Dinosaurs under the big sky

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nesting in colonies and traveling in large groups or herds.

Case Contents: Edmontosaurus skull (MOR 1626B);

discovered in Dawson County, Montana in 2005.


(MOR 1125)
An extraordinary Tyrannosaurus rex specimen was discovered by MOR’s Field Crew Chief Bob Harmon in the summer of 2000. It was named B-rex in honor of its discoverer, “Bob.” Although only 50 percent complete, the skeleton is one of the best preserved dinosaur specimens ever found and has yielded two great paleontology discoveries:
First discovery of soft tissue in a fossilized organism. Soft tissue blood vessels and

cells were found inside one of the B-rex femurs.

Evidence that the animal was female. Specialized tissue called “medullary bone”

was found lining the bone marrow cavity of both B-rex femurs.

The excavation was supported by the Catherine B. Reynolds Foundation. In recognition of B-rex being identified as female, the specimen became known as “Catherine” in honor of Catherine Reynolds.


In 2000, the MOR Hell Creek Field Crew discovered an unprecedented five specimens of Tyrannosaurus rex. B-rex was one of the five specimens. Field Crew Chief Bob Harmon was out searching for dinosaur remains. One day in July 2000, Bob climbed up a 20-foot cliff to have lunch. When he finished lunch, he turned to look at the wall of the 40-foot cliff behind him. Sticking out of the wall was a single bone he knew was a metatarsal (foot bone) of a Tyrannosaurus. Bob couldn’t reach high enough to touch the bone, so he climbed back down the lower cliff and hiked a mile back to his satellite camp where his crew was excavating the giant Edmontosaurus, “X-rex.” He walked back out to the site with a folding chair, climbed up the lower cliff, stacked up some large flat rocks, and placed the folding chair on the flat rocks. He climbed up on the chair and excavated the bone.

Jack Horner and the MOR crew decided to excavate a small portion of the cliff to see if more bone could be found. The excavation would require crew members to hang on ropes with jackhammers. Nels Peterson, an MSU engineering student with mountain climbing experience, got a climbing crew together and began a 30-foot high excavation. The four person crew spent three week jackhammering and shoveling until they reached the bone layer, 30 feet below. When the excavation was finished they had found both hind legs of B-rex.
At the end of the 2001 field season, the specimen was ready to be transported. The plaster jacket containing the bones weighed about 3,000 pounds, much more than could be hauled out by hand. The Windway Corporation from Wisconsin sent out a helicopter to help in our effort, but the jacketed specimen weighed too much. The jacket had to be split and the bones broken in order to be safely helicoptered out. The breaking of the bones would eventually lead to one of the greatest discoveries in paleontology.

B-rex Major Excavation: In May 2002, Nels Peterson set up the B-rex satellite camp for another season of excavation. Excavation lasted for more than a month. The crew jackhammered and shoveled over a thousand cubic yards of rock, going down more than 40 feet. When the crew finished digging, the quarry was huge. The back wall was more than 50 feet high. Most of the skull was found along with a few ribs, vertebrae, and parts of an arm. Although incomplete, the skeleton would prove to be one of the best preserved dinosaur specimens ever found. At the end of the 2002 season, the remaining bones were jacketed and ready for removal. Each jacket had to be kept to under 2000 pounds so that the helicopter could airlift them out. When the skeleton was prepared, it was found to be about 50 percent complete.

The Wankel T-rex skeleton provided the first evidence of cell-like structures in a dinosaur, but it was the femur of B-rex that yielded the biggest surprise of all. Inside one of the broken femurs, Research Associate Mary Schweitzer and her lab assistant, Jennifer Wittmeyer, discovered soft tissue blood vessels and cells. They etched away the hard tissues of bone with acid to reveal the soft, pliable vessels. It was the first discovery of soft tissue in a fossilized organism, and has the best chance of providing ancient DNA and proteins. The discovery is also making scientists rethink the way bones fossilize, because with our present understanding of the fossilization process, soft tissue should not exist. Research continues on the specimen, with the focus on looking for proteins and DNA. Note: Panel images compare the T-rex soft tissue to that of a modern-day ostrich. Similarities in the structures of the vessels and the osteocytes (single cells) can be seen.

The B-rex skeleton of Tyrannosaurus rex revealed not only pliable vessels and cells, but evidence that the animal was female. A very thin bone tissue, found in the marrow cavity wall of both thigh bones, turned out to be a specialized tissue called “medullary bone.” Before finding medullary bone in the B-rex thigh bones, scientists had only found it in living birds. Medullary bone is a highly specialized tissue that forms in the marrow cavities of bird bones just before egg-laying. It is the tissue that provides calcium for eggshell formation. Only female birds make medullary tissue. The medullary bone found in B-rex indicates that the animal was female, and the amount of it suggests that B-rex died during egg-laying. The production of medullary bone is a characteristic now known to be shared by both avian dinosaurs (birds) and non-avian dinosaurs.
Dinosaurs had two active oviducts and laid two eggs a day (birds have one active oviduct and lay one egg a day). Most dinosaurs laid between 12 and 30 eggs, so egg-laying took from 6 to 15 days. B-rex likely died during these couple of weeks of egg-laying. If she had finished, there would most likely not have been any medullary tissue left. Had her death been prior to egg-laying, more medullary tissue would have been expected. Note: Panel images show similarities in the medullary bone and cortical bone in the B-rex femur and a female ostrich femur.
Discover magazine listed Dr Schweitzer’s findings of soft tissue and medullary bone in B-rex as one of the 100 most important scientific discoveries in 2005 (Discover, January 2006).

