Dinosaurs under the big sky

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Many fossil specimens in this exhibit are genuine and some are replicas. Replicas, or

exact copies, are made for some fossils because they allow the Museum to both study and exhibit the fossil. Many fossils are found in pieces, and putting them back together with glue, screws, and pins, can actually harm the fossils. Scientists can often learn more from studying bones that haven’t been “fixed” or glued and tacked back together. By creating replicas of these specimens we can display entire skulls and skeletons while leaving the original bones undamaged for further study. Throughout the exhibit, specimens are labeled with a museum identification number, and then often a B for original bone, or an R for replica.

FOSSIL REPLICAS ARE MADE HERE AT THE MUSEUM OF THE ROCKIES FROM OUR ORIGINAL SPECIMENS! Replicas are made by creating a mold of the original bone, replacing the real bone with resin or plaster to create a cast, and then painting the cast to look like the original bone. Replicas are not sculptures or models, but exact copies of the original bones.


Some land on which we find dinosaurs is privately owned, some is state or federally owned, and some belongs to Native American Nations.

State and Federally Owned Specimens: The skeleton of “Big Al,” displayed in the Hall of Giants, was discovered by Swiss dinosaur collector Kirby Siber, who inadvertently began collecting the specimen thinking it was located on private land. When it was discovered to be on Bureau of Land Management (BLM) land, the Museum of the Rockies was called to recover the specimen. Like all other specimens collected from federal lands, the skeleton of “Big Al” resides in the Museum of the Rockies National Repository but remains the property of the people of the United States. The Museum of the Rockies cares for thousands of specimens belonging to numerous state and federal agencies, and also to some Native American Nations. A few specimens are owned by the museum, but as the MOR is a part of Montana State University, these specimens are also the property of the people of Montana.

The Dinosaurs Under The Big Sky exhibit leads visitors through a journey of discovery across 90 million years of dinosaur history in Montana (155 - 65 million years ago) during the Mesozoic Era. The Hall of Giants focuses on the first 70 million years of that history extending from the Late Jurassic to the Middle Cretaceous (155 – 85 mya).
The Hall of Giants presents fossil evidence and displays to highlight scientific discoveries about dinosaur physiology, growth rates, appearance, predator/prey relationships, and specialized claws and teeth. The hall also contains displays and panel discussions on the theory that dinosaurs are the ancestors of birds. In addition, visitors will gain an appreciation for what fossils tell us about the ecosystems within which these dinosaurs lived.

MESOZOIC ERA: During the Mesozoic Era, 245 to 65 million years ago (mya), the Rocky Mountains were being formed. A marine embayment expanded into Montana from the Arctic, and eventually the Intercontinental Cretaceous Seaway flooded the interior of the continent from the Gulf of Mexico to the Arctic Ocean. Shallow seas alternately covered and uncovered what is now Montana. Coastal plains provided habitat for dinosaurs. Dinosaurs existed for about 155 million years from 220 to 65 mya. The Mesozoic Era is divided into three time periods called the Triassic, Jurassic, and Cretaceous Periods. Note: The Intercontinental Cretaceous Seaway is also referred to as the Western Interior Seaway.

TRIASSIC PERIOD: The Beginning of the Age of Dinosaurs (230 - 208 mya). During this period, the continents were still mostly locked together forming the super continent of Pangea. Primitive conifers were the prominent trees; there were no deciduous plants. The mountain ranges in western North America had not yet formed. Most of Montana was low in elevation and experienced little or no deposition of sediments; therefore, no record of either sediments or life forms was preserved. No Triassic Period dinosaur fossils have been found in Montana (DBS, pp. 15, 60-61)

Dinosaurs first appeared at the beginning of the Triassic Period, known as the time of the little dinosaurs. The oldest known dinosaur fossils come from rocks dated to be from the beginning of the Triassic Period of South America. These early dinosaurs, although most likely uncommon, had several advantages over their relatives. One of the most important features was that dinosaurs walked with their legs directly under their bodies, as do birds and mammals. This allowed dinosaurs to move more quickly; an advantage for a predator during Triassic time. Other reptiles walked with their legs sprawled out to the sides.

