in productivity in the oceans during the Cambrian, thereby expanding the base of the food web during the lead up to the Ordovician Radiation. The trend of increasing skeletonization could be due to the evolution of more robust skeletons among reef dwellers in order to protect themselves from increased predation in reef environments. These trends suggest that, at least during the early Paleozoic, increases in biological diversity are not necessarily concurrent with the establishment of a framework reef.