B-rex Femur Display Case: The case located below the B-rex wall panel contains the B-rex discovery bone, the metatarsal (foot bone) first seen by Bob Harmon. The whitish area was all that was visible. The case also includes the B-rex broken femur (thigh bone) where soft tissues and medullary bone were extracted. Note: The displayed femur is half of the famous B-rex femur; Dr Schweitzer’s research was done on pieces from the other femur half. The red triangle points to the location of the medullary bone in the femur.

Montana and Wyoming are famous for their horned (ceratopsian) dinosaurs. The first specimen of Triceratops was found in Wyoming, and since then many specimens of this horned dinosaur have been found in Montana, the Dakotas, Colorado and Saskatchewan. We have found two different horned dinosaurs from the Latest Cretaceous in Montana: Triceratops and Montanaceratops.
Triceratops lived from 68 to 65 million years ago in the northern Rocky Mountain region of North America and fed on plants. Based on collected fossils, we can say that Triceratops was one of the most common dinosaurs during the Cretaceous period. A full-grown adult Triceratops was about 26 feet long. The live animal would have weighed about 20,000 pounds (Note: A large bull elephant weighs about 14,000 pounds).
The Museum of the Rockies has the largest collection of Triceratops fossils in the world including 100 skulls. The Triceratops has the largest skull of any known dinosaur. The MOR 1122 specimen in the exhibit’s Triceratops skull growth series is the largest dinosaur skull ever found The skull is nine feet long and six feet wide.
Triceratops have three horns, one called a nasal horn on the front of the skull, and one above each orbital (or eye socket) called orbital horns. Ceratopsians are the only dinosaurs to have a rostrum which is a beak shaped bone located at the front of the mouth. Keratin does not fossilize but scientists believe that horns found as part of fossilized ceratopsian skulls are actually horn cores that would have been covered by a keratin sheath, just like horned animals today. Close study of horn cores indicates a hollow base.

Ceratopsian Cladogram: Compare these two different ceratopsians. Observe their similarities. The ceratopsian cladogram is a relatedness diagram showing the evolutionary relationship among 12 different ceratopsians.

Skulls of Horned Dinosaurs: All dinosaurs have most of the same bones in their skulls, but the size and shape of each is what makes dinosaur heads look so different. Horned dinosaurs have a bone called the rostrum that other dinosaurs lack. The rostrum forms the upper beak in all horned dinosaurs. Compare some of the skull bones of Triceratops with the same bones on the skulls of dinosaurs such as Edmontosaurus and Tyrannosaurus. Look at the differences and similarities between the skulls.

Teeth of Horned Dinosaurs: Ceratopsians are plant-eaters. Compare the beaks of the ceratopsians with the beaks of other plant-eating dinosaurs such as the duck-bills and “hypsilophodontids.” Notice the narrow beak of ceratopsians. Animals with narrow beaks or mouths usually are very particular about the food they eat. Think about the difference between the beaks of a heron and a duck. Based on the position of the head, teeth and beak of Triceratops, it is likely that these animals ate low, tough plants, perhaps similar to a palmetto or other fan palms. It may also be that Triceratops ate some sort of flowering bush. Given the height of its head off the ground, it is unlikely that the Triceratops ate leaves off trees. Fan palms, ferns, and rushes are the shortest plants that we know existed when Triceratops roamed this part of North America. It makes sense that they ate one or more of these kinds of plants, but it may be that they ate some other kind of plant for which we have little or no evidence.


This display showcases two of the museum’s most recent discoveries. These specimens are unique because this amount of Triceratops body skeleton is rarely found. Very few full Triceratops skeletons are on display for the public. Because skeletons are rarely found, some museums have combined the bones of different individuals to represent a single skeleton. In this display, all the bones displayed in each skeleton belong to that individual Triceratops.
The original fossils were found in the Hell Creek Formation in Garfield County, Montana. The brown display bones are replica casts of the original fossils in the museum’s collection. The light gray display bones are not based on original fossils but derived from other Triceratops research. These skeletons were sculpted by Matt Smith who also created the other large-scale models in the Siebel Dinosaur Complex including the adult Triceratops, Deinonychus, sauropod and plesiosaur models. Replica casts are used for this display because the fossils are being researched and to make the overall display lighter.

The museum is actively conducting research on both specimens. Leg bones should allow researchers to determine the age of each individual and give an age to each skull. By comparing both skull and body growth, researchers should gain insight into the growth process of Triceratops as well as the rate of growth.

One difference between the two skeletons is the orientation of the orbital horns. Subadult and adult Triceratops orbital (eye) horns pointed forward, whereas juvenile orbital horns arced backward. This difference allowed the subadults and adults to recognize the juveniles and allowed the juveniles to recognize the subadults and adults. This feature is evidence that these dinosaurs were very social animals.

YOSHI’S TRIKE/Large Subadult Triceratops Skeleton (MOR 3027R)

This skeleton was discovered by Yoshi Katsura, a Japanese graduate student, in 2010. The specimen was located on a hillside which had to be removed to excavate the skeleton. The skull was excavated in 2010; the body in 2011. Once fully jacketed and ready for transport, a helicopter had to be used to lift the fossil jackets due to their size and weight. Photos on the panel show how the bones of the skeleton were found within the rock as well as the outline of the quarry boundaries where the skeleton was excavated.

