Biotic responses to oxygenation pulses during the Cambrian Radiation

Oxygenation during the Cambrian Radiation progressed via a series of short-lived pulses. However, the metazoan biotic response to this episodic oxygenation (and potentially productivity changes) has not been quantified, nor have the causal evolutionary processes been constrained. Here we present analyses of the dynamics of early Cambrian metazoan body size changes, habitat distribution, and ecological complexity on the Siberian Platform (525–510 Ma).

First, we quantify high-resolution changes in species body size in archaeocyath sponges, hyolith lophophorates, and helcionelloid molluscs, and brachiopods. Archaeocyath, hyoliths, and helcionelloids, show dynamic and synchronous trends over million-year timescales, with peaks in body size during the latest Tommotian/early Atdabanian (~521–519 Ma) and late Atdabanian/early Botoman (~519–516.5 Ma), and notably small body sizes in the middle Atdabanian and after the Sinsk anoxic extinction event, starting ca. 513 Ma. These intervals of body size changes are also mirrored in individual species and correlate positively with increased rates of origination and broadly with total species diversity. Calcitic brachiopods (rhynchonelliformeans), however, show a general increase in body size following the increase in species diversity through this interval; phosphatic brachiopods (linguliformeans) show a body size decrease that negatively correlates with diversity. Both brachiopod groups show a rapid recovery at the Sinsk Event. The synchronous changes in these metrics in archaeocyaths, hyoliths and helcionelloids suggest the operation of external drivers through the early Cambrian, coincident with two oxic or productivity pulses. But the trends shown by brachiopods suggests a differing physiological response. Together, these dynamics created both the distinct evolutionary record of metazoan groups during the Cambrian Explosion and determined the nature of its termination.

Second, during the oxic pulse at ~521–519 Ma, we quantify the expansion of archaeocyath sponge reef habitat 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). The 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.