Crocodiles Survived What Killed the Dinosaurs, and Not by Staying the Same
Crocodilians came through the asteroid that ended the dinosaurs. The reasons are specific, and the popular idea that they did it by never changing is wrong.
Sixty six million years ago an asteroid struck what is now the Yucatán Peninsula and roughly three quarters of species on Earth stopped existing. Every non-avian dinosaur went. So did the marine reptiles, the pterosaurs, and most of everything larger than a badger.
Crocodilians came through it. So the obvious question is what they had that a Tyrannosaurus did not.
They Are Not Lizards
Worth clearing up first, because it changes the picture. Crocodilians are not close relatives of lizards and snakes. They are archosaurs, the same group that contains dinosaurs and therefore birds.
On the family tree, a crocodile is more closely related to a sparrow than to a monitor lizard. The resemblance to lizards is a matter of body plan rather than ancestry. What that means for the extinction is that crocodilians were not a distant lineage watching from outside. They were dinosaur relatives sitting inside the same broad group, and they lived while their cousins did not.
Why They Made It
Several things stacked in their favour, and none of them are about toughness.
They eat almost nothing. A large crocodile can go months without a meal. Being ectothermic means not burning energy to hold a body temperature, which cuts food requirements by roughly an order of magnitude against a mammal or a bird of the same size. In an extinction whose central problem was that the food ran out, that is close to the single most useful trait available.
They lived in the right water. The impact winter collapsed photosynthesis, and food webs built on living plants collapsed with it. Freshwater systems are different: rivers and swamps run substantially on detritus, the decaying material washing in from land. That kept working when sunlight did not. Freshwater aquatic animals came through the K-Pg boundary at notably better rates than land animals, and crocodilians sat squarely in that refuge.
They were not fussy. A generalist that eats fish, carrion, amphibians, small mammals and whatever else presents itself does not depend on any particular prey species surviving. Specialists died with the things they specialised in.
The survivors were not the huge ones. Giant crocodilians existed before the impact and did not make it either. What came through were mid-sized, mostly freshwater generalists. Size was a liability across the board.
The traits that got crocodilians through are unglamorous: they barely eat, they lived in detritus-fed fresh water that kept functioning without sunlight, they ate anything, and the survivors were not the giants. Toughness had nothing to do with it.
The Living Fossil Part Is Wrong
Crocodiles get called living fossils constantly, with the implication that they hit on a design and stopped evolving. The fossil record does not support it.
Crocodylomorpha has been a varied group. There were fully marine forms with paddle-like limbs and tail flukes that never came ashore. There were small, long-legged terrestrial species built for running rather than ambush. There were herbivores, identified from complex multi-cusped teeth suited to processing plants rather than tearing flesh. There were the enormous ones, including Sarcosuchus and Deinosuchus, well beyond anything alive now.
What survives today is one narrow branch of that history: semi-aquatic ambush predators. The body plan looks ancient because it is conservative, not because the lineage stood still. It kept producing variety, and most of the variety is gone.
What They Are Now
Modern crocodilians are not simple animals either. Saltwater crocodiles produce the highest bite force ever directly measured in a living animal, around 16,000 newtons. They show real parental care, with females guarding nests and carrying hatchlings to water in their jaws, and hatchlings that call from inside the egg to trigger the mother to dig them out. Several species have been documented using sticks balanced on the snout during bird nesting season, which if it holds up is tool use.
That is a fairly sophisticated animal, and it is the current state of a lineage that has been experimenting for over 200 million years rather than the leftover of one that stopped.
Sources
- Brochu, C. A. (2003). Phylogenetic approaches toward crocodylian history. Annual Review of Earth and Planetary Sciences.
- Robertson, D. S., McKenna, M. C., Toon, O. B. et al. (2004). Survival in the first hours of the Cenozoic. GSA Bulletin.
- Erickson, G. M., Gignac, P. M., Steppan, S. J. et al. (2012). Insights into the ecology and evolutionary success of crocodilians. PLoS ONE.
- Dinets, V., Brueggen, J. C. and Brueggen, J. D. (2015). Crocodilians use tools for hunting. Ethology Ecology and Evolution.
Written by Mike
Mike is the founder of Beastly Facts and a lifelong reptile enthusiast. He shares his home with Dex, a bearded dragon with strong opinions about crickets and basking schedules. Mike writes in-depth care guides, animal facts, and the occasional short story about life with exotic pets.
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