Dinosaurs under the big sky

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The last five million years of the Mesozoic Era is called the Latest Cretaceous (70 – 65 mya). No dinosaur fossils have been found in rocks formed after the Cretaceous Period. The Hall of Horns and Teeth focuses on the dinosaurs that lived in the Hell Creek ecosystem during the last three million years of the Latest Cretaceous (68 – 65 mya). By this time, many of the continents had moved into positions similar to where we see them today. Inland seas that once inundated many of the continents, including North America, were beginning to recede.

All material in the Hall of Horns and Teeth is from the Hell Creek Formation except specimens in the last three cases at the end of the hall. This hall contains the remains of some of the last non-avian dinosaurs to have lived on earth. Dinosaur excavations have taken place in the Hell Creek Formation for over a hundred years. In 1902, Barnum Brown from the American Museum of Natural History conducted the first paleontological survey of sedimentary rocks which he would later name the Hell Creek Beds. Numerous types of dinosaurs have been collected from the Hell Creek Formation including the type specimen of Tyrannosaurus rex by Barnum Brown in 1903. The formation is now widely recognized as one of the world’s great dinosaur graveyards containing a diverse array of theropods, ornithopods, ankylosaurs, pachycephalosaurs and ceratopsids (Dinosaurs The Encyclopedia by Donald Glut, p. 302).
Hell Creek Project: The Museum of the Rockies has excavated fossils from the Hell Creek Formation for over 30 years. The museum led a six-year (1999-2004) investigation of the formation around Fort Peck Reservoir in eastern Montana. The investigation was conducted by a team of paleontologists, paleogeologists, paleobotanists, and other scientists from around the world resulting in a much better understanding of the Hell Creek ecosystem. During this project, a record eight Tyrannosaurus rex specimens were discovered as well as ten Triceratops and five duck-bill skeletons. Other discoveries included fossil remains of plants, mollusks, micro-vertebrates, and mammals. The goal of the project was to compile comprehensive collections of geological and paleobiological data that will be used to reconstruct the Hell Creek ecosystem. Nearly one-third of the Hall of Horns and Teeth is devoted to displays showcasing some of the results of the Hell Creek Project.

The Dinosaurs Under The Big Sky exhibit displays both real bones and replicas of dinosaur fossils. The museum has set up a process where real fossils on display can still be studied by researchers.
Cladistics: This hall also introduces visitors to cladistics, a newer method used to determine an organism’s family tree. The traditional classification system, the Linnaean Classification System created by Carl Linnaeus in the 18th century, groups organisms in ranks according to similar characteristics (e.g. kingdom, phylum, class, order, family, genus, and species). However, the Linnaean system cannot show how one organism is related to another as a result of evolution. Cladistics, a new classification system, organizes plants and animals by how closely they are related to one another based on characteristics acquired through evolution. Cladograms are diagrams that show relatedness. When two branches of a cladogram come together, it indicates that two groups are related by ancestry. The Hall of Horns and Teeth displays cladograms for ceratopsians, hadrosaurs and hypsilophodontids. (DBS, pp. 13-14)

The hall has four themes and key exhibit elements support each theme:

What Montana was like 68 to 65 million years ago

See fossil evidence of dinosaurs, plants, clams, turtles, etc. that lived in the

Hell Creek area, 68 to 65 mya

Changes in skull shape during growth

Tyrannosaurus rex and Triceratops Growth Series

Function of horns, thickened skulls, and body armor

Triceratops horns and frills

Pachycephalosaurus (Domed Dinosaurs)

Ankylosaurus armor
Determining what dinosaurs ate

Different types of teeth and jaws


LATEST CRETACEOUS (70 – 65 mya): In Montana, the shoreline of the intercontinental seaway began moving eastward early in the Latest Cretaceous. As the seaway receded, the climate near the Rocky Mountains became drier. By the end of the Cretaceous Period, the seaway was gone. Three rock units formed during this time preserve most of the Latest Cretaceous dinosaur fossils in the northern Rocky Mountain region. The Hell Creek Formation is located in eastern Montana, and the Saint Mary River Formation is located west of Hell Creek along the east flank of the Rocky Mountains. The Hoppers Formation was deposited in the area of Bozeman and Livingston and was associated with nearby volcanoes.
Sixty-five million years ago, a global disaster killed off all the non-avian dinosaurs. The geological level that records this event is called the Cretaceous/Tertiary, or the K/T Boundary. An extensive coal layer exists at this stratigraphic level, and the level also marks a color change from the grays of the Hell Creek Formation to the tans of the overlying, younger Paleocene sediments. Fossil remains of non-avian dinosaurs are found below the K/T Boundary/coal layer but not above it (DBS, pp. 77-80).
Map Panel: The World and North America during the Latest Cretaceous,

