Flipping plankton.

Sudden, global reversals in shell coiling direction are a striking and recurrent feature in the fossil record of planktonic foraminifera (marine zooplankton), yet their evolutionary significance has been a mystery. Because coiling direction is a simple, binary character, such shifts have often been interpreted as environmentally induced phenotypic responses rather than indicators of evolutionary change, although it is increasingly evident that genetic variants can have different coiling preferences. Here, we synthesize recent evidence from multiple case studies spanning the Eocene to the Recent (the last 56 Mya). Coiling flips occur on timescales of thousands of years or less, across diverse taxa and ocean basins, far too abruptly to be explained by gradual trait evolution. Instead, these rapid, synchronous coiling reversals may signal cryptic speciation and episodic population sweeps, associated with distinct habitat preferences and water mass distributions. In most cases such replacements would leave little trace in the fossil record, but when competing groups differ in coiling preference, a dramatic and geologically abrupt coiling reversal becomes visible. These findings challenge the assumption that reproductive isolation alone delimits species in planktonic foraminifera and instead supports a model of ecological speciation mediated by habitat partitioning in the open ocean. Shell coiling direction thus serves not as an adaptive trait, but as a fortuitous marker of hidden evolutionary dynamics shaping marine microplankton diversity.