Evolution

No Time for Rest

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Science  08 May 2009:
Vol. 324, Issue 5928, pp. 695
DOI: 10.1126/science.324_695b

In the history of life, the Ediacaran Period is marked by an enigmatic collection of macroscopic fossils that record the appearance of animals that often took the form of discs, bags, or quilted sheets. Simultaneously, highly ornamented microfossils of an unusual size (>100 µm) appeared in the geological record. Like that of their macro brethren, the origin of these microfossils has remained something of a puzzle.

Using a process of elimination, Cohen et al. argue that many of these ornamented microfossils were in fact cysts—the resting stages of multicellular animals. Their large size and distinct lipid content exclude dinoflagellates as the culprits, and their size and spiny surfaces similarly argue against their being the remains of prasinophyte algae. The microfossils have complex and layered outer walls, distinct from various modern algal forms but similar to the diapause cysts of present-day brine shrimp. Encystment is a self-preservation response to an inconstant and potentially lethal environment, which may have been a chronic problem in the anoxic seas of the early- and mid-Ediacaran. This idea is reinforced by the disappearance of the microfossils roughly 560 million years ago, a period which corresponds to the oxygenation of the sea floor. So, instead of recording the loss of life, the disappearance of these microfossils would reflect a renaissance.

Proc. Natl. Acad. Sci. U.S.A.106, 6519 (2009).

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