# Paleobiota of the Hell Creek Formation

This is an overview of the fossil flora and fauna of the Maastrichtian-Danian Hell Creek Formation.

## Invertebrates
### Insects

Insects from the groups Diptera, Zygoptera, and possibly Hemiphlebiidae have been unearthed in Hell Creek in amber. Fossils found in the Hell Creek Formation and the Fort Union Formation of these insects went extinct during the K-T Event.
| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
| Genus | Species | Notes |
| Cephaloleichnites | Cephaloleichnites strongi | Possible hispine beetle herbivory on Zingiberopsis attenuta |
| ?Hemiphlebiidae | Indeterminate | Possible Hemiphlebiid damselflies |
| Brachycera | Indeterminate | Undescribed brachyceran flies |
| Nematocera | Indeterminate | Undescribed nematoceran flies |
| Aeschnidiidae | Indeterminate | Indeterminate aeschnidiid dragonflies |

### Molluscs

| Genus | Species | Location | Notes |
| Plesielliptio | P. postbiplicatus | | Freshwater Unionid river mussels |
| Plesielliptio | P. gibbosoides | | Freshwater Unionid river mussels |
| Plesielliptio | P. whitfieldi | | Freshwater Unionid river mussels |
| Rhabdotophorus | R. aldrichi | | Freshwater mussel of family Unionidae. |
| Pleurobema | P. cryptorhynchus | | Freshwater mussel of family Unionidae. |
| Plethobasus | P. aesopiformis | | Freshwater mussel of family Unionidae. |
| Plethobasus | P. biesopoides | | |
| Quadrula | Q. cylindricoides | | Freshwater mussel of family Unionidae. |
| Proparreysia | P. verrucosiformis | | Freshwater mussel of family Unionidae. |
| Proparreysia | P. holmesiana | | |
| Proparreysia | P. barnumi | | |
| Proparreysia | P. percorrugata | | |
| Proparreysia | P. pyramidatoides | | |
| Proparreysia | P. letsoni | | |
| Proparreysia | P. retusoides | | |
| Proparreysia | P. corbiculoides | | |
| Proparreysia | P. paucinodosa | | |
| ?Obovaria | ?O. pyramidella | | |
| Corbicula | C. cf. subelliptica | | |
| Corbicula | C. sp | South Dakota | From a marine facies ("tongue"). Modern members of the genus live in fresh water |
| Sphaerium | S. beckmani | | "Pill clam". "Nut clam". "Fingernail clam". "Pea clam". Family Sphaeriidae. |
| Pleiodon | Indeterminate | | |
| Campeloma | C. sp | | Freshwater snail |
| Anomia | A. gryphorhyncha | | Bivalve. Family Anomiidae. From a marine facies ("tongue") in South Dakota. |
| Crassostrea | C. subtrigonalis | | Oyster. Family Ostreidae. Collected from a marine facies ("tongue") in South Dakota. |
| Granocardium | G. sp | | Bivalve. Family Cardiidae (cockle). Collected from a marine facies ("tongue") in South Dakota. |
| ?Hiatella | ?H. sp | | Bivalve. Present members of this genus are rock borers. Collected from a marine facies ("tongue") in South Dakota. |
| Leptosolen | indeterminate | | Bivalve. Family Cultellidae. Collected from a marine facies ("tongue") in South Dakota. |
| Sphenodiscus | S. lenticularis | | Ammonite. From a marine facies ("tongue") in South Dakota. |
| Discoscaphites | D. rossi | | Microconch of an ammonite. From a marine facies ("tongue") in South Dakota. |
| Scaphitidae | indeterminate | | Ammonite. From a marine facies ("tongue") in South Dakota. Other attributes: specimen has hooks on its shell. |

## Amphibians

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Anura (frog) | indeterminate | North Dakota South Dakota | Middle to upper Hell Creek Formation | 3 unassigned specimens | Anura indet. consists of material not currently assigned to any genus of frog. |
| Barbourula | Indeterminate | Montana | | | |
| Caudata (salamander) | indeterminate | North Dakota South Dakota | Lower to uppermost Hell Creek Formation | 149 unassigned specimens | Material of Caudata indet. is not currently assigned to any genus. |
| Eopelobates | Indeterminate | Montana | | | |
| Habrosaurus | H. dilatus | Montana North Dakota South Dakota | Middle to upper Hell Creek Formation | 6 specimens | Habrosaurus is a sirenid amphibian. |
| Lisserpeton | L. bairdi | Montana | | | |
| Opisthotriton | O. kayi | Montana North Dakota South Dakota | Lower to upper Hell Creek Formation | 22 specimens | Opisthotriton is classified as a Batrachosauroididae. |
| Paranecturus | P. garbanii | Montana | | | A member of Proteidae. |
| Proamphiuma | P. cretacica | Montana | | | |
| Prodesmodon | P. copei | Montana | | | |
| Scapherpeton | S. tectum | Montana North Dakota South Dakota | Lower to upper Hell Creek Formation | 144 specimens | Scapherpeton is a scapherpetonid that is very common in the Hell Creek Formation. |
| Scotiophryne | S. pustulosa | Montana | | | A small frog |

## Fish
### Bony fish

Bony fishes :
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
| Acipenser | A. amnisinferos | North Dakota (Tanis) | | | A sturgeon | |
| Acipenser | A. eruciferus | Montana | | | A sturgeon | |
| Acipenser | A. praeparatorum | North Dakota (Tanis) | | | A sturgeon | |
| Acipenser | A. cf. A. amnisinferos | North Dakota (Tanis) | | | A sturgeon | |
| Acipenser | cf. A. sp. | North Dakota South Dakota | Lower to upper Hell Creek Formation | 18 specimens are tentatively assigned to Acipenser sp. | Acipenser sp. is tentatively referred to the genus. | |
| Anchiacipencer | A. acanthaspis | Montana | | | A sturgeon, originally considered as indeterminate material | |
| Belonostomus | B. longirostris | Montana North Dakota South Dakota | Lower to upper Hell Creek Formation | 28 specimens | A long-snouted slender fish classified as an aspidorhynchid. | |
| Coriops | C. amnicolus | Montana | | | | |
| Cyclurus | C. fragosus | Montana North Dakota South Dakota | | | A small amiid fish (ubiquitous). Previously known as Amia fragosa or Kindleia fragosa. 2610 specimens have been assigned to Kindleia, making it an extremely common genus. | Cyclurus kehreri from Messel Pit, species within same genus as C. fragosus |
| "Lepisosteus" | "L. occidentalis" | Montana North Dakota South Dakota | Lower to uppermost Hell Creek Formation | 938 specimens are assigned to Lepidosteus | A lepidosteid that is very common in the Hell Creek Formation. Nomen dubium. | |
| Melvius | M. thomasi | North Dakota South Dakota | Lower to upper Hell Creek Formation | 6 specimens are assigned to Melvius | A large amiid fish. | |
| Phyllodus | P. paulkatoi | | | | Fish with columnar teeth | |
| Palaeolabrus | P. montanensis | Montana | | | | |
| Paleopsephurus | P. wilsoni | Montana | | | A paddlefish | |
| Paralbula | P. casei | Montana | | | | |
| Parapsephurus | P. willybemisi | North Dakota (Tanis) | | | A paddlefish | |
| Platacodon | P. nanus | Montana | | | Small teleost fish | |
| Protamia | Indeterminate | Montana | | | | |
| Pachyrhizodontoidei | Indeterminate | | | | Fish | |
| Polyodontidae | Indeterminate | Montana | | | Paddlefish | |
| Protoscaphirhynchus | P. squamosus | Montana | | | A sturgeon | |
| Pugiopsephurus | P. inundatus | North Dakota (Tanis) | | | A paddlefish | |

