The extinctions that claimed the dolphinlike marine reptiles known as ichthyosaurs (artist’s conception above) have always been mysterious, and they just got more puzzling. The group’s ultimate die-off didn’t really line up with any major mass extinctions, researchers note. Now, a detailed analysis of the fossil record reveals that the loss of these marine reptiles happened in two phases, not one. First, the ichthyosaur family tree suffered a massive pruning about 100 million years ago—a previously unrecognized event that reduced the formerly diverse group of predators to a remnant that included only top-of-the-food-chain creatures. The reasons for this preliminary phase of die-offs aren’t clear, the researchers report online today in Nature Communications, but they don’t seem to include increased competition from other sorts of marine creatures. Nor do they appear to be linked to overall changes in food supply. Small populations, among other factors, stifled evolution among the surviving species, rendering them vulnerable to a series of rapid and profound changes in climate that eventually claimed the entire group in a second extinction about 94 million years ago.