The two phases of the Cambrian Explosion.

ABSTRACT: 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 to Cambrian Stage 5 (~545 - ~505 Million years ago (Ma)) of the 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 early Cambrian Terreneuvian and again in Stage 2, but were devastated during the early Cambrian Stage 4 Sinsk extinction event (~513?Ma) never to recover previous diversity. Inferred crown group brachiopod and mollusc species (n?=?76) do not appear until the Fortunian, ~537?Ma, radiate in the early Cambrian 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 diversification 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 of the carbon cycle. We therefore propose two phases of the Cambrian Explosion separated by the Sinsk extinction event, the first dominated by stem groups of phyla from the late Ediacaran, ~542?Ma, to early Cambrian 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.

SUBMITTER: Zhuravlev AY 

PROVIDER: S-EPMC6226464 | biostudies-other | 2018 Nov

REPOSITORIES: biostudies-other