### Cartilaginous fish

Chondrichthyes reported from the Hell Creek Formation :
| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Chiloscyllium | C. sp. | Montana | | | A member of Hemiscylliidae. |
| Galagadon | G. nordquistae | South Dakota | | Isolated teeth | A carpet shark |
| Lonchidion | L. selachos | Montana North Dakota South Dakota | Lower to upper Hell Creek Formation | 40 specimens | A genus of prehistoric sharks in the family Hybodontidae. It makes up 0.4% of the remains of the vertebrates of the Hell Creek Formation. |
| Myledaphus | M. pustulosus | Montana North Dakota South Dakota | Lower to upper Hell Creek Formation. | 1677 specimens previously assigned to M. bipartitus. | Is an anacoracid batoid rajiform related to guitarfishes. Described on the basis of teeth formerly assigned to the species M. bipartitus. The material assigned to Myledaphus bipartitus and makes up 16.5% of the vertebrate remains. |
| Protoginglymostoma | P. estesi | Montana | | | A member of Ginglymostomatidae. Formerly assigned to the genus Brachaelurus. |
| Restesia | R. americana | Montana North Dakota South Dakota | Middle Hell Creek Formation | 5 specimens previously assigned to Squatirhina | A wobbegong-like shark. Formerly assigned to Squatirhina. The remains consist of 0.05% of the vertebrates. Also known from the Lance Formation. |
| Carcharhinidae indet. | Indeterminate | South Dakota | | An isolated tooth. | |
| Scapanorhynchus | .sp. | North Dakota | | | |

## Dinosaurs

A paleo-population study is one of the most difficult of analyses to conduct in field paleontology. Here is the most recent estimate of the proportions of the eight most common dinosaurian families in the Hell Creek Formation, based on detailed field studies by White, Fastovsky and Sheehan.
* Ceratopsidae 61%
* Hadrosauridae 23%
* Ornithomimidae 5%
* Tyrannosauridae 4%
* Hypsilophodontidae 3%
* Dromaeosauridae 2%
* Pachycephalosauridae 1%
* Ankylosauridae 1%
* Troodontidae 1% (represented only by teeth)
Outcrops sampled by the Hell Creek Project were divided into three sections: lower, middle and upper slices. The top and bottom sections were the focus of the PLoS One report, and within each portion many remains of Triceratops, Edmontosaurus, and Tyrannosaurus were found. Triceratops was the most common in each section, but, surprisingly, Tyrannosaurus was just as common, if not slightly more common, than the hadrosaur Edmontosaurus. In the upper Hell Creek section, for example, the census included twenty two Triceratops, five Tyrannosaurus, and five Edmontosaurus.
The dinosaurs Thescelosaurus, Ornithomimus, Pachycephalosaurus and Ankylosaurus were also included in the breakdown, but were relatively rare. Other dinosaurs, such as Sphaerotholus, Denversaurus, Torosaurus, Struthiomimus, Acheroraptor, Dakotaraptor, Pectinodon, Richardoestesia, Paronychodon, Anzu, Leptorhynchos and Troodon (more likely Pectinodon), were reported as being rare and are not included in the breakdown.
The dinosaur collections made over the past decade during the Hell Creek Project yielded new information from an improved genus-level collecting schema and robust data set that revealed relative dinosaur abundances that were unexpected, and ontogenetic age classes previously considered rare. We recognize a much higher percentage of Tyrannosaurus than previous surveys. Tyrannosaurus equals Edmontosaurus in U3 and in L3 comprises a greater percentage of the large dinosaur fauna as the second-most abundant taxon after Triceratops, followed by Edmontosaurus. This is surprisingly consistent in (1) the two major lag deposits (MOR loc. HC-530 and HC-312) in the Apex sandstone and Jen-rex sand where individual bones were counted and (2) in two thirds of the formation reflected in L3 and U3 records of dinosaur skeletons only.
Triceratops is by far the most common dinosaur at 40% (n = 72), Tyrannosaurus is second at 24% (n = 44), Edmontosaurus is third at 20% (n = 36), followed by Thescelosaurus at 8% (n = 15), Ornithomimus at 5% (n = 9), and Pachycephalosaurus and Ankylosaurus both at 1% (n = 2) are relatively rare.
Fossil footprints of dinosaurs from the Hell Creek Formation are very rare. As of 2017, there is only one find of a possible Tyrannosaurus rex footprint, dating from 2007 and described a year later. A trackway made by mid-sized theropod, possibly a small tyrannosaurid individual, was discovered in South Dakota in 1997, and in 2014 these footprints were named Wakinyantanka styxi.

### Ornithischians
#### Ankylosaurs

Indeterminate nodosaur remains have been unearthed in the Hell Creek Formation and other nearby areas.
Ankylosauria reported from the Hell Creek Formation :
| Genus | Species | State | Stratigraphic position | Material | Notes | Images |
| Ankylosaurus | A. magniventris | Montana | Upper | A partial skull, teeth, cervical vertebrae, dorsal vertebrae, caudal vertebrae, right scapulocoracoid, otic capsule, maxilla fragment, right jugal, left jugal and quadratojugal, sacral centra, additional fragments of the cervical half rings and a partial tail club handle. | An ankylosaur. Also found in the Lance, Frenchman, Ferris and Scollard Formations. | |
| Denversaurus | D. schlessmani | South Dakota | Lower | A complete skull without the lower jaw and associated plates (DMNH 468) | A nodosaurid ankylosaur whose remains have been also found in the Lance Formation | |

#### Pachycephalosaurs

An undescribed and unnamed pachycephalosaur is present in North Dakota. Pachycephalosaur remains have been unearthed in Montana as in the case of Platytholus and the now invalid genus Stenotholus kohleri, which is now a junior synonym of Pachycephalosaurus. Dracorex hogwartsia is agreed by paleontologists to be an ontogimorph of either Pachycephalosaurus or Stygimoloch, while the synonymy of Stygimoloch spinifer with Pachycephlosaurus remains uncertain due to difference in stratigraphic level.
Pachycephalosaurs reported from the Hell Creek Formation :
| Genus | Species | State | Stratigraphic position | Material | Notes |
| Pachycephalosaurus | P. wyomingensis | Montana South Dakota | | | A pachycephalosaur. Also found in the Lance Formation. |
| Platytholus | P. clemensi | Montana | Lower | A partial skull | A medium-sized pachycephalosaurine closely related to Acrotholus and Prenocephale |
| Sphaerotholus | S. buchholtzae | Montana | Lower to upper | "Skull material." | A pachycephalosaur whose remains have also been found in the Frenchman Formation. Genus also known from the Kirtland Formation, Dinosaur Park Formation and the Horseshoe Canyon Formation. |
| Sphaerotholus | S. triregnum | Montana | Middle | A left squamosal. | A pachycephalosaur distinguished from S. buchholtzae by its more ornamented squamosal. |
| Stygimoloch | S. spinifer | Montana South Dakota North Dakota | | | A pachycephalosaur, possibly synonymous with Pachycephalosaurus. Also found in the Ferris Formation and the Lance Formation. |

