When did Alligatorinae Evolve?

Source: Geological Society of America
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The Alligatorinae-Caimaninae split is actually one of the best-constrained divergence dates for vertebrates. The two clades diverged from one another somewhere between 71 and 64 million years ago (Ma). This was either shortly before, during, or just after the Cretaceous-Paleogene (K-Pg) boundary, which is currently placed at 66 Ma. This point in geologic time is best known as when the non-avian dinosaurs went extinct.

This means that alligatorines have been around for the entire (or almost the entire) Cenozoic, which takes up the leftmost column in the picture above. If they diverged from caimans shortly before the K-Pg boundary, they would have also been present in the Maastrichtian, which is the last age in the Mesozoic.

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Where did Alligatorines Live?

Source: www2.nau.edu
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Today, alligatorines can be found in the southeastern United States and in a very restricted area of the Yangtze River Valley in China. In the recent past, Chinese alligators ranged throughout more of China, but habitat loss has resulted in them being almost completely extinct in the wild.

In the past, alligatorines lived in a much wider area, with fossils found in the continental US, Canada (even Ellesmere Island), several western European countries, China, Taiwan, Thailand, and another place I’m not going to disclose because it’s someone else’s research and hasn’t been presented yet.

This trio of modern continents–North America, Europe, and Asia–used to make up a large supercontinent called Laurasia. During the course of Laurasia’s history, various parts have been separated as islands (especially true in Europe), and entire continents have been connected and disconnected as land bridges appeared and disappeared.

By the time alligatorines evolved, Laurasia was no longer one big mass, but given the continued close association between the continents, it’s still useful to think of alligatorines as having a Laurasian distribution.

What’s an Alligatorine?

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Taxonomy

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.

Alligatoroid cladogram

Modified from Brochu (2004). Asiatosuchus is a crocodile and Borealosuchus is its own thing.

Morphology

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).

Allognathosuchus jaw demonstrating deep curvature between the 4th and 10th dentary teeth

Allognathosuchus jaw demonstrating deep curvature between the 4th and 10th dentary teeth

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.

Not a gator, in spite of the snout

Dwarf crocodile Image source: Wikipedia

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.

interfingering teeth

Siamese crocodile at the croc farm in Bangkok, Thailand.