It’s been 200 million years, but crocodiles haven’t aged a day. In a new paper, scientists from the U.K. explain how this notorious “living fossil” has kept its rugged good looks since the heyday of the dinosaurs. In the study, the researchers used “advanced evolutionary modeling” to show why some animals have stayed exactly the same since just after the Triassic-Jurassic mass extinction.
In a corresponding piece in The Conversation, lead author Max Stockdale, from the University of Bristol, explains: “Relatives of crocodiles have been around for an extraordinary length of time. [T]heir fossil remains having been found in rocks from the early Jurassic period, around 200 million years old. Stranger still, these crocodiles from the age of the dinosaurs often look surprisingly like the crocodilians of today.”
To study the evolution of today’s crocodilians in more depth, the researchers dropped a ton of evolutionary data into a massive new mathematical model. They found that crocodiles, of which there are 25 very similar but different species today, evolved in an overall shape called punctuated equilibrium.
Think of this like the classic folk story about the tortoise and the hare. Yes, crocodile species flowered out in a wider, more fan-like shape at certain points, where some species evolved faster. But through the entire 200 million years of history, slower-evolving crocodiles—so slow that they look precisely the same 200 million years later—continued on the same slow trajectory. The “hares” were more likely to die out during climate events like ice ages, leaving the “tortoises” to continue to the present.
For crocodiles, the lucky coincidence is the slower-evolving species have the right mix of qualities to let them survive almost any normal-range Earth conditions during that gigantic amount of time. Like tardigrades or cockroaches, crocodiles have qualities closer to those of “extremophile” organisms than almost any other animal. They can survive a long time without eating, for example.
The scientists say punctuated equilibrium emerges more in organisms that face larger exterior pressures, like the mass extinction that directly preceded the last explosion of, er, “modern” crocodiles. Mass extinctions are interspersed with mass radiations of new species, which makes intuitive sense, although research finds the two are less causally linked than we previously believed.
Basically, when Earth is suddenly both almost emptied of life and in a new kind of climate or environment, the conditions are just right for new mutations to more easily take hold.
If today’s crocodiles are like a classic two-button suit, the also-rans of crocodile history could be the wide lapels we left behind in the ‘70s. They were fine then, but the more timeless crocodile has prevailed. And the scientists think the same model of punctuated equilibrium could apply to other living fossils like turtles.
Understanding which kinds of animals have lived unchanged and which have died out helps other researchers continue to piece together Earth’s extremely long evolutionary history.