Tracks in the Rocks
Mountains are nature’s mystery novel. Captured in layers of rock are stories about time, about life, about death. As I pass the Robledo Mountains just northwest of Las Cruces, New Mexico, I ignore the dry, sparsely vegetated cover, knowing an extraordinary story awaits me within the rock of the Prehistoric Trackways National Monument.
The monument, managed by the Bureau of Land Management, is relatively new, so infrastructure consists of a few interpretive panels, two dirt parking lots, and a hiking trail that switches up the hillside. I called ahead and signed up for a guided tour—the best way to discover the secrets of the Robledos.
On that bright, spring, Saturday morning, a small group of hikers gather around Colin Dunn, the BLM paleontologist. We’ve come to see the fossilized trackways of creatures that squelched through mud 280 million years ago. “We’re skipping the dinosaurs,” Colin says. “Mammals, flowers, birds. Just skip it.” These tracks were made by animals 60 million years before any flower bloomed or dinosaur roamed the earth.
The trackway eroding from the Robledo Mountains dates to the Early Permian. At that time, New Mexico was part of a single, massive continent known as Pangea. The rocks that surround us are sediments from a shallow, warm sea that lapped against a tropical shoreline.
We hike up a dry creek bed. Colin stops to show us long, thin sea urchin spines, round disks of crinoids (sea lilies), flat colonies of bryozoan, and ridged shells of clam-like brachiopods. These fossils create a layer of “shell hash”—a jumbled assortment of life in the Permian sea.
As we move inland from sea to shoreline, thick beds of red rock jut from the hillside. Colin scrambles to the first terrace. Crouched before a large, flat slab of siltstone he points out a series of tracks. About the size of my hand, the “palm” of each print is round and deeply impressed. The toes are long, thin, and sharply pointed.
“These are Dimetrodon tracks,” Colin tells us. Dimetrodon was the terrestrial terror of the Permian world. Lizard-like, with a bony sail fin down its back, the largest Dimetrodon was 13 ft. long and weighed 500 lbs. “This one was pretty small,” says Colin. “It was only about 100 lbs.”
Colin pulls a small rock out of his pocket. Clusters of three lines, each arranged like an arrow, cover the surface. “I was at White Sands and saw beetles making tracks just like this. I was totally geeking out!”
I am too. Fossilized bones fascinate me, but these tracks? Even better. Bones write the story of death. These footprints tell the story of life. I can picture an araeoscelid (go ahead and think lizard) scampering for cover, a beetle making its way across the mud, and the corkscrew twist of a worm as it drills a burrow. Raindrops dimple one rock; a mat of Walchia tree fronds from an inland forest is impressed on another. The fronds look like the potted Norfolk Pines I used to buy at the home improvement store.
By studying these tracks, paleontologists are unveiling twists to the mystery of Permian life. Initially, scientists believed Dimetrodon was a low-slung, lumbering thug. But the tracks—set close together and lacking belly scrapes or tail drag marks—suggest Dimetrodon was a sprinter rather than a plodder.
Colin tells me of petrified log jams and other track sites at the monument. He won’t share where they are though. “Have you heard of Fossil Cycad National Monument?” he asks. I shake my head, no.
“That’s the problem. Fossil Cycad had one of the world’s greatest concentrations of cycads. Then people came in and took them all. They shut down the monument because there wasn’t anything left to see. I don’t want that to happen here.”
I don’t either.
The Prehistoric Trackways National Monument is open to the public, but the best way to visit is with a knowledgeable guide. Tours to the Discovery Site are given once a month on Saturdays. Contact the Bureau of Land Management for more information.
To Learn More:
Traces of a Permian Seacoast is a wonderful, beautifully illustrated book about the fossil trackways of Prehistoric Trackways National Monument. Purchase it from the link below to help support this blog!