The two phases of the Cambrian Explosion

The dynamics of how metazoan phyla appeared and evolved – known as the Cambrian Explosion – remains elusive. We present a quantitative analysis of the temporal distribution (based on occurrence data of fossil species sampled in each time interval) of lophotrochozoan skeletal species (n= 430) from the terminal Ediacaran toCambrian Stage 5 (~545 – ~505 Million years ago (Ma)) ofthe Siberian Platform, Russia. We use morphological traits to distinguish between stem and crown groups. Possible skeletal stem group lophophorates, brachiopods, and molluscs (n= 354) appear in the terminal Ediacaran (~542 Ma) and diversify during the earlyCambrianTerreneuvian and again in Stage 2, but were devastated during the earlyCambrian Stage 4 Sinsk extinction event(~513 Ma) neverto recover previous diversity. Inferred crown group brachiopod and mollusc species (n= 76) do not appear untilthe Fortunian, ~537 Ma, radiate in the earlyCambrian Stage 3 (~522 Ma), and with minimal loss of diversity at the Sinsk Event, continued to diversify into the Ordovician. The Sinsk Event also removed other probable stem groups, such as archaeocyath sponges. Notably,this diversifcation starts before, and extends across the Ediacaran/Cambrian boundary and the Basal Cambrian Carbon Isotope Excursion (BACE) interval (~541 to ~540 Ma), ascribed to a possible global perturbation ofthe carbon cycle.We therefore propose two phases of the Cambrian Explosion separated by the Sinsk extinction event, the frst dominated by stem groups of phyla from the late Ediacaran, ~542 Ma,to earlyCambrian stage 4, ~513 Ma, and the second marked by radiating bilaterian crown group species of phyla from ~513 Ma and extending to the Ordovician Radiation