Dinosaurs under the big sky

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“Big Al” is an Allosaurus fragilis skeleton discovered in 1989 in Big Horn County, Wyoming. Allosaurus was a common meat-eater during the Jurassic Period, and its teeth are often found around the carcasses of sauropods. In some locations Allosaurus skeletons are found in abundance, suggesting that these meat-eaters were social feeders. Recent histological studies indicate that Allosaurus individuals the size of Big Al were from 13 to 15 years of age, and that the upper age limits of Allosaurus were between 22 and 28 years. Even though the specimen is called Big Al, it is not a particularly large or old individual. The skeleton above the display case is a replica skeleton of Big Al.

Allosaurus had short hornlike structures over each eye and a very narrow skull with laterally compressed teeth in its jaws. Allosaurus had relatively short arms with enlarged hand claws that are strongly recurved. The claws can be as large as a person’s hand. The claws were probably useful for hanging on to live prey or ripping flesh during feeding (DBS pp. 91-92).

Studies conducted on the Big Al Allosaurus skull suggest that Allosaurus might have been a “slash killer,” opening its mouth wide and slashing at its prey with its teeth. To determine this, the skull was CAT-scanned to create a meshwork of points that could be analyzed to learn how tension and compression would have affected the skull during feeding. This method, called Finite Element Analysis and frequently used by industrial designers, provided evidence to indicate that Allosaurus did not have a strong bite compared to other meat-eaters such as Tyrannosaurus rex. Scientists determined that Allosaurus had a weak bite, but a very lightly built, extremely strong skull. The hypothesis is that Allosaurus slashed at its prey with its mouth wide open.

Case Contents: Allosaurus fragillis skull and cast of maxilla (jaw)
DISCOVERY OPPORTUNITY: Observe the inside surface of the Allosaurus maxilla (upper jaw) and notice the cutaway bone that exposes the incoming teeth.
Dinosaur Teeth Replacement: Archosaurs, which comprise a group of reptiles

that includes alligators, crocodiles, and dinosaurs, replace their teeth

throughout their lives. As the teeth wear down, they are pushed out (shed) and

replaced by new teeth already growing. Dinosaurs replaced their teeth on a

regular basis throughout their lives. Most dinosaur teeth were in the animal’s jaw

for less than one year before being pushed out by the next tooth. Because

dinosaurs shed their teeth, they never had cavities or other kinds of tooth problems.

If a tooth broke during feeding it would simply be pushed out like all other teeth.


Like all other animals, dinosaurs occasionally got injured or contracted illnesses. The study of injury and disease in the fossil record is called paleopathology. Dr. Ewan Wolff, who received his PhD here at MSU, studied dinosaur illnesses. He concluded that many of the illnesses suffered by dinosaurs were similar to those contracted by their modern relatives—the crocodilians (including alligators), and birds.

Nineteen of Big Al’s bones are abnormal, and show that this animal survived multiple injuries, some of which became infected. When trauma or disease affects an animal’s skeleton, the bone cells create a record of it by (1) forming new bone; (2) eating away existing bone; or (3) combining bone growth and destruction.
Healed fractures are the most common problem seen in the fossil record. When an animal breaks a bone, the fracture is mended by new bone growth called a callus. The callus grows quickly and can sometimes be very large. Over time, the bone cells work to remodel the callus, making it smaller and smoother. If an animal dies right after it is injured, however, the bone cells will not have time to heal the fracture. Because bone abnormalities reflect life events, patterns of trauma and disease in the fossil record can be used to infer lifestyle and behavior of extinct animals.
Big Al may have been injured by another Allosaurus, or maybe in a fight with a sauropod or Stegosaurus. He/She survived the immediate causes of these injuries and died later in life. Rebecca Hanna, a former MSU student who studied Big Al, concluded that it is likely that some of the problems seen in Big Al’s skeleton caused this animal to die before reaching adulthood. Big Al would have walked with a limp, and had weeping sores in several places including the right side of his torso, right hand, and right foot. Infection places high demands on an individual and requires strong immune and inflammatory responses. Food and fluid consumption must be increased while battling an infection or healing from an injury. The multiple injuries and infections in his body may have prevented this animal from getting enough nourishment. Ultimately, these problems may have made Big Al more susceptible to additional injury, fatal disease, or even predation.
Case Contents: Normal and abnormal pes (foot) phalanges, manus (hand)

phalange, metatarsals, and ribs

Normal vertebrae with abnormal spine

Articulated abnormal right foot (cast)



This case contains replicas of a skull, back plate, and tail spikes of a Stegosaurus collected in 1886 by M. P. Felch in Fremont County, Colorado. The Stegosaurus was a plated plant-eater. Stegosaurus remains—like those of all armored dinosaurs—are not common. This suggests that Stegosaurus either lived away from the rivers and streams that would have carried sediment to preserve them, or that they were uncommon in their ecosystem.

Most Stegosaurus remains have been found in the Morrison Formation of Wyoming, Colorado and Utah. Stegosaurus remains in Montana are rare. However, MOR began excavating Stegosaurus fossils close to an existing sauropod dig site near Livingston, Park County, Montana, in 2015.

Case Contents: Casts of Stegosaurus tail spikes, skull, and back plate
Discovery Opportunity: Why did Stegosaurus have plates?

Over time, scientists have formed many hypotheses about the function of

Stegosaurus plates. One hypothesis was that the plates were used for defense.

Another was that the plates were used for thermoregulation (heating and cooling

blood to regulate body temperature). Studies conducted at the Museum of the

Rockies and at the University of California, Berkeley, have disproved these


A current hypothesis is that Stegosaurus plates were used for display. The

indentations on the Stegosaurus plate displayed here represent blood vessel

channels. These grooves indicate that the plates were covered with hard keratin

(KAIR-uh-tin). Evidence for this comes from studies of birds, which have similarly

indented vessels underneath the keratin on their beaks and claws. Keratin is a

substance that is very colorful in many living animals. It is likely that dinosaurs

were also very colorful and that, like birds, their color was used for display and

visual communication.

CRETACEOUS PERIOD: Second Great Age of Dinosaurs (144 – 65 million years ago). The Cretaceous Period is marked by the appearance of angiosperms or flowering plants and the increasing diversification of deciduous plants. These new plant resources led to the development of many new groups of dinosaurs. Most of the Mesozoic rocks of Montana accumulated between 112 to 65 million years ago during the Cretaceous Period. Sea levels changed rhythmically, rising and falling repeatedly. Most Cretaceous dinosaur remains come from nonmarine sediments, mainly on the east side of the Continental Divide (DBS, pp. 92-93).

EARLY CRETACEOUS (125 - 110 mya): By Early Cretaceous, Apatosaurus, Diplodocus, Allosaurus, and Camptosaurus were all extinct and had been replaced by Deinonychus, Tenontosaurus, and Sauropelta. The primary rock unit in Montana from this time is the Cloverly Formation found in south-central Montana along the Montana/Wyoming border and Pryor Mountains (DBS, pp. 63, 66).
Map Panel: The World and North America during the Early Cretaceous,

120 million years ago

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