Life in the SeaIn the late Cretaceous 80-75 million years ago, Chaco sat on the edge of an inland sea. Just offshore, fierce reptilian predators like mosasaurs and plesiosaurs hunted ammonites and fish, while bivalves carpeted the seafloor and mud shrimps burrowed beneath. The fossilized remains of this ancient ecosystem found at Chaco include plesiosaur bone, mosasaur jaw and teeth, shark teeth, ammonite, bivalve, and mollusk shells, and trace fossils of extinct shrimp burrows.
Life on LandThe regional climate during the Cretaceous was much different than it is today. On land, the first flowering plants began to evolve, and by the Late Cretaceous were fairly diverse. The temperature was much warmer, and this subtropical environment covered in lowland forests with species thought to be similar to modern day conifer, eucalyptus, ebony, palm, cypress, and magnolia. Upland areas contained hardwood forest species similar to oak, walnut, ash, and birch. These new plants provided new food sources and habitats for animals. The fossilized remains of this much warmer climate found at Chaco include petrified wood, dinosaur bones, turtle shells, and termite burrows.
Chaco’s Stratigraphic Storybook
The Western Interior Seaway came in and went out multiple times before eventually receding for the last time about 75 million years ago. During this time, thousands of feet of sand, mud, and other sediments were deposited by river deltas and ocean waves, burying and fossilizing dead plants and animals that lived on land and in the ocean.
Flowing rivers, lagoons, and coastal floodplain- Menefee Formation
One of the oldest formations visible at Chaco is the Menefee Formation. This section of shale, coal, and sandstone forms soft, eroding slopes and contains diverse plant and animal fossils from the Late Cretaceous. This sediment is thought to have deposited by rivers flowing through lowland swamp, lagoon, and floodplains. Fossils found in the Meneffe include petrified wood, coal, amber pellets, bivalves, crocodiles, dinosours, and termite burrows. Geologic features common to the Menefee era include coal seems, sandstone ledges, mudstone, calcareous & siderite (iron) concretions, and badlands terrain. The Menefee is visible on the bottom half of Fajada Butte and along the Una Vida great house trail.
Beaches and barrier islands - Cliff House Sandstone FormationThe steep cliffs of the Cliff House Formation are hard to miss. They are made of sturdy, dense sandstone and contains the remnants of an ancient inland ocean ecosystem. Occasionally large boulders break off the cliff edge as the softer Menefee erodes from underneath.The massive monolith that fell on Pueblo Bonito known as Threatening Rock was a large slab of the Cliff House.
Fossils found in the Cliff House Sandstone formations include petrified wood, bivalves, ammonites, gastropods, echinoids, shark teeth, fish, mosasaurs, plesiosaurs, turtle bones, vertebrate bones, and shrimp burrows.
Common geologic features seen in the Cliff House formations include ripple marks, cross bedding, honeycomb weathering, and sheer cliff walls.
The Cliff House formation surrounds the main wash of the canyon and looms over nearly all the structures at Chaco. Hike the Pueblo Alto and South Mesa trails to stand on top of it and look for embedded fossils, and even walk right across preserved ripple marks, formed by the changing tides. As you look down over the canyon, imagine a seashore and lapping waves extended out in front of you.
Frequently Asked QuestionsWhat is a fossil?
Fossils are the remains of ancient life that have been preserved by the process of fossilization over 10,000 years or more. Body fossils are in the form of an organism, such as bones, shells, molds (imprints) or casts (filled imprints). Trace fossils are not the organism itself, but evidence of behavior or presence of an organism, such as footprints, trackways, burrows, or coprolites (fossilized poop).
What should I do if I find a fossil in the park?
All fossils and archaeological artifacts in the park are protected under federal law. Fossils and archaeological artifacts should not be disturbed, collected, or moved. If you find one, you can help protect these resources by documenting where it is and reporting it to the Visitor’s Center. Click here to learn more about the Paleontological Resources Protection Act.
What is Paleontology?
Paleontology is the study of prehistoric life.
What makes CHCU an interesting paleontological site?
Fossils of ancient reptiles, fish, sharks, numerous plants, and even dinosaurs have been found at Chaco, however, very few of the fossils found here have been studied in detail. The diverse paleontological resources located within the park are important to understand and preserve, as they hold the potential for discovery of new species and answering questions about the history of life on Earth.
How do we know Chaco was on the seashore?
We can look for clues in the stratigraphy, or layers of sediments laid down over time, to understand what the environment was like in the past. Geologists are interested in what these layers are made of, and paleontologists examine what kinds of fossils are preserved within them. Together, this information tells a story of the Earth’s history.
Are you interested in learning more about Chaco's fascinating paleontolgical history? Click here to view virtual 3D models of a few of the park's paelontolgical treasures.
Last updated: August 12, 2020