Ancient Armor

In summer 2007, two geologists armed with rock hammers and a shotgun hiked through the Yukon, looking for fossils. For two weeks, Phoebe Cohen, a postdoc in MIT's Department of Earth, Atmospheric and Planetary Sciences, and Francis Macdonald, an assistant professor of geology at Harvard University, set up camp along the Alaska-Canada border in a remote mountain range accessible only via helicopter.

The shotgun came in handy: Macdonald fired it once to scare off a grizzly bear. And the rock hammers proved invaluable — the team worked them against mountainsides, chiseling out rock samples. They hauled the rocks back to Cambridge and made a surprising discovery: The ancient carbonate contained hundreds of incredibly well-preserved fossils resembling tiny, shield-like plates.

Cohen, who was a Harvard PhD student at the time, says single-celled organisms may have produced the plates as armor, in a process called biomineralization. Today, many organisms have evolved the ability to produce mineral structures: Mollusks generate shells, and mammals and birds form bone. The 700-million-year-old fossils Cohen found may be the oldest evidence of biomineralization; Cohen, Macdonald and co-authors reported the finding this week in the journal Geology. …

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