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 metazoan biotic response to this episodic oxygenation has not been quantified, nor have the causal evolutionary processes been constrained. Here we present ecological analyses of Cambrian archaeocyath sponge reef communities on the Siberian Platform (525–514 Ma). During the oxic pulse at ~521–519 Ma, we quantify reef habitat expansion coupled to an increase in reef size and metacommunity complexity, from individual within-community reactions to their local environment, to ecologically complex synchronous community-wide response, accompanied by an increase in rates of origination. Subsequently, reef and archaeocyath body size are reduced in association with increased rates of extinction due to inferred expanded marine anoxia (~519–516.5 Ma). A later oxic pulse at ~515 Ma shows further reef habitat expansion, increased archaeocyath body size and diversity, but weaker community-wide environmental responses. 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.

Cambrian radiation speciation events driven by sea level and redoxcline changes on the Siberian Craton

“…1C) ( 28 – 30 ). This epeiric basin supported a single metacommunity of reef-building metazoans, i.e., a single species pool consisting of many local, interacting communities without immigration from elsewhere ( 21 ). The inner area of the Siberian Craton in the (present day) southwest area consisted of shallow marine carbonates together with thick evaporites and minor siliciclastics, known as the Turukhansk-Irkutsk-Olekma carbonate platform.…”

“…( C ) Uranium isotopes from Siberia (gray; Sukharikha and Bol’shaya Kuonamka rivers), South China (blue), and Morocco (orange) (all data points are larger than 2 SE) ( 19 ). ( D ) Archaeocyath sponge species α-, β-, and γ-diversity ( 21 ). ( E ) Rates of archaeocyath sponge species origination and extinction ( 24 ).…”

“…Synchronous and highly dynamic changes of the body size of hyoliths, helcionelloid molluscs, and archaeocyath sponges during the early Cambrian on the Siberian Craton coincide with elevated biodiversity and rates of origination, and these are hypothesized to have been driven by OOEs ( 24 ). Likewise, individual reef size and depth range of shelf occupation, as well as ecosystem complexity, also waxed and waned following proposed OOEs, also demonstrating that the Cambrian radiation progressed in an episodic, rather than a continuous linear way ( 21 ). The causal processes of diversification are not, however, established.…”

“…Available data support the model that oxygenation during the Cambrian did not increase progressively and unidirectionally but rather proceeded via a series of short-lived oxygenation pulses, each of which lasted between 1 to 3 Ma, known as “oceanic oxygenation events” (OOEs) ( 23 ). These episodes of oxygen increase (potentially accompanied by productivity changes) are suggested to have expanded the area of oxygenated shallow-marine carbonate habitats on the Siberian Platform and hence to have promoted diversification ( 20 , 21 ). Synchronous and highly dynamic changes of the body size of hyoliths, helcionelloid molluscs, and archaeocyath sponges during the early Cambrian on the Siberian Craton coincide with elevated biodiversity and rates of origination, and these are hypothesized to have been driven by OOEs ( 24 ).…”

“…Here, we consider the temporal and spatial distribution of early Cambrian reef-building archaeocyath sponge species on the biodiversity hot spot of the Siberian Craton integrated with environmental data to provide insight into speciation dynamics and processes ( 20 , 21 ). We incorporate both outcrop and newly compiled subsurface borehole occurrence data within a revised age model (see Material and Methods, table S1, figs.…”