The Hell Creek Formation Fauna

1. #1

   Keys: Triceratops

   Along with Torosaurus and Leptoceratops, Ceratopsians make up more than half of the large dinosaur population of the Hell Creek Formation. Triceratops is represented by two chronospecies, with the older T. horridus giving rise to the younger T. prorsus by the end of the Cretaceous. As such, only T. prorsus is present in the Hell Creek Formation by the time {{char}} and {{user}} arrive.

   T. prorsus is a solitary herbivorous species with a massive, robust skull with two large brow horns and a smaller nose horn. A large bony frill shields it's neck, and when preyed upon serves as a nice chew toy for an adult Tyrannosaur. These features are present since birth, but experience a rapid growth spurt during puberty, indicating their use in sexual selection. Triceratops tend to scare easily, and will defensively display and threaten anything that gets too close.

2. #2

   Keys: Torosaurus

   Along with Triceratops and Leptoceratops, Ceratopsians make up more than half of the large dinosaur population of the Hell Creek Formation. Represented by a single species, T. latus, Torosaurus are rare in the Hell Creek Formation. Their frills are less sturdy than their Triceratops cousins, but their horns are equally as impressive. Rather than sturdy defense, Torosaurus specialize in using their frills for display. These features are both present since birth, but experience a rapid growth spurt during puberty, indicating their use in sexual selection. They tend not to be as skiddish as Triceratops, but are significantly more territorial. They will drive off any competition to the best of their ability, flushing blood to their frill to look as big and intimidating as possible- a threat it will back up with action if not heeded swiftly.
3. #3

   Keys: Leptoceratops

   Along with Triceratops and Torosaurus, Ceratopsians make up more than half of the large dinosaur population of the Hell Creek Formation. Leptoceratops is represented by a single species in the Hell Creek Formation, L. gracilis. A smaller dinosaur, members of L. gracilis tend to live in small family groups for protection. While eating, a matriarch will serve as a lookout, hooting if danger arises. When threatened, the group will form into a chevron and prostrate themselves with their sharp beaks level to the ground and the large bristles along the spine of their tails are held splayed wide and high, waving back and forth. Similar tactics are used in courtship, but facing the opposite direction displaying the cloaca.
4. #4

   Keys: Pachycephalosaurus

   Pachycephalosaurus is represented by a single species in the Hell Creek Formation, P. wyomingensis. A bipedal herbivore that experiences extensive ontogenetic growth, Pachycephalosaurus starts life with exaggerated spiked head ornamentation which gradually reduce until it reaches puberty, after which the dome on its head thickens to withstand ritual combat during mating season. Come mating season, pairs of males will take turns bashing their skulls together violently until one either taps out, is knocked unconscious, or dies. The victor wins mating rites.
5. #5

   Keys: Sphaerotholus

   Standing at about half the height of its better known cousin, Sphaerotholus is pachycephalosaur represented by two species, S. triregnum and S. buchholtzae, with only S. buchholtzae surviving into the latest Cretaceous of the Hell Creek Formation. These herbivorous bipeds spend their time grazing in small herds, competing among themselves for dominance and mating rites. Instead of bashing their skulls together like their larger relatives, these dinosaurs play chicken, with two charging each other and the one to veer off first forgoes the competition.
6. #6

   Keys: Ankylosaurus

   Represented by a single species, A. magniventris is a living tank. Armored head to toe with dense osteoderms with spikes along its shoulders and a stiff baseball bat like tail club as a weapon, Ankylosaurus doesn't have much to fear. These solitary gentle giants are relatively docile, given the only predator in the environment with a chance of harming them is an adult Tyrannosaur. Even then, all they have to do is lay down or shoulder slam their attacker in the shin. As such, most Tyrannosaurs tend to stay away from them. Their tail clubs appear to be excellent defensive weapons at first, and they certainly can be used as such, but they are more specialized for intraspecific combat, where individuals with smack trees and each other in the hips with their clubs. This is combined with their highly specialized calls, reminiscent of simple bird calls albeit significantly lower frequency and much louder.
7. #7

   Keys: Edmontosaurus

   Represented by a single species in the Hell Creek Formation, E. annectens was very common and very large. Rivaling T. rex in size, these massive herding dinosaurs could inhale any plant meterial they could get their beaks onto. Always on the move, herds migrate between the biomes to ensure that there is always fresh food to gorge themselves on. Although they may appear defenseless at first glance, a swift from an animal the size of a bus would shatter bones in just about any would-be assailant or pest.
8. #8

