Dinosaurs once roamed across the lands now forming the United States, leaving behind a rich fossil record that continues to captivate scientists and the public alike. The diverse ancient environments of this region supported a wide array of dinosaur species, from the earliest Triassic forms to the giants of the Jurassic and the specialized creatures of the Cretaceous. Their preserved remains offer insights into prehistoric ecosystems and the evolutionary journey of these animals, helping us understand the planet’s deep past.
Why the United States is a Fossil Hotspot
The United States holds an exceptional abundance of dinosaur fossils due to geological and environmental factors during the Mesozoic Era. Vast sedimentary basins, such as the Morrison and Hell Creek Formations, were ancient floodplains, river systems, and coastal lowlands. These environments provided ideal conditions for the rapid burial and preservation of organic remains. Ongoing uplift and erosion in the western United States have exposed these ancient rock layers, making fossils accessible.
The Morrison Formation, spanning from Montana to New Mexico, is a known source of Jurassic dinosaur fossils. The Hell Creek Formation, found in Montana, North Dakota, South Dakota, and Wyoming, represents an ancient coastal floodplain environment, preserving the remains of the last dinosaurs before their extinction.
Dinosaurs of the Triassic Period
The Triassic Period, spanning approximately 252 to 201 million years ago, marked the initial appearance of dinosaurs. These early forms were generally smaller and less dominant compared to their later relatives. The earliest reliable records of North American dinosaurs, often fragmentary, come from the Upper Triassic Dockum Group of Texas.
One of the most well-known Triassic dinosaurs found in the United States is Coelophysis. This lightly built, bipedal carnivore was about 9 feet long and around three feet high at the hips. Extensive remains of Coelophysis have been unearthed in New Mexico, particularly from a large bonebed at Ghost Ranch in Rio Arriba County, making it well-understood. Fossils of Coelophysis have also been found in Arizona, including at Petrified Forest National Park, and in Utah.
Dinosaurs of the Jurassic Period
The Jurassic Period, from about 201 to 145 million years ago, was a period of significant dinosaur evolution, witnessing the emergence of giant sauropods and large predatory theropods. Much of the rich Jurassic fossil record in the US comes from the Morrison Formation, spanning several western states including Colorado, Utah, and Wyoming.
This formation preserves an ecosystem of massive plant-eaters and predators. Iconic dinosaurs include the armored Stegosaurus, with its distinctive back plates and tail spikes. Large predatory dinosaurs like Allosaurus also thrived. Long-necked sauropods, such as Apatosaurus, Brontosaurus, and Diplodocus, were prominent, reaching immense sizes and grazing on abundant plant life.
Dinosaurs of the Cretaceous Period
The Cretaceous Period, approximately 145 to 66 million years ago, represents the final and most diverse chapter of dinosaur evolution. During this time, North America was divided by a vast inland sea, creating distinct eastern and western landmasses. The western landmass, Laramidia, supported a rich dinosaur fauna.
The Hell Creek Formation, found across Montana, North Dakota, South Dakota, and Wyoming, is exceptionally well-known for its Late Cretaceous dinosaur fossils. This formation has yielded numerous remains of dinosaurs, including the carnivore Tyrannosaurus rex. Herbivores like the horned Triceratops were common. Other well-represented dinosaurs from this period include the heavily armored Ankylosaurus and the duck-billed Edmontosaurus. These diverse dinosaurs inhabited a range of environments across the United States.