During the Early Cretaceous (130 million years ago), Big Bend National Park was underwater. The warm, shallow sea that covered Big Bend and most of Texas is called the Western Interior Seaway. This sea supported a magnificent assemblage of marine organisms including the mighty mosasaur and the monstrous fish, Xiphactinus. Alongside these “monsters of the sea” lived ammonites, turtles, sharks, sea urchins, oysters, and snails. The rocks from this time period yield many scientific treasures. For example, the oldest North American mosasaur was discovered in the limestones at Big Bend National Park.
130 to 83 million years ago, the Big Bend region was covered by a warm, shallow sea that divided North America from the Gulf of Mexico to the Arctic Ocean. The presence of the Western Interior Seaway led to the deposition of marine muds and calcareous oozes. These calcareous oozes formed from the shells of microscopic marine organisms called nannoplankton that flourished in the surface waters. These marine shales and limestones preserved the remains of sea creatures such as ammonites, oysters, gastropods (snails), and echinoids (urchins).
Block Diagram by Tom Lehman
Around 100 million years ago, the sea began its gradual retreat to the shoreline’s present day position at the Gulf of Mexico. This decrease in sea level was caused by the continental uplift during the building of the Rocky Mountains. At this time, the environment at Big Bend resembled the coastline of the Maldives Islands in the Indian Ocean. The marine limestones now contained more land sediments and were deposited on a continental shelf which preserved the remains of bivalves, oysters, sharks, fish, ammonites, and mosasaurs.
Mosasaurs were the top ocean predators 90 million years ago and are often called the “T. rex” of the sea. They were air-breathing marine lizards that could get up to 40 feet (13 meters) in length. They fed on fishes, sharks, turtles and even other mosasaurs. There are two species of mosasaur found in the Big Bend area: Tylosaurus, which was found west of the park, and Platecarpus, found within the park.
Xiphactinus is a large predatory fish that grew up to 18 feet (6 meters) long and lived around 90 million years ago. It fed on fish, small sharks and occasionally mosasaurs. Skull parts and teeth have been found at Big Bend National Park.
Inoceramus is a large clam that lived on the ocean floor. Some of the shells found at Big Bend are up to 3 feet (1 meter) across and can be mistaken for dinosaur tracks. This fossil is considered an index fossil in which it is used to identify the geologic age of the rock.
Allocrioceras hazzardi is an ammonite that lived in a shallow marine environment. Its open coil and distinctive spines distinguish it from other ammonites. Along with Inoceramus, Allocrioceras hazzardi is also an index fossil.
Other fossils found in these marine limestones include:
- Terlinguachelys (Cretaceous sea turtle)
- Other ammonites (Placenticeras, Texanites, and Baculites)
- Shark teeth (Squalicorax, Scapanorhynchus, and Cretoxyrhina)
- Echinoids (sea urchins)
- Exogyra oysters
- Rudist bivalves
Even though these marine limestones contain large vertebrates such as Squalicorax and mosasaurs, it is often the marine invertebrates that are the most scientifically important specimens. Without these fossils, it is difficult to determine the age of the vertebrate fossils and to reconstruct the past marine ecosystem accurately. Below are the significant fossil occurrences at Big Bend found in these Early Cretaceous limestones:
- Through the use of the invertebrate index fossils, Collignoniceras (ammonite) and Rotalipora (foraminiferan), paleontologists were able to determine the age of mosasaur fossils found with these invertebrates in the Boquillas Formation. From the age of the index fossils, it was determined that these Big Bend mosasaur fossils represented the oldest occurrence of mosasaurs in North America!
- Preservation of soft body parts of organisms is extremely rare in the fossil record and can only happen if the organism is covered fast enough to protect it. The discovery of the fossilized soft parts of a small squid in the Boquillas Formation is a rare find indeed!
Places in the Park
Places in the park to see these fossil-rich rocks include:
- Dagger Flat Auto Trail: Boquillas Formation, Buda Limestone, Del Rio Clay, and Santa Elena Limestone
- San Vicente Hot Springs Road: Boquillas Formation and Buda Limestone
- Hot Springs Trail: Boquillas Formation, Buda Limestone, Santa Elena Limestone
- The road to Boquillas Canyon (by the bridge that goes over Tornillo Creek): Geologic contact between Boquillas and Pen Formations
- Santa Elena Canyon Nature Trail: Glen Rose Limestone, Telephone Canyon Formation, Del Carmen Limestone, Sue Peaks Formation, Santa Elena Limestone and Del Rio Clay
- Marufo Vega Trail: Glen Rose Limestone, Telephone Canyon Formation, Del Carmen Limestone, Sue Peaks Formation, and Santa Elena Limestone.