   Keys: Thescelosaurus

   Thescelosaurus is a medium-sized, bipedal ornithopod, and is represented by two species in the Hell Creek Formation. T. garbanii and T. neglectus are both abundant species of dinosaurs in the Hell Creek Formation. Both species burrow burrow along the coast of the Western Interior Seaway, each specializing on different low lying plant life, partitioning themselves into separate niches. Not relevant yet, but an interesting fact nonetheless, their burrowing habits and ability to adapt into a general grazing lifestyle allowed them to be some of the last surviving non-avian dinosaurs in western North America, although they would eventually fall in the global ecological cascade.
9. #9

   Keys: Trierarchunucus

   Trierarchunucus prairiensis is a small alvarezsaurid that lives in the Hell Creek Formation. Using their muscular arms, chest, and single digited hands, they dig into substrates such as rotten logs and loose soils looking for insects. While they may rarely take small mammals and lizards as prey, they prefer to snack on insects. Trierarchunucus are burrowing animals, making shallow burrows to sleep safely at night.
10. #10

    Keys: Tyrannosaurus

    Represented by a single species in the Hell Creek Formation, T. rex, Tyrannosaurus is both the largest carnivore of its ecosystem as well as the largest terrestrial carnivore currently known. Its life stages are marked by dramatic ontogenetic shifts. Juveniles are downy, slender, agile pursuit predators specializing in smaller, quicker prey, while adults bulk up dramatically during puberty into powerful, robust ambush predators capable of taking down massive prey like Edmontosaurus and Triceratops. Though adults may scavenge opportunistically, their ecology primarily revolves around active predation and territorial dominance. Supporting this, they have some of the best senses of smell and eyesight of any animal ever as well as thick dampening pads on their feet to quietly stalk their massive prey. Juveniles and sub-adults occupy separate niches, helping to reduce competition within their own species and ensure ecological balance, and sub-adults compete directly with other mid-sized predators like Dakotaraptor and Nanotyrannus for the mass they need to bulk during this integral part of their lives, often bullying them away from their meals.

    The lips of Tyrannosaurus are extremely sensitive. Intraspecies face biting can range from competition displays to affectionate nips between mating pairs and their chicks. More than a single mating pair living in an area is rare, as the local ecosystems would struggle to provide enough food for the huge creatures while maintaining balance. Seasonally, mature members will gather for a lek, where males will size up against each other and engage in brief combat if absolutely necessary before leaving with a mate if they are successful.

11. #11

    Keys: Nanotyrannus

    Represented by two closely related species in the Hell Creek Formation, N. lancensis and N. lethaeus, Nanotyrannus are significantly smaller than their larger cousins T. rex. With their more slender build, Nanotyrannus are built for pursuit rather than ambush. Fulfilling a medium sized therapod niche alongside D. steini, teenage and subadult Edmontosaurus and Triceratops are certainly on the menu. These solitary predators will track prey by smell for miles, but are timid when an adult T. rex is around. They may be close to the top of the food chain, but they know they aren't the kings.

    Nanotyrannus are fuzzy dinosaurs with dark brown horny ridges on the crests of their eyes. They stand about 6ft tall at the hip and are coated with off-white and beige feathers with brown splotches to break up their form in the backdrop of their environment.

    Despite being a fraction of the size compared to their larger cousins, the arms of Nanotyrannus are slightly larger than those of Tyrannosaurus. Their longer arms along with a few other traits may point to Nanotyrannus being a more basal tyrannosaur plotting closer to the dryptosaurs on the eastern side of the Western Interior Seaway. This serves as significant evidence of potential faunal interchange across North America prior to the K-Pg.

12. #12

    Keys: Struthiomimus

    Represented by a single species in the Hell Creek Formation, S. sedens, Struthiomimus is a swift, ostrich-like ornithomimid abundant in the Hell Creek Formation. It occupies the ecological niche of a generalized omnivore, opportunistically feeding on plants, small animals, insects, and eggs. Ontogenetic change in Struthiomimus primarily involves increased speed and endurance with age, with juveniles more restricted to dense undergrowth and adults capable of swift escape into open habitats. Its long, slender legs, lightly feathered body, and sharp eyesight make it one of the fastest animals in the Hell Creek Formation, allowing it to evade predators through rapid bursts of speed.
13. #13

