Distinctly Montana Magazine

Time Traveling Through Montana's Mesozoic BY JACK AND VANESSA HORNER Park M of us travel in time. We time travelers are paleontologists and geologists who understand the meaning of the rocks and sediments that make up the modern-day countryside. Rec- ognizing rocks that represent sediments deposited in rivers, lakes, or oceans, allows us to imagine ourselves into the past, and visualize extinct habitats and ecosystems, each filled with exotic plants and animals. Unfortunately, the fossil record does not provide a complete history of life, but instead it gives us little snapshots that are recorded in geological units called for- mations. Here in Montana we have five primary geologi- cal formations that provide fossil remains from the age of the dinosaurs. We can imagine the dinosaurs and how they lived by examining their skeletons and other fos- sils, like nests, eggs, and footprints. Many of these types of fossils are on display in the museums scattered along Montana's Dinosaur Trail. ost people travel through Montana viewing the mountains or the plains, the forests or the wheat fields, as each exists today, but a few NORTHWEST MONTANA COVERED BY INLAND SEA Jurassic Period 155,000,000 YEARS AGO ited around 120 million years ago and contain fossils provided by the Cloverly Formation. Montana remained flat without mountains, while an inland sea continued to occupy the northern and east- ern regions of the state. But, the animals had undergone a signif- icant change. Most of the long- necked sauropods had gone extinct, and Stegosaurus had been replaced by an armored ankylosaur. The herbivorous Camptosaurus was replaced by a similar looking dinosaur called Tenontosaurus. The top predator of the early Cretaceous became We can imagine the dinosaurs and how they lived by examining their skeletons and other fossils, like nests, eggs, and footprints. The oldest dinosaur-bearing rock formation in Montana is called the Morrison Formation. It produces dinosaur re- mains from about 155 million years ago, or what geologists call the late JURASSIC PERIOD. During this period of time, Montana was flat. A warm, inland sea covered most of northern and eastern Montana. The land was covered with ferns, cycads, and coniferous trees. No flowering plants or hardwood trees had existed yet. Feeding in the lowlands near the seaway were the giant, long-necked sauropods like Diplodocus and Barosaurus. There were also other plant eat- ers like the plated Stegosaurus and the bipedal Camptosau- rus. The primary meat-eater was the ferocious Allosaurus. Fossils of these Jurassic age dinosaurs can be seen in the Museum of the Rockies in Bozeman and the Great Plains Dinosaur Museum in Malta. The next dinosaur snapshot is younger and takes us into the early CRETACEOUS PERIOD. These sediments were depos- DISTINCTLY MONTANA | DIGITAL Watch Jack talk about little dinosaurs, go to... www.distinctlymontana.com/horner124 www.distinctlymontana.com 53 Deinonychus, the dinosaur portrayed in the movie Jurassic Park as Velociraptor. While the dinosaurs underwent major changes, the most striking change occurred to the flora, or plant life. It was at this time when the flowering plants first arrived. The Museum of the Rockies holds a collec- tion of fossils from this time period. Rocks from the late Jurassic and early Cretaceous Peri- ods are rare in Montana. They are found mostly around the uplifted mountain ranges. The late Cretaceous Period contains the best dinosaur fossils, as there are many places in eastern Montana where the rocks from this time are exposed at the surface of the ground. About 100 million years ago, the inland sea that had ex- tended into northern and eastern Montana from the north, expanded westward and southward to meet another inland sea that extended up from the Gulf of Mexico. When these

• Cover