Prehistoric Trackways National Monument

Robledo Mountains - New Mexico


In the Robledo Mountains – a small range a few miles northwest of Las Cruces, New Mexico, and along the western bank of the Rio Grande – there lies a remarkable geologic chapter in the story of life on our restless planet. Its significance first realized by amateur paleontologist Jerry MacDonald, it now ranks as the world’s most important fossil record of its kind for its geologic time—the Permian period of the Paleozoic Era. In recognition of its scientific value, the U.S. Congress passed legislation in 2009 to make the site a national monument—the Prehistoric Trackways National Monument. It spans some 5,200 acres in the southeastern Robledo Mountains, and it encompasses at least 150 Permian period sites. The monument is managed by the U.S. Department of the Interior, Bureau of Land Management.

Discovery Site.

BLM sign located at the discovery site in the Prehistoric National Monument.
Medtrails, CC BY-SA 3.0, via Wikimedia Commons

Basic Geologic Setting and History

The story began some 250 to 300 million years ago, tens of millions of years before the appearance of dinosaurs. Shifting across our earth’s surface in a process that geologists call plate tectonics, the planet’s landmasses had largely merged, forming a supercontinent, Pangea, which covered more than 30 percent of the earth’s surface. It was surrounded by a global ocean, Panthalassa. According to Traces of a Permian Seacoast, written by Spencer G. Lucas and published by the BLM and New Mexico’s Museum of Natural History & Science, what is now New Mexico was located near the western, equatorial edge of Pangea. The southern part of the state lay beneath a shallow tropical sea called the Hueco Seaway. The northern part featured highlands ancestral to today’s Rocky Mountain chain. The land that would someday become the monument formed part of the coastline of the Hueco Seaway, with extensive tidal flats and conifer forests onshore and shallow, clear, warm and calm waters offshore.

Detail of a Paleozoic track in Prehistoric Trackways National Monument, New Mexico.
By United States Department of the Interior, Bureau of Land Management, Public Domain

The tidal flat and sea bottom muds effectively served as geologic guest books, recording a rich assemblage of imprints – often called “trace fossils” – of animals’ footsteps, body shapes and behaviors and of plants’ stems and foliage. (There is also an abundance of mineralized fossil forms in neighboring strata.) The imprints filled in with sediments, which shielded the shapes from erosion. Over time, the muds hardened into stone, which effectively “froze” the imprints in time. The offshore muds also became an ideal geologic matrix for massive tree trunks, which likely flushed out into the sea during violent storms, eventually sank into the muds, and petrified over time. So far, two dozen of the Trackway’s Permian sites have yielded petrified logs. Roughly 200 million years ago, with the record of Permian life cast in stone, Pangea began to fracture, says Encyclopedia Britannica. Landmasses separated, creeping like monumental rafts across the earth’s surface.

Tracks, including possible Dimetrodon tracks, found on a slab at the Discovery Site.

Tracks, including possible Dimetrodon tracks, found on a slab at the Discovery Site.

Sea waters flooded the widening spaces between the landmasses. Over millions of years, the landmasses became our modern continents, the sea waters, our modern oceans. Some tens of millions of years ago, according to the New Mexico Bureau of Geology and Mineral Resources, the North American continent’s Colorado Plateau and Great Plains began to separate, creating a great north/south fracture we call the “Rio Grande Rift.” It extends from central Colorado southward through the heart of New Mexico into western Texas. It defines the course of the river. It has triggered mountain building along its length. During this time, the Robledo Mountains, specifically, arose as a result of complex faulting with some magmatic intrusions, which, according to Lucas, produced the “many broken blocks of rock that now form the rugged canyons and steep ridges that characterize the...landscape” of the range. Many of its exposed strata and rocks, which geologists call the “Hueco Group,” embrace the animal and plant record of the “ancient sea bottom, seashore and land environments that existed in southern New Mexico about 280 million years ago.” It is the reddish-colored siltstone and sandstone strata and rocks that hold the most striking geologic records of the animals and plants along that Permian coastline.

Possible Dimetrodon track, found on a slab at the Discovery Site.

Possible Dimetrodon track, found on a slab at the Discovery Site.

