A Glimpse into Hell
How the Fossil Record of the Hell Creek Formation Tells the Story of the Final Days of the Dinosaurs
Welcome to Hell Creek
The Hell Creek Formation offers a remarkable glimpse into the final chapter of the dinosaurs. Located across Montana, North Dakota, South Dakota, and Wyoming, this fossil-rich deposit from the Late Cretaceous period puts the most infamous dinosaurs on full display. Witness how beasts like the Tyrannosaurus rex, Triceratops, and others navigated and thrived in the perils of ancient Earth. As you know, the age of the dinosaurs would come with a resounding bang. The Hell Creek Formation captures a glimpse of life shortly before the catastrophic asteroid impact that led to the dinosaurs' extinction, marking a pivotal moment in Earth's history and paving the way for the age of mammals.
The Lower Hell Creek Formation is outlined in yellow. The Middle and Upper Hell Creek Formation is shown in orange. Though Hell Creek extends across multiple states, the epicenter for paleontological discovery is focused in the areas depicted above.
The Geology of Hell Creek & Paleoecology of Maastrichtian North America
The supercontinent, Pangaea, is often associated with the age of the dinosaurs. Though the earliest dinosaurs did live on the supercontinent before its split up approximately 200 MA, the dinosaurs of Hell Creek lived on a prehistoric North America somewhat similar to our own. However, North America was at least partially split by a large inland sea known as the Western Interior Seaway.
Molleweide Projection of the Earth during the Cretaceous Period vs Today. In the Upper Cretaceous Period, the continents have already begun to become recognizable. Maps Courtesy of Geology Page
The dinosaurs of Hell Creek lived within the Upper (late) Cretaceous period, more specifically the Maastrichtian Age (72 MA - 66 MA). The Maastrichtian is associated with the pinnacle of dinosaur evolution, with the warm and humid global climate at the time enabling lush plant growth on every continent. Fossil tree evidence suggests that Maastrichtian Hell Creek were densely populated with forests, creating opportunities for life to flourish in the canopies and undergrowth. Furthermore, the discovery of both fossilized aquatic plants and animals suggests that Hell Creek's ecosystem was one that was rife with coastal regions, lakes, & rivers.
Stratigraphy, the way layers of rock are organized, suggests that floodplains were common in the area as well. The stratigraphic evidence shows that layers of deposition were distinct and varied, including depositional layers of silt, mudstone, sandstone, and shale. The presence of floodplains in the area could suggest why so many well-preserved fossils are discovered in Hell Creek. Generally, the process of fossilization occurs to very few organisms, as many barriers precede the process of fossilization.
How to Become a Fossil
The process of fossilization is an incredibly rare natural occurrence. It is believed that less than 1% of all organisms to have ever existed have been preserved as fossils.
There are many types of fossils: including molds, casts, trace fossils, etc. However, body fossils, wherein the actual remains of the organism are preserved, are what we will be looking at today.
Free Stock Photo
Step 1: Die
The organism dies in such a way that it avoids being scavenged or completely broken down by decomposers.
Image Courtesy of Jurassic Park
Step 2: Burial
The organism is buried by some sort of geologic substrate. The rapid covering of the organism by sediment prevents total decomposition from detritivores or microorganisms.
Rapid burial is often observed in the case of floods, thus suggesting that the presence of flood plains in Hell Creek is a major contributor to the formation's impressive fossil record.
Image Courtesy of Research Gate
Step 3: Preservation & Mineralization
Soft tissue will often quickly decay regardless of the depositional environment (with some extremely rare exceptions). However, the hard parts of the organism (teeth, shells, bark, or bones) may remain.
Mineralization is the process in which the organic material of the fossil is replaced by minerals. Mineralization may occur in various forms, but the bottom line is that eventually, none of the original organic material will remain.
Image Courtesy of Fossilera
Step 4: Compaction
Millions of years of sedimentary layer buildup puts immense pressure on the preserved organism, causing the substrate around it to compact into solid rock.