#### Ceratopsians

Indeterminate ceratopsid teeth and some identifiable bones from Triceratops can be extremely common. 8.31% of all vertebrate remains from the Hell Creek Formation are unassigned ceratopsids. In 2012, a new unidentified species of chasmosaur ceratopsian with noticeable differences from Triceratops was unearthed in South Dakota by a fossil hunter named John Carter.
| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Ceratopsians reported from the Hell Creek Formation :
| Genus | Species | Synonyms | State | Stratigraphic position | Material | Notes |
| Leptoceratops | L. gracilis | | Montana | | | A small primitive-looking ceratopsian. Fossils have also been found in the Lance Formation in Wyoming. |
| Tatankaceratops | T. sacrisonorum | | South Dakota | | | A controversial ceratopsian possibly synonymous with Triceratops |
| Torosaurus | T. latus | | Montana North Dakota South Dakota | Upper Hell Creek Formation | | A ceratopsian that was once proposed to be synonymous with Triceratops, but is now regarded as a valid and distinct ceratopsian. A rare ceratopsid. Fossils have been in the Lance Formation, Javelina Formation, North Horn Formation, Laramie Formation, El Picacho Formation, Frenchman Formation and Scollard Formation. |
| Triceratops | T. horridus | T. serratus Ugrosaurus olsoni | Montana North Dakota Wyoming South Dakota | Lowermost to the middle Hell Creek Formation | Four specimens are assigned to Triceratops horridus from the Hell Creek Formation. Isolated, shed ceratopsid teeth are incredibly common in the Hell Creek and Lance Formations, being by most collectors, with some being nothing more than worn down fragments up to superb teeth containing complete, preserved roots. Because the teeth of different ceratopsians are so similar to one another, its hard to differentiate between genera/species, but based on the abundance of identifiable bones belonging to Triceratops in Lancian-aged North American formations, especially the Hell Creek, isolated ceratopsid teeth from the lower and middle Hell Creek Formation have a high likelihood of originating from T. horridus. | A ceratopsian. Also found in the Evanston, Frenchman, Lance, Laramie, and Scollard Formations. |
| Triceratops | T. prorsus | | Montana South Dakota | Upper 1/3 of the Hell Creek Formation | Very common. Because the teeth of different ceratopsians are so similar to one another, its hard to differentiate between genera/species, but based on the abundance of identifiable bones belonging to Triceratops in Lancian-aged North American formations, especially the Hell Creek, isolated ceratopsid teeth from the upper 1/3 of the Hell Creek Formation have a high likelihood of originating from T. prorsus. | Also found in the Frenchman and Lance Formations. |

#### Ornithopods and relatives

Indeterminate hadrosaurid remains are very common in the Hell Creek Formation. Fossils of indeterminate lambeosaurs have been unearthed in Hell Creek, specifically in South Dakota and Montana.
Ornithopods and Thescelosaurs reported from the Hell Creek Formation :
| Genus | Species | Synonyms | State | Stratigraphic position | Material | Notes |
| Edmontosaurus | E. annectens | Anatosaurus annectens Anatotitan copei | Montana South Dakota North Dakota(Mummy Fossil) | | Very common. | A hadrosaur. Also found in the Denver, Frenchman, Lance, Laramie, and Scollard Formations. Hatchlings have also been unearthed. |
| Thescelosaurus | T. garbanii | Bugenasaura garbanii | Montana South Dakota | | | |
| Thescelosaurus | T. neglectus | | North Dakota South Dakota | Lower to upper Hell Creek Formation | 50 specimens | A small thescelosaurine. Also found in the Frenchman, Lance, Laramie, and Scollard Formations. Two species are known from Hell Creek; T. neglectus and T. garbanii. |

### Theropods

Theropod tracks have been found in South Dakota. A trackway from South Dakota, named Wakinyantanka, was made by a mid-sized theropod with three slender toes, possibly a small tyrannosaurid. A second footprint that may have been made by a specimen of Tyrannosaurus was first reported in 2007 by British paleontologist Phil Manning, from the Hell Creek Formation of Montana. This second track measures 72 centimeters (28 in) long, shorter than the track described by Lockley and Hunt. Whether or not the track was made by Tyrannosaurus is unclear, though Tyrannosaurus is the only large theropod known to have existed in the Hell Creek Formation, though in past albertosaurine remains have described here but its most likely that they are the remains of Tyrannosaurus rex. Theropod remains are very common in Hell Creek, some of which belong to indeterminate species on maniraptorans.

#### Alvarezsaurs

Alvarezsaurs reported from the Hell Creek Formation :
| Genus | Species | Synonyms | State | Stratigraphic position | Material | Notes | Images |
| "Ornithomimus" | "O." minutus | | | | | | |
| Trierarchuncus | T. prairiensis | | Montana | upper Hell Creek Formation. | | An alvarezsaur known from a partial post-cranial skeleton. | |

#### Tyrannosaurids

Tyrannosaurids reported from the Hell Creek Formation :
| Genus | Species | Synonyms | State | Stratigraphic position | Material | Notes | Images |
| Tyrannosaurus | T. rex | Albertosaurus megagracilis Aublysodon molnari Dinotyrannus megagracilis Nanotyrannus lancensis? | Montana North Dakota South Dakota | Lower to upper Hell Creek Formation. | | A tyrannosaur, known from several specimens including a juvenile nicknamed "Jane". Also found in the Denver, Frenchman, Hill Creek South, Javelina, Lance, Ferris, Livingston, McRae, North Horn, Scollard, Willow Creek Formation, and also found in Lomas Coloradas Formations. Isolated teeth in the Hell Creek are common enough to be dug commercially by collectors, but rare enough that they are often sold for very high prices with fragmentary teeth usually beginning at least in the hundreds of USD, and complete teeth in the thousands of USD. Perhaps the best known iconic dinosaur. | |
| Nanotyrannus? | N.lancensis | Stygivenator? Deinodon lancensis Albertosaurus lancensis Tyrannosaurus lancensis? | Montana Wyoming | Lower to upper Hell Creek Formation. | A few specimens are known | Invalid genus, now recognised as juvenile T. rex. | |

#### Ornithomimosaurs

Ornithomimid remains are not uncommon in the Hell Creek Formation. Fifteen specimens from the Hell Creek Formation are undetermined ornithomimids
| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Ornithomimids reported from the Hell Creek Formation :
| Genus | Species | State | Material | Notes |
| "Orcomimus" | unnamed | South Dakota | One partial skeleton. | An ornithomimid; nomen nudum. |
| Struthiomimus | S. sedens | Montana | AMNH 975, a foot claw UCMP 154569, a partial skeleton | A large ornithomimid similar to Gallimimus in size. Also found in the Lance Formation. |
| Ornithomimus | O. velox | Montana South Dakota North Dakota | Fragmentary specimens | An ornithomimid which was also found in the Denver Formation. |

#### Oviraptorosaurs

Oviraptorosaur fossils have been found at the Hell Creek Formation for many years, most notably from isolated elements until the discovery of Anzu. In the past, oviraptorosaur fossils found were thought to have belonged to Leptorhynchos, Caenagnathus, Chirostenotes, and Elmisaurus. In 2016, an undescribed large-bodied caenagnathid was unearthed in Montana.
| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Oviraptorosaurs reported from the Hell Creek Formation :
| Genus | Species | State | Stratigraphic position | Material | Notes |
| Anzu | A. wyliei | North Dakota South Dakota | Lower to upper Hell Creek Formation | 12 well-preserved specimens | One of the largest known oviraptorosaurs, and the largest known from North America. Material previously assigned to Caenagnathidae indet. is now placed in the genus Anzu. |
| Caenagnathidae | Indeterminate | Montana | | | Similar to Citipes and Elmisaurus. |
| Eoneophron | E. infernalis | South Dakota | | A partial right hindlimb | Closely related to Citipes and Elmisaurus. |

