The Late Cretaceous
The Late Cretaceous Period (~100-66 million years ago) was a time in Earth’s history that was fairly different from today. Although by this point most of the continents are in relatively recognizable positions, many of them are at least partially inundated by shallow seas, as during much of the Late Cretaceous there were no polar ice caps, resulting in much higher sea levels than exist today.
In addition, atmospheric carbon dioxide concentrations were much higher, and the world was far warmer, with subtropical to warm temperate climates extending to high latitudes (with places like Montana having a warm wet climate similar to the Carolinas). This was also a period of very high biodiversity for dinosaurs and other animals. But all of that came to an end about 66 million years ago, when an asteroid crashed into the Yucatán peninsula of Mexico, resulting in a global catastrophe that resulted in the mass extinction of an estimated ~75% of life on Earth, including all of the dinosaurs (with the exception of birds).
The Hell Creek Formation
The Hell Creek Formation is the name of a famous series of rocks found throughout Montana, North Dakota, South Dakota, and Wyoming. The ‘type section’ of the formation, where it gets its name, is around Hell Creek, a tributary of the Missouri River, and is located north of the town of Jordan, Montana.
Today, the landscape where the rocks of the Hell Creek Formation crop out are mostly badlands – highly arid terrain featuring steep valleys and coulees shaped by erosion of the soft sands and clays that make up the majority of the sedimentary rocks in the area. There is relatively minimal soil or dense vegetation, water from rain or runoff tends to drain from the area fairly quickly, and the preferential erosion of some rocks layers over others leads to the formation of striking mesas, hoodoos, and other geological forms.
The Hell Creek Formation is scientifically significant in that it preserves and extremely rich record of the animals, plants, and environments that existed near the very end of the ‘age of dinosaurs’.
Thousands of fossils of dinosaurs and other extinct species of animal and plant have been found in the Hell Creek Formation. This includes carnivorous dinosaurs such as Tyrannosaurus and Acheroraptor, herbivorous dinosaurs like Triceratops and Edmontosaurus, as well as a variety of other animals including frogs, turtles, small mammals, crocodilians, fish, and more.
Vertebrate Microfossil Bonebeds
One of the ways we can understand ancient ecosystems and environments is by studying vertebrate microfossil bonebeds. These sites are thought to mostly form in ancient wetlands, lakes, and ponds. As animals were living in these ecosystems die, their bones and teeth accumulate in local wetlands, ponds, lakes, and rivers. These concentrations eventually become fossilized, giving us a ‘snapshot’ of a relatively small area over a fairly short period of geological time.
One of the ways we can understand ancient ecosystems and environments is by studying vertebrate microfossil bonebeds. These sites are thought to mostly form in ancient wetlands, lakes, and ponds. As animals were living in these ecosystems die, their bones and teeth accumulate in local wetlands, ponds, lakes, and rivers. These concentrations eventually become fossilized, giving us a ‘snapshot’ of a relatively small area over a fairly short period of geological time.
When we find these sites during fieldwork (evidenced by many small bones eroding out on a surface), we can collect them by shoveling sediment from them into dozens and dozens of bags. We bring these bags back to the lab, run them through a series of different sieves to separate the fossils from the sediment, and can then begin working though the concentrated material to identify and count the fossils within.
Microsite Science
Collecting and studying vertebrate microfossil bonebeds (microsites) is a very big job though, and that’s where you come in. We want your help in sifting through the concentrated fossils from these sites and helping us to identify and count what animals lived in each site. By comparing this information for different animal groups within and between sites we can understand the ecology and environment of a particular time and place in the Hell Creek Formation. By expanding that out and comparing across many different sites from a wider period of time or broader area geographically, we can analyze just how the animals, plants, and ecosystems were changing during this important part of Earth’s history.
The figure above shows the kind of analysis that can be performed using data from these sites. By tracking the relative abundances of different species in each site (the colored rows), we can see how the ecosystems are changing over time and in relation to changes in the environment (as recorded in the rocks). In this particular case, looking at sites from Alberta from about 78-75 million years ago, observe how the proportion of dark blue bars (sharks and rays) changes as the environment becomes more terrestrial, with the green bars (amphibians) then becoming a larger part of the ecosystem. This pattern later reverses as sea levels in the region rose again, and the ecosystem become more coastal and marine-influenced. While this gives us an idea of big changes in these ecosystems, we are also interested in smaller scale changes, and changes within particular groups of animals like the dinosaurs. The data you help us collect from the Hell Creek Formation will help us answer those questions, and allow us to track changes in these ecosystems as they approach, and then go through the period of time corresponding to the asteroid impact and end-Cretaceous global mass extinction.