What killed the dinosaurs

The desert of northwestern New Mexico, in the vicinity of the Four Corners, is my special place. The high-altitude su...

Posted: May 28, 2018 11:46 AM
Updated: May 28, 2018 11:46 AM

The desert of northwestern New Mexico, in the vicinity of the Four Corners, is my special place. The high-altitude sun sparkles off the badlands, illuminating rocky pastels of red, green and brown that seem to extend indefinitely in all directions. No wonder that Georgia O'Keeffe -- who painted here for decades -- found this landscape as her muse.

Not many people live here, making it feel like a remote backwater within the world's most industrialized country. But that's the way I like it. I'm a paleontologist, and I visit here at least once a year, to hunt for fossils of dinosaurs and other long-extinct creatures. The fewer buildings, roads and houses to cover up the treasure we seek, the better.

Most of the candy-striped badlands in this part of New Mexico are carved from rocks laid down in rivers and lakes between about 84 and 56 million years ago. These were lush environments, teeming with life during a time when the Earth was much warmer and there were no ice caps on the poles. Bones, teeth, shells, and other parts of animals would often get buried in mud or sand and turn to stone, becoming the fossils that provide the only clues that these lost worlds ever existed.

You can find many dinosaurs here. We often come across the railroad spike teeth of T. rex and the gargantuan limb bones of long-necked sauropods of the Brontosaurus mold, some of which weighed more than a Boeing 737, easily making them the largest animals to ever thunder across the land.

We find the skull domes that horse-sized omnivores called pachycephalosaurs used to head butt each other, and the jaws that horned and duck-billed dinosaurs sliced up plants with. So many species, big and small, living together.

I usually prospect these colorful hills with one of my best friends in science, Tom Williamson, a curator at the New Mexico Museum of Natural History and Science in Albuquerque. Sometimes, we walk for days and can't get away from the dinosaur bones, because they are so common. By now we know the best places to find them: a layer-cake series of rock strata, formed during the very end of the Cretaceous Period (84-66 million years ago).

You can read the layers like the pages in a novel, and although the characters are fascinating, the story is fairly uneventful. During this whole stretch of time, dinosaurs were in control. History seemed to be standing still, and it appeared that dinosaurs would keep on ruling the world forever, as they had done for over 150 million years.

But then, suddenly, their bones disappear. We can pinpoint the exact place in the rock sequence. It's where the cyclical mudstones and sandstones, records of that stable Cretaceous world, abruptly give way to coarser boulder-strewn rocks characteristic of fast-moving currents and corrosive storms. Something dramatic happened to the local environment, and the dinosaurs were gone.

The same pattern is seen halfway around the world, in the chalky-colored limestones of Gubbio, Italy. Underneath a medieval aqueduct that clings to the sides of a deep gorge, the geologist Walter Alvarez noted that the Cretaceous rocks at the bottom of the canyon are chock full of small fossils of ocean plankton.

Above these rocks, however, are nearly barren limestones, sprinkled with a few tiny, simple-looking fossils. The knife-edge separation between these two rocks is a dainty strip of clay, only about half an inch thick.

The clay is the cockpit voice recorder that reveals the fate of the plankton, and the dinosaurs: it is full of iridium, an element common in outer space but rare on Earth. It was delivered by a 6-mile-wide asteroid the size of Mount Everest, which was moving faster than a jetliner when it collided with the Earth 66 million years ago, punching a crater more than 100 miles wide and causing a chain reaction of volcanoes, wildfires, tsunamis, earthquakes and climate change that wiped out some 70% of all living things.

The dinosaurs couldn't cope, and all of them (except for a few birds) died. They were soon replaced, and we see the evidence in New Mexico. The chaotic boulder-filled rock layer quickly gives way to the same types of mudstones and sandstones that had been formed during the Cretaceous, a sign that environments returned to normal within a few thousand years. But there are no dinosaur bones to be found in these newer Paleocene-aged rocks (66-56 million years old).

Instead, there are countless jaws, teeth, and skeletons of the things that took over from the dinosaurs, the species that went on to start the next great dynasty of Earth history: mammals.

It's a sobering story, and one of relevance to us today, as our climate and environment are changing rapidly.

Just within the last few months studies have shown that sea level is rising twice as fast as we thought, the Antarctic ice sheets are melting at alarming rates, and temperature is increasing so fast that humans may make the Earth warmer than it has been in more than 50 million years.

There are consequences to all of this upheaval: we are in the age of the so-called "sixth extinction," with species dying out at hundreds or thousands of times the usual rate. Faster, perhaps, than even during the five mass extinctions of Earth history, including the one that killed the dinosaurs.

Maybe dinosaurs can help save us. We're used to thinking of them as movie monsters, skeletons that wow tourists at museums, and objects of childhood fascination. But they are so much more than that. They were real living, breathing, evolving animals that had to deal with rising and fall temperatures, fluctuating sea levels, volcanoes and asteroids.

After all, none of the environmental changes going on today is new. The Earth has been through them before, and dinosaurs and other extinct animals can tell the story of what happened. What died, what survived, how long it took to recover.

Among the mammals that Tom Williamson has discovered in those dinosaur-free, post-extinction rocks in New Mexico is a skeleton of a puppy-sized creature called Torrejonia.

It had a slender body, gangly limbs and long fingers and toes, and you can almost envision it leaping through the trees. It is one of the oldest primates -- a fairly close cousin of ours, and a reminder that we humans had ancestors that were there on that terrible day, that saw the rock fall from the sky, that survived the cataclysm while the dinosaurs did not, probably because they were small, agile, adaptable and able to eat many types of food.

There is something almost poetic about it. In a sense, we are the dinosaurs. Before creatures like Torrejonia started the domino chain of evolution that led to humans, the dinosaurs ruled. They evolved superpowers like big brains, keen senses and the ability to grow to enormous sizes. There were probably many billions of them, living in all corners of the globe, that woke up on that day 66 million years ago confident of their undisputed place at the pinnacle of nature.

We humans now wear the crown that once belonged to the dinosaurs. We are confident of our place at the pinnacle of creation, even as our actions are rapidly changing the planet around us. It leaves me uneasy, and a troubling thought lingers as I walk through the New Mexico scrublands, seeing the bones of dinosaurs give way so suddenly to fossils of Torrejonia and other mammals.

If it could happen to the dinosaurs, could it also happen to us?

Dinosaurs, of course, had no way to prevent the asteroid that killed them. But we have a choice -- we can still stop, or at least slow down, pumping toxins into the atmosphere. Our choice will dictate whether we really are the dinosaurs: whether we go the way of T. rex and Triceratops, or whether we have learned from their sad story.

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