Category Archives: Fossils

Prehistoric Trackways National Monument

Tiny tracks from 280 million years ago are preserved in red siltstone.

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.

Colin Dunn, paleontologist with the Bureau of Land Management, shows us a rock loaded with fossils.Skip the Dinosaurs

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.

Fragments of sea urchin spines and shells make a jumble of shell hash in a rock.Shell Hash

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.

Tracks of a Dimetrodon are deeply impressed into red siltstone.


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!”

Walchia fronds from an upland forest washed downstream and pressed into the mud.Walchia Fronds

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.

A sign at the monument tells of the trackway discovery and warns people not to collect rocks.

Getting There:

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!


Also posted in Chihuahuan Desert, Geology, New Mexico, Travel

Ghosts of the Past: Stromatolites

The 1.25 billion year old Castner Formation is exposed in the Franklin Mountains near El Paso.

I believe in ghosts. They hide in plain sight, urging us to listen to the stories they have to tell.

It was my birthday and, feeling just a wee bit sorry for myself (I’m of that age), I decided to go in search of something older than I was—the stromatolites of the Castner Marble in the Franklin Mountains near El Paso. Now, these stromatolites aren’t just a little older than I am, they’re way older than I am. About 1.25 billion years older. Yes, that’s right: 1.25 billion.

Math isn’t the strongest of my academic skills, but let’s work our way through this. Let’s say you wanted to count to 1.25 billion—just to get a sense of how colossal that number is. If you keep up a steady rhythm of a number per second (and absolutely no stopping for dinner, a potty break, or Facebook), it will take you 40 years to reach 1.25 billion. If you wanted to restrict your counting to a mere 8 hour day, buckle down for 120 years. Hopefully, you don’t get confused, skip a number, and have to start all over. That would be depressing.

Tiny, single-celled cyanobacteria create layered sedimentary rocks.

So I’m in search of 1.25 billion-year-old stromatolites. In case you’re wondering, stromatolites are a type of sedimentary rock created by single-celled photosynthetic cyanobacteria (that’s blue-green algae). These microorganisms like to congregate in large groups, forming sticky mats in shallow water along a shoreline. I don’t think anyone knows why the stickiness is necessary, but the result is that the bacterial mats accumulate sediment that clogs the system up. The bacteria, being photosynthetic, need the sun’s energy to produce food, so they move upwards, forming another mat on top of the sediment. And so it goes. Layer after layer, eventually forming a cauliflower-shaped lump of rock in shallow water.

Now the amazing thing is that these single-celled cyanobacteria have been carrying on like this for over 3.2 billion years. In fact, stromatolites are some of the oldest fossils in the world. They reached their peak diversity just about the time the Castner Marble stromatolites formed. After that, they’re harder to find. The theory is that grazers evolved that found a stromatolite mat downright tasty and pretty much ate them to near-extinction.

Arizona Barrel Cactus (Ferrocactus wislizeni) have hook-shaped spines that can grab an unsuspecting ankle.

I cajole a geologist friend into going with me, and we head westward, driving past yucca-studded grasslands and mesquite-topped sand dunes. We skirt El Paso and climb the Trans-Mountain Highway through the Franklin Mountains. I navigate, reading directions from the Geological Excursions to a Transmountain Precambrian Adventure field guide that I’d downloaded from the internet. We’re looking for Stop 2—the Castner Marble Stromatolites. A gust of wind shakes our vehicle as we round the corner.

We pull over onto a wide gravel patch (obviously not the first geologists to investigate this location) and begin walking back down the highway to the end of the guardrail. Another gust of wind nearly knocks me off the mountain.

The authors of the field guide suggest that we follow a small trail down into the valley, and up the other side. The stromatolites will be there.

The trail is, indeed, narrow, and very steep. I cautiously make my way down, trying to avoid the baseball-sized round rocks that ensure a quick—and most likely painful—descent down the slope. I skirt the lechuguilla, prickly pear cactus, and a barrel cactus or two to arrive at a 2 to 3 ft thick band of light-colored rock.