Researchers are still analyzing materiel to determine the age of this individual recently estimated to be about 5-6 years old. The forward orientation of its horns indicates this individual was reaching maturity; however, histology studies show it was still growing and growing very fast. Its horns were getting longer and thicker. The skull is about six feet long; however, a fully mature adult Triceratops skull can reach nine feet in length.

JUVIE TRIKE 3/Juvenile Triceratops Skeleton (MOR 1199R)

Denver Fowler discovered this specimen in 2008. The specimen possesses one of (if not THE) most complete juvenile Triceratops skulls ever found. In addition, this skeleton is the only mounted juvenile Triceratops skeleton on display in the world based on actual fossil material.

The skull was excavated in 2008; the body in 2009. This specimen was named “Juvie Trike 3” because it was the third juvenile Triceratops discovered by Denver Fowler that day! Latest research indicates this individual may have been about one year old.

Panel photos show the Juvie Trike III field site and the MOR crew placing Plaster of Paris field jackets on the skeleton for safe travel back to the museum. The center photo shows the skull bones of Juvie Trike III which are disarticulated, meaning separated and not attached or joined. Juvenile skulls are often disarticulated because the bones have not grown together or fused.

Discovery Opportunity: Compare the juvenile and the large subadult Triceratops skeletons and look for differences.
Some of the museum’s latest research describes significant changes in skulls as well as body

size and shape as dinosaurs matured from juveniles to adults.


Hell Creek was the last dinosaur-dominated ecosystem in the world. The Cretaceous-Tertiary boundary is found at the top of the Hell Creek Formation. The Hall of Horns and Teeth showcases the dinosaurs that dominated this ecosystem in Montana.

Triceratops is the most common dinosaur found in the Hell Creek Formation. Skulls are very common, but they are generally found as isolated specimens. Associated skeletons such as the two on display within the hall are rare. All the Triceratops skulls in the exhibit were found in Montana. All are in the MOR collection except one, the baby Triceratops skull (UCMP 154452, University of Southern California at Berkeley).
Because the museum has 100 Triceratops skulls, latest research has concentrated on the evolution of the Triceratops over time and the growth changes in a Triceratops as it matures.
TWO SPECIES OF TRICERATOPS: Triceratops prorsus and Triceratops horridus

Panel: There are two species of Triceratops from the Hell Creek Formation: Triceratops prorsus (two skulls on the right – MOR 004 and MOR 2923) and Triceratops horridus (skull on the left – MOR 1120). These two species did not live at the same time. Triceratops horridus lived about a million years before Triceratops prorsus. Notice that Triceratops horridus has a small nasal horn and Triceratops prorsus has a large nasal horn. There is evidence that Triceratops horridus is the ancestor of Triceratops prorsus meaning that Triceratops horridus evolved into Triceratops prorsus over the course of about one million years. The increase in size of the nasal horn is one of the changes that occurred during Triceratops evolution.
Panel: Triceratops horridus Skull (MOR 1120R) – “GETAWAY TRIKE” – Small Subadult

The Getaway Trike was found in the lower third of the Hell Creek Formation. The skull was found disarticulated (pieces apart) in a mudstone sediment representing the floodplain of a river. The Getaway Trike represents a small subadult Triceratops. Compare it with the small subadult (MOR 2999, Situ But Sad) in the growth series to the right. Notice that the orbital horns of both specimens are pointed forward, but there is a slight backward arc near the tips of the orbital horns. Note: The skull is five feet long.

Panel: Triceratops prorsus Skulls (Notice their Large Nasal Horns)
MOR 004B – “MORT” – Large Subadult Skull: Photo at bottom right of panel shows the plaster jacketed skull of MORT at its excavation site in 1981. This skull was discovered in 1979 on the southeastern edge of Fort Peck Reservoir and collected in 1981. The Museum of the Rockies held a contest for Gallatin County students to name this skull. The winner was a fifth grader from Gallatin Gateway, Montana, and the winning name was MORT for Museum of the Rockies Triceratops.
MOR 2923B – “JOE’S TRIKE” – Subadult Skull: Photo at bottom left shows Jurassic Park III director, Joe Johnston, with the Triceratops skull (on his left) that he found in 2007. This subadult Triceratops skull was discovered by Joe Johnston, the director of Jurassic Park III and other films including Honey, I Shrunk the Kids, Jumanji, The Rocketeer, October Sky, The Wolfman, and Captain America: The First Avenger. He often joins the MOR crew on digs. In 2007, he found this specimen in Garfield County, Montana, and it was excavated in 1-2 weeks. The panel shows Joe at the site. The Triceratops skull is upside down in the rock to his left. All you can see is the base of the shield. Another photograph shows the skull being field jacketed prior to removal. The jacketed skull weighed 3500 pounds.
NOTE: Latest MOR research on the two displayed Triceratops skeletons indicate that the Juvie Trike III juvenile skeleton is a Triceratops prorsus specimen. Yoshi’s Trike, the large subadult skeleton appears to be a transitional specimen between Triceratops prorsus and early Triceratops horridus; however, the specimen shows no real characteristics of its own which would be needed to define another separate Triceratops species.

The Museum of the Rockies has the most extensive collection of Triceratops specimens in the world and the world’s most complete Triceratops skull growth series. The growth series includes skulls of Triceratops that show how the shield and horns changed shape as the animal grew up. For many years, scientists thought these different shaped skulls represented multiple species, but we now hypothesize that they represent the growth of one species. Note: All the skulls in the growth series are replica casts. The large subadult skull (MOR 004) is a replica; however, the actual fossil skull is located within the “Two Species of Triceratops” display area.

