Prehistoric Trackways National Monument is a national monument in the Robledo Mountains of Doña Ana County, New Mexico, United States, near the city of Las Cruces. The monument's Paleozoic Era fossils are on of land administered by the Bureau of Land Management. It became the 100th active U.S. national monument when it was designated on March 30, 2009. Fossils The Prehistoric Trackways National Monument site includes a major deposit of Paleozoic Era fossilized footprints in fossil mega-trackways of land animals, sea creatures, and insects. These are known as trace fossils or ichnofossils. There are also fossilized plants and petrified wood present, as well as plenty of marine invertebrate fossils including brachiopods, gastropods, cephalopods, bivalves, and echinoderms. Much of the fossilized material originated during the Permian Period and is around 280 million years old. Some of the animals who may have left tracks in the Robledo Mountains include Dimetrodon, Eryops, Edaphosaurus, and multiple other pelycosaurs. There are at least 13 major trace fossils found at the monument, including Selenichnites (sel-EEN-ick-NIGHT eez) or moon-shaped trace, Kouphichnium (koof-ICK-nee-um) or light trace, Palmichnium (pal-ICK-nee-um) or palm [frond] trace, Octopodichnus (oct-toe-pod-ICK-nuss) or eight-footed trace, Lithographus (lith-oh-GRAFF-us) or rock writing, Tonganoxichnus (tong-a-nox-ICK-nuss) or Tonganoxie [Kansas] trace, Augerinoichnus (aw-gurr-EE-no-ICK-nuss) or Augerino trace, Undichna (und-ICK-nuh) or wave-shaped trace, Serpentichnus (serpent-ICK-nuss) or snake-like trace, Batrachichnus (baa-track ICK-nuss) or frog trace, Dromopus (dro-MOE-puss) or running foot, and Dimetropus (die-MEET-row-puss) or Dimetrodon foot. The trackways can be difficult for the general public to find, as the monument is largely undeveloped with few facilities yet existing to aid fossil hunters. Many of the slabs pulled out by Jerry MacDonald are housed at the New Mexico Museum of Natural History and Science, although they are not on display at this time. Guided hikes are periodically offered by BLM interpretive staff. Geology The monument lies along the western portion of the Rio Grande rift and is within part of the Robledo Mountains. It is made up of Cenozoic alluvium and intrusive rock and Paleozoic (Hueco Formation) sediments. The Hueco Formation is Early Permian strata. Most of the monument is Permian and would have been underwater or along the coast of what was once the Hueco Seaway. The tracks can be found in the red rock of the Robledo Mountains Member of the Hueco Formation. Flora and fauna The monument is situated at the northern tip of the Chihuahuan Desert. Some examples of plants within