Want a take on something familiar that will (probably) turn your mental image of it on its head?
Picture dinosaurs. Not just the dinosaurs themselves, but where they are -- the terrain, plant life, and so on. I'm guessing you probably came up with something like this:
Lush, steamy, wet, sort of like today's Amazon rainforests. It's no surprise Jurassic Park was set on a (fictional) island, Isla Nublar, off the coast of Costa Rica.
And we know for certain that part of the "Age of the Dinosaurs" had a lot of these characteristics. The global climate from the mid-Jurassic to the end of the Cretaceous was largely warm and moist. But a new study, published last week in Science Advances, suggests that the dinosaurs may have come to prominence not because of their adaptation to warm climates, but because of their resistance to cold ones.
Just about everyone knows about the KT (Cretaceous-Tertiary) Extinction, that wiped out all the non-avian dinosaurs, and is now attributed with near certainty to the impact of the Chicxulub Meteorite sixty-six million years ago. Most people also have heard about the biggest mass extinction ever, the Permian-Triassic Extinction, that by some estimates wiped out between eighty and ninety percent of life on Earth, 252 million years ago. Surprisingly few people have heard about the End-Triassic Extinction -- surprising because it caused nearly as much decrease in biodiversity as the Cretaceous-Tertiary Extinction would 135 million years later.
One of the reasons that this event doesn't get much attention is that the wipeout seems to have been gradual rather than sudden and dramatic, as both the Cretaceous-Tertiary and Permian-Triassic Extinction were. "Gradual," of course, is in human terms; in geological or paleontological terms, it happened pretty damn quickly, over a period of about eight hundred thousand years or so. The cause isn't as well understood as either of the other aforementioned extinction events, but seems to have been because of a climatic rollercoaster that first cooled the climate dramatically, and then warmed it up even more. The cause is thought to have been the opening up of the Central Atlantic Magmatic Province, a line of enormous volcanoes that split what had been the supercontinent of Pangaea in half and opened up the Atlantic Ocean. Eventually the province became the modern Mid-Atlantic Ridge (which is still driving North and South America away from Europe and Africa at a rate of about 2.5 centimeters a year).
The climate had already been cooling during the late Triassic, and sea levels fell as seawater got locked up into polar ice caps and glaciers. The eruptions of the CAMP initially dropped the temperature even more, favoring cold-adapted animals and plants. But just as we've seen from modern volcanic eruptions, the "volcanic cold snaps" we get from sunlight-blocking effects of the ash and debris being launched aloft eventually rebound into a warming event because of the pulse of carbon dioxide injected into the atmosphere.
That's what happened here, only on a huge scale. The climatic teeter-totter tilted first toward significant cold and then into a warm, wet period, and the big winners in both scenarios were the dinosaurs. We know about their ability to tolerate heat; like I said, mostly that's the kind of environment we picture them living in. But their ability to weather a cold period seems to have been due to an adaptation their amphibian cousins didn't have: feathers.
We always tend to associate feathers with flight, for the very good reason that birds use them for that purpose. But what we have here is a great example of preadaptation (sometimes shortened to preaptation), in which a trait evolved in one context gains another, unrelated, function and experiences a whole bunch of different selective pressures. Feathers, which are modified reptilian scales (look at a snake scale under a microscope and you'll see the similarity), started out as heat-trapping devices; cold-adapted birds like penguins still use them that way. Once small arboreal dinosaurs began to use feathered limbs as aids to gliding when they jumped from branch to branch, all of a sudden they became seriously well-adapted for something else, and opened the road to modern birds.
The more well-preserved dinosaur fossils we find, the more species we find that had feathers -- including the ones that didn't fly. Even pterosaurs, which we usually picture as having leathery wings, were apparently covered with something very much like fur or fine down feathers. (In fact, one of the small pterosaurs of the late Jurassic is called Sordes pilosus, which roughly translates as "hairy devil.")
So the initial temperature drop at the end of the Triassic Period favored dinosaurs with insulation -- then when the temperature rebounded into jungle conditions in the early Jurassic, the competition (in the form of large amphibian species) were mostly extinct, and the dinosaurs really took off, one branch of them using their feathery innovations for something entirely different.
I always find it wryly funny when people think of dinosaurs as being some kind of "failed experiment" or "evolutionary dead end," when they were actually the dominant life form for 185 million years, which is almost six hundred times longer than modern humans have existed. In fact, most studies have flatly contradicted the notion that "dinosaurs were already declining and then the meteorite impact finished them off" -- all indications are that they were doing just fine when Chicxulub hit. Odd to think of it, but if it hadn't been for that catastrophic impact and horrifying extinction, our own ancestors would very likely never have thrived and spread -- and dinosaurs of some form might still be the dominant animal life on Earth.
But as far as the end-Triassic climate yo-yo goes, it just shows that when the external conditions change, what was a disadvantage can suddenly become an advantage, and what was an advantage can become a disadvantage -- or an advantage of another sort. If things change fast, so can the winners and losers.
In this case, favoring a group that would go on to rule the planet for another 135 million years.
Post a Comment