A layered band of stromatolites are exposed in the hillside.

“Are you sure these are stromatolites?” I ask Blaine. He’d gotten there before I had and was poking and prodding at the rock as geologists tend to do.

“Yeah, sure. Look.” He points to the thin, wavy layers in the rock. And there, right before me is evidence of the earliest life in the Chihuahuan Desert.

With just my naked eye I can distinguish layer after layer of sediments trapped by cyanobacteria over a billion years ago. To see the actual fossils, I’d need something slightly stronger, like a microscope. Most of the layers are flat-lying to slightly wavy—an indication that the mats collected sediments in relatively calm waters.

Stromatolites aren’t flashy like a trilobite fossil or spectacular like an ammonite. But all in all, they’re probably far more important. Stromatolites hint at the origin of life. When the cyanobacteria were trapping these sediments, the skies were pink—filled with methane and ammonia and other gasses that would kill us today. The land was barren. Not a single, living thing existed out of the water. But in the water, slimy mats of green bacteria clung together, creating their own little rock. The cyanobacteria were photosynthetic, and that nifty trick of converting the sun’s energy and some carbon dioxide into food while releasing oxygen as a byproduct was changing the world. A few billion years of lilliputian, single-celled bacteria releasing minute bubbles of oxygen created an oxygen-rich atmosphere necessary for complex life to evolve. Take a deep breath. That’s you I’m talking about.

Living stromatolites are pretty rare now, but they’re still around. In Cuatro Ciénegas, a desert wetland in the Chihuahuan Desert of northeastern Mexico, stromatolites can be found in the pozas or spring-fed pools of the marsh.

Scientists working at Cuatro Ciénegas cored one of the active stromatolites. The mat consisted of a complex community of microorganisms—not just cyanobacteria. The top layer was composed of diatoms (distinctive algae with transparent cell walls), below that was photosynthetic cyanobacteria. A third layer consisted of purple sulfur bacteria. These bacteria photosynthesize using hydrogen sulfide rather than water and produce elemental sulfur as their byproduct. Below that was a sulfur-reducing bacteria that takes the elemental sulfur and reduces it to hydrogen sulfide. “Each layer obtains and processes energy in a different way and produces different waste products, and yet it is an interactive community where each layer contributes to the survival of the mat as a whole,” the scientists concluded. Stromatolites are all about being good neighbors.

Stromatolite band exposed in the Franklin Mountains near El Paso.

I poke around the rocks, distracted by rainbow cactus with fat buds, and a lizard warming itself in the sun. None of this would be possible without those little cyanobacteria that lived in an ancient sea long, long ago. I wave at Blaine, who’s wandered off to look at a contact or something geological and we head back to the car. Finding rocks that are billions of years older than I am, makes me happy—all in all, a pretty spectacular birthday.

Also posted in Chihuahuan Desert, Geology, Texas

Places: Dinosaur National Monument, Utah

A two-story wall of dinosaur bones enthralls visitors at the Dinosaur National Monument in Utah.

I received the best news this morning. I feel kind of like the kid that really, really wanted a pony for Christmas. He unwraps his biggest, best-wrapped package to find nothing but a box full of horse manure. He’s insanely, wildly happy. “But you only got manure,” his sister points out.

“Yeah, but where there’s poop, there’s got to be a pony!”

My exciting package was an email that had been forwarded about a million times, but when I dug through the poop, I found the pony. A link—a simple little blue-underlined link—that led to a press release about Dinosaur National Monument in Utah.

Dinosaur National Monument and I go back a long way. My prized possession in the third grade was a fossil clam. Embedded in a limestone slab that I found in the creek on my great-aunt’s ranch in Texas, that clam and I moved around the world together.

