Dinosaurs under the big sky


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This rib measures nine feet in length! Ribs are difficult to identify, but the size of this one suggests that it belonged to a very large sauropod, probably an Apatosaurus (a-PAT-oh-SOR-uhss). An average adult Apatosaurus was about 80 feet long and weighed as much as 35 tons. In 1993, MOR collected this rib from north central Wyoming. Apatosaurus remains are most common in Colorado, Utah, and Wyoming. Only disarticulated remains, primarily from bone beds, have been found in Montana (DBS, p. 89).

Notice that this huge sauropod rib is not curved like the ribs of other dinosaurs. Straight ribs like this one are usually the shorter ribs—close to the first or last in the series. Scientists do not completely understand why sauropods had such straight ribs, but it may have to do with the way that they breathed. Because they would draw air through and across instead of into the

lungs, they would not need extra space in their rib cages for their lungs to expand. This is one of the largest ribs known from any sauropod dinosaur and therefore represents one of the largest land animals to have ever lived on Earth!
Note: Apatosaurus is the dinosaur that we used to call Brontosaurus. Its name has changed because of a scientific rule that says that the first name given to an animal has priority over any subsequent names. In 1877, Othniel Charles Marsh named Apatosaurus based on a sacrum and some vertebrae. In 1879, he named Brontosaurus on the basis of the major portion of a skeleton. Nearly 100 years later (1978), studies showed that Apatosaurus and Brontosaurus were the same species. So the name of Apatosaurus replaced the name Brontosaurus (DBS, p. 89).

DISCOVERY OPPORTUNITY: Compare Diplodocus baby, juvenile, and adult legs


Leg of a Baby Diplodocus Sauropod: Femur replica from the Science Museum of

Minnesota (fossil found in Wyoming) and reconstructed lower legs bones. The leg

belongs to an individual that had probably been out of its egg for about two weeks.

The baby probably weighed less than 20 pounds.

Leg of a Young Giant: This hind leg of a juvenile Diplodocus was discovered in

1994 at the Mother’s Day Site in Carbon County, Montana. This young Diplodocus

was only about 25 feet long and probably weighed about 2 tons (4,000 pounds).

Though a youngster, it was still large compared to all living land animals.

A WHOPPER OF A LEG! The Big Horn Thunder Lizard was an adult Diplodocus

about 70 feet long and would have weighed around 20 tons (40,000 pounds).

That’s about as much as three elephants! In 1993, MOR collected this hind leg in

north central Wyoming.

How Long Did It Take a Sauropod to Grow Up? The development of an organism from embryo to adult is called ontogeny (on-TOJ-uh-nee). An important part of ontogeny is understanding an animal’s physical growth. It took about 20 years for Big Horn Thunder Lizard to grow to an adult.

How do we know? The insides of dinosaur bones have rings like the insides of

trees, and to determine the age of a dinosaur we analyze the number of rings. To

figure out how long it took a dinosaur to grow up, we count the number of rings

from hatchling size to adult size. Bone histology is the study of the microscopic internal

structure of bone. Scientists at the Museum of the Rockies analyze thin bone slices to

better understand dinosaur growth and behavior. Note: The display panel includes a

slide showing the lines of arrested growth (LAGs) in an adult Diplodocus bone.


Often times, paleontologists have to think like detectives. For example, we have reconstructed a juvenile Diplodocus bonebed with the real bone fossils discovered in 1994 at the Mother’s Day Site in Carbon County, Montana. On the map of the Mother’s Day Site it looks like all the bones were strewn randomly around the quarry, but actually many of the bones of the feet and lower legs were in their life positions (e.g. articulated lower leg bones in proper skeletal positions as if animal was standing upright). The other bones of the skeletons were separated and strewn around.

DISCOVERY OPPORTUNITY: How did a group of young Diplodocus die?

The distribution of fossil bones was a clue to how the young sauropods died, and why there were no adults with them. The juveniles likely got their feet and legs stuck in the mud and couldn’t get out. Any adults would have been larger and therefore better able to free themselves from the mud. The following observations helped paleontologists develop hypotheses about how the Diplodocus group died.


Nearly all of the bones in this accumulation belong to juveniles of a long-necked

dinosaur named Diplodocus.

Most of the bones are from the feet and legs.

Along with the dinosaur bones, excavators found a few teeth of the meat-eater

Allosaurus, and fragments of conifer leaves, ferns, and mollusk shells.

The bones were found encased in a type of rock known as mudstone.

The original mud was deposited along the banks of a stream or river.

Many of the bones of the feet and legs were found articulated (in their proper

skeletal positions), some even standing upright. (Example: Look for a

juvenile leg and foot on the bonebed corner to the left of panel photo).

Other parts of the skeletons, such as ribs and vertebrae, were found disarti-

culated and strewn through the mudstone with no apparent orientation.

A study of the positions of the bones, known as taphonomy, indicates that the

articulated bones were not moved before they were covered with mud.

Many of the other bones, such as the vertebrae, show evidence of some

movement, and a few bones have bite marks.


Juvenile Diplodocid sauropods traveled together in groups.

This group of juvenile sauropods died after getting their feet and legs stuck in the

mud on the bank of a river.

The lower legs are best preserved because they were stuck in the mud before the

animals died. The parts of the bodies that laid on top of the mud were

disturbed by water currents and scavengers.

The meat-eater Allosaurus fed on the dying or dead bodies of the sauropods.


Diplodocus got their feet and legs stuck in the mud and died!


This dinosaur is a new, undescribed sauropod species. The specimen represents an adolescent individual that is thought to have been about 10 years old. Sauropod dinosaurs lived to be about 35 years old. The display case contains head and neck fossils. The skeletal drawing highlights bones in yellow on display above the case and some bones in red are in the display case. The remainder of the bones in red are in the museum’s paleo collection.
We can only identify different species of sauropod dinosaurs if we have the right parts of the skeleton (Remember the confusion over Apatosaurus and Brontosaurus). For most dinosaurs, the skulls are the easiest parts to identify. However, sauropod skulls look very similar. We generally use vertebrae to identify different species of sauropods. Research continues to determine if it is a new sauropod species (DBS, p. 89).
The specimen was discovered in 1989 in Park County, Montana, near Livingston. The skeleton was excavated by sculptor Matt Smith, and a group of volunteers from the MOR.
Case Contents: Sauropod tooth embedded in rock; caudal vertebra (from the

tail); dorsal vertebra (from the middle of the back)

Allosaurus was one of the largest meat eaters living during the Late Jurassic time in North America and was probably the primary predator of sauropods.

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