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.

Figure 1

Early Cambrian time scale for the Siberian Platform, Russia, with key radiometric dates (numbered; Siberian radiometric dates are in bold), international chronostratigraphy (ICS), and stages and zones accepted for the Siberian Platform. Radiometric dates from 1^,; 2; 3, 4; 5; 6^–; 7; 8. Right column shows numbered temporal units, each c. 2.5 Myr in duration. ED = Ediacaran. 3 = Cambrian Series 3, pars. Modified from.

Figure 2

Early and early middle Cambrian skeletal stem- (a–f,i–j) and crown group (g,h) lophotrochozoans from the Siberian Platform. (a) Aldanella attleborensis (Shaler & Foerste), stem mollusc, helcionelloid; shell (, Fig. 20A₁); (b) Camenella garbowskae Missarzhevsky, stem lophophorate, tommotiid; sclerite (, Fig. 37A); (c) Ceratoconus striatus Chen & Zhang, stem mollusc, helcionelloid; shell (, Fig. 26A₁); (d) Halkieria sp., stem lophotrochozoan; halkieriid; sclerite (, Fig. 46C₂); (e) Tannuolina pavlovi Kouchinsky et al., stem lophophorate, tommotiid; sclerite (, Fig. 2A₂); (f) Oymurania gravestocki Ushatinskaya, stem brachiopod; valve (, Fig. 8A); (g) Pelmanotreta neguertchenensis (Pelman), crown brachiopod, paterinate; valve (, Fig. 2i); (h) Pojetaia dentifera Kouchinsky et al., crown mollusc, bivalve; valve (, Fig. 3A); (i) Purella antiqua (Abaimova), stem lophotrochozoan, maikhanellid; valve (, Fig. 31B₂); (j) Khetatheca cotuiensis (Sysoev), stem lophophorate, hyolith; valve (, Fig. 50G). All photographs courtesy of Artem Kouchinsky.

Figure 3

Diversity of skeletal species through the Ediacaran – early Cambrian of the Siberian Platform. (A) Total diversity of all skeletal species, Trilobita, and Archaeocyatha. (B) Total diversity of skeletal lophotrochozoan species, and stem group and crown group representatives. Ediacaran and Cambrian chronostratigraphic subdivisions are scaled according to Fig. 1.

Figure 4

Schematic of hypothesised non-Bilaterian (total group Porifera, Cnidaria and Ctenophora) and Bilaterian diversification during the Ediacaran-Cambrian metazoan radaition, showing the fossil record of probable earliest metazoans (shown by a rangeomorph reconstruction), the Kotlin crisis, followed by two phases of Cambrian Explosion, separated by the Sinsk Event extinction (with a possible expanded interval of anoxia during Phase 1) and extending to the Ordovician Radiation through the SPICE extinction. Non-bilaterian stem group example is a stem group archaeocyath sponge; crown group is a crown group demosponge. Bilaterian stem group is shown by a tommotiid; crown group by a trilobite.