When the Ocean's Spiral Shells All Coiled the Other Way

For thousands of years, and sometimes for millions, marine plankton around the world built their spiral shells in one direction. Then the direction switched — suddenly, at the same time, everywhere — and later switched back again. The organisms carrying these shells are foraminifera, or forams, and they are not a local curiosity: found in all oceans from the tropics to high latitudes, they are the kind of thing that shows up in sediment cores on every continent's margins. What makes the pattern striking is the synchrony. A directional trait that reverses in the same direction, at the same time, across the whole ocean, and then reverses again later, is a global signal rather than a collection of local oddities — which is why it functions as a puzzle rather than a footnote.