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. 2013 Sep;14(6):344-54.
doi: 10.2174/13892029113149990011.

At the Origin of Animals: The Revolutionary Cambrian Fossil Record

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Free PMC article

At the Origin of Animals: The Revolutionary Cambrian Fossil Record

Graham E Budd. Curr Genomics. .
Free PMC article

Abstract

The certain fossil record of animals begins around 540 million years ago, close to the base of the Cambrian Period. A series of extraordinary discoveries starting over 100 years ago with Walcott's discovery of the Burgess Shale has accelerated in the last thirty years or so with the description of exceptionally-preserved Cambrian fossils from around the world. Such deposits of "Burgess Shale Type" have been recently complemented by other types of exceptional preservation. Together with a remarkable growth in knowledge about the environments that these early animals lived in, these discoveries have long exerted a fascination and strong influence on views on the origins of animals, and indeed, the nature of evolution itself. Attention is now shifting to the period of time just before animals become common, at the base of the Cambrian and in the preceding Ediacaran Period. Remarkable though the Burgess Shale deposits have been, a substantial gap still exists in our knowledge of the earliest animals. Nevertheless, the fossils from this most remarkable period of evolutionary history continue to exert a strong influence on many aspects of animal evolution, not least recent theories about developmental evolution.

Keywords: Animal evolution; Cambrian explosion; Developmental evolution.; Ediacara.

Figures

Fig. (1)
Fig. (1)
A broad outline of important aspects of the fossil record from the Ediacaran to Cambrian. Dating from e.g. Bowring et al. 2007. T. pedum is the trace fossil that formally marks the base of the Cambrian.
Fig. (2)
Fig. (2)
A typical fossil sponge from the Middle Cambrian Burgess Shale, Hazelia delicatula. The fossil is approximately 9mm in length. Photograph courtesy of Joe Botting.
Fig. (3)
Fig. (3)
Kootenia sp, a typical trilobite from the Middle Cambrian of Greenland, a product of the enormous radiation of trilobites that took place around 520 Ma. The body is approximately 87 mm long.
Fig. (4)
Fig. (4)
A simplified phylogeny of the Lophotrochozoa (or Spiralia), after [97]. Each of the numbered nodes subtends a number of phyla, and thus the developmental genetic state at each of them cannot have constrained future evolution within the total clade at which base each node lies. If any genetic constraints exist that steer future evolution of the phyla, then they must have evolved within the stem-groups to each phylum, and not be shared in common (e.g. like the kernels of [80]). However, then it is difficult to understand why there would be a persistent asymmetry in pairs of branches such as Branch A and Branch B, wherein only branches that give rise to phyla of today develop constraints.

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