    Keys: Ornithomimus

    Represented by a single species in the Hell Creek Formation, O. velox, Ornithomimus is another common ostrich-like ornithomimid found throughout the region. Ecologically similar to Struthiomimus, Ornithomimus has a narrower feeding niche, leaning more heavily toward herbivory, though it will occasionally take small animals and insects opportunistically. Juveniles are lightly built and rely on concealment within dense vegetation, while adults are strong, fast runners capable of sustained sprinting to evade predators. Their lightly feathered bodies, alert eyes, and slender build make Ornithomimus an exceptionally wary animal, quick to react to potential threats.
14. #14

    Keys: Anzu

    Represented by a single species, A. wyliei, Anzu is an unusual caenagnathid oviraptorosaur with a distinctly bird-like appearance. Moderately feathered with robust limbs and a toothless beak, Anzu specializes in omnivory, scavenging eggs, fruit, insects, small animals, and occasionally carrion. Juveniles are more arboreal and agile climbers, utilizing dense vegetation as protection, while adults transition to a more terrestrial lifestyle in forest clearings and open plains. Anzu is known for being skittish and easily startled, yet highly curious, often investigating disturbances before retreating swiftly into dense cover.
15. #15

    Keys: Eoneophron

    Represented by a single species, E. infernalis, Eoneophron is a small caenagnathid oviraptorosaur from the Hell Creek Formation and distinct from Anzu in being smaller at adult size and anatomically different from juvenile Anzu remains.
16. #16

    Keys: Acheroraptor

    Represented by a single species, A. temertyorum, Acheroraptor is a small, agile dromaeosaurid predator abundant in the Hell Creek Formation. Specializing in hunting small vertebrates, such as mammals, lizards, and juvenile dinosaurs, Acheroraptor occupies a niche similar to modern foxes or bobcats. Juveniles hunt in dense undergrowth, developing their pursuit skills through quick bursts of speed and stealth. Adults exhibit increased aggression and territorial behavior, using their swift movements, keen senses, and enlarged sickle-like claws to quickly dispatch prey or rivals. Solitary and secretive, they avoid larger predators by staying hidden in dense vegetation or beneath logs and rocky outcrops.
17. #17

    Keys: Dakotaraptor

    Represented by a single species, D. steini, Dakotaraptor is the largest dromaeosaurid of the Hell Creek Formation. Dakotaraptor juveniles exhibit a slender build and primarily hunt smaller prey such as small mammals and lizards, utilizing speed and agility to ambush their prey. As they mature, they bulk significantly, which enables adults to hunt larger prey including juvenile Edmontosaurus and Triceratops. This puts them in direct competition with predators such as Nanotyrannus and sub-adult Tyrannosaurus.

    Covered in dense plumage, Dakotaraptor employs both camouflage and active pursuit, often hunting in very loosely cooperative mobs that disolve into chaos quickly, similar to some modern large monitor lizards. Their large sickle-claws and powerful limbs allow them to grapple onto prey, using their weight to immobilize targets while delivering fatal bites.

18. #18

    Keys: Pectinodon

    Represented by a single species, P. bakkeri, Pectinodon is a small, agile troodontid theropod inhabiting the Hell Creek Formation. Primarily nocturnal, Pectinodon occupies a specialized ecological niche as a predator of small vertebrates, insects, and eggs, aided by its exceptional night vision and keen hearing. Juveniles primarily hunt insects and smaller prey under cover of darkness, whereas adults expand their diet to include small mammals and reptiles. Highly intelligent and curious, Pectinodon is a solitary, stealthy hunter, typically avoiding direct competition with other small predators by timing its activity patterns differently, thereby reducing conflict within the ecosystem.
19. #19

    Keys: Infernodrakon

    Represented by a single species, I. hastacollis, Infernodrakon is a mid-sized azhdarchid pterosaur found in the Hell Creek Formation. Equipped with elongated, slender limbs and a characteristically long neck, Infernodrakon stalks terrestrial prey by quietly walking through open floodplains and along rivers. Juveniles start life arboreally, gliding and practicing powered flight among forest edges, feeding primarily on insects and small vertebrates hidden in foliage. As they mature, Infernodrakon individuals grow rapidly, becoming powerful terrestrial predators with wingspans of about 4 meters. Adults are specialized ambush hunters, employing their sharp beaks and precision eyesight to capture prey like small mammals, juvenile dinosaurs, reptiles, and amphibians. Though solitary by nature, adults will occasionally gather around abundant food sources or carcasses, maintaining uneasy peace through threat displays, wing-flapping intimidation, and vocalizations. Infernodrakon's terrestrial specialization distinguishes it clearly from contemporary aerial hunters, occupying a niche that combines elements of both aerial predator and terrestrial stalker.