68 million years ago


The Hell Creek Formation is one of the most extensively exposed Mesozoic formations in Montana, cropping out over a vast area of eastern Montana. The formation consists mainly of tan sandstones, siltstones, and mudstones with occasional thin beds of coal or peat. Large meandering rivers flowing into the seaway deposited the Hell Creek Formation on a flat coastal plain in a warm and humid climate during the retreat of the inland seaway. The most famous dinosaurs to live in this area were Tyrannosaurus, Triceratops, Edmontosaurus, and Pachycephalosaurus. One of the last dinosaurs to exist in Montana was the Tyrannosaurus rex. Some fossil evidence indicates Tyrannosaurus may have been the scavenger of the coastal plain, following great herds of Triceratops or Edmontosaurus.
The Saint Mary River Formation is located in northwestern Montana around Browning. One of the smaller dinosaur fossil-bearing formations, this formation has yielded Montanaceratops, Hypacrosaurus, and Pachyrhinosaurus specimens.
The Hoppers Formation: Volcanic activity around the Elkhorn Mountains continued until the end of the Cretaceous Period. Volcanoes spewed ash into areas now known as Gallatin and Park Counties. The Hoppers Formation preserved volcanic ash where Tyrannosaurus, Edmontosaurus, and protoceratopsian specimens have been found (DBS, p. 77, 127-128).


Tyrannosaurus rex (tie-RAN-o-sore-us rex): The Tyrannosaurus rex (“tyrant reptile king”) reached nearly 40 feet long and weighed around 6 tons. T-rex was a carnivorous dinosaur that probably ate mainly Triceratops and Edmontosaurus (DBS, p. 135).
Triceratops (try-SARE-a-tops): The Triceratops (“three horned face”) was about 20 to 26 feet long and weighed 4 to 5 tons. Skulls of Triceratops are very common in the Hell Creek Formation but they are generally found isolated (DBS, pp. 130-131).
Edmontosaurus (ed-MONT-o-sore-us): The name refers to the Edmonton Formation. Edmontosaurus is the most common hadrosaur from the Hell Creek Formation of Montana. The skull can be as much as four feet long. An adult Edmontosaurus may have reached nearly 50 feet in length and weighed as a much as 5 tons (DBS, p. 132).


Tyrannosaurus rex was the largest land dwelling meat-eater that we know to have existed in North America. Its remains have been found in most states and provinces along the east side of the Rocky Mountains, but Montana has yielded more specimens than any other. For many years, Tyrannosaurus was considered to be a very rare dinosaur, but specimens are being discovered at a rate that suggests T-rex may have been common. The cladogram is a relatedness diagram and shows the evolutionary relationships of Tyrannosaurus.

Tyrannosaurus rex reached nearly 40 feet long and weighed around 12,000 pounds. T-rex was a carnivorous dinosaur that probably ate mainly Triceratops and Edmontosaurus. T-rex bones are relatively easy to identify because they are very large and very dense. An average T-rex femur measures more than 5 feet long and 10 inches in diameter (DBS, p.135).

Tyrannosaurus rex had a rather large skull with a mouth full of powerful teeth. An unbroken T-rex tooth can be up to 11 inches long and as much as 2 inches in diameter. Serrations cover the front and back of each tooth except for the front teeth, which only have serrations on the back sides (DBS, p.136). Like all other dinosaurs and crocodilians, T-rex replaced its teeth throughout its life. New teeth began growing in the jaw underneath the functioning tooth. When T-rex was young, it had laterally compressed sharp teeth, which were useful for cutting flesh. When T-rex was nearly full-grown, the replacement teeth were round and blunt, and were more conducive to crushing bones. The bone-crushing teeth took up more room in the jaw, so T-rex went from having 17 meat-slicing teeth in each jaw as juveniles and young adults to having 12 bone-crushing teeth as full-grown adults. This shift in tooth shape suggests that T-rex went from being a flesh-slicer to being a bone-crusher.

Predatory animals are very rare in any ecosystem, but T-rex remains are plentiful. Its commonness and its tooth structure supports the hypothesis that is was not a predator, but a scavenger. Eight Tyrannosaurus rex specimens were discovered in the six-year Hell Creek Project from 1999 to 2004. The specimens were not found together and did not live at the same time. These specimens will further the Museum’s research into the evolution of the Tyrannosaurus.

