Sunday, June 28, 2015

Ghosts in the Landscape

Photo by author
Not long ago, I was driving across the Colorado high plains when a pair of pronghorn crossed the road. When they saw me coming they ran, and I pulled over to admire their retreat. I was thinking, "Those things might be faster than deer." And then, as if affronted by the comparison, they decided to stop messing around and run. I've never seen any animal run that fast before. Later, I looked them up and found that they're the second-fastest land animal in the world. Only the cheetah is faster.

And that's odd, if you think about it. Why do pronghorn need to be that fast--far faster than any predator on the plains? Nature doesn't usually give its creatures abilities beyond their needs, so the pronghorn's excessive speed is a puzzle.

The poet Robinson Jeffers offers a hint of an answer:
What but the wolf’s tooth whittled so fine
The fleet limbs of the antelope?
Wolves were certainly important predators of pronghorn, but they probably aren't the whole story. Pronghorn can run 20 miles per hour faster than wolves, which means they may be built to outrun an even faster predator. And just such a predator once existed: the American cheetah, or false cheetah. Until about 12,000 years ago, there was a big cat in North America that fit into the same "turbocharged grassland predator" niche that modern African cheetahs occupy. As the name "false cheetah" suggests, it wasn't closely related to African cheetahs. It's an example of convergent evolution--it resembled a cheetah because it evolved to fill a similar ecological niche. It's also an example of what the science writer Connie Barlowe called a "ghost of evolution": a species that's now gone, whose legacy we can still see reflected in other species.

Photo by Greg Hume,
Wikimedia Commons
Ghost species aren't just reflected in other animals. You can also see their spectral outlines in the shape of some plants. For example, honey locust trees are covered with vicious, branched thorns as long as daggers. Why? Why do they need that kind of armament? The answer, most likely, is that they don't need it anymore, but they once did--when mastodons and sloths the size of cars used to eat their seeds. In fact, that may be why honey locust seeds are honey-sweet--to attract large mammals, which would eat the seeds, and then disperse them later along with a nice dollop of fertilizer. The trees needed to encourage the big beasts to eat their seeds, but they needed to discourage them from pushing them right over in their zeal. So they grew thorns big enough to repel a mastodon.

Some of the other ghosts in the landscape come from much farther back in time. If you look closely at certain rocks at the edge of the Rocky Mountains, near the Mile High City of Denver, you may get lucky and find a shark tooth. Of course, actual sharks are rare these days in the Rockies. The fossilized teeth are actually older than the mountains. They're reminders that Colorado was once covered by a sea, where sharks patrolled alongside giant aquatic reptiles that went extinct along with the dinosaurs.

Cretaceous Shark Tooth from Colorado. Photo by Author
Some of nature's most influential ghosts were never actually alive, but they left a far more visible legacy than the sharks and mastodons did. The other day I hiked into the mountains outside Denver and saw the incredible wall of rock in the picture below. The trees clinging to its left side give an idea of its scale--it's hundreds of feet tall; sweeping up in a great curve from the valley floor. Why is it shaped like that? Why isn't there just a tree-covered slope like you might find in the Appalachians? The answer is that it was carved by glaciers--a whole series of glaciers that crept down the valley every when the climate got really cold every hundred thousand years or so. The glaciers are gone from Colorado today (except for a few small ones in sheltered cirques) but their handiwork is visible everywhere; in sharp ridges, glacial lakes, and walls of bare rock framing U-shaped canyons. Their work is as alive today as Michelangelo's, and unlike Michelangelo, they may come back someday.

Glacial Cliffs near Chicago Lakes, Colorado. Photo by Author
When you look closely, you find ghosts everywhere in the landscape. In fact, you can even see entire phantom landscapes if you know how to look for them. Many geologists do their job by learning to see such ghost landscapes; looking out across the modern terrain and seeing outlines in their mind's eye of long-gone features like volcanoes and mountain ranges. In the picture below, for example, we see a mesa near Golden made of ancient lava that once flowed from volcanoes into a valley. The valley walls and the volcano eroded away, but the hardened lava resisted erosion, and now stands above the surrounding plains. It's a beautiful scene, but it's more than that, too: It's also a reflection of an older, equally beautiful landscape; a landscape that's now nothing but a ghost.

Table Mountain, Golden, Colorado. Photo by Ross Mays, ancient landscape drawing based on Drewes, 2008


More details on giant sloths, locust trees, and the mesa above:

Thorn Trees, Avocados, and VLS's (Very Large Sloths)

How a Valley Became a Mesa, and Why a River Runs Through It

Table Mountain Shoshonite Porphyry Lava Flows and Their Vents,Golden, Colorado / Harald Drewes, 2008