MIDDLE CRETACEOUS: During the Middle Cretaceous, sea levels rose and inundated most of Montana. About 97 mya, the Intercontinental Cretaceous Seaway flooded the interior of the continent from the Gulf of Mexico to the Arctic Ocean and bisected North America into western and eastern land masses. The Rocky Mountains were just beginning to form. Oceanic sediments deposited in the seaway included numerous marine formations. Clams, cephalopods and reptilian plesiosaurs were the dominant lifeforms of the seaway. Dinosaurs were confined to areas within the mountain ranges or along narrow strips of coastal plains. Dinosaur fossil remains are rare and until recently had not been found in Montana.
Map Panel: The World and North America during the Mid-Cretaceous,
92 million years ago.
Panel: THE MID CRETACEOUS, 100 – 95 MILLION YEARS AGO
Elevated atmospheric concentrations of carbon dioxide, globally warm temperatures and high sea levels marked the mid Cretaceous, 100 to 95 million years ago. In Montana and Idaho, the rising seas left only a very narrow coastal plain along the eastern flank of a volcanically active, north-south trending mountain range. Remnants of this coastal plain are preserved today in the Wayan Formation in eastern Idaho and the Blackleaf Formation in southwestern Montana. These geologic units preserve a unique dinosaur assemblage that appears dominated by a small burrowing dinosaur, Oryctodromeus, and eggs of an unknown gigantic oviraptor.
BURROWING DINOSAURS EXHIBIT:
DIGGING DEEPER INTO DINOSAUR BEHAVIOR FIRST-EVER MID-CRETACEOUS DINOSAUR FOSSILS FOUND IN MONTANA AND THEY MADE BURROWS!!
Oryctodromeus cubicularis – “Digging Runner of the Den”
An exhibit featuring the research of MSU paleontologists in southwestern Montana
and eastern Idaho. Sculptures by Matt Smith and Tammy Payne Smith.
Display Overview: Model of adult and two juveniles in burrow. An adult Oryctodromeus cubicularis on top of display is a replica that shows a skeleton on one half and a flesh-covered version on the other side. A small burrowing mammal is located to the left of the burrow display to represent other burrowing animals that could have lived in the vicinity of the Oryctodromeus burrow. It is also a good reminder that other animals lived at the same time as dinosaurs. (You will need a flashlight to illuminate the small mammal so visitors can see it)
Panel: THE BURROWING DINOSAUR
In 2005, Dr Yoshihiro Katsura made an unusual discovery in the 95 million year old rocks of the Blackleaf Formation in southwestern Montana. Note: The site is located about 15 miles from Lima, Montana, in Beaverhead County on US Forest Service land. At the time of discovery, Katsura was a MSU graduate student on a paleontology field crew led by David Varricchio.
Sediment filling an ancient burrow preserved the remains of an adult and two juveniles of a new small ornithopod dinosaur named Oryctodromeus cubicularis (“digging runner of the den”). It was a small (about 70 pounds) bipedal ornithopod dinosaur with a length of about 10 feet, with much of that consisting of a long tail. The long legs of this herbivore indicate it could run fast. The occurrence of an adult and two juveniles within a den-like burrow suggests Oryctodromeus had extensive parental care of young.
The Oryctodromeus bones occurred jumbled and elevated within the sediment filling the burrow. The preserved portion of the S-shaped burrow measured nearly seven feet long with portions of the entrance and the expanded chamber lost to erosion. A one-foot wide tunnel slopes down and makes two sharp turns before entering into a 1.5 foot-wide chamber. Smaller accessory burrows branched off the main tunnel and likely represent cohabitation by insects and mammals.
Modern burrowers such as aardwolf, striped hyena and rabbits dig tunnels that are just big enough to fit through. This helps maintain the distinct environmental conditions of the den and importantly keeps out any bigger, unwanted visitors like a predator. Based on modern burrowers, the dimensions of the Blackleaf burrow fit the body size of Oryctodromeus. Panel Question: Given the disarticulated arrangement of bones, how can we be sure that the burrow really belonged to Oryctodromeus? Experimental work by former MSU undergraduate, Cary Woodruff, used a scale burrow model, a rabbit skeleton, and a sediment-water slurry to simulate the infilling of the burrow chamber. He demonstrated that similar distributions of bones could be produced by flooding skeletal remains sitting within a burrow chamber. Images show the rabbit bones prior to flooding and the bones elevated in sediments after the flooding of the test burrow with sediment slurry. This resulting arrangement of bones is similar to the arrangement observed in the original fill. The final image shows part of the original fill where the Oryctodromeus bones (dark feature) are buried and elevated above the base of the burrow fill.