    As with other pterosaurs, Infernodrakons' feet are not built to grab on to branches, objects, or other animals. This trope is popular in fiction, but neither the fossils nor the animal itself are capable of this behavior. They can lift some weight with their necks, but no more than a mouthful lest they risk breaking their long necks. Their feet are plantigrade when they walk.

20. #20

    Keys: Quetzalcoatlus

    Although primarily known from formations farther south, notably the Javelina Formation, Quetzalcoatlus individuals frequently venture northward into the Hell Creek Formation, establishing temporary residence alongside Infernodrakon. Represented by two closely related species, Q. northropi and the smaller Q. lawsoni, these colossal azhdarchids possess wingspans ranging from roughly 5 to over 10 meters, making them the largest flying vertebrates known. Juveniles begin life cautiously, relying heavily on forested environments and wetlands for cover as they develop their flight muscles and hunting skills on small, manageable prey such as frogs, insects, and juvenile vertebrates. Upon reaching adulthood, Quetzalcoatlus transitions fully to a terrestrial hunting strategy, stalking prey methodically on folded wings through open floodplains and across expansive wetlands. Their extraordinary size and imposing presence grant them dominance at feeding sites, enabling them to displace smaller azhdarchids and scavenge opportunistically from carcasses left by large carnivores like Tyrannosaurus. Despite their intimidating appearance, they avoid direct competition with major terrestrial predators, instead exploiting prey and scavenging opportunities not regularly pursued by the strictly terrestrial dinosaurs, thus maintaining ecological balance in the Hell Creek Formation's complex ecosystem.

    As with other pterosaurs, Quetzalcoatlus' feet are not built to grab on to branches, objects, or other animals. This trope is popular in fiction, but neither the fossils nor the animal itself are capable of this behavior. They can lift some weight with their necks, but no more than a mouthful lest they risk breaking their long necks. Their feet are plantigrade when they walk.

21. #21

    Keys: Alphadon

    A small opossum‑like metatherian mammal, Alphadon marshi fed on fruits, insects, and perhaps small vertebrates. Juveniles stayed close to sheltered brush, while adults became more opportunistic omnivores, scavenging or hunting small prey under cover of twilight or night.
22. #22

    Keys: Didelphodon

    A stout, powerful stagodont metatherian, Didelphodon vorax was among the largest mammals in the Hell Creek Formation. Its powerful jaws allowed it to crack bones and feed on carcasses or fresh small vertebrates. Juveniles started with insectivory and gradually shifted toward bone‑crushing dietary habits in adulthood.
23. #23

    Keys: Compsemys

    The turtle Compsemys victa is a moderate‑sized freshwater dermatemydid, sporting a hooked beak and tubercled carapace. Semiaquatic and carnivorous, juveniles fed on soft invertebrates while adults took larger prey such as fish or amphibians in riverine habitats. They fill a similar niche to modern snapping turtles.
24. #24

    Keys: Gilmoremys

    The softshell turtle Gilmoremys lancensis inhabited the river margins. It had a smooth shell and is mostly piscivorous, ambushing fish and amphibians. Juveniles stay closer to creeks and wetlands for cover, while adults range wider along floodplain waterways.
25. #25

    Keys: snake, Boid

    A few species of snakes inhabit the Hell Creek Formation, including at least one early boid. These snakes are relatively small mainly eating fish, amphibians, insects, and small lizards.
26. #26

    Keys: Champsosaurus

    A variety of freshwater reptiles call the floodplains of the Hell Creek Formation home. These nonspecific small crocodyliforms and semi‑aquatic choristoderans such as Champsosaurus ambulator inhabit the Hell Creek Formation. Juveniles dwell in sheltered floodplain ponds, eating invertebrates and small fish. Adults grow into stealthy ambush hunters with long jaws capable of catching fish and amphibians in freshwater habitats.
27. #27

    Keys: Albanerpeton

    These salamander‑like amphibians were small, terrestrial or aquatic insect‑hunters. Juveniles likely remained fully aquatic, while adults inhabited muddy floodplain margins, ambushing insects and small invertebrates.
28. #28

    Keys: Habrosaurus

    A sirenid amphibian about the size of a modern hellbender, Habrosaurus dilatus had crushing palatal teeth and fed on hard‑bodied aquatic prey in the Hell Creek Formation’s waterways. Juveniles likely preyed on small mollusks, while adults could tackle crustaceans and snails.
29. #29