Panel: Baby Triceratops Skull (UCMP 154452R)

This specimen shows that the orbital horns existed when Triceratops was very young. The presence of orbital horns in this size individual suggests that the horns were not for sexual display or defense, but most likely for species recognition. A nasal horn wasn’t found with this specimen, so we don’t know if this size Triceratops had a nasal horn or not. We added one to show its size and what it would have looked like if it did exist. The baby has large eyes and a short snout. The orbital horn is tiny but has the distinctive blood vessel impressions seen in larger specimens, indicating a covering of hard keratin.

Discovery Opportunity: Compare the edge of the baby’s squamosal to the squamosal of a larger juvenile or adult.
The baby’s squamosal bone is part of the shield and has triangular projections.

The larger individuals have extra bones, called epoccipitals, on the outer edge of

their shield. The purpose of these structures is unknown.

Panel: Small Juvenile Triceratops Skulls (MOR 2569R and MOR 1199R)

Small juvenile Triceratops are extremely rare. Those on display, with the exception of the baby skull which is in the collection of the University of California at Berkeley, are part of the MOR collection. Juvenile Triceratops are rare in other museum collections because of selective collection of large specimens. In the past, most museums were interested in large adult skulls

and often left juvenile specimens behind. At MOR, we study dinosaur growth and have a major interest in specimens that depict growth stages of different species.
In July 2006, the small juvenile Triceratops skull (MOR 2569) was unearthed in the Hell Creek Formation near Jordan, Montana. The orbital horns are only about 3-4 inches long. Jack Horner thinks this animal was probably less than a year old when it died. Small juvenile Triceratops are identified by having backward-curving orbital horns and delta-shaped epoccipital bones. Like the baby Triceratops, these young Triceratops retain juvenile characteristics such as the large eyes, backward curving horns and unfused nasal bones. The nasal horn was an individual bone, separate from the nasal bones.

Panel: Medium Juvenile Triceratops Skull (MOR 2951R) - “Juvie Trike III”

Like all the younger and the next two older individuals in this growth series, the Juvie Trike III specimen was found disarticulated in a mudstone sediment representing the floodplain of a river. Disarticulation of cranial elements is indicative of an animal’s young age. As Triceratops skulls matured, many of their skull bones fused together, very similar to our human skulls. Juvie Trike III is the most complete juvenile Triceratops skeleton known. The mounted skeleton is on display in this hall.

Panel: Large Juvenile Triceratops Skull (MOR 1110R)

The bones of this specimen were found disarticulated (separated), indicating its youthfulness. Like the younger skulls, the orbital horns curved backward, and the nasal horn was loose and not firmly attached to the nasal bones. Also, the nasal bones were not yet fused together. Unlike other ceratopsians, the nasal horn of juvenile Triceratops was an individual bone, separate from the nasal bones. As an animal grew, the horn became fused to the nasal bones. This specimen was found among a large concentration of mollusk shells indicating the site was a mature river bed where mollusks grew to very large sizes. Apparently, the young Triceratops died and fell into the river.

Discovery Opportunity: Compare the two small juvenile skulls to the large juvenile skull. Why did Triceratops horns and epoccipital bones change shape as the animals grew older?
Notice that the horns of all the juveniles arc backwards, and that the epoccipital

bones of the larger juvenile are more flattened against the frill. These bones

became more flattened with age, until they merge with the frill in adulthood.

Scientists do not know the purpose of these structures.

Dinosaurs are known to have been very social creatures, very much like their living

descendants – birds. Dinosaurs nested in colonies and traveled in herds. Among social

animals today, it is important for the adults to be able to recognize when young

individuals reach sexual maturity, as this also signals adulthood. It is theorized that

Triceratops individuals that had backward-pointing horns and triangular-shaped

epoccipitals were recognized as juveniles, and that individuals with forward-facing

horns and flattened epoccipitals were recognized as adults.

Panel: Small Subadult Triceratops Skull (MOR 2999R) – “Situ But Sad”

As Triceratops matured, it lost its juvenile characteristics. The orbital horns that had grown arcing backward during its juvenile stage began to grow forward, and the triangular bones along the edge of the frill (epiossifications) began to flatten against the parietal shield. These changes may mark the beginning of sexual maturity of Triceratops.

NOTE: At this stage of growth, the horns over the eyes have shifted forward, and the nose horn has fused to the nasal bones. The nasal bones have also fused together. These features are similar to adults, suggesting that at this size the animal had reached maturity, even though it would continue growing larger.
Discovery Opportunity: What are some of the adult characteristics seen in the small subadult Triceratops skull?
This specimen clearly shows adult characteristics. As Triceratops adults grew older, their

faces got longer in proportion to their shields. Blood vessel grooves are more prominent

than in the skulls of younger, smaller individuals. These grooves indicate that the keratin

had hardened, and was pushing the vessels into the bone as the skull continued to grow.

The nasal bones and nasal horns of a large juvenile and small subadult Triceratops are in the

floor case behind you. Compare the unfused nasal bones of the large juvenile to the fused

nasal bones of the small subadult. Fusion of the horn occurs sometime during the subadult

stage when the skulls are around five feet long.

