"Petrified Footprints: A Puzzling Parade of Permian Beasts", Smithsonian, July 1992
The following is a transcript from the article that appeared in the Smithsonian Magazine. Note: it is copyrighted and MUST be credited.
"It's a clear, chilly morning in southern New Mexico's desolate Robledo Mountains, and Jerry MacDonald, panting as much from excitement as exertion, is hot on the trail of wild game. He's not quite sure what he's tracking but he knows he want it.
"Look at that little guy go!"
Too late. MacDonald's four-footed prey has vanished. It seems to have scurried straight into the sheer rock cliff rising up next to us. Only its tracks remain in the mud behind it. MacDonald, sweating in the 40-degree desert air, is off in pursuit.
"He's realy moving. Let's see if we run into him up this way."
That the trail is some 280 million years old, and the mud now mudstone, doesn't dampen the thrill of the chase for MacDonald--a thrill that verges on bloodlust. The spritely creature he's hunting is an amphibian whose tracks he's never seen before; it may well be that no one has ever seen them before. With a loud grunt, he jams a six-foot pry bar into a fissure between two layers of red mudstone at knee level and starts rocking the end of the bar. Then he clasps his two meaty hands on the broken-off slab and begins to yank.
"I want this," he whispers to himself, breathing hard, as he struggles to free the rock. "Come on!"
At layer 25 of excavation, MacDonald brushes off some unidentified prints that appear curiously bipedal.
The slab finally comes free, a smooth chunk of heavy red-brown mudstone about the size of a small suitcase. Gasping, MacDonald heaves the slab up onto one side. Running the length of the top edge is a series of fine, parallel cracks. He pokes a chisel into the most prominent of the cracks and thwunks it with a small sledgehammer. The stone pops open.
The early-morning sun highlights a crisp series of footprints much larger than we were anticipating. They're almost an inch deep and several inches long from the back of the heel to the vicious-looking ends of the widely splayed toes. On this exact spot, long before the age of dinosaurs, a sail-backed pelycosaur known as Dimetrodon grandis, king carnivore of the Permian period, swaggered through this patch of now petrified mud. To MacDonald, panting and chortling as he sweeps rock debris from the impressions with his fingers, it's as though the reptile had just crawled out of sight, snorting and hissing.
Most museums would be happy with a single slab bearing a half-dozen Permian footprints. MacDonald has dug up and assembled trackways up to 27 feet long that contain an entire food chain's worth of footprints, whole feeding frenzies in stone. Until this morning, I've always regarded fossil hunting as slow, tedious and uneventful work. No longer. MacDonald is digging up footprints faster than I can jot down their descriptions in my notebook.
"It's so rich they wouldn't believe me"
"Jeez Louise! Another pelycosaur--a big one!" We're both laughing now. "Man, look at those claws! This layer is going to be spectacular." MacDonald wasn't expecting this; he was just clearing rock to get down to his target layer. I comment that he doesn't have to wait long for gratification. "Uh-uh. Not her." He stops prying and straightens up. "That's why people wouldn't believe me," he says, fixing me with an exhilarated glare. "It's so incredibly rich, they just wouldn't believe me!"
Jerry Paul MacDonald was headed for a PhD in sociology when he struck pay dirt in the Robledos years ago. Since then, his life has been consumed by paleontology--or, more specifically, ichnology, the collection and study of animal traces, including footprints. To say that hunting for fossilized footprints is MacDonald's hobby is like saying that Norman Mailer like to type. MacDonald is obsessed with footprints. He dreams about them at night and fantasizes about them during the day. Though he remains essentially an amateur among professionals, the tracks he's unearthed single-handedly at three dozen sites in and around the Robledos have attracted worldwide attention and aroused new scholarly interest in the Permian period, the last age of the Paleozoic era.
