Patterns in alligatorine evolution

Alligatorines—the American alligator and every species more closely related to it than to the spectacled caiman—are a diverse clade of crocodylians with a fossil record going back almost 66 million years, the time of the K–Pg mass extinction. They are suited to answer a variety of questions with far‐reaching impacts due to physiological features such as salt intolerance and “cold‐bloodedness”, as well as their high preservation potential in the rock record. They have been the subject of several analyses studying how they’re related to one another, how they came to be in the geographic regions they’re found in, and how their diversity has changed over time. However, prior analyses of how the various species are related included many inconclusive results and several potential alligatorines have never been included. And analyses of biogeography and diversity would be more informative and accurate if viewed through the context of evolutionary history. Here, I analyze their “family tree”, biogeography, how their shape changes based on what they’re eating, and how their diversity correlates (or doesn’t) with climate change. An almost fully resolved “family tree” shows two groups within Alligatorinae and includes several species not previously analyzed. Alligatorinae originated in North America and dispersed to Europe and Asia between three to five times via at least three different paths at high latitudes when climate—and potentially ocean salinity—were favorable, likely including the Lomonosov Ridge, which today crosses the North Pole and is entirely underwater. The modern American alligator is a dietary generalist, eating anything and everything it can. But it evolved from species specialized to crack hard shells. This evolutionary direction runs contrary to the intuitive reasoning of the “Law of the Unspecialized”, which states that specialists go extinct when the environment shifts, while generalists are able to make it through unfavorable changes by being more adaptable. Alligators were able to break it by entering the generalist dietary niche vacated by now‐extinct crocodile‐relatives after the latter went extinct in mid‐latitude North America due to their inability to tolerate cold. Rock record bias, wherein apparent diversity is due to rock exposure rather than actual diversity signals, should always be considered as a possible influence in diversity studies. This is fortunately not the case for alligators, with diversity tracking climate rather than rocks. Alligators are more diverse in number of species and in range of ecological niches, as well as being more likely to disperse, in warmer, wetter times than in cooler, drier times.