The group of organisms I’m studying is called ‘Alligatorinae’. It contains the American alligator (Alligator mississippiensis) and all species closer to it than to the spectacled caiman (Caiman crocodilus). There are two alligatorines alive today: the American alligator and the Chinese alligator. The group both alligatorines and caimanines are in is ‘Alligatoridae’. That, in turn, is inside a larger group called “Globidonta”, which also contains several extinct species. And, lastly, Globidonta is inside a large group called “Alligatoroidea”. Alligatoroidea is “alligators” in the largest sense. Its sister group (closest relative) is Crocodyloidea, which contains crocodiles, tomistomines, and possibly gharials.
If it’s morphology you’re interested in, alligatorines ancestrally have two unambiguous synapomorphies (features that distinguish them from their relatives) lower jaws that curve strongly between the 4th and 10th teeth, and nares (those are the nose holes) that open up and forward. But because nature likes to be difficult, those traits are reversed (returned to a prior state) in the two extant species (that part of their jaw is straighter and their nares point straight up).
Alligators vs. Crocodiles
Something I get asked a lot when I tell people I study gators is how to tell the difference between an alligator and a crocodile. The thing you’ve probably heard is to look at the snout shape–alligators have broad snouts, crocodiles have pointy snouts. Forget it; it’s wrong.
It only works if you only look at the species alive today and exclude certain ones you’re not liable to encounter in the US, where there is one native alligator and one native crocodile which fit that description well. It doesn’t even work perfectly here anymore–some spectacled caimans have pointy snouts, and former pet caimans have been found out-and-about in Florida. On the other side of the tree, if you look at the dwarf crocodiles of Africa, they have broad snouts. And things get even more complicated when you include extinct species.
The best way to tell whether the crocodylian contemplating taking a bite out of you is an alligator or crocodile is to look at how the teeth fit together (i.e., occlude). If it has an overbite, it’s an alligator. If the upper and lower teeth interfinger with one another, it’s a crocodile. Sometimes tooth occlusion can get weird if you’re looking at a particularly aberrant spectacled caiman or a captive individual raised in poor conditions, but it holds up in most cases.