In the 5th grade, I spent my time after school sifting through the bulldozed soil of construction sites in our neighborhood looking for pretty rocks and fossils. I found a trilobite one time. I was enthralled by this marble-sized, dark grey animal, curled up like a pillbug. I took it to school the next day and my 5th grade science teacher tried to take it from me. “It will go into a museum,” he said. Even as a 5th grader, I was skeptical of this story. I’d seen the gleam in his eye, the way his hand curled possessively around the trilobite. The trilobite returned home with me and I never showed it to another adult.

After graduating from high school, I traveled with my grandparents from Oregon to Texas. We stopped briefly somewhere in Wyoming or Utah where they were excavating a dinosaur. “Do you want to stay?” my grandfather asked.

“Yes.” I began unloading my luggage from the car.

“No, no,” my grandfather grabbed my arm. “I was just teasing. Let’s go.”

I would have stayed. I probably should have stayed. I think I pouted all the way through Colorado.

My first real brush with Dinosaur National Monument was in 2006 when a friend and I stopped to see the famous wall of bones. We didn’t get far. The quarry was closed for renovations. Come back later. Have a nice day! Damn.

For 150-million years, these dinosaur bones have been sealed in layers of rock.

For 150-million years, these dinosaur bones have been sealed in layers of rock.

Even though I’ve had a life-long obsession with fossils and dinosaurs and things preserved in rock, it wasn’t until 2015 that I actually got to see the Carnegie Quarry at Dinosaur National Monument. I arrived promptly at 9:00 in the morning, road the first tram of the day to the (newly renovated) Quarry Exhibit Hall, and stepped from the bright sunshine to face The Wall. I felt like I was in the presence of God. My heart beat rapidly and I couldn’t quit smiling.

The Wall is a 2-story rock face preserving the story of life approximately 150 million years ago. The rock from nearly 1,500 fossilized bones of long-necked sauropods, predatory theropods, spike-tailed Stegosaurus, and small, but speedy ornithopods was painstakingly chiseled away so the bones would be easier to see.

Although the dinosaur bones get the most attention, it’s the details that tell the story the best. To get the big picture, scientists looked at microfossils, invertebrate, small vertebrates, fossil soils, trace fossils and pollen to understand the environment in which the dinosaurs lived. Was it humid and hot? Or hot and dry? People assumed that the long necks of the sauropods were for browsing in tall forests of conifer trees. The fern fossils were believed to be indicators of a very wet environment.

But by looking at the details as individual pieces to a larger puzzle—rather than the entire puzzle itself—a different story emerged. Over 225 different types of pollen and spores were recovered from the sediments surrounding the dinosaur bones, indicating a much greater diversity of plants than was first believed to exist. Yes, there were conifer trees, but there was also an understory of ginkgo, cycad-like plants, horsetails, ferns, rushes, and clubmosses.

Dense thickets of horsetails still grow along the watercourses of Dinosaur National Monument, Utah.

It turns out that the long-necked sauropods really couldn’t raise their heads much above hip-level, so the idea of dense forests of conifers with sauropods nipping branches from the treetops lost favor. The Jurassic environment was revealed as a Serengeti-like plain with vast semi-arid fernlands interspersed with clumps of conifers and dense riparian vegetation along the watercourses.

At least, that’s what the scientists think now. The thing about science—the exciting thing about science—is that the story changes and gets refined, as we learn more. Chemist Albert Szent-Gyorgyi is credited with my favorite definition of science: “Science is to see what everyone else has seen, but think what no one else has thought.”

That press release that I received this morning opens the door wide for some new thinking about the dinosaurs of Dinosaur National Monument. For the past two years, paleontologist Dr. Dan Chure and a team of volunteers have photographed the fossils of the Carnegie Quarry, illustrated them, digitized historical records, and consolidated data to create a publicly-accessible Digital Quarry.

For armchair paleontologists like me, or serious academic types, this is an incredible resource. It’s days like these that make paying taxes a pleasure. Congratulations, National Park Service. Well done.



Also posted in Dinosaur National Monument, National Parks, Utah Tagged , , |