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.

The Scaly-foot Snail genome and implications for the origins of biomineralised armour

“…Since then, a long history of body plan modifications, reversals, and convergences has obscured the relationships among some animal lineages 2 . Understanding the genomic toolkit that enabled innovations in skeletons and armour is critical to reconstructing the early radiation of major clades 3,4 . In particular, Lophotrochozoa (including annelids, brachiopods, molluscs and others) presents a significant and unresolved problem for phylogenetic reconstruction, and controversies over the affinities of key fossils 5 .…”

Was the Kalkarindji continental flood basalt province a driver of environmental change at the dawn of the Phanerozoic?

“…Global diversity in the early Palaeozoic began with an initial radiation during the first 20 million years of the Cambrian., commonly known as the "Cambrian Explosion" (Sepkoski, 1979;Zhu et al, 2006;Bambach et al, 2007;Servais et al, 2008) occurring within two distinct phases, separated by significant environmental and biotic events (Zhuravlev and Wood, 2018). Despite the evident proliferation of life, two critical intervals of major genera-level losses and taxonomic turnover appear in apparent synchrony with the Kalkarindji (Zhuravlev and Wood, 1996;Faggetter et al, 2017).…”

“…Extended intervals of marine anoxia are often associated with warmer ocean temperatures due to the diminished capacity for warmer water to store dissolved oxygen (Hough et al, 2006). Records of warm sea surface temperatures (Wotte et al, 2019), a global greenhouse climate (Hearing et al, 2018;Zhuravlev and Wood, 2018) and potential global warming indicated by deposition of extensive carbonate and evaporite deposits (Keller et al, 2012;Schmid, 2017) are evident during the window of time for the Kalkarindji emplacement. During this period of the spread of ocean anoxia may have rendered shallow marine environments potentially vulnerable to CH4 and/or H2S poisoning during transgression.…”

Early Palaeozoic diversifications and extinctions in the marine biosphere: a continuum of change

“…The Cambrian Explosion is distinguished by the apparent origination of bilaterian body plans, essentially phyla, the endemism of its biotas and a dominance in many faunas of predators. The explosion was partitioned by the Sinsk extinction event, at least in Siberian sections (Zhuravlev & Wood, 2018); prior to the event many stem groups radiated but few survived, while crown group brachiopods and molluscs diversified. A critical part of the 'explosion' was the Cambrian substrate revolution (Bottjer et al 2000).…”