Increases in reef size, habitat and metacommunity complexity associated with Cambrian radiation oxygenation pulses

Abstract

Oxygenation during the Cambrian Radiation progressed via a series of short-lived pulses. However, the biotic response of metazoans to this episodic oxygenation has not been quantified, nor have the causal evolutionary processes been constrained. Here we present high-resolution ecological metrics of archaeocyath sponge reef communities on the Siberian Platform through Cambrian stages 2 to 3 (525–514 Million years ago (Ma)). During the oxic pulse at ~521–519 Ma, we quantify an expansion of reef habitat both on- and offshore, together with an increase in both reef size and the ecological complexity of the metacommunity, from communities where individual species responded to their local environment (Gleasonian), to more ecologically complex where groups of species show a synchronous community-wide response (Clementsian). These changes were accompanied by an increase in individual archaeocyath body size, total species diversity, and rates of origination. But during the succeeding phase of expanded marine anoxia (~519–516.5 Ma), reef and archaeocyath body size reduced as did diversity, driven by increased rates of extinction. A later oxic pulse at ~515 Ma shows further reef habitat expansion, increased archaeocyath body size and diversity, but a return to less complex, quasi-Clementsian communities. These metrics confirm that oxygenation events created temporary pulses of evolutionary diversification and enhanced ecosystem complexity, potentially via the expansion of habitable space, and increased archaeocyath individual and reef longevity in turn leading to niche differentiation. Most notably, we show that progression towards increasing biodiversity and ecosystem complexity was episodic and discontinuous, rather than linear, during the Cambrian Radiation.