During Late Triassic time, dinosaurs diversified very quickly. By the end of the Triassic Period, there were several kinds of dinosaurs including the early ancestors of the sauropods, the carnosaurs, and the plant-eating ornithischians.

Map Panel: The World and North America during the Triassic Period,

220 million years ago.


JURASSIC PERIOD: First Great Age of Dinosaurs (208 – 144 mya). The giant dinosaurs evolved in the Jurassic Period. A subtropical to temperate climate existed in North America. Ferns and conifers, some gigantic in size, were the dominant forms of vegetation. By the Late Jurassic Period, the continents were beginning to separate and the North Atlantic Ocean began to form. About 155 mya, a marine embayment had expanded into Montana from the Arctic and left a broad coastal plain. Rivers flowed into the embayment and deposited the stream sediments called the Morrison Formation. Dinosaur fossils found in Montana from this period include Diplodocus, Apatosaurus, Allosaurus, Stegosaurus and Camptosaurus. (DBS, pp. 62-63).

Map Panel: The World and North America during the Jurassic Period,

150 million years ago


Jurassic fossils from Montana include fossils from the Sundance Formation (160 million years old) and the Morrison Formation (155 million years old).
The Sundance Formation was deposited in the Sundance Sea—a long, shallow inland extension of the Arctic Ocean. There is no evidence that any plants or animals dwelled on the bottom of this sea. Fossils found in the thinly bedded siltstones include one species of fish, and a variety of aquatic and flying insects, none of which were land-based. The intact bodies of the fossilized fish and insects indicate that the sea floor was probably stagnant and oxygen-poor.

The Morrison Formation represents the coastal plain of the Sundance Sea. It was deposited as sandstones and mudstones by rivers and streams. In most of Montana, the Morrison Formation is red or green, indicating alternating episodes of oxidation and reduction, and a relatively dry climate.”

Case Contents:

Sundance Formation in MT - Fossil fish, flying insect, insect larva

Morrison Formation in MT - Allosaurus tooth and metatarsal (foot bone)

Diplodocid vertebra and Diplodocus humerus (upper arm bone)

Morrison Formation in WY - Apatosaurus tooth; Camptosaurus vertebra;

Diplodocid skin impression; Camptosaurus skull (cast) from CO


Sauropods (SOR-uh-podz) were one of the most successful groups of dinosaurs, existing for more than 100 million years. Sauropods represent the most abundantly found dinosaur remains from the Upper Jurassic sediments of North America. The commonness of their remains indicates that these dinosaurs spent a great deal of time in areas near rivers, streams, and lakes, where they had the best chances of being covered by sediment. Sauropods were large, quadrupedal, plant-eating dinosaurs with long necks and included Diplodocus and Apatosaurus.
Diplodocus (dih-PLO-doe-kus) and Apatosaurus (ah-PAT-o-sore-us): Remains of Diplodocus and Apatosaurus are often found in the same deposits, suggesting that they lived together. Diplodocus (“double beam” – refers to split or double neural spine or beam) was a more slender and gracile sauropod that could reach 88 feet in length. Researchers think it weighed about 11 tons. Most of the Diplodocus remains from Montana are juveniles in bonebeds. The Apatosaurus (“deceptive reptile”) was more stoutly built. An average adult Apatosaurus was about 80 feet long and weighed as much as 35 tons. Apatosaurus remains are most commonly found in the Morrison Formation of Colorado, Utah, and Wyoming. Only disarticulated remains, primarily from bonebeds, have been found in Montana (DBS, pp. 89-90).
Allosaurus (AL-oh-SOR-uhss): The Allosaurus (“different reptile”) was a common theropod dinosaur that lived during the Late Jurassic time. The word “theropod” refers to meat-eating dinosaurs and their descendants – birds. Allosaurus was a bipedal dinosaur that reached 26 to 29 feet long and may have weighed 1 to 2 tons. Allosaurus remains are uncommon in Montana, consisting mainly of isolated teeth and bones, commonly in sauropod bonebeds. Scientists think that Allosaurus fed on sauropods (DBS, pp. 91-91).
Stegosaurus (STEG-oh-SOR-uhss): Stegosaurus (“roof reptile”) was a plant-eating, armored dinosaur with large plates on its back that it used for display and possibly for defense. It had a very small skull for its body size (25 feet long) and a particularly small brain – no larger than a couple of walnuts. Most Stegosaurus remains have been found in the Morrison Formation of Wyoming, Colorado, and Utah. Stegosaurus fossils have been discovered in Montana but are rare (DBS, p. 88).
Camptosaurus (CAMP-toe-SOR-uhss): Camptosaurus (“bent reptile”) was a medium, bipedal plant-eater that grew to about 17 feet long. It was closely related to Iguanodon and shares an ancestry with hadrosaurs. Camptosaurus remains have been found in Wyoming, Colorado, Utah, and South Dakota. Their remains are very rare in Montana, probably because so little exploration has been conducted in the Morrison Formation. A single fragmentary juvenile skeleton has been collected in Montana (DBS, p. 87).



