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National Park Service
U.S. Department of the Interior

2014 Mesozoic Partner Highlight

Petrified log within Big Bend National Park. Cowboy hat for scale. NPS photo.

Big Bend National Park

Texas-sized fossils!

Article by Cassi Knight (National Fossil Day coordinator) with information from Steve Wick (Big Bend National Park).

The rocks in Big Bend National Park, Texas, tell a long story. The story starts about 100 million years ago, when the dinosaurs roamed across the United States, and reads almost uninterrupted into the age of mammals about 60 million years ago. It is illustrated by all of the fossils found in these special rocks—fossils of more than 40 dinosaur species, and several species of fish, frogs, salamanders, turtles, crocodiles, and lizards. In addition to all of these reptiles and amphibians, almost 60 species of plants and some early mammals have been discovered in the park, making Big Bend one of the most complete stories of an ancient ecosystem on earth!

Of all the Mesozoic rocks in Big Bend National Park, there are three rock units that best describe three major chapters in Big Bend's story: the Boquillas Formation (100-95 million years ago), the Aguja Formation (80-75 million years ago), and the Javelina Formation (75-60 million years ago).

From Sea to Land


click to enlarge...

Paleogeographic maps of North America. Red star indicates location of Big Bend NP. Basemaps by Ron Blakey (Colorado Plateau Geosystems).

The Boquillas Formation was deposited during the Cretaceous when most of the midwestern US was covered by an ocean, about 100 million years ago. Marine reptiles, like 30-foot long mosasaurs (featured on the National Fossil Day 2011 artwork), that swam in this ancient sea are now found as fossils in the limestone that was deposited on the ocean floor.

The Aguja Formation contains most of the vertebrate fossils found in Big Bend National Park. We know that the sea had retreated by this time because the rocks and the fossils had changed from limestone to sandstones and shale, and from marine reptiles to dinosaurs and terrestrial plants. Twenty-eight different species of dinosaurs, including the duck billed hadrosaur (likely Kritosaurus), have been found in these rocks! Fossils of palms and conifers are found in the Aguja Formation, and these plants tell us that the dinosaurs were wandering around in a humid swampy environment. The overall global climate at this time was warm and the tropical swamp was most likely very close to the coast.

The Javelina Formation was deposited as the sea retreated further, and this area started to dry out, 70-66 million years ago. More than 80 species of plants are preserved in the Javelina formation. Plants are an excellent indicator of the environment and climate that they were growing in, and this flora suggests that the land was drier than it was when the Ajuga Formation was being deposited. The area was a floodplain inhabited by many different types of conifers, and some flowering trees (angiosperms). But not only plants lived there. There are 11 species of dinosaurs that have been found in the Javelina Formation too. These represent unique species that give us clues as to which dinosaurs ruled the land during the last 35 million years of their existence. Above the Javelina Formation lies the Black Peaks Formation, which marks the extinction of the dinosaurs at the Cretaceous/Paleogene boundary. There are many Paleocene mammal fossils that have been found in this layer.

Big Fossils In Texas

Quetzalcoatlus northropi is the largest flying organism ever discovered. NPS photos.

The primitive crocodilian Deinosuchus reached lengths of up to 36 feet, and had impressive 6 inch long teeth. Much like modern-day crocs, they were most likely ambush predators, laying in the swamp shallows waiting to grab a dinosaur for lunch! There are many dinosaur bone fossils that show major damage and bite marks from these predators.

One particularly special—and large—find from the Javelina formation is the pterosaur Quetzalcoatlus northropi. The enormous, hollow, wing bones were found in 1971 by Douglas A. Lawson, a graduate student at University of Texas Austin. Only the wing bones of this specimen were found, but another more complete pterosaur was found elsewhere in the park, and allowed paleontologists to estimate that the wingspan of Quetzalcoatlus northropi was 36 to 39 feet across! This impressive span makes Quetzalcoatlus the largest known flying organism to have ever lived.

Alamosaurus adds to the gigantic fossils found in the Javelina Formation. This dinosaur was found in 1999 by another graduate student, Dana Biasatti, from University of Texas Dallas. Pelvic bones and nine giant vertebrae were excavated, just part of the 80 foot long, 35 ton dinosaur.

Excavation of Alamosaurus vertebrae by Tony Fiorillo and the Perot Museum field crew within Big Bend National Park. NPS photo.
Alamosaurus belonged to the sauropod group of dinosaurs, which are large herbivores with long necks and tails. Can you imagine how many plants Alamosaurus would have had to eat every day to feed itself?

These Texas-sized fossils are featured in a activity within the Junior Paleontologist booklet.

Visit Big Bend National Park

Once a remote and seemingly inhospitable area reached only by miles of dirt roads, Big Bend has become one of the most popular vacation destinations in the state of Texas visited by an average of 300,000 visitors each year. Scenic vistas, diverse wildlife, historic sites, and border culture rank among the features visitors enjoy in Big Bend. Plan your visit today!

2014 Mesozoic Ecosystem Partner feature articles:

| January: Fossils of the 2014 National Fossil Day Artwork | February: Petrified Forest National Park | March: Garden Park Paleontology Society | April: Big Bend National Park | May: Fossil Cycad National Monument | June: Alaskan National Parks | July: Dinosaur State Park |

Last updated: April 1, 2014