Scientists Come to Study
In 1998, paleontologists and geologists from around the country descended on the Tracksite to study this intriguing site. The scientists from the University of Wyoming, Dartmouth College, Department of Geological Sciences - Indiana University, Kansas State University, BLM National Science & Technology Center, South Dakota School of Mines and Technology and the Smithsonian Institution formed the Red Gulch Dinosaur Tracksite Science Team. These scientists are working at the Tracksite under a BLM Paleontology permit.
They are mapping, measuring and comparing the rocks and fossils at this site with other previously studied tracksites. Working as a group, the team is breaking new ground in the study of the Middle Jurassic in central North America.
What We Know
Scientists have been working for the past four years, trying to unlock the Red Gulch Dinosaur Tracksite (RGDT) puzzle. They had many questions and now have some of the answers.
What dinosaur made the tracks? All the tracks identified so far were formed by two-legged (bipedal) dinosaurs. Some, and perhaps all, of the tracks appear to have been made by meat-eating dinosaurs (theropods). They weighed between 15-400 pounds. Because Middle Jurassic dinosaurs are so rare, it is very difficult to match the tracks to any particular dinosaur.
Typically, a well-preserved theropod dinosaur track is three-toed and nearly symmetrical, exhibits tapering toes and preserves a slightly "S" shaped impression of the middle toe. Identifiable theropod trackways preserve prints that are slightly "pigeon-toed" having an inward rotation of the feet.
However, many other tracks and trackways do not exhibit such features. Although clearly made by two-legged dinosaurs, these less well-defined prints may have been made by a different type of dinosaur such as a plant-eating ornithopod. In many cases, it is impossible to identify the trackmaker as to ornithopod or theropod.
How many tracks are there? 1,000 tracks have been located, 600 of them are in the Ballroom. Nearly 600 dinosaur tracks have been located by surveying instruments. A network of one-meter-square grids has been surveyed in the "Ballroom and Discovery" area. Scientists estimate there are at least 1000 tracks in the "Ballroom" area of the Tracksite.
How big are the tracks? The tracks are 8-28 cm. long, have three distinct toes, and may also show the heel and claws.
Where were the dinosaurs going? Most of the hundreds of identifiable trackways go in the same south-southwesterly direction. The orientations of most of the tracks indicate that the dinosaurs were moving to the south-southwest.
This could indicate herding or migratory animal behavior. Or it could indicate the presence of a physically constrained pathway (such as a tidal flat or beach next to an open body of water).
A logical interpretation would be that the dinosaurs may have been moving parallel to the shoreline. However, the shapes of the ripples associated with the RGDT surface shows this was not the case. The presence and orientation of the ripples indicate that relatively deeper water conditions existed to the southwest. Since the dinosaurs were moving in a southwesterly direction, it appears the dinosaurs were moving perpendicular to the shoreline and not parallel. The ripples reveal the dinosaurs may have been moving toward the water.
|Cross sectional view of a ripple showing an asymmetric shape. The steep side to the right of the ripple crest is the down-current side. Flow was therefore left to right. Figure modified from Reineck and Singh, 1980.|
What do the ripples tell us? The Tracksite exhibits a well-developed rippled surface. Ripples can be used to determine the direction of current movements. The coated-grain limestone consists of tiny sand-sized spheres of calcium carbonate and fossil shell fragments cemented together. The ripple surface formed before the grains were cemented together. The ripples are very similar in shape to those formed by relatively gentle waves in very shallow water. The dinosaur trackways appear to have formed shortly after the formation of the ripples.
Some tracks have been cross-sectioned or "sliced" to find out what the rock looks like within the footprint. The geologists are studying how the soft, limy mud was deformed as the weight of the dinosaur pushed it down.
What was the geography and climate like? During the Red Gulch Dinosaur Tracksite time, large portions of the western interior of North America were inundated by a shallow sea. To the west, a volcanic arch extended north from Mexico to southwestern Canada. To the east, the sea was bounded by very shallow water, low-lying coastline conditions that extended from central Wyoming into the present day Dakotas.
The climate during this time was extremely arid, at least seasonally.
The geology team has traveled to Florida, Texas, New England and elsewhere to visit other ancient environments. The ichnology team has visited sites throughout Colorado and Utah. Track expert Beth Southwell, UW, has been to Dinosaur State Park in Connecticut and even to China!
How old is the Tracksite? The Red Gulch Dinosaur Tracksite surface dates to approximately 167 million years old. This puts it in the mid-Bathonian Stage.
How do we know this? Scientists used several factors to determine the date:
The occurrence of the oyster Gryphea nebrascensis, just above the track bed;
The occurrence of a complete specimen of the ammonite Cadoceras muelleri just above the track layer;
Two microfossils of marine planktonic protozoans called "Dinoflagellates."
The ages of these fossils have been well established in many places around the world. By finding these fossils here in the RGDT area, scientists can deduce the age of the surrounding area including the Tracksite.
The Tracksite preserves an ancient tidal environment. Scientists have traveled around the world to get clues to the Tracksite puzzle. Even though the daily rise and fall of the tide is small, because the tidal flat slopes at a very low angle towards the sea, a very broad expanse (several kilometer wide) is exposed at low tide. This is similar to the type of tidal flat preserved in the Tracksite area.
Why were the footprints preserved? Team members are studying modern environments where footprints may be preserved and trying to find out why such delicate structures were not immediately washed away but instead preserved and turned to rock.
What is a trace fossil? A fossil is any physical evidence of ancient life. A body fossil would include bone material or shell. A trace fossil, however, is evidence of the activity of ancient animals or plants. The physical evidence in the rock record of burrowing, crawling, walking, etc., constitutes a trace fossil. Therefore, the footprints at the Tracksite are vertebrate trace fossils.
What other kinds of trace fossils were found at the Tracksite? The older trace fossils on the Tracksite are classified as belonging to a Skolithos burrow type. The primary example are round, vertical tubes approximately 0.5 cm in diameter, similar to those made by modern annelid worms. These were probably made at the same time the dinosaurs were making their tracks.
The second, younger generation of invertebrate faunal traces is also present. These were made after the dinosaur tracks had been buried by sediment. The younger traces consist of U-shaped burrows ranging from 5 cm to greater than 12 cm in width and 2 cm to 3 cm in diameter. These may have been made infaunal crustaceans, bivalves such as clams or lugworms.
What are ABs? Shallow, irregularly shaped depressions are also present on the Tracksite. They have been nicknamed "AB's" for "amorphous blob."
The Tracksite AB's appear to have formed through a combination of both biological and physical erosional processes. The depressions are very similar to features observed on modern intertidal flats.
What else has been found? Samples of pollens, collected by scientists during the summer of 1998 have been processed in the laboratory. So far, it appears that there are pollens of cycads and several different conifers, fern spores and a variety of one-celled organisms in the layers directly overlying the Tracksite. The pollen is the only remains we have of these plants and could have been blown or washed in from vegetated areas tens to hundred of miles away.
PowerPoint Presentation from NARSC