Thirteen partial Tyrannosaurus skeletons are in the MOR paleontology collection. Some of the field names were given using the first initial or last name of the finder of that T-rex. The four most significant T-rex specimens include:
MOR 980, Montana’s T-rex: Formerly known as Peck’s Rex before going on display at the Museum of the Rockies in April 2015. Discovered by Louis Tremblay in the Hell Creek Formation, McCone County, Montana in 1997. Initially prepared and reconstructed by Fort Peck Paleontology, Inc, while on loan from the Museum of the Rockies. Original skeleton on display in the Hall of Horns and Teeth.
MOR 1125, “Catherine” (B-rex): Found by MOR crew chief Bob Harmon in 2000 and collected during the 2002 MOR field season. First discovery of soft tissue in a fossilized organism. Evidence animal was female. Excavation was significantly funded by the Catherine B. Reynolds Foundation. “Catherine” (B-rex) is named for Catherine Reynolds. Some B-rex fossils and a replica cast of the B-rex skull are on display in the Hall of Horns and Teeth.
MOR 008, Custer T-rex: Found by Montana State College geology professor in 1965 near Custer, Montana. The World’s Largest T-rex Skull. Original specimen is on display in the Hall of Horns and Teeth as part of the T-rex growth series of skulls.
MOR 555, Wankel T-rex: Found by Kathy Wankel in 1988 and collected by MOR in 1990. First discovery of T-rex lower arm bones. Used to cast the bronze “Big Mike” skeleton in front of the Museum of the Rockies. A replica cast of the skull is on display in the Hall of Horns and Teeth as part of the T-rex growth series of skulls. The original skeleton was on display in the Hall of Horns and Teeth, 2005-2011. In April 2014, the specimen was shipped to the Smithsonian Institution in Washington DC on a 50-year loan. Renamed the “Nation’s T-rex”, the skeleton will become the centerpiece of a new $35 million dinosaur hall at the National Museum of Natural History. The exhibit is scheduled to open in 2019.

BIG MIKE: The bronze T-rex skeleton in front of the Museum of the Rockies is a cast of the Wankel T-rex, one of the most complete T-rex skeletons ever found. The bronze skeleton is 38 feet long, 15 feet high, and weighs 6,000 pounds. As a live animal, this T-rex probably weighed about 10,000 pounds. Big Mike is the very first bronze skeleton of a dinosaur anywhere and was cast by Research Casting International, Ltd. of Ontario, Canada. The bronze cast was named “Big Mike” in honor of Dr. Michael (Mike) Malone, historian, author, Museum of the Rockies board member, and president of Montana State University from 1991 to 1999. Big Mike was dedicated October 3, 2001.



The Museum of the Rockies’ exhibit, The Tyrant Kings, showcases the Museum’s world-class T.rex fossil collection and related scientific research. The T.rex skeleton dominates the Hall of Horns and Teeth while directly facing visitors entering from the Hallway of Growth and Behavior. The real bone skeleton is one of only two on display in the United States. The other one is “Sue” on display at The Field Museum in Chicago, Illinois. A third, the “Wankel T.rex” from MOR, will go on display at the National Museum of Natural History in Washington, DC, in 2019. The Tyrant Kings exhibit opened April 11, 2015.

At the base of the skeleton is an amazing growth series of T.rex skulls ranging from one of the smallest skulls to be discovered to the world’s largest T.rex skull. The skeleton and all six skulls were found in Montana and all are in the MOR collection except one skull. A series of panels surrounding the base describe information about T.rex which has been derived from scientific research. Each panel cites the published scientific paper used as the source of information.
Exhibit Plaque: MONTANA’S T.REX

On April 11, 2015, the Museum of the Rockies at Montana State University fulfilled its promise to the people of Montana to display a real T.rex skeleton (MOR 980) in the Siebel Dinosaur Complex. Formerly named Peck’s Rex, MOR 980 was discovered in 1997 by Louis Tremblay and collected by Dr. Keith Rigby, Kraig Drestler and their field crews.

The Museum of the Rockies gratefully acknowledges the United States Army Corps of Engineers, Whitney and Betty MacMillan, the Board of Trustees of the Museum of the Rockies, Inc., Research Casting International and Smith Studios for their roles in bringing Montana’s T.rex to the Museum of the Rockies for all Montanans and the world to enjoy.
DINO STATS: This T.rex skeleton is approximately 12 feet tall and 38 feet long. When alive, it weighed nearly 7 tons. Skeleton is about 60% real bone, one of the most complete specimens ever discovered. Only T.rex skeleton found with intact gastralia, the floating ribs in the animal’s abdominal cavity. Found in 1997 in McCone County near Fort Peck, Montana.


