The two phases of the Cambrian Explosion

Rachel A. Wood,Andrey Yu. Zhuravlev

doi:10.1038/s41598-018-34962-y

Rachel A. Wood, Andrey Yu. Zhuravlev

Open Access

https://doi.org/10.1038/s41598-018-34962-y

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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.

Highlights

The Cambrian Explosion is a phenomenon that encompasses the dramatic appearance of diverse metazoans with biomineralized skeletons, an increase in metazoan complexity and behaviour, a substrate revolution that re-organised the sedimentary record, and the development of biodiverse marine ecosystems with complex food webs[1,2,3,4,5]
The combined body and trace fossil record suggests the Cambrian Radiation of bilaterians may have followed a progressive two-stage diversification: the terminal Ediacaran (~560 Ma) to early Cambrian Stage 2 to 3 interval dominated by stem groups, and after Cambrian Stage 2 to 3 when definitive crown group representatives of phyla appeared[18]
The former problematic fossil Tumulduria is reinterpreted as a detached central portion of the ventral interarea of a paterinid brachiopod[47]

Summary

Introduction

The Cambrian Explosion is a phenomenon that encompasses the dramatic appearance of diverse metazoans with biomineralized skeletons, an increase in metazoan complexity and behaviour, a substrate revolution that re-organised the sedimentary record, and the development of biodiverse marine ecosystems with complex food webs[1,2,3,4,5]. The combined body and trace fossil record suggests the Cambrian Radiation of bilaterians may have followed a progressive two-stage diversification: the terminal Ediacaran (~560 Ma) to early Cambrian Stage 2 to 3 (mid-Tommotian to Atdabanian) interval dominated by stem groups, and after Cambrian Stage 2 to 3 when definitive crown group representatives of phyla appeared[18]. Placing extinct fossil taxa in phylogenetic order through the application of stem- and crown group concepts allows the order of character acquisition to be considered in both time and environmental context[18]. This consists of the primitive relatives of the crown group, along the phylogenetic line up to, but not including, the last common ancestor of the crown group and their nearest living relatives[19]. Mass extinctions may operate quite differently, as they can remove taxa selectively based on particular ecological or other traits[21] and lead to long-lasting changes in taxonomic composition and ecosystem functioning[22]

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