"On a scale of one to ten," says James Farlow, an Indiana University fossil-footprint expert, "I'd give the Robledos findings about a hundred. They've got everything: reptiles, amphibians, insects, gorgeous plant fossils, even some bone beds, all in the same rock. It's one of the best footprint faunas of any kind, any age, anywhere." The slabs that MacDonald has been splitting open and hauling out may contain evidence of more than 100 different animal and plant genera, many of which appear to be new to the world of paleontology.
Bony fossils reveal what animals looked like, but they only hint at how they behaved. Fossilized footprints can offer an exact glimpse of what animals were doing at a given instant in time, almost as though a hidden camera had been running. What makes the Robledos footprints especially valuable is that they're not simply tracks, they're trackways--long, unbroken trails of animals of all kinds, sizes and dispositions, walking, darting, swerving, pouncing. Many of the footprints are so sharp, the mud appears still wet. Yet they are almost unimaginably old: they were imprinted not long after (in geological terms) the first animals slithered out of the sea.
The discovery of Pelycosaur Heaven
Back in 1987, the year MacDonald began discovering trackways, his key site, now dubbed Pelycosaur Heaven, was just a smooth-sloping expanse of gravel and undistinguished rock debris. Since then, MacDonald reckons, working with simple hand tools and his bare hands, he's dug, scraped and pulled 100 tons of rock out of the hillside. Twenty-five of those tons were collectible trackway specimens that he tied to his back and lugged out for eventual cataloguing and analysis. (All of his excavations are on federally controlled land. None of the sites are accessible by road, nor has he publicized their exact whereabouts, the better to deter fossil thieves.) "The biggest slab I ever carried out weighed 175 pounds," he says. "That was a real killer." The slab contained pristine footprints of what was probably a ten-foot sail-backed pelycosaur. MacDonald roped the slab to his back and spent two hours staggering the half-mile to his Jeep. "For the final climb out, I was on my hands and knees." As a result of his one-man quarrying operation, MacDonald now suffers from permanent ligament damage in his shoulders, fallen arches and so much nerve damage to his heels that he can't feel a pinprick in either one.
MacDonald is obviously a highly motivated individual. At 5 feet 8 inches tall, he's not a large man, but he's sturdily built. He has the untamed look of a modern-day mountain man: broad, weathered face, unkempt beard, intense wide-set eyes. A former high school gymnast (springs, flips and handstands were his specialty) and later a recreational weight lifter, MacDonald approaches his fossil hunting with the mind-set, the training and the solitary self-discipline of a heavy lifter. Yet to take him for a blue-collar workhorse would be a mistake. He's both a dreamer and a detective, a quiet observer who, in his mind's eye, pictures the Permian shards he finds and constantly tries to complete them. He may at times be unsure of his taxonomy, and he may stumble over the requisite paleojargon, but as a crafty hunter of Permian wildlife he is unsurpassed. "Jerry's intelligent, tenacious and very honest," says Nicholas Hotton, a curator in the Smithsonian's paleobiology department. "He hasn't just hauled the rocks out. He's curated them. The catalog of trackways that he's completing is just as good as one you'd get from a professional." The Smithsonian's Museum of Natural History (which now houses a 20-foot trackway that MacDonald excavated) has given him a collaboratorship, and the federal Bureau of Land Management and the Los Angeles County, Carnegie and New Mexico museums of natural history support his fieldwork. To display his finds, the BLM has plans to put up a Permian trackways museum near his main site. There is even talk of designating as a national monument a five-square-mile region encompassing his excavations.
Despite the widespread excitement his discoveries have generated, MacDonald continues to do his fieldwork alone for the most part, as he always has. Most mornings, he rouses himself before dawn, packs a canteen and some trail mix, and heads from his home in Las Cruces into the mountains. On a recent visit to his key excavation site, we leave his battered 1958 Jeep near a flagstone quarry and cover the last half-mile on foot, picking our way through arid ravines studded here and there with sagebrush, yucca and prickly pear.
