The discovery of fossils in gypsum on Earth may help scientists zero in on fossils containing past life on Mars.
Scientists find microfossils in gypsum formed when the Mediterranean Sea dried up six million years ago.
Scientists didn't know the soft mineral could preserve microscopic fossils of ancient life.
The discovery raises the prospect that Mars' rich deposits of gypsum may hold fossil records as well.
A common mineral widely believed to be a poor vessel for fossils actually contains a treasure trove of ancient life, a discovery that may lead scientists searching for life on Mars to the planet's sweet spot.
Both Earth and Mars have the mineral gypsum, though it hasn't been of much interest to scientists studying ancient life forms because no one believed tiny fossils could last in the soft, water-soluble mineral, also known as calcium sulfate.
But scientists at an astrobiology conference in Houston will report today that microscopic remnants of a diverse collection of algae and phytoplanktons have been found in samples of gypsum that formed six million years ago when the Mediterranean Sea went dry.
"It came as a surprise to me," Bill Schopf, a paleobiologist at the University of California in Los Angeles, told reporters on Wednesday. "There's been almost no work ever done here on Earth looking for fossils in gypsum because we all assumed that there wouldn't be anything in there and we were wrong."because Schopf began looking for fossils in gypsum after being contacted by Mars scientists studying a variety of sulfate deposits found by the NASA rover Opportunity in 2004.
Later, orbiting spacecraft discovered vast deposits of gypsum around the north pole of Mars and near Valles Marineris, a massive canyon in the planet's equatorial region.
"We now know this is a good place to look for evidence of fossil life on Mars," Schopf said. "If we can find the organic matter then we have a real reason to think that there might once have been life there."
"We're on this path in Mars exploration right now to see if there are any preserved ancient biosignatures in the rocks that we've found there and sulfates are a huge target," added Arizona State University astrobiologist Jack Farmer.
"This is helping close the loop a little bit on what the potential might be for preserving biosignatures in those rocks."