Sedimentary columns found within the Hell Creek Formation are approximately 200-300ft thick. Therefore, many fossil specimens in the area have yet to be discovered.
Image Courtesy of World Atlas
Step 5: Exposure
Through processes of erosion, plate tectonics, or human excavation, the fossil is exposed. Prolonged exposure could result in the destruction of the fossil from the elements. Therefore, when a significant fossil is discovered, paleontologists go to great lengths to protect and excavate it.
Image Courtesy of CBC News
Paleobiology of the Hell Creek Formation
Though Hell Creek is most widely known for its impressive repertoire of culturally significant and infamous dinosaurs, it's important to realize that they were only one part of a thriving ecosystem.
The Dinosaurs of Hell Creek, ranging in sizes from the chicken-sized Trierarchuncus to the 43ft long Edmontosaurus. Image courtesy of Wikipedia
The Fauna of Hell Creek
Dinosaurs were the dominant life forms of Hell Creek. They existed in two primary groups: Saurischians ("lizard-hipped" dinosaurs) & Ornithischians ("bird-hipped" dinosaurs).
Saurischian dinosaurs were primarily carnivorous bipeds, though some exceptions to this rule exist outside of Hell Creek (the long-necked Sauropods, though herbivorous and quadrupedal, are saurischians, as well as the bipedal herbivores such as Deinocheirids.) Ironically, birds are considered theropods, and thus technically saurischian-avian dinosaurs.
Ornithischians were typically herbivorous or possibly omnivorous, with both bipedal (Ornithomimids & Pachycephalosaurs) and quadrupedal (Ceratopsians & Ankylosaurs) specimens being discovered in Hell Creek.
Alongside the non-avian dinosaurs, Hell Creek was home to a plethora of animals, including insects, lizards, turtles, crocodilians, pterosaurs, amphibians, fish, mollusks, birds, and mammals. All organisms in the Hell Creek played significant roles in the ecosystem. Whether as predators, prey, parasites, or decomposers, each creature dutifully performed its role in upholding intertwined food webs and environmental sustainability.
The Flora of Hell Creek
Though often overlooked when it comes to paleontology in popular culture, plant fossils are just as important to understanding the earth's paleoenvironment as any dinosaur. The Hell Creek formation was home to a diverse array of plants, including mosses, ferns, cycads, conifers, palms, ginkgoes, and angiosperms. Plants are an excellent indicator of paleoenvironment, based upon the fact that various plants are often associated with specific climactic conditions. Furthermore, plants remain largely unchanged from their prehistoric counterparts, thus allowing researchers to more easily understand prehistoric plants though study of their living counterparts.
The K-Pg Boundary Event
66 million years ago, a massive asteroid impacted Earth in an event that would signal the end of the Mesozoic Era. With an estimated 75% of all life on Earth being wiped out, the K-Pg Boundary Event was the 5th and most recent mass extinction. Though the Chicxulub Impactor was the crowning event of the K-Pg Boundary, additional stressors such as the Deccan Traps & plummeting sea levels likely contributed to ending the reign of the dinosaurs (Renne, 2013).
Deccan Traps
Located in present-day India, the Deccan Traps were regions of significant chronic volcanic activity that is hypothesized to be one of the contributing factors to decline of life in the Cretaceous.
The Deccan Traps created a hostile hellscape of basalt spanning around 500,000 squared kilometers over the course of hundreds of thousands to millions of years. Constant eruptions facilitated the vast expulsion of harmful atmospheric gasses such as sulfur dioxide and carbon dioxide.
Sulfur dioxide, a major contributor to acid rain, may have had vast impacts on global plant life, causing food chain collapse. Additionally, the presence of the compound in the atmosphere in aerosol form (combined with water molecules) could have acted like a reflective blanket, deflecting sunlight away from the surface of the earth and contributing to short-term global cooling.