The excavation comes into view as a horizontal red scar gouged into the base of a steep ocher-colored hillside. Up close, it's a man-made, neatly terraced cliff 20 feet high and 120 feet wide, with alternating layers of mudstone and siltstone. "Of the major layers, this is layer 25," MacDonald tells me as we walk out onto the stone floor of the excavation area. Plainly visible between our feet are footprints almost an inch deep that look uncannily like large human hands. A giant reptile or amphibian of some kind left the tracks, but so far, no one who's had a chance to see them knows what it was.
Among the mesquite bushes below the quarry he's pieced together mosaics of track-bearing slabs that are too heavy to left. They await the arrival someday of an all-terrain vehicle. MacDonald scrambles over the trackways, giving me a guided tour of his alfresco art gallery. "See this little sweetheart here?" he says, crouching to run a finger along a trail. "You can see its tail drag. That's probably belly drag over here because this little deary is quite a bit wider." MacDonald is as smitten by the tiny critters as by the big ones. Crawling over the mosaics, he delightedly traces dozens of faint centipede tracks and points out landing traces made at random by flying insects. Like carbon paper, the fine-grained mudstone records every detail. Even the weather is preserved: one slab is pitted with hundreds of shallow craters--raindrops.
The smooth, almost wet-looking, surfaces of all these slabs were once the tranquil shoreline of a vast inland sea, MacDonald explains. The waning tide would leave water bugs and small shellfish exposed on the mud flat, where small reptiles and amphibians would feed on them. Larger predators, ranging from carnivorous pelycosaurs a couple of feet long up to Dimetrodon grandis, 12 feet in length from fangs to tail, would then pick off anything smaller and slower than they were. "It was like a restaurant," MacDonald says, "the hottest one in town. Everybody came."
Incoming tides and rain-swollen streams would regularly bury the still-fresh footprints of prey and predators in new layers of mud or silt. (Skeletal remains are rarely found in the same sediments. Bones are best preserved by a quick, catastrophic burial, Pompeii-style.) The layered rock outcrop 20 feet high that MacDonald has excavated here is not a steady accretion of sediments over 280 million years, but rather a record of a relatively brief period of calm shoreline deposition; sedimentologists surmise that the bottom layer was laid down 10,000 years at most before the top, a blink of an eye in geological terms. Much later, after the mud had turned to rock, violent tectonic upheavals twisted and buckled the sediments, eventually thrusting them nearly a mile high. "It's rare to find the ancient shoreline unbroken like this," MacDonald says. "That's what you need to get these rich deposits of trackways." Of the 25 consecutive layers he has excavated here, he's found tracks of some kind in every single one--an unabridged encyclopedia in stone.
Today, after five hours of tracking his "sweethearts," MacDonald slides three of the day's finds into his knapsack, and we hike to the Jeep. Later, at an outdoor Mexican restaurant on the outskirts of town, he tells me about how he came to discover Pelycosaur Heaven. In the mid-1980s, MacDonald was a late-blooming earth sciences major at New Mexico State University here in Las Cruces. (A college dropout, he'd worked as a cabinetmaker in Oklahoma for ten years, raising three children with his wife, Pearl, before moving to Las Cruces in 1983.) As he knew from college field trips into the Robledos, rocks with a few eroded footprints sometimes washed down from the hillsides after heavy rains, but the tracks were typically small and indistinct. No one had ever tried to dig out tracks systematically, let alone whole thoroughfares of them. "I was new to the area," MacDonald recalls. "I wanted to find a footprint for my collections." He talked to local fossil hunters and visited quarries where workers often discovered rocks containing well-defined footprints. In fact, they set them aside for ornamental jobs--patios and fireplaces especially.
The restaurant we're sitting in, Guacamole's Patio Pick-Up, is a case in point. MacDonald points out an impressive set of claw marks from a prehistoric reptile in the flagstones next to the takeout window. The low afternoon sun is right for track hunting, as it happens, and MacDonald interrupts his story for a little preprandial sleuthing. Wandering among the diners, he notices something on the floor that would be invisible if the sun were any higher: a series of unusually thin four-toed footprints an inch long. "I don't think I have that one," he says, squatting excitedly between the picnic tables; "those are really nice." A diner offers to move if MacDonald plans to dig them up. "No, but I'm going to get my camera," he replies.