Panel Images: Dr Katsura and Dr David Varricchio at dig site. Eve Morschhauser at the burrow with a plaster jacket containing the remains of Oryctodromeus. Drawing of S-shaped burrow as it descended through various types of rock in the Blackleaf Formation.
Panel: BUILT TO DIG?
The occurrence of Oryctodromeus remains within a burrow fill suggests that this dinosaur dug its own den. However, the burrow may have belonged to some other animal. The bones of Oryctodromeus could have been dragged in by a predator or scavenger or simply washed in with the sediment that eventually filled the burrow. Experimental work suggests the skeletons were in the burrow prior to being buried. If the burrow really belonged to Oryctodromeus, the skeleton could potentially show similar adaptations like those found in modern scratch-diggers like the armadillo, who use powerful retraction of the forelimb to dig while bracing their body with a solid hip region.
The anatomical features of Oryctodromeus all coincide with muscles used in scratch-digging mammals. Both animals show a down curved expansion at the end of the scapula and a large scapular (acromion) spine. These two areas would have supported enlarged retractors of the forelimbs and stabilizers of the shoulders. Based on comparisons with mammals, it appears that Oryctodromeus is adapted for digging burrows.
Panel Images: MSU graduate student Jamie Fearson tested the digger capabilities of Oryctodromeus by comparing the bones of its shoulder and arm to those of other closely related dinosaurs. She wrote her MSU master’s thesis on the results of this research. The panel also includes a drawing of specific Oryctodromeus arm bones and muscles adapted for digging.
Display Case on Panel: Comparison of left arms of two diggers: The Oryctodromeus cubicularis dinosaur and the modern armadillo mammal (both arms are replicas).
MIDDLE CRETACEOUS PLANTS: New fossil evidence often adds new information which changes scientific hypotheses. Although some fossil evidence of late Jurassic/early Cretaceous flowering plants may have been found elsewhere in the world, no flowering plants were included in the Hall of Giants displays because no fossil evidence of Cretaceous flowering plants had been found in Big Sky sediments. However, new plant fossil remains discovered in the Wayan Formation in eastern Idaho provided the first physical evidence of flowering plants in the region by the Middle Cretaceous.
Panel: Plant fossils from a locality in the Wayan Formation of Bonneville County, Idaho, occur in a two inch-thick zone of gray mudstone directly overlying an ancient river channel. The plants retain their original three dimensions with fern fronds possibly still being in the growth position, suggesting abrupt burial along the margins of a river. Fossils found at this site include the ferns Gleichenia and Anemia, as well as angiosperm leaves that resemble a Magnolia.
Panel Images: Images show the geologic horizon containing plant fossils from a locality in the Wayan Formation of Bonneville County, Idaho. The plant fossil specimens were collected by MSU doctoral student, L. J. Krumenacker.
Display Case on Panel: Contains plant fossil specimens: Conifer, Anemia, angiosperm and Gleichenia.
GIANT MYSTERY EGGS
Macroelongatoolithus Partial Egg Clutch (replica) Panel: Excavations along Jackknife Creek in eastern Idaho conducted in 2007 produced the first examples of giant, elongate dinosaur eggs known as Macroelongatoolithus from North America. Several localities in China and South Korea have produced similar eggs in large circular clutches with eggs arranged in pairs, but only eggshell fragments had been found in North America.
US Forest Service paleontologist, Steve Robison, discovered the egg site. MSU paleontologist, Dr Franke Jackson, excavated the Macroelongatoolithus locality and recovered a pair of nearly 1.5 foot long eggs. MSU graduate student Jade Simon studied these eggs. She showed that the Idaho specimens compared very closely to those from Asia in their overall shape, surface ornamentation of parallel ridges, and microstructure. These eggs like those of the Asian oviraptors have a two-layered structure with a lower mammillary layer (ML) and an upper, thicker continuous layer (CL) as observed in plane and polarized light (panel images provide comparison of observation in different light).
Panel Case: Replica of Macroelongatoolithus eggs and actual fossil eggshell fragments.
These eggs, except for their much larger size, closely match those assigned to oviraptor dinosaurs based on both embryonic remains and adults found in brooding postures. Presence of these eggs in Idaho and Montana suggests that a giant oviraptor, perhaps weighing a ton or more and standing over six feet tall at the hips, inhabited the mid Cretaceous of the region. Interestingly and curiously, the two most common remains from this time period, Macroelongatoolithus eggs and Oryctodromeus skeletons may have originally been placed underground.
Panel Image: The oviraptor that produced the Macroelongatoolithus eggs from Idaho and Montana 100-95 million years ago may have been similar to another oviraptor from Inner Mongolia called, Gigantoraptor. Image artist: Raul Martin.
MARINE LIFE IN MONTANA: THE WESTERN INTERIOR SEAWAY