Why Did Crocodiles Survive the K/T Extinction?

Stomatosuchus, a crocodile of the middle Cretaceous period (Dmitri Bogdanov).

You already know the story: at the end of the Cretaceous period, 65 million years ago, a comet or meteor struck the Yucatan peninsula in Mexico, triggering extreme changes in the global climate that resulted in what we call the K/T Extinction. Within a short period of time--estimates range from a few hundred to a few thousand years--every last dinosaur, pterosaur and marine reptile had disappeared off the face of the earth, but crocodiles, oddly enough, survived into the ensuing Cenozoic Era.

Why should this be surprising? Well, the fact is that dinosaurs, pterosaurs and crocodiles are all descended from archosaurs, the "ruling lizards" of the late Permian and early Triassic periods. It's easy to understand why the earliest mammals survived the Yucatan impact; they were small, tree-dwelling creatures that didn't require much in the way of food and were insulated by their fur against plunging temperatures. The same goes for birds (only substitute "feathers" for fur). But some Cretaceous crocodiles, like Deinosuchus, grew to respectable, even dinosaur-like sizes, and their lifestyles weren't all that different from those of their dinosaur, pterosaur or marine reptile cousins. So how did crocodiles manage to survive into the Cenozoic Era?

Theory #1: Crocodiles Were Exceptionally Well-Adapted

Whereas dinosaurs came in all shapes and sizes--huge, elephant-legged sauropods, tiny, feathered dino-birds, towering, ravenous tyrannosaurs--crocodiles have stuck with pretty much the same body plan for the last 200 million years (with the exception of the very first Triassic crocodiles, like Erpotosuchus, which were bipedal and lived exclusively on land).

Perhaps the stubby legs and low-slung posture of crocodiles allowed them to literally "keep their heads down" during the K/T upheaval, thrive in a wide variety of climatic conditions, and avoid the fate of their dinosaur pals.

Theory #2: Crocodiles Lived Near the Water

As stated above, the K/T Extinction wiped out land-dwelling dinosaurs and pterosaurs, as well as sea-dwelling mosasaurs (the sleek, vicious marine reptiles that populated the world's oceans toward the end of the Cretaceous period).

Crocodiles, by contrast, pursued a more amphibious lifestyle, perched halfway between dry land and long, winding freshwater rivers and saltwater estuaries. For whatever reason, the Yucatan meteor impact had less of an impact on freshwater rivers and lakes than it did on saltwater oceans, thus sparing the crocodile lineage.

Theory #3: Crocodiles Are Cold-Blooded

Most paleontologists believe that theropod dinosaurs were warm-blooded, and thus had to constantly eat in order to fuel their metabolisms--while the sheer mass of sauropods and hadrosaurs made them slow to both absorb and radiate heat, and thus able to maintain a steady temperature. Neither of these adaptations would have been very effective in the cold, dark conditions immediately following the Yucatan meteor impact. Crocodiles, by contrast, possess classically "reptilian" cold-blooded metabolisms, meaning they don't have to eat very much and can survive for extended periods of time in severe darkness and cold.

Theory #4: Crocodiles Grew More Slowly than Dinosaurs

This is closely related to theory #3, above. There's an increasing amount of evidence that dinosaurs of all types (including theropods, sauropods and hadrosaurs) experienced a quick "growth spurt" early in their life cycles, an adaptation that better enabled them to avoid predation.

Crocodiles, by contrast, grow steadily and slowly throughout their lives, and would have better been able to adapt to the sudden scarcity of food after the K/T impact. (Imagine a teenaged Tyrannosaurus Rex experiencing a growth spurt suddenly needing to eat five times as much meat as before, and not being able to find it!)

Theory #5: Crocodiles Were Smarter than Dinosaurs

This is probably the most controversial hypothesis on this list. Some people who work with crocodiles swear that they're almost as smart as cats or dogs; not only can they recognize their owners and trainers, but they can also learn a limited array of "tricks" (like not biting their human trainer in half). Crocodiles and alligators are also fairly easy to tame, which may have allowed them to adapt more readily to the harsh conditions after the K/T impact.

The problem with this theory is that some end-Cretaceous dinosaurs (like Velociraptor) were also fairly smart, and look what happened to them!

Even today, when numerous mammal, reptile and bird species have gone extinct or are seriously endangered, alligators and crocodiles around the world continue to thrive (except for those targeted by shoe-leather makers). Who knows--if things keep on going the way they have been, the dominant forms of life a thousand years from now may be cockroaches and caimans!

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Strauss, Bob. "Why Did Crocodiles Survive the K/T Extinction?" ThoughtCo, Jun. 21, 2017, thoughtco.com/why-did-crocodiles-survive-the-kt-extinction-1092137. Strauss, Bob. (2017, June 21). Why Did Crocodiles Survive the K/T Extinction? Retrieved from https://www.thoughtco.com/why-did-crocodiles-survive-the-kt-extinction-1092137 Strauss, Bob. "Why Did Crocodiles Survive the K/T Extinction?" ThoughtCo. https://www.thoughtco.com/why-did-crocodiles-survive-the-kt-extinction-1092137 (accessed March 23, 2018).