Montana’s T.rex (MOR 980) was discovered by Louis Tremblay in the Hell Creek Formation, McCone County, Montana, on July 4, 1997. Keith Rigby, Jr., Kraig Drestler, and their field crews collected the skeleton (formerly known as Peck’s Rex) in subsequent years. The skeleton was prepared and reconstructed by Fort Peck Paleontology, Inc, while on loan from the Museum of the Rockies.

In 2010, the skeleton was returned to the Museum of the Rockies where it was reconditioned by the Museum paleontology staff. In 2014, the United States Army Corps of Engineers granted permission to the Museum of the Rockies to mount the real skeleton in its dinosaur halls. The Museum exhibits and paleontology departments designed and fabricated The Tyrant Kings exhibit.
Montana’s T.rex is an adult that stands approximately 12 feet high and is 38 feet long. The skeleton is approximately 60% real bone. In this mount, the real bone is brown and reconstructed elements are lighter in color. The skull on the skeleton is a full replica with brown representing the real elements of the skull, and a lighter color indicating elements that were reconstructed. The skeleton was mounted by Research Casting International and the skull was reconstructed by Smith Studios.
The real skull of Montana’s T.rex can be viewed in the growth series display to the left of the skeleton. This skull is the third largest of the adult skulls in the growth series.



Chomper Rex, Jane Rex, B-Rex, Montana’s Rex, Wankel Rex, and Custer Rex reveal the changes that occur to the heads of Tyrannosaurus rex as they grow up. One of the most important changes is the reduction in the number of teeth in their lower jaws. Juveniles have 17 teeth, but by the time they reach adulthood, they only have 11 or 12 teeth. Notice that B-rex has 13. Chomper Rex and Jane Rex are juveniles, and B-rex is a subadult or teenager. Montana’s Rex, Wankel Rex, and Custer Rex are adults.

Six skulls in the T.rex Growth Series:

Chomper Rex (MOR 6625) - Juvenile

Jane Rex (BMRP 2002.4.1) - Juvenile

B-Rex (MOR 1125) – Subadult or Teenager

Montana’s Rex (MOR 980) – Formerly Peck’s Rex - Adult

Wankel Rex (MOR 555) - Adult

Custer Rex (MOR 008) - Adult

Case Label: Chomper Rex – T.rex Juvenile – 13.5 inches long - MOR 6625 (3D Print)

Nicknamed Chomper. One of the smallest T.rex skulls ever found. Partial elements of this smaller juvenile skull were discovered in 2010 by Lee Hall, a MSU student, on Federal land administered by the Bureau of Land Management in eastern Montana. The specimens were collected by a Museum of the Rockies field crew supervised by Denver Fowler. The site is currently being researched by MOR. Special thanks to Larry Wilmer, Ryan Ridgely, and Joseph Jollick at Ohio University for the 3D modeling and printing of Chomper, and Scott Williams from the Burpee Museum of Natural History for his help.

Background on 3D printing of Chomper replica cast: The Witmer Lab at Ohio University put CT scan data of Jane, the older juvenile skull, into a 3D modeling and animation software package to “down-size” or “de-age” Jane digitally to an animal the age of Chomper. Because baby tyrannosaurines probably all resembled each other very closely, the lab also used scanned data of a Chomper-sized Asian cousin of T-rex called Tarbosaurus. Finally, the lab scanned the original Chomper fossils with a micro CT scanner. The modeled Chomper fossils fit perfectly with the composite of the other two specimens. The lab took the 3D file to Ohio University’s 3D printing facility to print out the Chomper replica cast. It took 40 hours to print. The printed skull is plastic and was originally bright white until painted.

Case Label: Jane Rex – T.rex Juvenile – 28 inches long - BMRP 2002.4.1 (Replica)

Commonly known as Jane. A larger juvenile skull. The site containing this juvenile skull and associated skeletal material was discovered in 2001 by staff and volunteers from the Burpee Museum of Natural History, Rockford, Illinois, on Federal land administered by the Bureau of Land Management in southeastern Montana. The Burpee Museum field crew excavated the site in 2002. NOTE: Jane was an older juvenile and was starting to develop some more mature characteristics such as ornamental hornlets and roughening on the bone surfaces.