Panel: Large Subadult Triceratops Skull (MOR 004R) – “MORT”

Compared to the smaller, younger skulls of Triceratops, the bones of this skull are highly fused together, indicating that it was clearly a mature adult. Also, note that the orbital horns point completely forward in this size individual, differently than those of the younger skulls.

Specimen Comparison: Note the shortness of the frill and the long facial region. Also notice the size of the nasal horn, and the length of the rostrum anterior to the nasal horn. Compare these proportions with other Triceratops specimens. Young adult Triceratops have a short frill whereas a full mature Triceratops adult has a long frill and a long face.
Discovery Opportunity: Compare the large subadult skull to the smaller, younger skulls of Triceratops.
The bones of this skull are tightly fused together, indicating that it was a mature adult. Also note that the orbital horns point completely forward in this size individual, differently than those of the younger skulls. Note the shortness of the frill and the long facial region. Also notice the size of the nasal horn and the length of the rostrum anterior to the nasal horn. Compare these proportions with other Triceratops specimens. Young adult Triceratops have a short frill whereas a fully mature Triceratops has a long frill and a long face.


Panel: Adult Triceratops Skull (MOR 1122R)

The MOR 1122 adult Triceratops skull was found at the base of a 20-foot high sandstone cliff. The MOR crew dug back into the cliff, forming a cave, rather than taking off the cliff above the

specimen. Adult Triceratops had a skull nine feet long and six feet wide. Note that the adult skull looks very similar to the large subadult skull except that the adult skull has a much larger frill, and it is penetrated by two large holes. As Triceratops matured, its shield expanded in length and breadth, but thinned in thickness. The thinning of the frill allowed the frill to be

larger in length and width but still have the same weight. In addition, note the gnarly nasal horn and the deep grooves in the orbital horns of the adult skull compared to the juvenile and subadult specimens.

For many years, this animal was called “Torosaurus” because paleontologists originally thought that a horned dinosaur with holes in its frill must be a different species, but research here at the Museum of the Rockies showed that “Torosaurus” was simply a full-grown Triceratops. Large and small adult Triceratops appear to have very similar skulls, except that a large adult Triceratops has two large holes in its frill.

Discovery Opportunity: Why did the adult Triceratops have holes in its neck shield but the subadult and juveniles didn’t?
Research at the Museum of the Rockies shows the Triceratops developed these large holes in

its frill when it was a full-grown, very mature Triceratops. Research continues on the

purpose of the holes. One hypothesis suggests the holes formed to make the overall skull


Panel: The Neck Shield (Parietal Bone) of Triceratops horridus (MOR 1122B)

The lower shield is the original fossil facing forward, and the shield above is a cast showing the rear side. Note the pattern of blood vessel grooves on the front and rear of the shield. The neck shields of Triceratops, and all other horned dinosaurs, are indented with branching blood-vessel channels. On the front side of the shield, large indented vessels seem to be directed into the hole openings, while on the rear side, these vessels seem to pass through the holes, terminating at the outer rim of the shield. The blood supply appears to have come from the head and gone out to the epoccipital bones on the outer rim of the shield.

Discovery Opportunity: Why are blood vessels indented into the skull bones of Triceratops? How do we know the Triceratops skull was covered with hard keratin?
Show cast of indented blood vessels from a portion of the MOR 1122 shield. The Museum of the Rockies used histology to study microscopic structures within the shield. Histological sections (paper-thin slices) cut from the inside of the bony shield revealed tiny bundles of Sharpey’s fibers. These microscopic structures are often observed in bone where muscle, ligaments or hard keratin attaches to bone.
Studies of ceratopsian shields by Jack Horner and post-doctoral student Cynthia Marshall provided evidence to suggest that bones with indented vessels and internal Sharpey’s fibers were overlain by hard keratin. In birds, indented vessels and internal Sharpey’s fibers are found under keratin beaks and on the bones of keratin-covered claws. Keratin beaks in birds are often very colorful. Yellow, black, and gray are common colors of keratin.

Panel: The Back of the Skull of Triceratops horridus (MOR 1122B)

This specimen was discovered by hunters in Fergus County. The skull is lying on its right side. Notice the large ball (occipital condyle) that connected the skull to the neck vertebrae. The ball was located halfway between the front of the skull and the back of the skull, which allowed the head to be balanced. The head would have been very heavy, but connected by massive neck muscles. The ball on the back of the skull connected to a special neck bone called the “atlas/axis complex.” In horned dinosaurs, this complex was made up of three tightly connected vertebrae that supported the weight of the head. (You can see this connection on the skeletal side of the Triceratops sculpture). NOTE: Triceratops skulls balanced on the occipital condyle. Research shows that as the shields got bigger, the front of the skull lengthened so that overall skull balance was maintained.

Discovery Opportunity: How did Triceratops use its horns and shield?
For defense? Not likely! For many years, researchers thought that horned dinosaurs used their horns and shields for defense, but careful examination indicated that this was highly unlikely. The horns over the eyes actually become hollowed out on their undersides as they grow larger, so that when the animals are fully grown, the horns have very thin walls, and are therefore weak. Also, as you can see in this exhibit, the horns of juveniles curved backward, useless for any kind of combat. And, the shield became much thinner as the animals grew to adulthood, so much so that full-grown Triceratops frills had openings in them.
To attract a mate? Not likely! Many paleontologists have suggested that Triceratops used its horns and shield for display to attract mates or to challenge rivals. This is unlikely because there is no apparent difference between males and females as both sexes had horns and frills. Among animals that use horns and antlers in the mating ritual, usually only the males have them, or there is a distinct difference between the males and the females.
To be recognized by other Triceratops? Most likely! Juvenile Triceratops heads looked very different than adult Triceratops heads, suggesting that the head-gear was important for recognition of age. Most animals that live in social groups change their looks as they grow older. Because of this, juveniles can recognize adults and adults can recognize juveniles. We call this species recognition, and it probably explains all of the bizarre structures in dinosaurs, including plates, spikes, horns and shields.

