The Javelina and Aguja Formations: A Journey Through Time in Texas
The Big Bend region of West Texas is a paleontological treasure trove, home to some of the most important Late Cretaceous fossil discoveries in North America. The rocks that preserve this ancient world are primarily found in two major geological units: the older Aguja Formation and the younger Javelina Formation.
The Aguja Formation: The Coastal Past
When It Formed: The Aguja Formation dates back to the Campanian stage of the Late Cretaceous, approximately 80 to 76 million years ago. This was a time when a vast sea, the Western Interior Seaway, extended through the middle of North America, and West Texas was a swampy, coastal floodplain on its western shore.
What the Environment Was Like: Imagine a landscape of slow-moving rivers, swamps, and marshes, all leading to a shallow sea. The climate was warm and humid, and the air was thick with the sounds of ancient life. The Aguja Formation is characterized by its interbedded layers of marine shales (from the sea) and continental sandstones and mudstones (from the rivers and floodplains). This mix of environments is why it holds such a diverse fossil record.
Key Fossil Finds: This formation is famous for a variety of dinosaurs and other prehistoric creatures:
Horned Dinosaurs: The horned dinosaur Agujaceratops is one of the most notable discoveries.
The Giant Crocodile: Fossils of the immense crocodile Deinosuchus have been found here. This terrifying predator could grow to be over 30 feet long!
Duck-billed Dinosaurs: Hadrosaurs like Kritosaurus are also a common find, along with small teeth from juvenile dinosaurs, which suggests the area was a nesting ground.
Other Life: The Aguja has also yielded fossils of turtles, small mammals, and an incredible diversity of ancient plant life, including petrified wood and fossilized leaves.
The Javelina Formation: An Inland World
When It Formed: As the Cretaceous Period drew to a close, the Western Interior Seaway began to retreat, leaving behind a more inland environment. The Javelina Formation was deposited on this new landscape, dating from roughly 70 to 66.5 million years ago, right up to the very end of the dinosaur age.
What the Environment Was Like: The Javelina Formation represents a drier, more open inland floodplain far from the coast. The rock layers are often distinctively colored, with shades of maroon, deep red, and purple, indicating well-drained, oxidized soils. This was a river-filled basin surrounded by forested areas.
Key Fossil Finds: The Javelina Formation is famous for some of the largest and last dinosaurs of the Cretaceous:
The Last Sauropod: This formation is the only place in the world where you can find Alamosaurus, the giant long-necked sauropod, alongside fossils of Tyrannosaurus rex. This discovery helped to prove that sauropods did not go extinct before the end of the Cretaceous.
The "Texas Pterosaur": Quetzalcoatlus, the largest flying creature ever discovered, was found here. This immense pterosaur had a wingspan of up to 40 feet.
Late-Cretaceous Life: The Javelina also preserves fossils of a variety of other animals, including the horned dinosaur Torosaurus, armored dinosaurs, and smaller theropods, as well as an abundance of petrified wood.