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.

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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.

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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.

Theropods

Characterized by their strong jaws, sharp talons, & saurischian hip structure, theropods occupied predatorial niches in Hell Creek. Tyrannosaurus rex, perhaps the most infamous dinosaur of all time, was the apex predator of Maastrichtian North America.

Other theropods, such as pack-hunting dromaeosaurids like Dakotaraptor sp. & and the colloquially dubbed "Hell Chicken", Anzu wyliei, occupied other ecological niches in the Hell Creek food web.

Image Courtesy of Prehistoric Planet

Ceratopsians

A group of herbivorous dinosaurs infamous for their large frills and face horns. Fossil evidence suggests that ceratopsians were social herbivores that gathered in hierarchal herds for safety. Fossilized ceratopsian frills show signs of healed bite marks & puncture wounds, suggesting that these dinosaurs used their massive horns and frills to ward off predators like Tyrannosaurus rex & compete amongst themselves for dominance or mating rights.

Once thought to be the same dinosaur at different life stages, Torosaurus latus & Triceratops sp. were the two dominant ceratopsian genera in Hell Creek.

Image Courtesy of Science Photo Library

Ornithopods

Ornithopods are a diverse group of herbivorous dinosaurs that occupied various prey niches in Hell Creek. Their specialized flat grinding teeth and broad Jaws suggests that these dinosaurs were grazers.

Some genera were thought to exist in eusocial groups, with fossil evidence of trackways, eggs, & grouped nests suggesting that these dinosaurs were dedicated parents. Hadrosaurs, also known as duck-billed dinosaurs, are perhaps the most recognizable of the ornithopods.

The hadrosaur, Edmontosaurus annectens, was the largest dinosaur of the Hell Creek formation, with the largest specimens discovered measuring over 40ft long.

Image Courtesy of Dinopedia

Ankylosaurs

Covered in an armored suit bony osteoderms, these low to the ground, herbivorous dinosaurs relied on their impressive defensive capabilities to avoid predation.

The largest of the ankylosaurs, Ankylosaurus magniventris, represents the pinnacle of ankylosaurid evolution. Its massive club tail served as a deadly defense against apex predators such as the T. rex, possessing the ability to shatter bones though sheer blunt trauma.

Additional genera of ankylosaurids existed in Hell Creek, inhabiting the floodplains of the region, grazing on ground plants such as ferns and cycads.

Image Courtesy of Prehistoric Planet

Basal Mammals

The mammals of the Hell Creek formation were small, rodent-like, burrowing omnivores that co-existed with the dinosaurs during the late Cretaceous.

Due to their small size, subterranean dwellings, and omnivorous diet, basal mammals such as Alphadon sp. were able to survive in the post-apocalyptic environment following the asteroid impact.

The ecological power vacuum left behind from the death of the dinosaurs provided opportunity for mammals to eventually leave their underground dwellings without fear of predation. These early mammals would go on to eventually become the dominant terrestrial life forms on Earth.

Image Courtesy of Dinopedia

Early Birds

Contrary to popular belief, chickens are not the direct descendants of the mighty Tyrannosaurus rex. Though birds are considered avian-theropod dinosaurs, their co-existence with non-avian dinosaurs goes much farther back than the age of the T. rex.

The appearance of 'modern' birds in the fossil record goes back as far as the Early Cretaceous (95 MA), but fossil evidence of the ancestors of modern birds such as the Archaeopteryx have been found as far back as the Jurassic (150 MA).

Though fossil remains of birds in Hell Creek are few, significant findings have nevertheless been uncovered. Fossil specimens of early birds such as Styginetta lofgreni have been found in geologic strata both before and after the asteroid impact, meaning that it was one of the few to survive the K-Pg extinction event.

Image Courtesy of Prehistoric Planet

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.

Angiosperms

Angiosperms (flowering plants) make up the majority of the flora fossil record (90%) of the Hell Creek Formation. Common angiosperms include ground flowers, grasses, and flowering trees.

Fossilized magnolias have been found at Hell Creek, as they are some of the oldest known flowers. (Magnoliids actually appeared around 30 million years earlier than Hell Creek, a testament to their survival). Additionally, aquatic angiosperms such as water lilies have also been found at Hell Creek.

The presence of flowers allows paleontologists to infer the environment and climate of the era. In the case for Hell Creek, the widespread existence of Magnoliids is evidence of warm and humid climate (Magnoliids in particular require a warm & humid climate to flourish today, so it can be inferred that the principle applies to the Cretaceous as well).