#### Eumaniraptorans

Historically, numerous teeth have been attributed to various dromaeosaurid and troodontid taxa with known body fossils from only older formations, including Saurornithoides, Zapsalis, Dromaeosaurus, Saurornitholestes, and Troodon. However, in a 2013 study, Evans et al. concluded that there is little evidence for more than a single dromaeosaurid taxon, Acheroraptor, in the Hell Creek-Lance assemblages, which would render these taxa invalid for this formation. This was seemingly disproven in 2015, when DePalma et al., described the new genus Dakotaraptor, a large species of dromaeosaur. However, Dakotaraptor’s validity has been called into question since, as the holotype may be a chimeric assemblage of various non-dromaeosaurid coelurosaurians. Fossilized teeth of various troodontids and coelurosaurs are common throughout the Hell Creek Formation; the best known examples include Paronychodon, Pectinodon and Richardoestesia, respectively. Teeth belonging to possible intermediate species of Dromaeosaurus and Saurornitholestes have been unearthed at the Hell Creek Formation and the nearby Lance Formation.
Eumaniraptorans reported from the Hell Creek Formation :
| Genus | Species | State | Stratigraphic position | Material | Notes |
| Acheroraptor | A. temertyorum | Montana North Dakota? South Dakota? | Lower? to upper Hell Creek Formation | ROM 63777, a maxilla and tooth ROM 63778, a partial dentary isolated teeth | A velociraptorine dromaeosaurid. Teeth previously referred to various Campanian dromaeosaurids Saurornitholestes and Dromaeosaurus, frequently found throughout the formation, probably belong to this one species. Evans et al. conclude that there is little evidence for the former two taxa being present in the Hell Creek-Lance assemblages. |
| Avisaurus | A. archibaldi | Montana | Middle Hell Creek Formation | UCMP 117600, holotype, a tarsometatarsus PU 17324, a tarsometatarsus | An avisaurid. |
| Avisaurus | cf. A. archibaldi | Montana | Uppermost Hell Creek Formation | YPM 57235, a coracoid | An avisaurid tentatively referred to A. archibaldi based on its size. |
| Avisaurus | A. darwini | Montana | | DDM 1577.730, a right tarsometatarsus | |
| Avisaurus | A. sp. | Montana | | distal tarsals, metatarsus (juvenile) MOR3070, a partial right tarsometatarsus | |
| Brodavis | B. baileyi | South Dakota | | UNSM 50665, a left tarsometatarsus missing proximal end, trochleae II and III. | A primitive hesperornithiform. |
| Dakotaraptor | D. steini | South Dakota | Upper Hell Creek Formation | PBMNH.P.10.113.T, a partial skeleton. PBMNH.P.10.115.T, a tibia. PBMNH.P.10.118.T, an astragalocalcaneum. isolated teeth. | A large dromaeosaurid. Possibly chimeric. |
| Magnusavis | M. ekalakaensis | Montana | | CCM V2019.5.1, partial right tarsometatarsus and toe bone | A large enantiornithine closely related to avisaurids. |
| Paronychodon | P. caperatus | North Dakota, South Dakota, and Montana. | | YPM 10624, a fossilized tooth. Teeth of this genus have been found too. | A troodontid theropod who is known from fossil teeth. Fossils have also been found in the Lance Formation in Wyoming. |
| Pectinodon | P. bakkeri | Montana | | UCM 38445, a fossilized tooth. Teeth of this genus have been found too. | A troodontid theropod who is known from fossil teeth. Fossils have also been found in the Lance Formation in Wyoming. |
| Potamornis | P. skutchi | Montana | | UCMP 117605, a tarsometatarsus | A hesperornithiform also found in the Lance Formation. |
| Richardoestesia | R. sp. | Montana. | | Teeth. | A coelurosaur that is known from teeth and from two species Richardoestesia gilmorei and Richardestesia isosceles, which have also been unearthed in the Lance Formation in Wyoming. |
| "Styginetta" | "S. lofgreni." | Montana. | | | A Presbyornithid, it is notable for being one of the few birds known to have survived the Cretaceous–Paleogene extinction event. |
| "Unnamed enantiornithine B" | Unnamed | Montana | | YPM 57823, a partial coracoid | An unnamed enantiornithean. |
| "Unnamed hesperornithiform A" | Unnamed | Montana | | UCMP 13355, a tarsometatarsus | A primitive hesperornithiform. The Hell Creek specimen was referred to the same unnamed taxon as RSM P 2315.1 from the Canadian Frenchman Formation. RSM P 2315.1 was later made the holotype of Brodavis americanus. May be a synonym of Potamornis. |
| "Unnamed ornithurine B" | Unnamed | Montana | | UCMP 129143, a partial coracoid | An ornithurine possibly similar to Cimolopteryx |
| "Unnamed ornithurine C" | Unnamed | Montana South Dakota | | SDSM 64281A, a partial coracoid SDSM 64281B, a partial coracoid UCMP 175251, a partial coracoid MOR 2918, a partial coracoid | An ornithurine, also present in the Lance Formation and Fort Union Formation, one of the few individual bird species known to have survived the Cretaceous–Paleogene extinction |
| "Unnamed ornithurine D" | Unnamed | Montana | | UCMP 187207, a partial coracoid | An ichthyornithean also present in the Frenchman Formation |

## Pterosaurs

Undescribed pterosaur remains were reported from North Dakota. Undescribed pteranodontian specimen has also been mentioned in the supplementary material of Longrich et al. (2018).
Pterosaurs of the Hell Creek Formation :
| Taxa | Species | State | Stratigraphic location | Material | Notes | Images |
| Infernodrakon | I. hastacollis | Montana | | | A single azhdarchid neck bone originally considered to belong to the genus Quetzalcoatlus. | |

## Crocodylomorphs

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Crocodylomorphs reported from the Hell Creek Formation :
| Genus | Species | State | Notes |
| Borealosuchus | B. sternbergii | Montana North Dakota South Dakota | Extinct genus of crocodylians that lived from the Late Cretaceous to the Eocene in North America. |
| Brachychampsa | B. montana | Montana South Dakota North Dakota | Extinct genus of alligatoroid. |
| Thoracosaurus | T. neocesariensis | Montana | Extinct genus of gavialoid crocodilian which existed during the Late Cretaceous and early Paleocene. |

## Turtles

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Turtles reported from the Hell Creek Formation :
| Genus | Species | State | Material | Notes |
| Adocus | Indeterminate | Montana | | Extinct genus of aquatic turtles belonging to the family Adocidae. |
| Axestemys | A. infernalis | Montana North Dakota South Dakota Wyoming | | A turtle belonging to the family Trionychidae. Its fossils from the Hell Creek Formation were formerly assigned to the late Campanian species Axestemys splendidus. |
| Compsemys | C. victa | Montana | | A relative of Dermatemydidae. |
| Peckemys | P. brinkman | Montana North Dakota | | A relative of Baenidae. |
| Emarginachelys | E. cretacea | Montana | | A relative of chelydrids. |
| Eubaena | E. cephalica | Montana | | Baenid turtle |
| Gamerabaena | G. sonsalla | North Dakota | | Extinct genus of baenid turtle. |
| Palatobaena | P. cohen | North Dakota | | A relative of extinct family of cryptodiran turtles. |
| Cedrobaena | C. putorius | South Dakota North Dakota | | A relative of Baenidae. |
| Gilmoremys | G. lancensis | Montana North Dakota | | Trionychidae related to the softshell turtle. |
| Hoplochelys | H. clark | North Dakota | | A kinosternoid related to the Central American river turtle. |
| Hutchemys | H. walkerorum | North Dakota | A shell | A Plastomeninae related to the softshell turtle. |
| Plastomenus | P. sp | | | Trionychidae turtle. |
| Basilemys | B. sinuosa | | | Largest dermatemydid land tortoise. |
| Trionyx | Indeterminate | Montana | | A genus of softshell turtles belonging to the family Trionychidae. |
| Aspideretoides | A. foveatus | | | Trionychidae turtle. |
| Helopanoplia | H. distincta | | | Trionychidae turtle. |
| Judithemys | J. backmani | | | Thin-shelled macrobaenid turtle. |
| Plesiobaena | P. antiqua | | | Baenid turtle. |
| Stygiochelys | S. estesi | | | Baenid turtle. |
| Neurankylus | N. eximius | | | Largest baenid turtle in Hell Creek Formation. |
| Saxochelys | S. gilberti | North Dakota | A population of over 30 individual skeletons | A member of the family Baenidae. |
| Thescelus | T. insiliens | | | Baenid turtle. |
| Chelydridae | Indeterminate | | | Chelydrids-like turtle. |