Case Label: B.Rex – T.rex Subadult or Teenager – 47 inches long - MOR 1125 (Replica)

Commonly known as B.rex. The B.rex site containing this skull with associated skeletal material was discovered in 2001 by Bob Harmon (MOR Paleo staff member) on Federal land administered by the United States Fish and Wildlife Service in eastern Montana. Museum of the Rockies field crews supervised by Bob Harmon excavated the B.rex site from 2001-2004. NOTE: When found, the B.rex skull was perfectly disarticulated. The 40+ pieces were exceptionally well-preserved.

Case Label: Montana’s T.rex – T.rex Adult - 54 inches long - MOR 980

(Real Bone with Reconstructed Materials)

This skull of Montana’s T.rex (formerly known as Peck’s rex) with associated skeleton was discovered in 1997 by Louis Tremblay on Federal land administered by the United States Army Corps of Engineers in eastern Montana. Montana’s T.rex was excavated by Dr Keith Rigby, Kraig Drestler and their field crews. Special thanks to Matt and Tammy Payne Smith of Smith Studios for creating the armature and reconstruction of Montana’s T.rex skull. NOTE: This is the original fossil skull. The skull used in the T.rex skeleton display is a replica cast. The original skull was placed in the growth series display so visitors could get a close look at the skull and compare it to the other T.rex skulls.

Case Label: Wankel T.rex – T.rex Adult – 56 inches long - MOR 555 (Replica)

Commonly known as the Wankel T-rex. This site containing the skull with skeletal material was discovered in 1988 by rancher Kathy Wankel on Federal land administered by the United States Army Corps of Engineers in eastern Montana. Museum of the Rockies field crews supervised by Pat Leiggi excavated the Wankel T.rex site from 1989-1990. On April 11, 2014, the Wankel T.rex was sent to the National Museum of Natural History in Washington, D.C. on a fifty-year loan. A bronze skeleton of the Wankel T.rex stands in front of the Museum of the Rockies and is known as “Big Mike.”

Case Label: Custer T.rex - Tyrannosaurus rex Adult - MOR 008

THE LARGEST T.REX SKULL IN THE WORLD, 60 inches or 5 feet long!

(Real Bone with Reconstructed Materials)

Measuring five feet in length, the Custer T.rex is the largest T.rex skull in the world. The skull was discovered and collected by William McMannis in 1969 on private land in south central Montana. The skull was initially prepared by Carrie Ancell and reconstructed by Michael Holland, MOR staff members.

Background on Custer T-rex Skull (MOR 008): MOR 008 was among the museum’s initial 10 fossils (current collection over 35,000 specimens). In 1965, a geology professor from Montana State College (now MSU) discovered the skull in the Hell Creek Formation east of Billings, Montana.
The specimen arrived at the museum with two big jaw fragments and hundreds of other pieces. Over the years, Jack Horner, preparators, and graduate students spent about 2000 hours preparing the pieces for research and display. In 2005, MOR Artist in Residence, Michael Holland, began the daunting task of assembling the skull. He took the skull, about 80 percent original material, and filled in the missing pieces with epoxy (easily distinguished from real fossil pieces). The skull is displayed with the jaws wide open. The lower jaws are not connected for two reasons. In life, the jaw is connected by cartilage rather than fused as in humans or other mammals. The clear post between the jaws is necessary to properly support the skull’s weight and structure and to allow maximum exposure of the roof of the mouth for study. He spent about eight months reconstructing the skull which went on display in April 2006.
The Largest T-rex Skull: The largest and most complete T-rex specimen, named “Sue,” was found in South Dakota in 1990 and is part of Chicago’s Field Museum’s collection. A scientific paper described Sue’s skull as 55.4 inches long. The MOR 008 skull is 59 inches long, from the tip of the snout to the back of the skull. (Note: MOR 008 is not considered the largest T-rex found because only the skull and a few small additional fossils were found from this animal).
An Elderly T-rex: MOR 008 may be the most elderly T-rex found, over 25 years old when it died. The extremely rugose nasal and postorbital bones give the skull a bumpy nose and Groucho Marx-style eyebrows which may be signs of age. In younger tyrannosaurs, these areas of the skull are smoother. Research continues to determine the specimen’s age. Scientists calculate age by counting the annual lines of arrested growth in bone (similar to counting tree rings).
Teeth: A tooth near the front of the lower jaw shows a great deal of wear, perhaps from hard food items. In the roof of the mouth on the maxilla, a portion of bone is broken away, revealing the root structure of some teeth. You can see a small tooth crown in this area, the replacement tooth that was forming to replace the larger tooth once it was shed.
Jaw Problems: The lower jaws show a number of pits, holes and lumps. These pathological areas may represent infection, injury or disease. The square outline visible at the back of the right lower jaw shows a section that was removed for histology studies that could provide information about these pathologies.

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