Carbon dioxide was likely a major contributor to long-term global warming. Similarly to what is observed today, the presence of atmospheric carbon contributes to global warming via the Greenhouse Effect. Additionally, the release of carbon from the earth and into the oceans contributes to ocean acidification, proving detrimental to the planktonic marine organisms that upheld the marine food web.
It is thought that these cycles of short-term cooling with an upward trend in global warming contributed to environmental factors that put extreme stress on life on a global scale. Though the Chicxulub Impactor was the final nail in the coffin for the dinosaurs, the Deccan Traps were nevertheless a likely culprit to their decline.
Image Courtesy of HAWAII Magazine
Falling Sea Levels
The Western Interior Seaway split modern-day North America in two during the Cretaceous period. Depicted on the right is how the Western Interior Seaway would have looked in the Middle Cretaceous, around 100 million years ago.
By the time of the K-Pg boundary event (66 million years ago), the seaway had pretty much dried up. This reduction of sea levels is largely associated with geological and climactic events.
The primary cause of the Western Interior Seaway's regression is attributed to the Laramide Orogeny, wherein tectonic activity thrusted Western North America upwards, reducing the continent's capacity to retain an inland sea. This shifting of tectonic boundaries in combination with cyclic global cooling and the reconstitution of polar ice caps caused the gradual reduction of the seaway, until it was almost dried up by the end of the Cretaceous.
Such an event would have lasting implications for both marine and terrestrial life.
The drying of the seaway resulted in the habitat reduction and displacement of prehistoric marine fauna such as marine reptiles, fish, & ammonites. The absence of such creatures meant that terrestrial animals that relied on fish as a food source (mainly pterosaurs & some dinosaurs) also went on the decline in certain areas.
The decline of the seaway also resulted in significantly increased temperatures & decreased humidity in inland North America, as large bodies of water often serve as thermal regulators and contributors to water cycles. Such a drastic change in paleoclimate resulted in significant changes of terrestrial food webs, as once-costal ecosystems gave way to plains and grasslands.
Though life had certainly adapted well to the change in climate brought forth by the reduction of the seaway (the paleoenvironment of Hell Creek took place after significant seaway reduction), drastic changes such as these had lasting impacts on prehistoric life.
Image Courtesy of Alchetron
Chicxulub Impactor
Measuring an estimated 6 miles in diameter, the Chicxulub Impactor, upon collision with the Yucatan peninsula, released a shockwave billions of times more powerful than that of the atomic bomb. The impact likely caused global earthquakes, volcanic eruptions, & tsunamis. (Pälike, 2013).
Ejecta catapulted into the atmosphere by the impactor resulted in mass global cooling and blockage of sunlight, causing significant changes in atmospheric and ocean chemistry. Whatever life that wasn't obliterated by the initial impact or resulting fallout was slowly withered away by mass ecosystem collapse. Without sunlight, plants were unable to photosynthesize, causing the degradation of terrestrial food webs. Ocean acidification dissolved the shells of zooplankton & phytoplankton, destroying the ocean ecosystem as well.
The Chicxulub Impactor officially brought forth the end of the Mesozoic era. Though life in the Late Cretaceous was under significant stress due to several volcanic, climactic, and atmospheric factors, if the asteroid were to miss earth, who is to say that the non-avian dinosaurs wouldn't have evolved and pulled through?
The entirety of life on earth as we know it is the direct result of the miraculous and catastrophic turn of events that unfolded eons ago.
Image Courtesy of Fine Art America
Scientific Applications
The discoveries made in Hell Creek greatly advanced our understanding of prehistoric ecosystems in the Maastrichtian age. The rich fossil record of the formation has allowed paleontologists to reconstruct a thriving ancient ecosystem.
