According to a recent analysis, published in Meteoritics & Planetary Science, a set of fossil clams from a quarry in Florida preserve dozens of small glass balls, probably remnants of an ancient meteorite.
These remains are that they are microtektitas, particles that form when the explosive impact of an extraterrestrial object sends molten debris into the atmosphere, where they cool and recrystallize before falling back to Earth. These spheres have traces of exotic metals.
So, we are probably the first to recover from fossil shells. Most of them had been sealed inside the fossil Mercenary campechiensis. Researchers want to date microtektites, but the working guess is that they are "around 2 to 3 million years old."
According Mike Meyer, assistant professor of Earth Systems Science at the University of Harrisburg in Pennsilvania, microtektites are the product of one or more small impacts of previously unknown meteorites, potentially on or near the Florida Platform, the plateau that holds The Florida Peninsula.
But How do they end up inside clams? As the clams die, the fine sediments and particles are washed inside. As more sediments settle on the clams over time, they close, becoming excellent long-term storage containers. According to Meyer:
A rarity is that they contain high amounts of sodium, a characteristic that differentiates them from other impact wastes. Salt is highly volatile and generally evaporates if it is pushed into the atmosphere at high speed. This high sodium content is intriguing because it suggests a very close location for impact. Or at least, any impact that created it probably affects a large reserve of rock salt or the ocean. Many of these indicators point to something close to Florida.