## Squamata

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Squamates reported from the Hell Creek Formation :
| Genus | Species | State | Notes |
| Cemeterius | C. monstrosus | Montana | A platynotan lizard of uncertain phylogenetic placement, also known from the Lance Formation. |
| Cerberophis | C. robustus | Montana | An alethinophidian snake of uncertain phylogenetic placement. |
| Obamadon | O. gracilis | Montana | A polyglyphanodontian lizard of uncertain phylogenetic placement. Also known from the Lance Formation. |
| Peneteius | P. aquilonius | Montana | A chamopsiid polyglyphanodontian lizard. |
| Haptosphenus | H. placodon | | Teiidae lizard. |
| Leptochamops | L. denticulatus | | Small Teiidae lizard. |
| Chamops | C. segnis | | Largest Teiidae lizard in Hell Creek Formation |
| Contogenys | C. sloani | | Scincidae? lizard. |
| Exostinus | E. lancensis | | xenosaurid lizard. |
| Proxestops | P. jepseni | | Anguidae lizard. |
| Parasaniwa | P. wyomingensis | | Necrosaurid lizard. |
| Paraderma | P. bogerti | | Helodermatidae? lizard. |
| Palaeosaniwa | P. canadensis | | A large Monstersauria lizard, closely related to today's varanid lizards. It was the largest lizard in the Hell Creek formation. |
| Boidae | Indeterminate | | Snake. Earliest-known boid. |
| Mosasaurinae | Indeterminate | North Dakota | Indeterminate mosasaurine remains have been unearthed in North Dakota; they may belong to a mosasaur measuring 11 m (36 ft) in length. |

## Choristoderans

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Choristoderans reported from the Hell Creek Formation :
| Genus | Species | State | Stratigraphic position | Material | Notes | Images |
| Champsosaurus | C. sp. | Montana | | | A champsosaur. | |

## Mammals
### Multituberculates

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Multituberculates reported from the Hell Creek Formation :
| Genus | Species | State | Notes |
| Cimexomys | C. minor | Montana | A multituberculate of uncertain phylogenetic placement. |
| Cimolodon | C. nitidus | Montana North Dakota | A cimolodontid multituberculate. |
| Cimolodon | C. cf. nitidus | South Dakota | A cimolodontid multituberculate. |
| Cimolodon | C. sp. | North Dakota | A cimolodontid multituberculate. |
| Cimolomys | C. gracilis | Montana | A cimolomyid multituberculate. |
| Cimolomys | C. cf. gracilis | South Dakota | A cimolomyid multituberculate. |
| Essonodon | E. browni | Montana | A cimolomyid multituberculate. |
| Meniscoessus | M. conquistus | South Dakota | A cimolomyid multituberculate. |
| Meniscoessus | M. robustus | Montana North Dakota South Dakota | A cimolomyid multituberculate. |
| Meniscoessus | M. cf. robustus | South Dakota | A cimolomyid multituberculate. |
| Meniscoessus | M. sp. | North Dakota | A cimolomyid multituberculate. |
| Meniscoessus | ?M. sp. | North Dakota | A cimolomyid multituberculate. |
| Mesodma | M. formosa | Montana South Dakota | A neoplagiaulacid multituberculate. |
| Mesodma | M. cf. formosa | Montana | A neoplagiaulacid multituberculate. |
| Mesodma | M. hensleighi | Montana South Dakota | A neoplagiaulacid multituberculate. |
| Mesodma | M. cf. hensleighi | South Dakota | A neoplagiaulacid multituberculate. |
| Mesodma | M. thompsoni | Montana North Dakota South Dakota | A neoplagiaulacid multituberculate. |
| Mesodma | M. cf. thompsoni | Montana | A neoplagiaulacid multituberculate. |
| Mesodma | M sp. | Montana | A neoplagiaulacid multituberculate. |
| Mesodma | ?M sp. | Montana | A neoplagiaulacid multituberculate. |
| ?Neoplagiaulax | ?N. burgessi | Montana | A neoplagiaulacid multituberculate. |
| Paracimexomys | P. priscus | Montana | A multituberculate of uncertain phylogenetic placement. |
| Paressonodon | P. nelsoni | | A cimolomyid multituberculate. |
| Stygimys | S. kuszmauli | Montana | It was a member of the extinct order Multituberculata. |

### Metatherians

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Metatherians reported from the Hell Creek Formation :
| Genus | Species | State | Notes |
| Alphadon | A. marshi | Montana North Dakota | An alphadontid. genus of small, primitive mammal that was a member of the Metatheria, a group of mammals that includes modern-day marsupials. |
| Alphadon | A. cf. marshi | Montana South Dakota | An alphadontid. |
| Alphadon | A. wilsoni | Montana | An alphadontid. |
| Alphadon | A. cf. wilsoni | Montana | An alphadontid. |
| Alphadon | A. sp. | North Dakota South Dakota | An alphadontid. |
| Didelphodon | D. padanicus | South Dakota | A stagodontid. |
| Didelphodon | D. vorax | Montana North Dakota | A stagodontid. genus of Stagodontidae marsupials from the Late Cretaceous of North America. |
| Didelphodon | D. cf. vorax | South Dakota | A stagodontid. |
| Didelphodon | D. sp. | North Dakota | A stagodontid. |
| Didelphodon | cf. D. sp. | North Dakota South Dakota | A stagodontid. |
| Glasbius | G. twitchelli | Montana | A glasbiid. |
| Glasbius | G. cf. twitchelli | Montana | A glasbiid. |
| Leptalestes | L. cooki | Montana | A pediomyid. |
| Leptalestes | L. krejcii | Montana South Dakota | A pediomyid. |
| Nanocuris | N. improvida | | A deltatheridiid. |
| Nortedelphys | N. jasoni (= N. intermedius) | Montana South Dakota | A herpetotheriid. |
| Pediomys | P. elegans | Montana South Dakota | A pediomyid. |
| Protalphadon | P. foxi | Montana | An alphadontid. |
| Protalphadon | P. lulli | Montana | An alphadontid. |
| Protolambda | P. florencae | Montana North Dakota South Dakota | A pediomyid. |
| Protolambda | P. hatcheri | Montana South Dakota | A pediomyid. |
| Protolambda | P. mcgilli | Montana | A pediomyid. |
| Turgidodon | T. rhaister | Montana | An alphadontid. |

### Eutherians

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Eutherians reported from the Hell Creek Formation :
| Genus | Species | State | Notes |
| Altacreodus | A. magnus | Montana North Dakota | a possible creodont, formerly a species of Cimolestes |
| Alostera | A. saskatchewanensis | Montana | A eutherian of uncertain phylogenetic placement. |
| Ambilestes | A. cerberoides | Montana | A eutherian of uncertain classification, formally a species of Cimolestes |
| Batodon | B. tenuis | Montana | A cimolestid eutherian. |
| Cimolestes | C. incisus | Montana | A cimolestid eutherian. |
| Cimolestes | C. stirtoni | Montana | A cimolestid eutherian. |
| Gypsonictops | G. hypoconus | Montana South Dakota | A gypsonictopsid eutherian. |
| Gypsonictops | G. illuminatus | Montana North Dakota | A gypsonictopsid eutherian. |
| Gypsonictops | G. cf. illuminatus | Montana | A gypsonictopsid eutherian. |
| Gypsonictops | G. sp. | Montana | A gypsonictopsid eutherian. |
| cf. Paranyctoides | cf. Paranyctoides sp. | Montana | A nyctitheriid eutherian. |
| Protungulatum | P. coombsi | Montana | A stem-placental. |
| Purgatorius | P. ceratops | Montana | A genus with four species believed to be either stem-placentals or stem-primates. |
| Scollardius | S. propalaeoryctes | Montana | A eutherian of uncertain classification, formally a species of Cimolestes |

