Tracks of Time
Prehistoric Trackways National Monument preserves
the passage of life before the dinosaurs.
by Jay W. Sharp
In the Robledo Mountains — a small range a few miles northwest of Las Cruces 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 the geologic time span called the Permian Period of the Paleozoic Era. In recognition of the site's scientific value, the US Congress passed legislation in 2009 to make it a national monument: the Prehistoric Trackways National Monument. The monument spans some 5,200 acres in the southeastern Robledos, and it encompasses at least 150 Permian Period sites. The monument is managed by the US Department of the Interior's Bureau of Land Management.
Possible Dimetrodon track, found on a slab at the Discovery Site.
This remarkable site's 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% 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 our 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.
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 this record of Permian life cast in stone, Pangea began to fracture. Landmasses separated, creeping like monumental rafts across the earth's surface. 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." The reddish-colored siltstone and sandstone strata and rocks hold the most striking geologic records of the animals and plants along that Permian coastline.
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 Quaternary Period.
For a few examples, along Permian shorelines like those of the Robledos, vertebrate animals with some mammal-like features preyed on other animals. These included, according to science teacher Michael David Viney, creator and editor of The Virtual Petrified Wood Museum, 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, for a few examples, various pine tree species as well as club mosses, horsetails and ferns.
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