Later, MacDonald recalls how his whim became an obsession. In the spring of 1987 he was taking night classes at NMSU toward a master's degree in sociology, so his days were free for prospecting. He'd collected a few scattered tracks by then, and had resolved to begin a methodical search for rock formations that suggested a meeting of sea (limestone) and land (sandstone), reasoning that the ancient shoreline was the likeliest repository of buried footprints. Three or four mornings a week for six weeks he trekked through the hills chipping samples, finding a small track here, a small track there. "At the end of each day, I was ready to give up." But at night, MacDonald dreamed of sensational trackways stampeding across the arroyo floors, trackways as perfectly inscribed as the day they were made. The next day, he'd be out cracking rocks again.
"It was like opening an Egyptian tomb"
Finally, one morning in June, he dragged a 120-pound slab of coffee-colored mudstone out of a hillside and cracked it open with his rock hammer. "It was like finding and opening the door of an Egyptian tomb," MacDonald says. Inside were five consecutive pelycosaur tracks, exquisitely preserved, each seven inches long. That slab turned out to be a small part of the longest trackway of Lower Permian pelycosaur footprints ever found. "I didn't even know what a pelycosaur was, but I knew I had something of worldwide significance," he says.
That day, the search became an excavation. Excited though he was, he resolved to operate methodically. That meant conscientiously excavating the 12 feet of rock lying on top of the discovery layer, starting at the top rather than simply diving into the hillside. "I started getting other trackways right away," he says. "Layer 3 had good stuff. Layer 4 was spectacular." He worked every day that summer, even weekends, when Pearl and the children would join him. They spent the next school year at the University of Virginia, where MacDonald had arranged to begin work toward his doctorate in sociology, but his heart was in the Robledos. That spring he returned to trackway hunting full time. Nineteen months passed before he was able to pick up the pelycosaur's trail in what turned out to be layer 10 (a trail that continued for 25 feet).
After lunch MacDonald takes me to the Southern New Mexico State Fairgrounds just outside Las Cruces. There, most of the rocks he has collected since 1987 are stored in three small buildings. One of them is an old padlocked Quonset hut where every inch of floor space is covered with rocks. The rocks, in turn, are covered with tracks--hundreds and hundreds of waddling, scurrying, slithering tracks. "Aren't these things neat?" MacDonald asks. "I'm just so in love with this stuff. They're like my children. Look at this little guy." He picks up a loose slab. "See the suction cups on the ends of his toes?" In one corner is a large tray he uses for comparative trackway research: he spreads baking soda over the bottom, drops live lizards, beetles or centipedes inside, then scoops them up and studies the patterns they leave.
The fossil tracks that MacDonald has collected include a number of what paleontologists like to call "problematica." On one trackway, for example, a three-toed creature apparently took a few steps, then disappeared--as though it took off and flew. "We don't know of any three-toed animals in the Permian," MacDonald points out. "And there aren't supposed to be any birds." He's got several tracks where creatures appear to be walking on their hind legs, others that look almost simian. On one pair of siltstone tablets, I notice some unusually large, deep and scary-looking footprints, each with five arched toe marks, like nails. I comment that they look just like bear tracks. "Yeah," MacDonald says reluctantly, "they sure do." Mammals evolved long after the Permian period, scientists agree, yet these tracks are clearly Permian.
Rumors that he'd carved the tracks himself
MacDonald feels there must be a plausible explanation. These may be creatures whose gaits are unknown; or an animal's back feet may have obliterated its front footprints; or a running five-toed animal may have grazed the mud with only its middle three digits, then been gobbled up on the hoof, as it were. MacDonald himself believes that there were neither birds nor bears in the Permian period (although he tries to stay open-minded about such things). He suspects, however, that conventional theories about precisely who was walking around in Permian times, and how they did so, will end up being revised, perhaps extensively, once these tracks are studied in detail.