## Flora

The Hell Creek Formation was a low floodplain at the time before the sea retreated, and in the wet ground of the dense woodland, the diversity of angiosperms and conifers were present. An endless diversity of herbaceous flowering plants, ferns and moss grew in the forest understory. On the exposed point bars of large river systems, there were shrubs and vines. The evidence of the forested environment is overwhelmingly supported by petrified wood, rooted gley paleosols, and ubiquitous tree leaves. The presence of the simple and lobed leaves, combined with an extremely high dicot diversity, extinct cycadeoid Nilssoniocladus, Ginkgo, many types of monocots, and several types of conifers is different from any modern plant community. There are numerous types of leaves, seeds, flowers and other structures from Angiosperms, or flowering plants. The Hell Creek Formation of this layer contains over 300 tablets, of which angiosperms are by far the most diverse and dominant flora of the entire population, about 90 percent, followed by about 5% of conifers, 4% of ferns, and others. Compared to today Hell Creek's flora which is prairie, then Hell Creek's flora was hardwood forest mixed with deciduous and evergreen forest. In sharp contrast to the Great Plains today, the presence of many thermophilous taxa such as palm trees and gingers meant the climate was warmer and wetter then.
The plants of the Hell Creek Formation generally represent angiosperm-dominated riparian forests of variable diversity, depending on stratigraphic position and sedimentary environment. There appears to be floral transitions visible on a stratigraphic range from the lower to the upper Hell Creek Formation. For this reason, Kirk Johnson and Leo Hickey divided it into five zones and described them as HCIa, HCIb, HCIIa, HCIIb, and HCIII as a reflection of floral change through time. For example, the HCIa zone is dominated by "Dryophyllum" subfalcatum, Leepierceia preartocarpoides, "Vitis" stantonii, and "Celastrus" taurenensis, and is located 55 to 105 meters below the K-Pg boundary layer. Although the HCIb zone is a very thin layer, about 5 meters of rock, it bears unusually high diversity of herbaceous and shrubby plants, including Urticaceae, Ranunculaceae, Rosaceae, and Cannabaceae.
There is evidence of transitional floras in the middle of the Hell Creek Formation as shown by HCII and HCIII zones. The HCII flora represents a transitional period where taxa from the lower Hell Creek are replaced by the HCIII flora. The diversity of the HCIII zone is very high, and its composition is more uniform than that of HCII, many of which were rare or absent from the zones below, and some others that used to be common below became rarer in the HCIII zone. These forms include Elatides longifolia, "Dryophyllum" tennessensis, Liriodendrites bradacii, and many members of the Laurales including Bisonia niemii, "Ficus" planicostata, and Marmarthia trivialis, while "Celastrus" taurenensis, Leepierceia preartocarpoides, and many cupressaceous conifers became rarer. This phenomenon suggests that the global temperature was warming during the last 300,000-500,000 years of the Cretaceous period.
Johnson claims that there are no grasses, oaks, maples, beeches, figs, or willows in the Hell Creek Formation. There is no evidence of fern prairie either. However, there was an extremely high angiosperm diversity — common plane trees, "Dryophyllum" subfalcatum, Leepierceia preartocarpoides, and palm trees — along with extinct cycadeoid Nilssoniocladus, Ginkgo, araucariaceous, podocarpaceous, and cupressaceous conifers. This represents the mixed deciduous and evergreen broad-leaved forest as the Hell Creek landscape. The nature of these forests is uncertain because Johnson found that the majority of the angiosperm and conifer genera are now extinct. He also believes that, very roughly 80% of the terrestrial plant taxa died out in what is now Great Plains at the K-Pg boundary. On other hand, there is a great increase in the abundance of fossil fern spores in the two centimeters of rock that directly overlies the impact fallout layer (the famous K-Pg boundary layer). This increase in fern spore abundance is commonly referred as "the fern spike" (meaning that if the abundance of spores as a function of stratigraphic position were plotted out, the graph would show a spike just above the impact fallout layer).
Many of the modern plant affinities in the Hell Creek Formation (e.g., those with the prefix "aff." or with quotes around the genus name) may not in reality belong to these genera; instead they could be entirely different plants that resemble modern genera. Therefore, there is some question regarding whether the modern Ficus or Juglans, as two examples, actually lived in the Late Cretaceous.
Compared to the rich Hell Creek Formation fossil plant localities of the Dakotas, relatively few plant specimens have been collected from Montana. A few taxa were collected at Brownie Butte Montana by Shoemaker, but most plants were collected from North Dakota (Slope County) and from South Dakota. Among the localities, the Mud Buttes, located in Bowman County, North Dakota, is probably the richest megaflora assemblage known and the most diverse leaf quarry from the Hell Creek Formation. "TYPE" after the binomial means that it is represented by a type specimen found in the Yale-Peabody Museum collections. "YPM" is the prefix for the Yale-Peabody Museum specimen number; "DMNH" is for the Denver Museum of Nature & Science; "USNM" is for Smithsonian National Museum of Natural History; and so on. The majority of Hell Creek megafloral specimens are collected at the Denver Museum of Nature & Science.

### Overview (from Johnson, 2002)

302 plant morphotypes based on leaf only, including:
* 1 bryophyte (mosses and liverworts)
* 11 ferns
* 1 sphenopsid
* 10 conifers
* 1 ginkgo (uncommon)
* 278 angiosperms (roughly 92% of all taxa found)

### Paleoflora

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |

#### Liverworts

| Genus | Species | Location | Stratigraphic position | Abundance | Notes | Images |
| Marchantia | M. pealii | | | | Only known liverwort in Hell Creek. | |

#### Ferns

| Genus | Species | Notes |
| Equisetum | E. sp | Rare in the Hell Creek Formation. |
| Polypodiaceae? | indeterminate | |
| Salvinia | S. sp | Floating aquatic plant. |

#### Cycadophytes

| Genus | Species | Location | Stratigraphic position | Abundance | Notes | Images |
| Nilssoniocladus | N. comtula | | | | Unlike N. yukonensis, its leaves are pinnatisect pinnules. Common. | |
| Nilssoniocladus | N. yukonensis | | | | The only Hell Creek Formation cycadophyte. A simple leaf. Common. | |

#### Ginkgoales

| Genus | Species | Location | Stratigraphic position | Abundance | Notes | Images |
| Ginkgo | G. adiantoides | | | | The only ginkgoalean in the Hell Creek Formation; uncommon | |

#### Conifers

| Genus | Species | Notes |
| Araucaria? | indeterminate | Casts of Monkey-puzzle leaves. |
| Cupressinocladus | C. interruptus | Casts of cupressaceae foliage. Uncommon |
| Dammarites | D. sp | A conifer seed cone. It's likely to belong to Elatides longifolia. |
| Ditaxocladus | D. catenulata | A cupressaceous conifer closely related to Cupressinocladus. Common. |
| Elatocladus | Elatocladus sp, | A taxodioid leaf morphogenus |
| Elatides | E. longifolia | Araucaria-like or Cunninghamia-like conifer. Less common in the lower 2/3 but more common in the upper 1/3 of the Hell Creek Formation. |
| Glyptostrobus | G. europaeus | Uncommon conifer. |
| Metasequoia | M. occidentalis | M. occidentalis seed cones are known from the Hell Creek. |
| Sequoia | S. dakotensis | A sequoia known from cones |
| Platyspiroxylon | Platyspiroxylon sp. | A cupressaceaeous wood morphogenus |
| Podocarpoxylon | Podocarpoxylon sp. | A possibly podocarpaceous wood morphogenus |
| Sequoiaxylon | Sequoiaxylon sp. | A cupressaceaeous wood morphogenus |
| Taxodioxylon | Taxodioxylon sp. | A cupressaceaeous wood morphogenus |
| Taxodium | T. olrikii | Related to today's bald cypress. |
| Unidentified | Unidentified | Unspecified cheirolepidiacous fossils |

