Paleontology at Yucca House

Artist’s reconstruction of the Western Interior Seaway ecosystem, with various prehistoric creatures swimming, hunting, and diving through an ocean environment
Artist’s reconstruction of the Western Interior Seaway ecosystem

NPS/Karen Carr Studio

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. The long human history of the area has been shaped and influenced by the dynamic geologic features and processes present in the region. The Ancestral Puebloans likely settled at YUHO because of the presence of natural springs. In particular, the larger Aztec Spring was situated in the center of the archeological structures. Although the springs currently have very low flows, as indicated by the presence of a small wetland, at the time the Ancestral Puebloans occupied the area, the springs must have flowed enough to sustain the population. The presence of springs was most likely a contributing reason that the Ancestral Puebloans settled in the area.

Yet, the geologic features and processes in the region began influencing life long before human habitation. Millions of years ago—well before humans occupied the area—communities of ancient organisms thrived. This multimillion-year history of geology and life is recorded in the rock units and fossils that are present in the monument; thus, although the monument is known for is archeological resources, it has rich paleontological resources as well.

The main bedrock unit at YUHO is the fossiliferous Mancos Shale, which was deposited during a time when the region looked much different than it does today. Today, YUHO sits more than a mile above sea level in the Colorado Plateau, but 90 million years ago (during the Cretaceous Period) the Rocky Mountains did not yet exist and the area was submerged by the Western Interior Seaway. The seaway hosted a variety of marine organisms. These organisms are preserved as fossils in the rocks at YUHO. There are fossils of clams, oysters, ammonites, and marine vertebrates. There are two species of clams, two species of oysters, six species of ammonites in three genera (plural of genus), and one vertebrate species (just a small unidentifiable bone fragment) that have been found in the rocks at YUHO. The three genera of ammonites are all easy to identify by non-paleontologists. Specimens of the genus Scaphites have loosely coiled shells shaped like the number nine. Specimens of the genus Baculites have straight or slightly bent shells; and specimens of the genus Prionocyclus have the tightly coiled or spiraled shells that even non-paleontologists often recognize as belonging to an ammonite.

The majority of the fossils found at YUHO are in fossiliferous slabs of rock that were used as building stones by the Ancestral Puebloans. They likely collected the slabs from nearby outcroppings of the fossiliferous rocks, which are a half mile west of the structures they built. Presumably, the Ancestral Puebloans deemed these rocks important enough to collect and transport to use as building materials. This is an example of fossils in a cultural resource context; and it illustrates the interconnectedness of geologic history and human history.

Fossils are non-renewable resources that have scientific and educational information and provide insight into what Earth was like in the past. Park staff and paleontologists work together to maintain fossils for scientific study and public education. It is exciting to find a fossil, but important to protect it.

Report by Victoria Crystal
Geologic Resources Division
National Park Service
An artist's map depicting North America at the time of the Western Interior Seaway, with an ocean passage cutting through the middle of the continent from the Arctic to the Gulf of Mexico

Base map by Scott D. Sampson, Mark A. Loewen, Andrew A. Farke, Eric M. Roberts, Catherine A. Forster, Joshua A. Smith, Alan L. Titus, 2010, and licensed under Creative Commons Attribution 4.0 International. Graphic annotated by Victoria Crystal (NPS).

Map of the Western Interior Seaway, approximately 75 million years ago. The red star indicates the position of Yucca House.
Three different genera of fossil ammonites found at Yucca House
Ammonites found at Yucca House

NPS/Victoria Crystal

Ammonite genera found at Yucca House. Scale bars are all 1 cm. Fossils of Prionocyclus are recognized by the characteristic tightly coiled or spiraled shells that even non-paleontologists often recognize as belonging to an ammonite. Fossils of Scaphites are loosely coiled and vaguely resemble a number 9. Fossils of Baculites have straight or slightly curved shells.

Four photos of fossilized genera of clams and oysters found at Yucca House
Species of clams and oysters found at Yucca House

NPS/Victoria Crystal and Samuel Denman

Species of fossilized clams and oysters found at Yucca House. Scale bars are all 1 cm.

Last updated: April 25, 2022

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