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.

Highlights

  • The Cambrian explosion continues to excite interest from a wide range of biologists interested in morphological, ecological and developmental evolution

  • The popular view, inspired by accounts such as Gould’s Wonderful Life [1], sees an almost instantaneous appearance of the phyla, an approach that long proved attractive to evolutionary developmental biologists, while molecular clocks continue to place the origin of the animals and even the bilaterians well before the Cambrian in the Cryogenian, i.e. before 635 Ma [2,3]

  • This had the effect of smearing the appearance of the extant phyla upwards into the Cambrian and even later, without denying the likely appearance of the largest clades of animals in the (Late) Ediacaran

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Summary

Introduction

The Cambrian explosion continues to excite interest from a wide range of biologists interested in morphological, ecological and developmental evolution. This could in principle be tested by examining groups that did not survive to form a modern crown group—the striking prediction here would be that these groups would have slower rates of initial morphological diversification, and had less chance of avoiding extinction before the present This raises a subtle and important question: did the extant phyla all diversify rapidly at around the same time because they are all total-group animals, and given their survival must have appeared rapidly after their total group did, or does this apparently correlated rapid diversification imply a true ecologically mediated event? We turn to the Cambrian fossil record itself to interrogate its utility in establishing the order and ecological significance of these early clades

Cambrian worlds: the sequence of faunal change in the Cambrian
Ecological diversification in the Cambrian
A ‘U-tube’ theory for early lophotrochozoan ecology
88. Zhang ZF et al 2014 An Early Cambrian
Findings
91. Zhang ZF et al 2013 A sclerite-bearing stem group
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