#### Angiosperms

| Genus | Species | Notes |
| "Artocarpus" | "A." lessigiana | Abundant at Brownie Butte, Montana. |
| Annona? | A?. robusta | Abundant at Brownie Butte, Montana. |
| Araliaephyllum | A. polevoi | A lobed leaf. Closely relating to Bisonia. Fairly common. |
| Averrhoites | cf. A. affinis | An uncommon taxon with compound leaves. |
| Bisonia | B. niemi | A broad leaf, probably in the Laurales. A common taxon. Type specimen was found in South Dakota. |
| Browniea | B. serrata | In the Nyssaceae, closely relating to extant Camptotheca. Less common. |
| Cannabaceae? | indeterminate | Incertae sedis. It's not likely in the Cannabaceae. |
| Carpites | C. ulmiformis | Though this fossil fruit is abundant in the Early Paleocene, it's also found in Hell Creek. It may belong to Apiaceae. |
| "Celastrus" | "C." taurenensis | Incertae sedis. It's common in the lowermost to the middle Hell Creek Formation, but less common in the upper 1/3 Hell Creek Formation. |
| "Cinnamomum" | "C." lineafolia | Included in Ficus affinis by L. Hickey. Belongs in Rhamnaceae (modern buckthorns and Ceanothus). Some other specimens referred to Cinnamomum sezanensis(?) sp.), a real cinnamon bush. Its affinity is questionable. |
| Cissites | cf. C. acerifolia | This morphotype was first described from the Cenomanian Dakota Group. |
| Cissites | C. insignis | This form represents of the group of Cenomanian leaves from Dakota Formation. |
| Cissites | C. lobata | A lobed leaf with half-naked basal lateral veins. Common in the upper 1/3 of the Hell Creek Formation. |
| Cissites | C. puilasokensis | A palmately lobed leaf with 5 primary veins. Common in the upper 1/3 of the Hell Creek Formation. |
| Cobbania | C. corrugata | A prehistoric species of water lettuce, previously assigned to the genus Pistia. |
| Cobbania | C. hickeyi | Another Cobbania species from pond sediments known as "Licking Leaves." |
| Cornophyllum | C. newberryi | The majority of leaves are entire-margined but some may develop a few teeth. |
| "Cypercites" | "C." sp | A reed-type plant. |
| Dryophyllum | D. subfalcatum | This taxon is extremely common in the Hell Creek Formation, but is rare in Paleocene sediments. It is believed to be in Sabiaceae, closely related to the living Meliosma. |
| Dryophyllum | aff. "Dryophyllum" subfalcatum | see above. |
| Dryophyllum | "D." tenneseensis | This taxon is similar to D. subfalcatum]] but with extremely high L/W ratio and craspedodromous venation. |
| Erlingdorfia | E. montana | Johnson, 1996. In the Platanaceae (related to today's Sycamore). A very common taxon. |
| "Ficus" | "F." planicostata | Despite the genus name, it's in Lauraceae. |
| Grewiopsis | G. saportana | Another generic Platanaceae. |
| Harmsia | H. hydrocotyloidea | Incertae sedis. May be related to lotus. An uncommon taxon. |
| Humulus? | aff. Humulus sp. | May be related to the extant genus Humulus. |
| Hydropteris | H. pinnata | Floating aquatic fern |
| Limnobiophyllum | L. scutatum | Floating aquatic monocot, closely related to Pistia. |
| Liriodendrites | L. bradacii | Johnson, 1996. In the Magnoliidae: a common taxon. |
| "Liriodendron" | "L." laramiense | Unlobed leaf. May be related to today's tulip tree (yellow poplar). An uncommon taxon. |
| Liriodendron | L. sp | Four-lobed leaf. May be related to today's tulip tree (yellow poplar). An uncommon taxon. |
| Leepierceia | L. preartocarpoides | Incertae sedis but possibly in Proteales. Johnson, 1996. |
| Laurophyllum | L. wardiana | Large leaves. Closely related to "Dryophyllum" subfalcatum. |
| "Magnolia" | M. pulchra | Occurs in southern the Wyoming flora, Leo Hickey claims it is found further north in Montana and the Dakotas. |
| Marmarthia | M. johnsonii | A new Marmarthia species described in Peppe et al. 2007. |
| Marmarthia | M. pearsonii | Johnson, 1996. In the Lauraceae: a very common taxon. |
| Marmarthia | M. trivialis | Johnson, 1996. In the Lauraceae: a very common taxon. |
| "Myrica" | "Myrica" torreyi | Incertae sedis. Not actually a bayberry. |
| Nelumbo | N. sp | One of the most common aquatic plants in Hell Creek. |
| Nelumbium | N. montanum | An aquatic angiosperm, closely related to lotus. Uncommon. |
| Nordenskioldia | N. borealis | A fossil fruit likely to belonging to Zizyphoides flabella. |
| Paranymphaea | P. hastata | Despite the name, it's not related to extant genus Nymphaea. |
| Palaeoaster | P. porosia | A papaveraceous with fruits and seeds visually similar to Romneya though it's dubious. |
| Penosphyllum | P. cordatum | May be related to Sterculioideae. A common taxon. |
| Platanites | P. marginata | Johnson, 1996. In the Platanaceae. A common taxon. |
| Platanites | P. raynoldsii | An uncommon taxon. |
| Porosia | P. verrucosa | A fossil fruit that may belong to Rutaceae. |
| Rhamnica | R. cleburnii | Incertae sedis. A Tetracera look-alike leaf. |
| Sabalites | S. sp | Coryphoid palm tree. Very common. |
| Spinifructus | S. antiquus | A fruit seed that may belong to the palm family (Arecaceae), closely related to the genus Astrocaryum. |
| Trapago | T. angulata | A water caltrop look-alike. |
| Trochodendroides | T. arctica | A fossil fruit that may belong to Trochodendroides nebrascensis. |
| Trochodendroides | T. ellipticum | A katsura look-alike. An uncommon taxon. |
| Trochodendroides | T. genetrix | A katsura look-alike. A common taxon. |
| Trochodendroides | T. nebrascensis | A very common taxon. |
| "Vitis" | "V." stantonii | Possibly a member of the Platanaceae rather than Vitaceae |
| "Ziziphus" | "Z." fibrillosus | A common taxon. |
| Zingiberopsis | Z. attenuata | Related to today's ginger plant. Its closest living relative is the Asian genus Alpinia. Some Hell Creek Formation specimens show damage from hispine beetles ("leaf beetles" (Wilf et al., 2000)). |
| Zingiberopsis | Z. magnifolia | Another Zinigberopsis species, previously assigned to Canna? magnifolia. |
| Zizyphoides | Z. flabella | An uncommon taxon. |

