Yucca House National Monument: Paleontological resource inventory (public version)
Authors: Crystal Victoria , Tweet Justin , Santucci Vincent 
Published: May 2022

Yucca House National Monument (YUHO) in southwestern Colorado protects unexcavated archeological structures that were constructed by the Ancestral Puebloan people between 1050 and 1300 CE. It was established by Woodrow Wilson by presidential proclamation in 1919 and named “Yucca House” by archeologist Jesse Fewkes as a reference to the names used for this area by the local Ute, Tewa Pueblo, and other Native groups. It was originally only 3.9 ha (9.6 ac) of land, but in 1990, an additional 9.7 ha (24 ac) of land was donated by Hallie Ismay, allowing for the protection of additional archeological resources. Another acquisition of new land is currently underway, which will allow for the protection of even more archeological sites. The archeological resources at YUHO remain unexcavated to preserve the integrity of the structures and provide opportunities for future generations of scientists. One of the factors that contributed to the Ancestral Puebloans settling in the area was the presence of natural springs. These springs likely provided enough water to sustain the population, and the Ancestral Puebloans built structures around one of the larger springs, Aztec Spring. Yet, geologic features and processes were shaping the area of southwest Colorado long before the Ancestral Puebloans constructed their dwellings. The geologic history of YUHO spans millions of years. The oldest geologic unit exposed in the monument is the Late Cretaceous Juana Lopez Member of the Mancos Shale. During the deposition of the Mancos Shale, southwestern Colorado was at the bottom of an inland seaway. Beginning about 100 million years ago, sea level rose and flooded the interior of North America, creating the Western Interior Seaway, which hosted a thriving marine ecosystem. The fossiliferous Juana Lopez Member preserves this marine environment, including the organisms that inhabited it. The Juana Lopez Member has yielded a variety of marine fossils, including clams, oysters, ammonites, and vertebrates from within YUHO and the surrounding area. There are four species of fossil bivalves (the group including clams and oysters) found within YUHO: Cameleolopha lugubris, Inoceramus dimidius, Inoceramus perplexus, and Pycnodonte sp. or Rhynchostreon sp. There are six species of ammonites in three genera found within YUHO: Baculites undulatus, Baculites yokoyamai, Prionocyclus novimexicanus, Prionocyclus wyomingensis, Scaphites warreni, and Scaphites whitfieldi. There is one unidentifiable vertebrate bone that has been found in YUHO. Fossils within YUHO were first noticed in 1875–1876 by W. H. Holmes, who observed fossils within the building stones of the Ancestral Puebloans’ structures. Nearly half of the building stones in the archeological structures at YUHO are fossiliferous slabs of the Juana Lopez Member. There are outcrops of the Juana Lopez 0.8 km (0.5 mi) to the west of the structures, and it is hypothesized that the Ancestral Puebloans collected the building stones from these or other nearby outcrops. Following the initial observation of fossils, very little paleontology work has been done in the monument. There has only been one study focused on the paleontology and geology of YUHO, which was prepared by paleontologist Mary Griffitts in 2001. As such, this paleontological resource inventory report serves to provide information to YUHO staff for use in formulating management activities and procedures associated with the paleontological resources. In 2021, a paleontological survey of YUHO was conducted to revisit previously known fossiliferous sites, document new fossil localities, and assess collections of YUHO fossils housed at the Mesa Verde National Park Visitor and Research Center. Notable discoveries made during this survey include: several fossils of Cameleolopha lugubris, which had not previously been found within YUHO; and a fossil of Pycnodonte sp. or Rhynchostreon sp. that was previously unknown from within YUHO.