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At the southern tip of the Robledo Mountains just northwest of Las Cruces, New Mexico, secrets of ancient life along a Permian tidal flat are documented in Prehistoric Trackways National Monument. They are revealed in a series of sandstones, siltstones and mudstone red beds sandwiched between the grayish yellow limestones of a Permian sea. These rocks are collectively known in this desert landscape today as the Robledo Mountain Formation of the Hueco group. In this area, the movements of animals have been frozen in the sands, silts and muds of the ancient tidal flat for almost 300 million years. Jumping trails of wingless insects have been preserved. Activities of amphibians, reptiles, a variety of arthropods including horse shoe crabs, and insects are recorded in the red beds. Even impressions of the animals themselves have been documented by recent discoveries of the features of sea anemones and jelly fish. "Acre for acre, this is one of the best fossil records on the planet." Dr. Spencer Lucas
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During the week of May 24 – 28, 2010, in Albuquerque, New Mexico, researchers presented findings from nearly two decades of work at the Prehistoric Trackways National Monument during A Decade of Discovery, a science symposium celebrating the 10th anniversary of the National Landscape Conservation System sponsored by the Bureau of Land Management and the New Mexico Museum of Natural History and Science. Dr. Spencer Lucas, paleontology curator at the New Mexico Museum of Natural History and Science, tells us, "The tracks at the Prehistoric Trackways National Monument rewrote scientific understanding of Permian footprints. They represent an instant in time almost 300 million years ago, and you can look at it and understand how animals back then were behaving. Acre for acre, this is one of the best fossil records on the planet." He adds, "The number, the quality, the diversity is amazing, and we're still finding new things. Scientists have come from all over the world to study them." Lucas and his colleagues made several presentations at the symposium on the significance of these findings. The fossils at this site are called trace fossils (ichnofossils) and can be used to develop a picture of what life was like on the tidal flat many millions of years ago, before the age of dinosaurs. Leaf impressions and petrified wood tell us what was growing on the landscape; some specimens are new to science. When the sea level rose, the tidal flats were inundated by marine waters, and the limestones were deposited. These marine limestones contain a variety of invertebrate body fossils such as shells of brachiopods, clams, and gastropods. Many of the tracks and traces were made by small animals the size of a salamander or small lizard. The largest animal during the time period was Dimetrodon, a mammal-like reptile that grew to a maximum of 11 feet and produced tracks about the size of a hand. Eryops was a large amphibian whose swimming traces are seen in some places. The Monument was established in 2009 to protect the unique and nationally important paleontological, scientific and other resources found there. The area was discovered and brought to the attention of paleontologists in the 1980s by a local Las Cruces citizen, Jerry MacDonald. He was at the symposium to talk about its history and about the fossil collection he initiated that is housed at the New Mexico Museum of Natural History and Science.
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The site is called a “megatrackway” because each red bed layer contains numerous surfaces where tracks can be found. Such an area is also called Lagerstätte (from the German words Lager and Stätte; literally "place of storage”), a sedimentary deposit that exhibits extraordinary fossil richness or completeness. The condition of the sediment when the tracks were made created a great variety of impressions which scientists had previously named individually. After studying the Robledo Mountain tracks site, however, paleontologists could see that many of the tracks were made by the same animal.
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Sedimentary structures such as ripple laminations, mud cracks and various trace fossils can be used to define life zones on the tidal flat, shallow water zones, and tidal channels, allowing paleontologists and sedimentologists to refine models of Permian ecosystems. The interesting thing about animal tracks, as opposed to bones or bodies, is that they reveal how an animal lived and what its life was like and, perhaps, who was food for whom. One way of documenting the tracks and traces is through the use of a controlled method of digital photography. Stereo photographs are taken of the features and processed by a computer program which generates a three dimensional image. This technique can be used to document tracks in museum collections or in an outside environment to monitor what changes occur over time as the fossils weather. This method, which has been used in other NLCS units, was highlighted in High-Tech Digital Photography of World-Class Fossil Footprints. Documenting the delicate and small tracks with this technique allows for long-term preservation of data and could be used to “virtually” display the features on web sites, exhibits and other venues. The southern New Mexico landscape has changed dramatically in 300 million years and what once was a tropical sea is now desert canyon country. Amazingly, some relatives of the track makers are still hanging around as scorpions and lizards. For a time traveler to the Permian, you need to keep an eye open to see the subtle stone tracks in the rocks, evidence of life in the lush, wet environment of prehistoric times. To learn more about Prehistoric Trackways National Monument, please click here. To read more features and learn more about A Decade of Discovery Science Symposium, please click here.
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