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Review
. 2008 Apr 27;363(1496):1425-34.
doi: 10.1098/rstb.2007.2232.

The Earliest Fossil Record of the Animals and Its Significance

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Free PMC article
Review

The Earliest Fossil Record of the Animals and Its Significance

Graham E Budd. Philos Trans R Soc Lond B Biol Sci. .
Free PMC article

Abstract

The fossil record of the earliest animals has been enlivened in recent years by a series of spectacular discoveries, including embryos, from the Ediacaran to the Cambrian, but many issues, not least of dating and interpretation, remain controversial. In particular, aspects of taphonomy of the earliest fossils require careful consideration before pronouncements about their affinities. Nevertheless, a reasonable case can now be made for the extension of the fossil record of at least basal animals (sponges and perhaps cnidarians) to a period of time significantly before the beginning of the Cambrian. The Cambrian explosion itself still seems to represent the arrival of the bilaterians, and many new fossils in recent years have added significant data on the origin of the three major bilaterian clades. Why animals appear so late in the fossil record is still unclear, but the recent trend to embrace rising oxygen levels as being the proximate cause remains unproven and may even involve a degree of circularity.

Figures

Figure 1
Figure 1
Provisional time scale for events around the Precambrian–Cambrian boundary. 1, range of large, acanthomorphic ‘Ediacaran’ acritarchs (a genus that contains metazoan-like embryos is found from close to the bottom of their range just above the Marinoan glaciation rocks); 2, possible range of the Doushantuo embryos and cnidarian-like fossils according to Barfod et al. (2002); 3, possible range of the same according to Condon et al. (2005) (which is correct is uncertain, but the former is favoured here); 4, the ‘Ediacaran’ biota; 5, trace fossils; 6, Cloudina and Namacalathus; 7, classical small shell fossils; 8, trilobites. The alphabets correspond to the key dated points in metazoan evolution in Peterson & Butterfield (2005) based on minimum evolution: A, origin of crown-group Metazoa; B, total-group Eumetazoa; C, crown-group Eumetazoa; D, crown-group bilateria (here equivalent to Protostomia+Deuterostomia); E, crown-group Protostomia. The ‘formative interval’ during which distinctive bilaterian features were assembled according to this dating is marked by arrows.
Figure 2
Figure 2
The Ediacaran acanthomorphic acritarch Tanarium pluriprotensum from the Tanana Formation, in the Giles 1 drillcore, Officer Basin, Australia; 100×. At least some Precambrian acanthomorphic acritarchs may be the eggs of animals. Courtesy of S. Willman.
Figure 3
Figure 3
Basal metazoan fossils. (a) Eiffelia globosa from the Middle Cambrian Burgess Shale (ROM 57023); 3.0× (Botting & Butterfield 2005). As well as the prominent hexaradiate spicules typical of calcarean sponges, rows of smaller, hexactinellid-like tetraradiate spicules are also visible (arrowed). (b) A section of Sinocyclocyclicus guizhouensis from the Ediacaran Doushantuo Formation (Xiao et al. 2000; Liu et al. in press); 75×. This small, branching tabulate fossil has been interpreted as being a potential stem-group cnidarian. (a) Courtesy of N. J. Butterfield. (b) Courtesy of Shuhai Xiao.

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