Discovery Opportunity: Show an original Triceratops fossil horn and a cast of a horn sheath with hollow horn core. What did Triceratops use its horns for?
Jack Horner believes evidence of a hollow horn core indicates a weakness in the horn that would have caused it to break off if the Triceratops used its horns for defense. Hollow horn cores are, therefore, one argument against this defensive behavior by Triceratops. Other arguments against use of horns for defense include the backward curvature of the horns for part of its life and the general premise that the skull is logically the last place on your body you want a defense mechanism. Paleontologists now believe that Triceratops likely used its horns for species recognition. Because ceratopsians are similar in body shape, differences in skull ornamentation would allow for differentiation between species.


Hadrosaurian dinosaurs were another very abundant plant-eating dinosaur that lived in this region. The hadrosaurian cladogram is a relatedness diagram showing the evolutionary relationships among 19 different hadrosaurs.

Two kinds of duck-billed dinosaurs are represented in our Latest Cretaceous collections: Edmontosaurus from the Hell Creek Formation and Hypacrosaurus from the St Mary River Formation. Edmontosaurus and Hypacrosaurus are in the group Hadrosauridae. Hadrosauridae includes the crested and non-crested duck-billed dinosaurs. Edmontosaurus is a non-crested duck-bill, and Hypacrosaurus is a crested duck-bill.
Teeth of Duck-Billed Dinosaurs: Duck-billed dinosaurs have a wide, duck-shaped bill and batteries of teeth similar to horned dinosaurs. Unlike the horned dinosaurs who sliced their food, duck-bills had the capability of chewing, or grinding their food into smaller pieces. The wide duck-bill beak suggests that duck-bills were not as particular about food items as were the horned dinosaurs.


Edmontosaurus is the most common hadrosaur from the Hell Creek Formation and was the largest of all the duck-billed dinosaurs. Edmontosaurus may have been over 39 feet long, weighed as much as 5 tons, and ate plants. The skull can be as much as four feet long. It lacked a nasal crest, a feature present on all other duck-billed dinosaurs. Eggs of Edmontosaurus have not been found, and baby skeletons are extremely rare.
Edmontosaurus skeletons have been found in giant bone-beds in South Dakota and Wyoming, suggesting that they traveled in large herds like other species of hadrosaurs. Edmontosaurus may have migrated along the eastern front of the Rocky Mountains from Montana to Alaska and back. Specimens of Edmontosaurus are found in sandstone sediments that were deposited by large meandering rivers in lowland areas very near the inland seaway. Edmontosaurus may have been semi-aquatic, taking advantage of plants that grew near the rivers and sea.

This specimen of the Edmontosaurus (MOR 1142B) represents one of the largest duck-billed skeletons found in North America. The tail is 26 feet long. If the entire skeleton had been preserved, it would have been nearly 50 feet in length, larger than the biggest T-rex on record!

But why is this duck-billed dinosaur called X-rex? Ah, therein lies a tale. In 1999, the MOR field crew was out looking for dinosaurs in Getaway Coulee in Garfield County. Three female Portuguese geology students, nicknamed the Portugirls, wandered up a ravine where they found a site with thousands of tiny bone fragments weathering off the side of a small ridge. The Portugirls knew it was something important so they called Jack Horner to come identify the dinosaur. When Jack examined the site, he could see that the knees of a dinosaur were weathering out. The bones were extremely dense and had a layered look to them.

Tyrannosaurus rex has large leg bones with dense layered looking bones, so Jack declared the specimen a T-rex and instructed the crew to begin an excavation. After six weeks of digging, they reached the skeleton and discovered it was the back half of a giant duck-billed dinosaur named Edmontosaurus. The Portugirls christened the specimen X-rex, calling attention to the fact that Jack had originally misidentified the specimen as a T-rex. Specimens are often misidentified when they are still in the rock and sometimes even when they are prepared.

Skin Impressions: This specimen is extremely well-preserved and reveals skin impressions on the underside of the tail, the pelvic area and the hind foot. Most skin impressions are found in fine-grained siltstones or mudstones, but the impressions on this specimen are preserved in sandstone. Sometimes specimens like this are called mummies, but that is an incorrect term. Skin is not actually preserved; instead, the so-called “skin” is actually an impression that was made in the fine sand and mud in which the animal died.
Examination of the skin impressions of this specimen suggests that the hide was not particularly thick or tight, but rather loose and wrinkly. The surface texture of this Edmontosaurus tail was similar to the skin surface of crocodiles and alligators, and the featherless areas of birds. Skin impressions from the underside of the foot reveal that the skin ossicles (rounded bumps) were somewhat smaller than those on the side of the tail. Evidence from this specimen also reveals the fleshy frill that probably ran down the middle of the back and tail. Above each vertebral spine is a rounded flap of skin separated from the next by a low depression.
Leaf Impressions: Leaf impressions were also found with this specimen. The leaf impressions displayed below the tail represent a number of deciduous species that apparently lived near the shore line of the inland sea.