Life in the Permian Period

The complex and abundant community of life in the Permian period would have seemed exotic, although not always entirely unfamiliar, to us in our own geologic time, or the Quanternary period. For example, along Permian shorelines like those of the Robledos, vertebrate animals with some mammal-like features preyed on other animals. These included, said science teacher Michael David Viney, the famous 10- to 13-foot-long Dimetrodon, or pelycosaur, which had a sail-like “fin” along its back. Other vertebrates fed on the vegetation. These included the 10-foot long, thick-skulled, “armored” reptiles known as the Pareiasaurs, which weighed more than a thousand pounds. One reptile, the Coelurosauravus, even took flight—the first vertebrate with wings. Amphibians such as frogs, toads and salamanders preyed on wildlife both in terrestrial and aquatic environments. Some amphibians reached six feet or more in length. Arthropods (invertebrate animals with external skeletons, segmented bodies and jointed appendages) proved highly adaptable to the Permian environment. They expanded across much of Pangea. These included some that would have looked similar, for instance, to our modern dragonflies, damselflies, grasshoppers, beetles, roaches, spiders and scorpions. Just inland from the coastline, coniferous forests flourished. They included various pine tree species as well as clubmosses, horsetails and ferns. Just offshore, in the shallow coastal waters, the living community included numerous species of fish such as primitive sharks, jawless fishes and bony fishes as well as corals and sponges and other reef-forming species.

Small reptile tracks, each less that one half inch in length.

Small reptile tracks, each less that one half inch in length.

Clues to Permian Life at the Robledo Trackways

The imprinted records of 280-million-year-old tracks, body shapes, behaviors and movements embedded in the exposed Hueco Group rocks of the Robledos offer a kaleidoscopic insight into the life of the Permian period. Some imprints, it should be mentioned, are depressed, just as they were left in the mud by the life form that caused them. Other imprints are raised, a product of the sediments that filled in the depressions, solidified in place and then separated from the matrix. The depressed imprints are described as “negative,” and the raised ones, as “positive.” The imprints often offer an insight into an animal’s behavior and character. For instance, some Robledo trackways, said Lucas, provide a record of “more than 50 footfalls of a single animal and show pelycosaurs walking with a surprisingly upright posture.” Other imprints, faint but undeniable, provide only a subtle indication of the animal that produced them.

Tracks, noticeable only after close inspection, may be “circular, elongate, two-toed (didactyl) or three-toed (tridactyl).” Those are often attributed to tarantula-like spiders or to scorpions. Still other imprints, even if distinct, at first seem baffling. For instance, a linear row of regularly spaced imprints of an insect’s body, including mouthparts, head, legs, abdomen and tail – with no interconnecting footprints – raised questions about locomotion. How did the insect move from imprint to imprint? It appears, scientists now think, that the imprints represent perhaps the oldest records of an insect that must have progressed forward by jumping.

Pine frond impressions found on a slab at the Discovery site.

Pine frond impressions found on a slab at the Discovery site.

Strangely-shaped imprints may also give insight into an animal’s behavior. A small, perfectly formed spiral, less than an inch in diameter, points to burrowing in the tidal flat muds by a tiny worm-like creature. Countless other imprints in the Robledo Permian sites suggest a range of behaviors, from fish swimming along the bottom in tidal-flat shallows, to horseshoe crabs moving and, intermittently, resting in sediments along the shoreline; from snake-like amphibians slithering over the mud of tidal pools, to jellyfish-like animals stranded and dying on beaches. Imprints of pine fronds and cones washed from inland forests can also be found among the traces of the animals. Collectively, the Robledo trackways monument speaks to a rich and diverse community of life 280 million years ago. It holds a treasure trove of information still awaiting discovery and interpretation.

The Mass Extinction of Permian Life

For reasons not fully understood, the Permian period drew to a close some 250 million years ago with perhaps the greatest mass extinction of life in the last 600 million years, according to the Smithsonian National Museum of Natural History. According to “Science News,” some “95 percent of all species on Earth became extinct...” For example, as the Smithsonian said, “insects suffered their greatest mass extinction in Earth history.” Additionally, some 90 percent of the marine animal species disappeared. Major groups vanished entirely. The extinctions, which played out over some millions of years, may have been attributable to a series of events that turned earth’s atmosphere into a toxic stew.