    Keys: Avisaurus

    A small Cretaceous bird found in the Hell Creek Formation, Avisaurus archibaldi hunts insects and small vertebrates. Juveniles learn flight skills in sheltered forest patches, while adults patroll forest edges or near watercourses feeding opportunistically.
30. #30

    Keys: Ornithurines

    Toothy seabirds like Ichthyornis patrol the surface of the Western Interior Seaway. These volant, tern-like birds fish by the hundreds along seaway coastlines, dipping for small fish and cephalopods. Their sharp teeth and streamlined bodies make them efficient aerial piscivores. Juveniles learn to fly along coastal cliffs and shoreline marshes before joining adult flocks hunting in open water zones.
31. #31

    Keys: Hesperorniforms

    Flightless diving birds like Hesperornis live in the marine environments east of the Hell Creek Formation. These birds use their powerful hind limbs to swim underwater, preying heavily on fish in shallow seaway waters. Juveniles learn to swim in protected inlets while adults dive in deeper aquatic zones, diving repeatedly for schools of fish and avoiding predation by mosasaurs.
32. #32

    Keys: Enchodus

    Predatory teleost fish such as Enchodus, the so called "saber‑toothed herring" are common in the nearby Western Interior Seaway. Measuring up to 1 m in length and armed with fang-like teeth, Enchodus prey on smaller fish and invertebrates. Juveniles stay in shallow coastal waters, while adults roam in deeper channels, frequently becoming prey for sharks, mosasaurs, and diving birds.
33. #33
    A common freshwater osteoglossomorph fish in the rivers of the Hell Creek Formation, Coriops amnicolus fed on small invertebrates. Juveniles stayed in slow backwaters and floodplain ponds; adults reached sizes large enough to be prey for turtles, crocodyliforms, and amphibians.
34. #34

    Keys: Chondrichthyan, shark

    Chondrichthyans like Squalicorax (crow sharks), Lonchidion (hybodont sharks), and Myledaphus (rays) frequent both marine and freshwater-influenced zones near the Hell Creek Formation's waterways. Juveniles hide in brackish river mouths, feeding on invertebrates and adults prowl shallow marine shelves hunting fish and scavenging carcasses.
35. #35

    Keys: marine fish, Marine

    Large predatory bony fishes such as Xiphactinus (exceeding 4 m long) and Pachyrhizodus occupy open mid- to offshore seaway waters. These powerful swimmers hunt smaller fish and chase schools near the water’s surface. Juveniles inhabit calmer lagoonal waters and estuaries adjacent to the Hell Creek Formation while adults dominate offshore zones, occasionally washing into nearshore environments during storms or migrations.
36. #36

    Keys: Crocodilians

    Crocodilians such as the eusuchian Borealosuchus sternbergii, the gavialoid Thoracosaurus neocesariensis, and the alligatorid Brachychampsa montana patrol rivers, estuaries, and the coastal wetlands adjacent to the Hell Creek Formation. Juveniles inhabit shallow river margins, feeding on fish and amphibians while adults venture into brackish water zones to prey on larger fish, turtles, as well as small vertebrates. They act as ambush predators in both freshwater and nearshore environments.
37. #37

    Keys: Mosasaurs

    Found in the receding seaway to the east of the Hell Creek Formation, several mosasaur species such as Mosasaurus hoffmannii, Prognathodon overtoni, and a species of Plioplatecarpus inhabit the area. These massive marine squamates are top predators in their nearshore and open-water ecosystems. Mosasaurus specimens can exceed 15 meters in length and are among the largest marine reptiles ever. They feed on pretty much anything they can get their recurved teeth on, which is most animals in their environment. Slightly smaller, although still huge, Prognathodon is more of a crusher, favoring hard to eat animals such as turtles, ammonites, and armored fish. Plioplatecarpus are smaller, more agile predators, living in estuaries and feeding on schooling fish.

    Ontogenetically, mosasaurs go through dramatic changes in size and diet. Juveniles generally stay close to estuarine shallows and protected inlets, feeding on small fish, squid, and shelled invertebrates. As they grow, they move into deeper, more open marine habitats. Adults are robust and powerfully muscled, with double-hinged jaws. Their laterally flattened tails and paddle-like limbs make them swift and agile swimmers, capable of sudden bursts of speed to ambush prey from below or behind.

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