Scientists have compared fossils from Hell Creek in with modern examples of similar creatures to better reconstruct the paleoenvironment. In Paleoenvironmental Reconstruction of the Upper Cretaceous Hell Creek Formation of the Williston Basin, Montana, USA: Implications from the Quantitative Analysis of Unionoid Bivalve Taxonomic Diversity and Morphologic Disparity by Schols et al., researchers used modern ecosystems of freshwater bivalves (clams) to infer living conditions of prehistoric clams found in Hell Creek:
"The quantitative evaluation of the taxonomic composition and the relative abundance of individual species in different samples (Dodd and Stanton, 1990) were also used to meet the palaeoecological objectives of his study. Warren (1991, 1992) demonstrated the importance of such analysis of freshwater bivalve communities for the reconstruction of modern environments. Similarly, species presence-absence and abundance data from numerous samples are analyzed for paleoenvironmental reconstruction" (Schols, 2007).
This study is one of many that incorporates specimens from the Hell Creek Formation in reconstructing paleoenvironments.
Cultural Significance
The dinosaurs discovered in Hell Creek are perhaps some of the most infamous in popular culture. T. rex cemented itself as the posterchild of Jurassic Park, and others such as Triceratops & Ankylosaurus are basically tattooed on the brains of young children. The popularity of these dinosaurs in the common eye facilitates discussion and appreciation for all of paleontology. The bottom line is that gigantic, scaly (or sometimes feathered), prehistoric creatures get children excited to learn about science & the world around them. Furthermore, this excitement and passion for dinosaurs assists in furthering the largely underfunded field of paleontological research.
The infamous T. rex scene from Jurassic Park
Unfortunately, the answers to so many questions about the earth's history remain locked behind financial barriers. Funding opportunities such as research grants are often allocated to other more lucrative sciences. Without the people's love for dinosaurs, paleontology would have no ground to stand upon.
Takeaways from Hell Creek
The term 'uniformitarianism' was popularized by 19th-century Scottish geologist Charles Lyell. Though the term had been previously coined by scientists William Whewell & James Hutton, Lyell's Principles of Geology greatly expanded upon the general definition of uniformitarianism we still use to this day.
"The present is the key to the past" - Charles Lyell
In the most basic terms, uniformitarianism states that the same laws of nature that governed the past govern the present. For example, a volcanic eruption that took place 70 million years ago is just as deadly and catastrophic as an eruption of the same magnitude today. Atmospheric carbon dioxide released by the Deccan Traps 66 million years ago produces the same greenhouse effect as atmospheric carbon dioxide released by the burning of fossil fuels has in 2024. Lyell further elaborates on uniformitarianism, and how it continues to shape our world:
"From the earliest period at which plants and animals can be proved to have existed, there has been a continual change going on in the position of land and sea, accompanied by great fluctuations of climate." (Lyell, 1830)
Lyell recognized that life on earth was the product of billions of years of evolution, and that life adapted to the changes the earth threw at it. Understanding uniformitarianism is key to understanding the world we live in today, and the world we will live in tomorrow.
Discoveries made in Hell Creek provide a glimpse into what life was like in an era too long ago to physically comprehend. Humanity exists within such a small timescale, that without paleontological discovery, our perspective becomes human-centric & short-sighted. Perhaps the most important takeaway from Hell Creek is that dinosaurs were not just bumbling reptiles, but rather complex and intelligent organisms that navigated and shaped their environments much like modern animals. Their displays of predation & eusociality are definitive proof of the existence of ancient thriving ecosystems & complex intelligent behavior. Analyzing ancient earth through this lens provides scientists with the tools needed to better understand our ecosystems today.
The K-Pg Boundary Event signaled a pivotal moment in the history of our planet, and eventually paved the way for mankind. The end of the dinosaurs should be a pervasive reminder of our own vulnerability. Life on Earth exists in a complex but delicate balance, and failure to understand and help preserve the world we live in will bring forth the 6th mass extinction. It is important to remember that we were not the first life forms to dominate the Earth, but will we be the last?