On our way out of the Quonset hut, I linger over a tiny reptile trackway. I can see where mud has squooshed up between the animal's toes. The footprints are so fresh and finely detailed, I remark, they look sculpted from clay. "Please don't say that," MacDonald says. When he first began showing a few of his finds around New Mexico, he recalls, some of the local rock-and-fossil hounds were uninterested, even dismissive. He later learned that rumors were circulating that he was exaggerating his finds, that he might be clumsily destroying fossil beds, even that he'd carved all the tracks himself. "It was really discouraging," he says. "Then Pearl said, 'Jerry, take off your paleontologist's hat and put on your sociologist's hat.'" MacDonald is familiar with paleontology's rich history of feuding and backbiting. Instead of giving himself ulcers, he resolved to use the experience as data for his eventual doctoral thesis in sociology (a discipline he intends to return to before he cripples himself for good, hauling rocks).
Over the next few days, in other canyons north of town, it becomes clear to me that MacDonald hasn't been carving these things by hand. I watch him repeatedly pull slabs of mudstone from eroded outcrops and then, with astounding frequency, erupt with glee as he finds biological time capsules buried inside. At a site he calls Insect Hill, I see revealed in the space of 20 minutes what is perhaps a diadectid, a formidable, 700-pound land animal with large, blunt feet; a 12-member centipede party; a mysterious scurrier whose long claws somehow leave S-shaped furrows ("I have no idea what that is"); and a gorgeous trackway, which might be from an eryopid, an alligator-like amphibian, complete with tail drag. "Too many goods things," MacDonald laments. "They're everywhere."
During a later prospecting trip through the nearby Dona Ana Mountains, MacDonald leads me up a dry arroyo and points out a long row of ancient petroglyphs. Local scholars believe they were carved by native desert dwellers about the time of Columbus. We scramble up the slope to examine a turtle, a rabbit, a deer, a hunter with a bow and arrow--all simplified but clearly identifiable and anatomically correct, opposable thumbs and all. On a single block of sandstone is a large figure MacDonald calls Godzilla. Its face is featureless except for a beaklike nose and mouth. Below its head are patterns of mountain peaks and a spiral shape that, according to local lore, has spiritual significance. Inside the spiral is a mysterious footprint.
"Four toes," MacDonald says. "There's no animal around like that now." Some of the Permian amphibians, however, had four toes. MacDonald claims no expertise in petroglyph interpretation, but he's learned that local Indian legends spoke of spirit-animals that could walk inside mountains. He's found what he thinks are eryopid tracks within a stone's throw of here; the canyon is full of them. He suspects that the natives of 500 years ago may have ben a enraptured by the magical subterranean trackways as he is and wove legends to explain the creatures who made them.
For MacDonald, too, this fascination with trackways has its spiritual side. "When you split open a slab and find tracks, you're the first human being to see them," he says as we hike back to the Jeep. "These animals are all extinct now." When he studies his trackways, he sometimes feels he has set foot alone in a vanished world. He knows it wasn't simply luck or the grace of the gods that led him to his discoveries; he used clever detective work, a well-trained eye and a lot of muscle power to roll back the door of the tomb. But he knows he's been lucky, too. He points to our footprints on the sandy floor of the arroyo. "Imagine how likely it is that any one of them will be preserved 280 million years from now." Yes, he says, he's been awfully lucky.
Doug Stewart, a regular contributor, reports that for sheer excitement he now ranks paleontology right up there with race-car driving and slam dancing."
COPYRIGHT © 1992, Smithsonian Institution
"Petrified Footprints: A Puzzling Parade of Permian Beasts" by Jerry MacDonald, Smithsonian, July 1992, Vol. 23, Issue 4, p. 70-79