Dinosaurs under the big sky


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The Early Cretaceous Cloverly and Kootenai Formations of Montana are exposed around mountain ranges or in areas pushed up by other tectonic activities. The Cloverly Formation was deposited along the banks of rivers and streams and is exposed in Carbon and Wheatland Counties. The Kootenai Formation was mostly deposited in an ancient lake and along the shore of an inland bay of the interior seaway. It is exposed in numerous counties throughout western and central Montana.
Many different kinds of fossils can be found in the Cloverly Formation including dinosaurs, turtles, crocodiles, primitive mammals, fish, invertebrates (animals without backbones) and plants. There are also some highly polished rocks thought to be gastroliths (GAS-truh-liths).
Gastroliths are rocks that have been inside the digestive track of an animal. They are common in plant-eating birds, which use them to help grind up their food. These rocks get extremely polished from rubbing against one another as the stomach churns during digestion. The rocks end up looking like they have been in a rock tumbler. Scientists think that some dinosaurs may have used gastroliths to help digest their food. Whether the shiny rocks in the Cloverly Formation are genuine gastroliths is still unresolved. Some may be, but some so-called gastroliths are larger than would have fit through the mouths of the animals who are thought to have used them.
Case Contents:

Cloverly Formation Fossils:

Sauropelta armor plate; Tenontosaurus left jaw

Titanosaur (sauropod) caudal (tail) vertebra, tooth, and ungula

phalange (foot claw)

Asphideretes (turtle) partial plastron (bottom shell)

Ceratodus (lung fish) tooth; Zephyrosaurus vertebra

Polished rock (gastrolith?); Crocodile coprolite (fossil dung)

Kootenai Formation Fossils: Hybodont shark spine


Most of the plants living during the Early Cretaceous were conifers, cycads, ginkgo (“gymnosperms”), and ferns left over from the Jurassic, but it was during this time that flowering plants (angiosperms) first appeared. Flowers enabled plants a much wider range of evolutionary relationship, which in turn broadened their ecological niches.

In Montana, we have found evidence of a variety of ferns, including giant tree ferns, various cycads, and ginkgo. It is unknown which specific plants each of the herbivores might have eaten, but judging from the height of their mouths the Tenontosaurus probably ate shorter plants while sauropods could eat taller plants. Note: The first fossil evidence of flowering plants in Montana is from the Middle Cretaceous and is part of the Burrowing Dinosaur display at the end of the Hall of Giants.


Tenontosaurus (ten-ON-to-SOR-uhss): Tenontosaurus (“sinew reptile”) specimens are probably the most common dinosaur remains found in the Cloverly Formation in Montana. It was a plant-eating dinosaur closely related to Iguanodon. Tenontosaurus was bipedal but could put its hands down to help when in a slow walk. It had a very long tail for the size of animal. Tenontosaurus reached nearly 26 feet long and may have weighed close to 1 ton although an average Tenontosaurus was only about 16 feet long (DBS, p. 93-94).
Deinonychus (die-NON-uh-kus): Deinonychus (“terrible claw”) was a small theropod, meat-eating dinosaur very closely related to Velociraptor. An adult Deinonychus reached eight feet long and weighed about 175 pounds. Deinonychus was probably one of the fiercest animals to have ever lived. It killed its prey by seizing it with razor-sharp hand claws and then ripping the animal open with the slashing claws on its hind feet. Evidence from sites in Montana support Deinonychus hunted in packs of three to five individuals (DBS, p. 97).
Sauropelta (sore-o-PELT-a): Sauropelta (“small shield reptile”) was a quadrupedal, plant-eater, armored dinosaur measuring about 18 feet long and weighing about 3 tons. It had a massive, gnarly skull and bony armor plate covered its body. Sauropelta armor is relatively common in the Cloverly Formation. (DBS, p. 96)


Using fossil evidence and scientific research, paleontologists frequently become “Dinosaur Mythbusters.” For over a hundred years after the discovery of dinosaur remains in England in the mid-1800s, the public and most paleontologists believed dinosaurs were big, cold-blooded, stupid overgrown lizards that plodded around dragging their tails. In the 1960’s, John Ostrom from Yale University pioneered work that launched a “dinosaur renaissance” of discovery and research and led to the “busting” of several myths about dinosaurs (DBS, pp. 53-54). John Ostrom is the man who provided the first solid evidence that dinosaurs did not drag their tails, that some dinosaurs had high metabolisms and therefore were at least partially warm-blooded, and that birds are the descendants of dinosaurs. Deinonychus is the dinosaur Ostrom used as evidence in these hypotheses.

In 1964, John Ostomy of Yale University discovered a new kind of meat-eating dinosaur in southern Montana . He named his new dinosaur Deinonychus (die-NON-uh-kus), which is Latin for “terrible claw.” Ostrom gave the dinosaur this name because of a sharp, strongly curved claw on each hind foot.
Deinonychus was a small but powerful predator, only about eight feet long from head to tail. Scientists think its long, muscular forelimbs gave it the ability to grip—and even scale—its prey. Its long, stiff tail helped it to balance while running, kicking, or leaping. The sharp, recurved claw on the second toe of each hind foot was likely used for ripping flesh and made Deinonychus a terrifying killer. This three-inch-long talon was held off the ground while walking and could be deployed in a powerful slashing kick.
The site where Ostrom found the dinosaur was excavated from 1964 to 1967, and produced bones of several individuals, suggesting that these dinosaurs were very social animals. Ostrom even hypothesized that they ate and may have hunted in groups, or “packs.” The Velociraptors in the Jurassic Park movies are based on Deinonychus findings.
Ostrom stopped excavating the site at the end of the 1967 season, reasoning that he had collected most of the bones and that the thickness of rock over the top of the bone-producing layer was too great to warrant further excavation. The site was abandoned for 26 years, until 1993, when Museum of the Rockies Curator Dr. Jack Horner decided to reopen the quarry to see if anything was left. The Deinonychus bones in this case are some of the pieces that were found in the 1993 excavation. Ostrom was right in that few fossils remained, but MOR crews did find important parts of the skull, hands, and feet.
The discovery of Deinonychus changed the direction of dinosaur paleontology. It helped scientists understand the relationship between theropod (meat-eating) dinosaurs and birds. It also changed the idea that dinosaurs were cold-blooded, sluggish, and slow lizards, which had prevailed since the early 20th century.
The display case contains the hand, foot, tail vertebrae, cervical vertebra, and jaw fossils of a Deinonychus specimen collected by MOR in Carbon County, Montana.

Case Contents: Jaw with one tooth (other teeth had fallen out)

Cervical (neck) vertebra – very similar to that of a bird

Caudal (tail) vertebra showing bony rods that held the tail


Reconstructed hand of Deinonychus

Foot showing ripping claw

Hand showing recurved, climbing claws

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