Why Study Alligatorine Evolution?

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Many people think of alligators and other crocodylians as “living fossils”, which is completely untrue (The phrase “living fossil” really just needs to die in a fire because it’s completely useless, misleading, and damaging, but that’s a discussion in and of itself). They have changed quite a lot in the course of their evolution and that’s what I’m studying. My research is on quite a few evolutionary concepts: phylogeny & taxonomy, ecomorphology, biogeography, and diversity in relation to climate change.

Phylogeny & Taxonomy

All of the “big picture” questions I’m looking should be approached in a phylogenetic context (how species are related to one another). Taxonomy (what species and higher units are named) goes hand in hand with that because taxonomy can change if our understanding of phylogeny does. Phylogeny is the documentation of how lineages split and gives scientists a framework to ask other questions.

Ecomorphology

One of these other questions is how shape and size change, and what that has to do with ecology. Modern American alligators are really weird–if you look at basal alligatorines and other species just outside the alligator-caiman split, they’re all these short-snouted, small (~6ft) animals with big, bulbous, crushing back teeth. I’m quantifying how shape and size change over time and through lineages and how this relates to changes in ecological niche.

Biogeography

Modern alligators have a disjunct distribution, with one species in the southeastern US and the other in southeastern China. Crocodiles have disjunct distributions too, but they also have physiological attributes that make them tolerant of saltwater which alligators lack. Alligators are mostly restricted to freshwater. They can enter saltwater for small periods of time and will be fine if they can make it back to freshwater before dehydrating, and often do (there’s a set of studies on Georgia island gators on this). But these are mostly adult or large juvenile individuals–and remember that most extinct gator species were small as adults. Small individuals lose water through their skin at a much quicker rate because of the surface area to volume ratio.

This all means that dispersing across continents is hard for gators. They need land bridges or island chains with freshwater supplies. But these would only be present in high latitudes during alligators’ existence, so climate is an additional limiting factor. I’m trying to figure out what routes they may have taken and when.

Diversity & Climate Change

As “cold-blooded” animals, alligators are geographically restricted by ambient air temperature. As crocodylians go, they’re actually pretty cold-tolerant, and can survive freezing temperatures by staying underwater with only their noses above the surface, but this can only last for so long. They need warmer temperatures to feed, breed, and hatch.

This means that they’ve been used as “paleothermometers”–indications that the climate in the place and time their fossils are found fell above certain climatic parameters. They’ve been around for all or nearly all of the Cenozoic, and the climate has changed substantially since it began. The early Cenozoic, the Paleogene, was much warmer and wetter. At this time, alligatorines ranged as far north as Ellesmere Island. But when it cooled off and dried out in the Neogene, their range became much more restricted and their number of species fell. It’s been a while since anyone examined how their diversity changed over time and they used simple species counts rather than considering phylogeny (which accounts for unsampled lineages that had to of been around based on who’s most closely related to whom). The taxonomy has also been revised quite a lot since this was last done. I’ll be updating it and analyzing how closely alligatorine diversity matches climate change through time.

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