Sauropod dinosaurs are abundant in nearly every Upper Jurassic fossil site. At Dinosaur National Monument in Utah and Colorado, thousands of sauropod bones have been found. Here in Montana there are two major sites that have produced large accumulations of juvenile individuals, and numerous other sites where abundant sauropod bones have been found in ancient river deposits.

Sauropod dinosaurs are among the most bizarre of animals to have ever lived on Earth, due to the size of their heads in proportion to their bodies. If you were proportioned like a sauropod, you would have a body similar in size to the one you have now, and a head the size of a peanut stuck out on the end of a neck the size of a three-foot-long straw! How a sauropod got enough food into its belly, or how it managed to get blood to its head—which could have been as high as 30 feet in the air—are still mysteries to scientists. Despite these mysteries, sauropods existed successfully for more than 100 million years. Their success makes them key to understanding the biology (anatomy, physiology, and behavior) of dinosaurs.

Sauropod fossils are huge! Some adult sauropods grew over 80 feet long and weighed over 30 tons. To help visitors appreciate the sheer size of these giants, we have displayed a line of huge sauropod fossils including an adult sauropod vertebra; a probable Apatosaurus rib; and three Diplodocus legs.


This vertebra is from the base of an adult sauropod tail. In 1993, it was collected by MOR in north central Wyoming. In life, this vertebra was completely encased in muscle and tendons. Studies of tendon attachment sites (locations where tendons attached to bone) indicate that sauropod tails were held parallel to the ground, not at a downward angle as early renderings indicate.

Dinosaur Vertebra Aided in Breathing: Although adult sauropod vertebrae are
enormous bones, in life they were very light for their size because they were filled
with air pockets. Sauropod and theropod vertebrae were constructed of very thin
blades of bone that protected the nerve cord and also held air chambers thought to
have been connected to the lungs. The air chambers found in the vertebrae of
these dinosaurs are similar to those found in bird vertebrae, so scientists
hypothesize that sauropods and theropods breathed like birds rather than like
other reptiles or mammals.

Reptiles and mammals have very different ventilation systems, but both breathe

by drawing air into the lungs in some way. This means that there is always some

static air in mammal and reptile lungs. When birds breathe, however, they rely on

a set of flexible air sacs that act like bellows to move air through the lungs. Airflow

across the respiratory surfaces of bird lungs is unidirectional and nearly

continuous. The ventilation system in birds is much more efficient than that of

mammals and reptiles. The air sacs found in bird vertebrae make their skeletons

much lighter than reptile and mammal skeletons, and scientists think that the

skeletons of sauropod and theropod dinosaurs were much lighter than those of

other reptiles and mammals as well.

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