Discovered in 1976 in Yellowstone County. Like other dinosaurs, the skull of Edmontosaurus underwent many changes during growth. The young juvenile had large eyes and a shortened snout, or duck-like bill. As Edmontosaurus grew, its bill expanded and its eyes got proportionally smaller. It also added teeth. As a baby, it had only 200 teeth; but when it was an adult, it had an average of 400 active teeth, with hundreds more forming in its dental batteries.


These jaws are from a juvenile and a giant adult Edmontosaurus. The juvenile jaw has

30 rows of small teeth; and the giant adult jaw has 66 rows of large teeth. For comparison,

the adult maxilla of the MOR 003 skull has 59 rows of small teeth. All dinosaurs

replaced their teeth on a regular basis so as the animals grew larger, the replacement

teeth also became larger. The giant maxilla is the largest duck-bill jaw known and

represents an animal more than 50 feet long with a skull more than five feet in length.

This duck-bill was larger than the largest known Tyrannosaurus rex.

Case Contents: Juvenile Edmontosaurus maxilla

Gigantic adult Edmontosaurus maxilla

“Hypsilophodontids” were primitive plant-eaters who walked on their hind legs and were closely related to the animals that gave rise to duck-billed dinosaurs. Dinosaurs that scientists call “hypsilophodontids” do not share a common ancestor. Therefore, the name is invalid but used for ease in identifying a group of similar animals. The “Hypsilophodontid” cladogram is a relatedness diagram showing the evolutionary relationships among 10 “hypsilophodontid” dinosaurs.
Thescelosaurus is the only known “hypsilophodontid” from the Hell Creek Formation, but these kinds of dinosaurs are represented in nearly every terrestrial formation in Montana. There are four different “hypsilophodontid” species represented in the Siebel Dinosaur Complex. The Thescelosaurus was about 10 to 13 feet long and may have weighed 500 pounds as an adult.
“Hypsilophodontid” Skulls: Thescelosaurus had a very long head, with cheek teeth and beak teeth. Compare the tooth arrangement of Thescelosaurus with the tooth and beak arrangement of Edmontosaurus.
“Hypsilophodontid” Teeth: The teeth of Thescelosaurus are very similar to those of Stegosaurus and many other plant-eaters in that they have coarse serrations on their front and back sides. These structures suggest that Thescelosaurus ate plant material that had to be shredded. These dinosaurs probably ate plants that were low to the ground.

Thescelosaurus neglectus Skeleton (MOR 979R)

This is a cast of the best skeleton of a “hypsilophodontid” ever found. The fossil is still under preparation and study and may be for several more years as the rock in which it is encased is extremely hard and the bone is very fragile. Associated with this specimen are patches of skin impressions. Also, there are odd bony projections on the lower arm bones. It is not known whether these are bone pathologies (due to illness or trauma) or if they were a useful structure for the animal. No other animals have been found with such projections.

Similar to the Wankel T-rex skeleton, this skeleton of Thescelosaurus was found in a river sandstone. Unlike the T-rex skeleton, however, the Thescelosaurus skeleton remained intact and as a result is the best Thescelosaurus skeleton ever found. Around the foot and chest areas are skin impressions showing that these animals had diamond-shaped bumps in their skin.

NOTE: Researchers tentatively identified this specimen as a new dinosaur and called it Bugenasaura. Additional study has determined that there is not enough information to call it a new dinosaur and that the specimen is a Thescelosaurus.



Other dinosaurs from the Hell Creek Formation include Ornithomimus, Ankylosaurus, and Pachycephalosaurs. Specimens of these three dinosaurs are displayed in the wall case to the left of the Thescelosaurus display.

Ornithomimus was a very lightly built saurischian dinosaur closely related to Tyrannosaurus. Unlike Tyrannosaurus, however, Ornithomimus had a beak instead of teeth. Ornithomimus was about 13 feet long and may have only weighed about 100 pounds. Ornithomimus skeletons are rarely found in the Hell Creek Formation, but their toe bones are quite common. The rareness of most Ornithomimus remains may have to do with their bones having been very lightly built and hollow like bird bones. Most Ornithomimus bones are found crushed and broken.
Case Contents: Skull (replica), vertebrae, pelvic bone, partial hand and foot

Sculpture by Ken Olson

Discovery Opportunity: What did Ornithomimus eat?
The claws on both the hand and foot are nearly straight and not curved like most meat-eaters. These animals were not catching or grasping prey. Some researchers think that Ornithomimus may have been a plant-eater.

Ankylosaurus was a large armored dinosaur with a huge boney club on the end of its tail. These animals grew to be around 30 feet in length and may have weighed as much as 20,000 pounds. Best specimen was found by Barnum Brown in Montana in 1910 (American Museum of Natural History, NY collection).
Case Contents: Tail club (replica), 3 armor fossils, sculpture by Ken Olson

Skull (cast) in separate exhibit case in front of the wall case

Discovery Opportunity: What did Ankylosaurus use its armor for?
Scientists think that Ankylosaurus used its armor and tail club for defense although the boney plates may have served in species recognition as well. Scientific studies of the armor of Ankylosaurus reveal that it is made of a special kind of tissue called “metaplastic bone” derived from cartilage. This suggests that the juveniles probably had soft, cartilaginous armor that hardened when the animals were adults.

