Experiment of the month

Natural History, May, 2003 by Stephan Reebs

Amid all the fanfare that has accompanied recent discoveries of fossilized Precambrian invertebrate eggs found in China and elsewhere, a few grumbles of disbelief have been heard. After all, invertebrate eggs are made of soft tissue, so shouldn't they decompose long before mineralization begins?

The answer is: not necessarily. Derek Martin, Derek E.G. Briggs, and R. John Parkes, all of the University of Bristol in England, dropped lobster eggs into vials containing seawater and natural sediments, then sealed the vials and incubated them at 59 degrees F. After three weeks the intact eggs were coated with a thin layer of calcium carbonate, which stabilized their shape. Mineralization had begun.

The process depends on two key factors: the lack of oxygen (a gas that speeds decomposition) and the presence of anaerobic bacteria (whose metabolic activity helps make minerals available). Paleontologists had previously thought that invertebrate eggs couldn't be fossilized unless the exoskeleton of a relatively large animal (the mother, for example) lay close enough to serve as a ready source of calcium or phosphorus for the compounds that would constitute the fossil. But the Bristol experiment suggests that orphaned eggs, including those produced by small, soft organisms, could still have become naturally fossilized. ("Experimental mineralization of invertebrate eggs and the preservation of Neoproterozoic embryos," Geology 31:39-42, January 2003)

Stephan Reebs is a professor of biology at the University of Moncton in New Brunswick, Canada, and the author of Fish Behavior in the Aquarium and in the Wild (Cornell University Press).