Those events may have been triggered by massive volcanic eruptions in Siberia, suggested Penn State geoscientist Lee R. Kump, quoted in “Science News.” These eruptions, said the Smithsonian, produced “massive outpourings of lava called flood basalts,” which would have blanketed an area two-thirds the size of the continental United States. The resulting volcanic carbon dioxide, said Kump, caused atmospheric warming, which, in turn, caused oceanic warming, depleting the waters of oxygen. This slowed the decomposition of marine animals and plants. It opened the door for the proliferation of bacteria that take their oxygen, not directly from the water, but by stripping it from organic compounds.

In turn, these bacteria produced hydrogen sulfide – a lethal poison, which dispersed into the water and subsequently into the atmosphere, with deadly consequences for Permian marine and terrestrial life. Compounding the process, the hydrogen sulfide in the atmosphere may have damaged earth’s protective ozone layer, admitting excessive ultraviolet radiation, which could have caused deformities in the animal life. Moreover, methane discharged by Permian period swamps rose to become part of the lethal atmospheric mix. Ultimately, the scientific question may be not why did so many species die out at the end of the Permian, but rather how did any species survive in an environment awash in hydrogen sulfide, ultraviolet radiation and methane? Those that did survive, however, would, as Viney said, “usher in a new era represented by different flora and fauna evolved from the small percentage of survivors who were, at first, cosmopolitan in their distribution.”

Scientific Significance of the Robledo Trackways

The Prehistoric Trackways National Monument provides an extraordinary insight into the community of life on an equatorial coastline 280 million years ago. As Jerry MacDonald and his associates have suggested, the Robledo imprints represent what is perhaps the most scientifically significant and abundant record of exceptional trace fossils of the past half billion years. It has yielded unparalleled insights into the Permian tidal flat animal population, including, for example, classifications and relative abundance of species, spatial and temporal variations of species, patterns of behavior, and interactions with the environment. “Despite intensive collecting and study, much new information... continues to be discovered,” said MacDonald and his associates. On a more sobering note, it has been suggested that the Permian record may serve as a preview of what could happen in our own geologic period, the Quaternary, if we continue to disperse excessive carbon dioxide into our atmosphere, accelerating the trend of global warming.

Trackways exhibit at Las Cruces' Museum of Nature and Life Science. Rock slabs bear the actual tracks of a Dimetrodon, which is shown as a life-size cast above the tracks.

Trackways exhibit at Las Cruces' Museum of Nature and Life Science. Rock slabs bear the actual tracks of a Dimetrodon, which is shown as a life-size cast above the tracks.

Visits to the Site

Ideally, if you are interested in exploring the Prehistoric Trackways National Monument, you would begin with a visit to the Jerry MacDonald Paleozoic Trackways site at Las Cruces’ Museum of Nature and Science. It's located in the heart of old downtown, at 411 North Main Street. There you will find, as the museum says, “two spectacular fossil trackways from the Permian Period, a life-sized cast of a Dimetrodon, two media programs on the trackways, and touchable trace fossils.” After visiting the museum exhibit, you would be well prepared for a hike into the monument, to the Discovery site. It was here that MacDonald excavated some 2,500 slabs with trace fossils in the 1980s, as he, almost single-handedly, focused national attention on the scientific value of his finds. Guided by a BLM representative, the hike follows an uneven and sometimes rocky trail for about a round trip of about two miles. BLM cautions that you should “Be prepared with water, closed toed shoes, snacks, sunscreen, cameras and whatever else you need for a healthy, happy hike.”

Contact the BLM in Las Cruces at 1-575-525-4300 or BLM Prehistoric Trackways National Monument Ranger McKinney Briske at 1-575-525-4334 for more information and schedules. Map PDF

After the museum visit and the guided hike, you will be prepared to begin exploring the Prehistoric Trackways National Monument on your own, although you should note the precautions posted on various signs along the trails. You would not want to run the risk of winding up as a fossil 280 million years from now.

By Jay W. Sharp

The Organ Mountain-Desert Peaks National Monument lies adjacent to the Prehistoric Trackways National Monument. Read more here: Organ Mountain-Desert Peaks NM

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