For many years, these three skulls were thought to represent three different “dome-headed” dinosaurs (Dracorex, Stygimoloch, and Pachycephalosaurus), but research here at the Museum of the Rockies provided evidence that they were actually growth stages of a single dinosaur named Pachycephalosaurus. Similar to the way that Triceratops skulls changed shape as they reached adulthood, Pachycephalosaurus skulls also changed. Young Pachycephalosaurus lacked domes, but had spikes on the back of their skulls. As they approached adult size, the dome began to inflate and the spikes began to resorb or shrink in size. When Pachycephalosaurus reached adult size, the dome was huge and the spikes had reduced to short knobs.
We think that this change in skull shape was for the purpose of recognition. Adults could recognize juveniles and subadults, and the juveniles and subadults could recognize the full-grown adults. This type of growth recognition is important to animals that live in social groups. Humans are very similar in that juveniles have very different characteristics than adults. We can always identify children, regardless of their size.
Panel images: Comparative size of Pachycephalosaurus adult, subadult and large juvenile skulls.
Case Contents: Pachycephalosaurus wyomingensis adult skull (cast)

Pachycephalosaurus wyomingensis subadult skull (cast). This growth stage

was originally named “Stygimoloch spinifer.”

Pachycephalosaurus wyomingensis juvenile skull (cast). This growth stage

was originally named “Dracorex hogwartsia.”


Teeth and claws, baby dinosaurs, and other interesting fossils from the Hell Creek Formation are displayed in the wall case to the left of the Women’s Restroom.

Teeth give us clues as to what animals ate. Meat-eating dinosaurs, such as Tyrannosaurus and dromaeosaurids, have pointed teeth with serrated edges, good for piercing and slicing meat. Adult T-rex had large, blunt teeth good for bone crushing. Triceratops had sharp slicing teeth, and Edmontosaurus had grinding teeth. Thescelosaurus, Ankylosaurus, and Pachycephalosaurus all had leaf-shaped teeth that were good for cutting leaves into small pieces. Of all these dinosaurs, only Edmontosaurus could chew its food. The crocodile Borealosuchus had pointed teeth good for eating fish; the alligator Brachychampsa had short conical teeth possibly for crushing mollusks and tough fishes such as the gar. The fish Myledaphus had flattened teeth for crushing mollusks.
Claws also provide clues concerning the behavior of animals. Curved claws are good for grabbing onto prey, or climbing. Long, pointed claws that are not curved might be good for defense or digging. Look at the claws of these animals and imagine what they might have been used for. The curved claws of the meat-eaters were likely for grabbing but what about the short, blunt claws or hoofs of horned dinosaurs? Were its hoofs of any special use? What about the claws on the hind feet of T-rex?
Case Contents: Edmontosaurus toe bone from hind foot, 2 teeth, juvenile tooth

Tyrannosaurus hand claw, 2 teeth; Triceratops hoof, 2 teeth

Brachychampa (alligator) tooth; Ornithomimus hind foot claw

Thescelosaurus hind foot claw
Six teeth under magnifying glass: Thescelosaurus, Myledaphus

(mollusk eating fish), Ankylosaurus, Borealosuchus (crocodile),

?Paronychodon, Dromaeosaurid

The Hell Creek Formation yields abundant remains of adult dinosaurs but the remains of eggs, babies, and juveniles are extremely rare. Nesting grounds have not been found in the Hell Creek Formation. In rare situations, such as microsites, a few isolated eggshell fragments and baby bones have been found. These rare finds indicate that nesting grounds did exist in the region, but for some unknown reason, they have not been preserved.
Case Contents: Baby Triceratops femur; Baby Edmontosaurus toe bones

Baby Thescelosaurus toe bone

2 eggshell fragments of unidentified dinosaur

Cast of a baby Edmontosaurus skeleton


A lot of fossil remains are found lying on the ground, having weathered out of the sandstone or mudstone of the Hell Creek Formation. In this display case, you see a number of fossil specimens that were found as isolated bits. Like skeletons, these isolated elements provide information about the kinds of animals that lived and died in the Hell Creek ecosystem.

Case Contents: Turtle – jaw, armor from leg, section of shell, femur

Triceratops – jaw fragment with tooth grooves, toe bone,

epoccipital bone (edge of shield)

Edmontosaurus – toe bone, fragment of maxilla,

cast of spinal cord from pelvis

Hesperonis (toothed bird) – foot bone

Thescelosaurus – toe bone, pelvic bone, shoulder girdle

T-rex – vertebra; Unidentified theropod – vertebral centrum

Lepisosteus (Gar fish) – scale

Elmisaurid (toothless dinosaur) – foot bone

Bird femur; Amiid fish vertebra

The center hall exhibit case to the right of the two Triceratops skeletons displays fossil specimens of non-dinosaurian reptiles and skeletal remains from microsites in the Hell Creek Formation.

When we think about the reptiles of the Mesozoic Era, we usually think about dinosaurs, but they were not the only reptiles around. Crocodiles, alligators, many varieties of lizards, turtles, and even a few snakes lived with the dinosaurs. The small crocodile Borealosuchus, a variety of soft-shelled turtles, and the lizard Champsosaurus were very common in the Hell Creek rivers and ponds. These animals probably fed on many of the fishes and amphibians that also inhabited the streams.
Case Contents: Borealosuchus (Crocodile) skeleton; cast of Borealosuchus

Aspideretoides (soft-shelled turtle) skeleton

Emarginochelys (baenid turtle)

Trionychoidea (soft-shelled turtle) carapace (shell)

Champsosaurus (aquatic lizard) skull; 5 vertebrae and 1 rib

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