It’s well known that pearls form when an irritating object, such as a grain of sand, gets inside a mollusk’s shell. Some ancient clams would probably have gone beyond irritation to outright fury when the products of a nearby asteroid impact entered their shells, if only they'd had brains.

As an undergraduate student at the University of South Florida undertaking summer fieldwork in 2006, Mike Meyer wasn’t expecting to discover traces of an asteroid. He was collecting fossil clams from a remarkable quarry in Sarasota County that holds several million years' worth of bivalve remains. Sediment trapped inside the clams after they die provides a record of the conditions at the time, and therefore of Florida’s geological history.  

This record is usually composed of foraminifera, single-celled organisms whose dominant species changes depending on climatic conditions.

Instead, Meyer noticed balls of almost microscopic translucent glass, clearly too close to spherical to be sand grains. Neither his supervisor nor any of the scientists Meyer emailed could identify them, so he held onto 83 in a private collection. 

Meyer has since established himself as a scientist with other research, earning a doctorate and position at Harrisburg University. He has specialized in extinctions in the Ediacaran era, but had unfinished business. "It wasn't until a couple years ago that I had some free time," he said in a statement. "I was like, 'Let me just start from scratch.'"

After comparing the balls to every sort of geological object he could think of, Meyer concluded they are microtektites, the tiny fragments of glass thrown up when a substantial object plows into the Earth, a result he has published in Meteoritics and Planetary Science. 

Powerful collisions can shoot microtektites enormous distances, but when this happens sodium burns off, so the high sodium concentrations in Meyer's find suggests a local impact.

The clam-preserved microtektites appear to date to between 2 and 3 million years ago, although more precise dating is planned. No other evidence has been found of an asteroid impact in the Florida area around that time, but a modest-sized crater might have eroded away in that time.

The more puzzling observation, which may lead some to question Meyer's conclusion, is that he and his fellow students found these glass balls in four layers within the quarry, indicating the clams were absorbing them over an immense span of time. It is hard to believe that one location could have been hit by multiple large space rocks, none of which we know about from other means, but it is Florida, so anything's possible.

More likely, however, is that a single collision deposited microtektites somewhere else, from which they were slowly eroded and washed to the quarry to find their way into clams.