Ecological innovations in the Cambrian and the origins of the crown group phyla

Abstract

Simulation studies of the early origins of the modern phyla in the fossil record, and the rapid diversification that led to them, show that these are inevitable outcomes of rapid and long-lasting radiations. Recent advances in Cambrian stratigraphy have revealed a more precise picture of the early bilaterian radiation taking place during the earliest Terreneuvian Series, although several ambiguities remain. The early period is dominated by various tubes and a moderately diverse trace fossil record, with the classical 'Tommotian' small shelly biota beginning to appear some millions of years after the base of the Cambrian at ca 541 Ma. The body fossil record of the earliest period contains a few representatives of known groups, but most of the record is of uncertain affinity. Early trace fossils can be assigned to ecdysozoans, but deuterostome and even spiralian trace and body fossils are less clearly represented. One way of explaining the relative lack of clear spiralian fossils until about 536 Ma is to assign the various lowest Cambrian tubes to various stem-group lophotrochozoans, with the implication that the groundplan of the lophotrochozoans included a U-shaped gut and a sessile habit. The implication of this view would be that the vagrant lifestyle of annelids, nemerteans and molluscs would be independently derived from such a sessile ancestor, with potentially important implications for the homology of their sensory and nervous systems.

Keywords: Lophotrochozoa; Spiralia; crown group; gut; nervous systems; trace fossils.

Figure 1.

A typical simulation of a tree with a birth–death model (see text for details). Typical crown groups (numbered) are marked in grey; the divergence time of the total groups that generated them is marked with the respective asterisked number. Note that the time scale is arbitrary and that patterns of diversification over long periods of time may not be accurately reflected in simulations [35], although this will not affect the results herein.

Figure 2.

Results of simulations of varying speciation (S) and extinction (E) rates where S = E (i.e. where diversification rate (D) = 0): error bars = ±2 s.d. y-axis represents fraction of total time at which crown group arises; x-axis represents S/E.

Figure 3.

The effect of increasing speciation relative to extinction rates on the time of emergence of crown groups relative to their total groups (y-axis). Each line represents an extinction rate with the x-axis providing the corresponding speciation rate. Note that some values are slightly offset from their x-value to avoid blurring of error bars; the only real positions on the x-axis are 0.1, 0.3, 0.5, 0.7 and 0.9.

Figure 4.

The effect of increasing diversification rates (x-axis) on the emergence time of crown groups relative to the total group (y-axis). The data points used here are the values from figure 3.

Figure 5.

Representative intervals of Cambrian time for trace and body fossils (see text for details). The first two trace fossil intervals represent ‘Proterozoic II’ and ‘Proterozoic III’ of Jensen [51], the last the Lower Cambrian zones of Macnaughton & Narbonne [52].

Figure 6.

Representative early Cambrian (spiralian?) fossils, with the oldest at the top. (a–c) ‘Tube world’ fossils appearing before about 536 Ma. (a) The protoconodont (total-group chaetognath) Prothertzina unguliformis, Kuanchuanpu Formation, China. (b) Anabarites tristichus, Siberia. (c) Anabarites hexasulcatus, Siberia. (d–f) ‘Sclerite world’ fossil lophotrochozoans (presumably total group molluscs) from about 536 Ma onwards. (d) Barskovia hemisimmetrica, Tajmyr, Siberia. (e) Anabarella plana, Bol'shaya Kuonamka, Siberia. (f) Purella antiqua, Bol'shaya Kuonamka, Siberia. (g) A ‘brachiopod world’ representative, Tumuldaria incomperta, a very early paterinid brachiopod from the Pestrotsvet Formation of Siberia [61]. Scale bars, (a,d) 200 µm, (b) 500 µm, (c) 300 µm, (e) 2 mm, (f) 100 µm and (g) 1 mm. Photocredits, (a) Jean Vannier, (b–f) Artem Kouchinsky and (g) Christian Skovsted.

Figure 7.

The phylogenetic scenario for spiralian evolution envisaged by the ‘U-tube’ theory in an illustrative spiralian phylogeny. Some taxa are grouped, so that ‘Lophophorata’ = Phoronida + Brachiopoda; ‘Polyzoa’ = Cycliophora, Ectoprocta and Entoprocta; ‘Platyzoa’ = the various smaller taxa (including e.g. Gnathostomulida and Syndermata) treated by Laumer et al. [78]. Some of these groups, especially the Polyzoa, are controversial, but their absence may not materially affect the argument herein. The position of Nemertea is unclear, but they may be the sister group to Lophophorata [78].