Additionally, due to the fact that flowers require pollinating insects such as beetles to reproduce, the evidence for Hell Creek's warm & humid climate is further supported.

Furthermore, the presence of aquatic plants would suggest the existence of floodplains and freshwater aquatic environments, thus contributing to painting a picture of the paleoenvironment of Hell Creek.

Image Courtesy of Wikipedia

Ferns

Ferns were a dominant form of vegetation for the majority of the Paleozoic & Mesozoic. Though they have given way to angiosperms in the Hell Creek Formation, ferns, both ground sprouting and tree ferns, were a core dietary sustenance for a wide variety of herbivorous dinosaurs.

The earliest ferns appeared in the fossil record far before the dinosaurs themselves in the Devonian period (360 MA). Like bryophytes and lycophytes, ferns predate seeds, instead electing to reproduce via spores.

Image Courtesy of Wikipedia

Fruits

The presence of fossilized fruits in Hell Creek gives researchers a glimpse into the evolution of flowering plants. Fruiting bodies of serve as seed carriers for plants, allowing for dispersal following consumption.

The fossil fruits of hell creek are crucial to understanding the evolution of plants alongside animals in the Mesozoic. The evolution of fruiting bodies is direct evidence of interaction with herbivorous dinosaurs, and it shows how plants developed a mutualistic relationship with animals dating back tens of millions of years.

Image Courtesy of Wikipedia

Trees

Tress existed in multiple forms in Hell Creek. Fossil evidence of conifers, palms, and angiosperms have all been found in Hell Creek.

Wood fossilizes relatively nicely compared to animal remains, thus, specimens are often well preserved and can be studied in great depth. The pervasiveness of petrified wood in Hell Creek suggests that at least parts of it were densely forested.

Fossilized charcoal has also been found at Hell Creek, thus indicating the presence of wildfires, which shaped the landscape.

Image Courtesy of Wikipedia

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).

1

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

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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

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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?

References:

Scholz, H., & Hartman, J. H. (2007). 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. PALAIOS, 22(1), 24–34.  http://www.jstor.org/stable/27670392  

Fastovsky, D. E. (1987). Paleoenvironments of Vertebrate-Bearing Strata during the Cretaceous-Paleogene Transition, Eastern Montana and Western North Dakota. PALAIOS, 2(3), 282–295.  https://doi.org/10.2307/3514678  

Rigby, J. K., Newman, K. R., Jan Smit Sander Van Der Kaars, & Sloan, R. E. (1987). Dinosaurs from the Paleocene Part of the Hell Creek Formation, McCone County, Montana. PALAIOS, 2(3), 296–302.  https://doi.org/10.2307/3514679  

Fricke, H. C., & Pearson, D. A. (2008). Stable Isotope Evidence for Changes in Dietary Niche Partitioning among Hadrosaurian and Ceratopsian Dinosaurs of the Hell Creek Formation, North Dakota. Paleobiology, 34(4), 534–552. http://www.jstor.org/stable/20445613 

Renne, P. R., Deino, A. L., Hilgen, F. J., Kuiper, K. F., Mark, D. F., Mitchell, W. S., Morgan, L. E., Mundil, R., & Smit, J. (2013). Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary. Science, 339(6120), 684–687.  http://www.jstor.org/stable/23365920  

Pälike, H. (2013). Impact and Extinction. Science, 339(6120), 655–656.  http://www.jstor.org/stable/23365909  

Lovegrove, B. G., Lobban, K. D., & Levesque, D. L. (2014). Mammal survival at the Cretaceous–Palaeogene boundary: metabolic homeostasis in prolonged tropical hibernation in tenrecs. Proceedings: Biological Sciences, 281(1796), 1–7. http://www.jstor.org/stable/43602294 

Stidham, Thomas Allen (2001). The origin and ecological diversification of modern birds: Evidence from the extinct wading ducks, Presbyornithidae (Neornithes: Anseriformes) (Thesis). OCLC 892837810. ProQuest 304684225.

Berry, E. W. (1924). Fossil fruits and seeds of the Cretaceous and Tertiary. American Journal of Science, 7(41), 201-212.

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BBC Studios. (2022). Prehistoric planet [Documentary series]. Apple TV+.

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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, ranging in sizes from the chicken-sized Trierarchuncus to the 43ft long Edmontosaurus. Image courtesy of Wikipedia

The infamous T. rex scene from Jurassic Park