The Cambrian "explosion": slow-fuse or megatonnage?

Abstract

Clearly, the fossil record from the Cambrian period is an invaluable tool for deciphering animal evolution. Less clear, however, is how to integrate the paleontological information with molecular phylogeny and developmental biology data. Equally challenging is answering why the Cambrian period provided such a rich interval for the redeployment of genes that led to more complex body plans.

Figure 1

Principal events across the Vendian-Cambrian boundary, spanning an interval of approximately 60 Myr (570–510 Myr), in the context of the early evolution of metazoans. On the left are denoted a series of important fossil assemblages, e.g., Burgess Shale, and various other significant events, e.g., cessation of ice age. To the right is the carbon isotope curve (redrawn from ref. 4), which provides an independent tool for correlation by chemostratigraphy and may also indicate substantial changes in ocean state with possible implications for evolutionary diversification. The evolutionary framework is largely based on molecular data (12, 19, 20), but the available fossil record not only gives a temporal perspective but also indicates major anatomical transitions that mark the emergence of distinct bodyplans. The sister-group of the Metazoa are the Fungi (35), and a possible time of divergence was ≈650 Myr ago. No fossil evidence for this event is yet available, and the early history of animals (≈650–570 Myr) is also cryptic. This is presumably because the earliest metazoans were microscopic and too fragile to fossilize readily. The most primitive animals in the fossil record may be represented by the vendobionts (36). Metazoans are otherwise divided into various major groupings, of which the most significant depends on the number of germ layers: respectively, two in the diploblasts and three in the triploblasts. The Ediacaran faunas postdate episodes of major glaciation and, with the exception of a few mineralized taxa (e.g., Cloudina), lack hard-parts. These Vendian-age assemblages comprise the problematic vendobionts, various coelenterates, and stem-group representatives of the three main groups of triploblast, referred to, respectively, as the ecdysozoans, lophotrochozoans, and deuterostomes. The process of exoskeleton molting, known as ecdysis, is a characteristic of the ecdysozoans. The most important group is the arthropods, and, possibly, they derive from a priapulid-like worm. Lophotrochozoans derive their name from a tentacular feeding organ (lophophore) found in some groups and the most widespread occurrence of a type of ciliated larva known as the trochophore. Lophotrochozoans are a diverse group encompassing the molluscs, annelids, and brachiopods. The ancestral form was probably rather slug-like. The deuterostomes are notably disparate and include the echinoderms and chordates.

Figure 2

Representative Cambrian animals from Burgess Shale-type deposits (all except d) and an example of early phosphatization (d). (a) The agnathan chordate Myllokunmingia fengjiao from the Lower Cambrian (lower Botomian) Chengjiang lagerstätte, at Haikou near Kunming, Yunnan, China. The photograph is courtesy of D. Shu (North-West University, Xilan, People's Republic of China). (b) The holotype and only known specimen (part and counterpart) of the ctenophore Fasciculus vesanus from the Middle Cambrian Burgess Shale lagerstätte, at Field in British Columbia, Canada. (c) The halkieriid Halkieria evangelista from the Lower Cambrian (upper Atdabanian) Sirius Passet lagerstätte in Peary Land, North Greenland. (d) Phosphatized embryos, possibly of a halkieriid, and referred to as Markuelia secunda from the Pestrotsvet Formation (Tommotian) of south-east Siberia. The figures show views of two embryos that are wrapped around the surface. The photograph is courtesy of S. Bengtson (Swedish Museum of Natural History, Stockholm). [Reprinted (abstracted/excerpted) with permission from ref. . Copyright 1997, American Association for the Advancement of Science.] (e) The Ediacaran survivor and presumed anthozoan (Cnidaria) Thaumaptilon walcotti from the Middle Cambrian Burgess Shale lagerstätte, at Field in British Columbia, Canada. (f) The lobopodian Hadranax augustus from the Lower Cambrian (upper Atdabanian) Sirius Passet lagerstätte in Peary Land, North Greenland. The photograph is courtesy of G. Budd (Uppsala University, Uppsala). (g) The primitive arthropod Kerygmachela kierkegaardi from the Lower Cambrian (upper Atdabanian) Sirius Passet lagerstätte in Peary Land, North Greenland. The photograph is courtesy of G. Budd. (h) The posterior trunk of the priapulid worm Ottoia prolifica from the Middle Cambrian Burgess Shale lagerstätte, at Field in British Columbia, Canada. The specimen shows the intestine and three hyoliths, interpreted as ingested prey. (a, ×2.0; b, ×0.3; c, ×0.8; d, ×55; e, ×0.3; f, ×0.8; g, 0.9; h, ×1.9.)