### Palynology

| Genus | Species | Location | Notes |
| Abietineaepollenites | Abietineaepollenites foveoreticulatus | Montana | A conifer palynomorph |
| Abietineaepollenites | Abietineaepollenites microalatus | Montana | A conifer palynomorph |
| Abietineaepollenites | Abietineaepollenites varius | Montana | A conifer palynomorph |
| Acanthotriletes | Acanthotriletes levidensis | Montana | |
| Alnipollenites | Alnipollenites verus | Montana | |
| Appendicisporites | Appendicisporites tricornitatus | Montana | |
| Aquilapollenites | Aquilapollenites amplus | Montana | |
| Aquilapollenites | Aquilapollenites attenuatus | | |
| Aquilapollenites | Aquilapollenites collaris | | |
| Aquilapollenites | Aquilapollenites conatus | Montana | |
| Aquilapollenites | Aquilapollenites delicatus | Montana | |
| Aquilapollenites | Aquilapollenites marmarthensis | | |
| Aquilapollenites | Aquilapollenites polaris | Montana | |
| Aquilapollenites | Aquilapollenites pulvinus | Montana | |
| Aquilapollenites | Aquilapollenites pyriformis | Montana | |
| Aquilapollenites | Aquilapollenites quadricretaeus | | |
| Aquilapollenites | Aquilapollenites quadrilobus | | |
| Aquilapollenites | Aquilapollenites reductus | Montana | |
| Aquilapollenites | Aquilapollenites reticulatus | Montana | |
| Aquilapollenites | Aquilapollenites senonicus | | |
| Aquilapollenites | Aquilapollenites turbidus | | |
| Aquilapollenites | Aquilapollenites striatus | | |
| Azolla | Azolla cretacea | Montana | A mosquito fern palynomorph |
| Balmeisporites | Balmeisporites sp. | | |
| Calamospora | Calamospora mesozoica | Montana | |
| Camarozonosporites | Camarozonosporites heskemensis | Montana | |
| Cicatricosisporites | Cicatricosisporites carlylensis | Montana | |
| Cicatricosisporites | Cicatricosisporites dorogensis | Montana | |
| Cingulatisporites | Cingulatisporites dakotaensis | Montana | |
| Cingulatisporites | Cingulatisporites scabratus | Montana | |
| Clavatricolpites | Clavatricolpites prolatus | Montana | |
| Concavisporites | Concavisporites rugulatus | Montana | |
| Concavisporites | Concavisporites rugulatus | Montana | |
| Converrucosisporites | Converrucosisporites sp. | Montana | |
| Corylus | Corylus granilabratus | Montana | A hazelnut palynomorph |
| Cupanieidites | Cupanieidites major | Montana | |
| Cyathidites | Cyathidites foveolatus | Montana | A tree fern palynomorph |
| Cyathidites | Cyathidites minor | Montana | A tree fern palynomorph |
| Cyathidites | Cyathidites australis | North Dakota | A tree fern palynomorph |
| Cyathidites | Cyathidites diaphana | North Dakota | A tree fern palynomorph |
| Cycadopites | Cycadopites scabratus | Montana | |
| Deltoidospora | Deltoidospora diaphana | Montana | |
| Dicotetradites | Dicotetradites granulatus | Montana | |
| Ephedripites | Ephedripites ovatus | Montana | |
| Ephedripites | Ephedripites undulatus | Montana | |
| Erdtmanipollis | Erdtmanipollis cretaceus | Montana South Dakota | |
| Gleicheniidites | Gleicheniidites excelsus | Montana | |
| Gleicheniidites | Gleicheniidites senonicus | Montana | |
| Gnetaceaepollenites | Gnetaceaepollenites eocenipites | Montana | |
| Haloragacidites | Haloragacidites quadratus | Montana | |
| Hamulatisporis | Hamulatisporis hamulatis | Montana | |
| Hymenophyllumsporites | Hymenophyllumsporites parvus | Montana | A fern spore palynomorph |
| Hymenophyllumsporites | Hymenophyllumsporites pseudomaximus | Montana | A fern spore palynomorph |
| Ilexpollenites | Ilexpollenites compactus | | |
| Inaperturopollenites | Inaperturopollenites rugulatus | Montana | |
| Interpollis | Interpollis cf. I. supplingensis | | |
| Kurtzipites | Kurtzipites trispissatus | Montana | |
| Kurtzipites | Kurtzipites trispissatus | Montana | |
| Laevigatosporites | Laevigatosporites anomalus | Montana | |
| Laevigatosporites | Laevigatosporites discordatus | Montana | |
| Laevigatosporites | Laevigatosporites gracilis | Montana | |
| Laevigatosporites | Laevigatosporites ovatus | Montana | |
| Liliacidites | Liliacidites variegatus | Montana | |
| Liliacidites | Liliacidites sp. | Montana | |
| Lycopodiumsporites | Lycopodiumsporites austroclavatidites | Montana | |
| Momipites | Momipites circularis | Montana | |
| Momipites | Momipites parvus | Montana | |
| Monosulcites | Monosulcites carpentieri | Montana | |
| Monosulcites | Monosulcites crescentus | Montana | |
| Monosulcites | Monosulcites latus | Montana | |
| Monosulcites | Monosulcites tectatus | Montana | |
| Monosulcites | Monosulcites sp. | Montana | |
| Myrtipites | Myrtipites granulatus | Montana | |
| Myrtipites | Myrtipites scabratus | Montana | |
| Nyssapollenites | Nyssapollenites analepticus | Montana | |
| Nyssapollenites | Nyssapollenites pseudocruciatus | Montana | |
| Osmundacidites | Osmundacidites wellmanii | Montana | |
| Pachysandra | Pachysandra cretaceae | Montana South Dakota | |
| Palmidites | Palmidites maximus | Montana | |
| Peromonolites | Peromonolites granulatus | Montana | |
| Phyllocladidites | Phyllocladidites mawsonii | Montana | |
| Phyllocladidites | Phyllocladidites ruei | Montana | |
| Podocarpidites | Podocarpidites otagoensis | Montana | |
| Pinuspollenites | Pinuspollenites labdacus | North Dakota | A pine palynomorph showing evidence of the genus Pinus in Hell Creek. |
| Pinuspollenites | Pinuspollenites sp | North Dakota | A pine palynomorph showing evidence of the genus Pinus in Hell Creek. |
| Piceapollis | Piceapollis sp | North Dakota | A spruce palynomorph |
| Polyadopollenites | Polyadopollenites psilatus | Montana | |
| Polycolpites | Polycolpites granulatus | Montana | |
| Polypodiidites | Polypodiidites inangahuensis | Montana | |
| Proteacidites | Proteacidites retusus | Montana | |
| Proteacidites | Proteacidites retusus | Montana | |
| Proteacidites | Proteacidites thalmannii | Montana | |
| Psilatricolporites | Psilatricolporites prolatus | Montana | |
| Pterocaryapollenites | Pterocaryapollenites stellatus | Montana | |
| Reticuloidosporites | Reticuloidosporites dentatus | Montana | |
| Schizosporis | Schizosporis complexus | Montana | |
| Schizosporis | Schizosporis parvus | Montana | |
| Spheripollenites | Spheripollenites subgranulatus | Montana | |
| Spinamonoporites | Spinamonoporites typicus | Montana | |
| Stereisporites | Stereisporites antiquasporites | Montana | |
| Stereisporites | Stereisporites psilatus | Montana | |
| Striainaperturites | Striainaperturites ovatus | Montana | |
| Styx | Styx major | Montana | |
| Styx | Styx minor | Montana | |
| Taxodiaceaepollenites | Taxodiaceaepollenites hiatus | Montana | |
| Triatriopollenites | Triatriopollenites granilabratus | Montana | |
| Tricolpites | Tricolpites bacustriatus | Montana | |
| Tricolpites | Tricolpites delicatulus | Montana | |
| Tricolpites | Tricolpites foveolatus | Montana | |
| Tricolpites | Tricolpites interangulus | | |
| Tricolpites | Tricolpites parvistriatus | Montana | |
| Tricolpites | Tricolpites psilascabratus | Montana | |
| Tricolpites | Tricolpites reticulatus | Montana | |
| Tricolpites | Tricolpites striatus | Montana | |
| Tricolpopollenites | Tricolpopollenites clavireticulatus | Montana | |
| Tricolpopollenites | Tricolpopollenites megaexactus | Montana | |
| Tricolpopollenites | Tricolpopollenites microreticulatus | Montana | |
| Tricolpopollenites | Tricolpopollenites microscabratus | Montana | |
| Tricolpopollenites | Tricolpopollenites sp1, sp2 | Montana | |
| Tricolporopollenites | Tricolporopollenites elongatus | Montana | |
| Tricolporopollenites | Tricolporopollenites foveotectatus | Montana | |
| Tricolporopollenites | Tricolporopollenites granustriatus | Montana | |
| Tricolporopollenites | Tricolporopollenites megaexactus | Montana | |
| Tricolporopollenites | Tricolporopollenites prolatus | Montana | |
| Tricolporopollenites | Tricolporopollenites striatus | Montana | |
| Triplanosporites | Triplanosporites sinuosus | Montana | |
| Triporopollenites | Triporopollenites rugatus | Montana | |
| Ulmipollenites | Ulmipollenites undulosus | Montana | |
| Ulmipollenites | Ulmipollenites verrucatus | Montana | |
| Ulmoideipites | Ulmoideipites tricostatus | Montana | |
| Wodehouseia | Wodehouseia spinata | Montana | |
| Zlivisporis | Zlivisporis blanensis | Montana | |