''Acheroraptor'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

dromaeosaurid Dromaeosauridae () is a family of feathered theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek ('), meaning ...

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

known from the latest

Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval from ...

Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation stretches over portions of ...

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...

, United States. It contains a single species, ''Acheroraptor temertyorum''. ''A. temertyorum'' is one of the two geologically youngest known species of dromaeosaurids, the other being ''

Dakotaraptor ''Dakotaraptor'' (meaning “thief from Dakota”) is a potentially chimaeric genus of large dromaeosaurid theropod dinosaur that lived in North America during the Late Cretaceous period. The remains have been found in the Maastrichtian stage of ...

'', which is also known from Hell Creek. A basal cousin of ''Velociraptor'', ''Acheroraptor'' is known from upper and lower jaw material.

Discovery and naming

''Acheroraptor'' was first described and named by David C. Evans, Derek W. Larson and Philip J. Currie in

2013 File:2013 Events Collage V2.png, From left, clockwise: Edward Snowden becomes internationally famous for leaking classified NSA wiretapping information; Typhoon Haiyan kills over 6,000 in the Philippines and Southeast Asia; The Dhaka garment fact ...

and the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...

is ''Acheroraptor temertyorum''. The generic name is derived from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

Ἀχέρων, ''Acheron'', "underworld", in reference to the provenance from the

, and the

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

''raptor'', "thief". The specific name honours James and Louise Temerty, the chairman of Northland Power and the

ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality, a medical classification to estimate the likelihood of death for a patient * Rupture of membranes, a term used during pregnancy to describe a rupture of the amniotic sac * ...

Board of Governors A board of directors (commonly referred simply as the board) is an executive committee that jointly supervises the activities of an organization, which can be either a for-profit or a nonprofit organization such as a business, nonprofit organi ...

and his wife, who have supported the museum for many years. ''Acheroraptor'' is known from the

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

ROM 63777, a complete right

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

with several maxillary teeth (some isolated), and from a referred dentary (

lower jaw In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

) ROM 63778, both housed at the

Royal Ontario Museum The Royal Ontario Museum (ROM) is a museum of art, world culture and natural history in Toronto, Ontario, Canada. It is one of the largest museums in North America and the largest in Canada. It attracts more than one million visitors every year ...

, Canada. Both specimens were collected approximately four metres from one another, from the same mixed faunal bonebed that occurs in the upper part of the Hell Creek Formation of

, dating to the latest

stage of the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...

, immediately prior to the

Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...

. The holotype specimen was collected on August 28, 2009, by commercial fossil hunters, one of whom also collected the dentary several years later, and who were later able to provide detailed geographic data from

GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...

and photographs of the specimen ''

in situ ''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...

'' in the ground on the day of discovery. Both specimens were subsequently purchased by the Royal Ontario Museum from a private collector.

Classification

The

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

position of ''Acheroraptor'' was explored by Evans ''et al.'' (2013) using several data matrices. Both specimens of ''Acheroraptor'' were coded as a single taxon into Turner ''et al.'' (2012) data matrix, an extensive phylogenetic analysis of theropods that focuses on

maniraptora Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the non-avian dinosaurs that were more closely related to them than to ''Ornithomimus velox''. It contains the major subgroups Avialae, Deinonychosauria, Oviraptoros ...

ns. ''Acheroraptor'' was recovered as a member of the clade containing

Eudromaeosauria Eudromaeosauria ("true dromaeosaurs") is a subgroup of terrestrial dromaeosaurid theropod dinosaurs. They were relatively small to medium-sized, feathered hypercarnivores (with diets consisting almost entirely of other terrestrial vertebrates) ...

and

Microraptorinae Microraptoria (Greek, μίκρος, ''mīkros'': "small"; Latin, ''raptor'': "one who seizes") is a clade of basal dromaeosaurid theropod dinosaurs. The first microraptorians appeared 125 million years ago in China. Many are known for long feather ...

, confirming its referral to the

Dromaeosauridae Dromaeosauridae () is a family of feathered theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek ('), meaning ...

, and possibly to Eudromaeosauria. Within that clade, however, most taxa were recovered in a large

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...

, due to the limited codings available for ''Acheroraptor''. Evans ''et al.'' (2013) also coded the specimens of ''Acheroraptor'' (together and separately) into an updated version of the smaller, dromaeosaur-specific dataset from Longrich and Currie (2009). ''

Velociraptor osmolskae ''Velociraptor'' (; ) is a genus of small dromaeosaurid dinosaur that lived in Asia during the Late Cretaceous epoch, about 75 million to 71 million years ago. Two species are currently recognized, although others have been assigned in the pa ...

'' and ''

Balaur bondoc ''Balaur bondoc'' is a species of theropod dinosaur from the late Cretaceous period, in what is now Romania. It is the type species of the monotypic genus ''Balaur'', after the ''balaur'' (), a dragon of Romanian folklore. The specific name ''b ...

'' were added, ''

Itemirus ''Itemirus'' is a genus of dromaeosaurid theropod dinosaur from the Turonian age of the Late Cretaceous period of Uzbekistan. Discovery ''Itemirus'' is known from a single small damaged fossil braincase or neurocranium, in 1958 found near the ...

'' was excluded (following its identification as a

tyrannosauroid Tyrannosauroidea (meaning 'tyrant lizard forms') is a superfamily (or clade) of coelurosaurian theropod dinosaurs that includes the family Tyrannosauridae as well as more basal relatives. Tyrannosauroids lived on the Laurasian supercontinent b ...

by Miyashita and Currie (2009)), and following Turner ''et al.'' (2012) the codings for '' Adasaurus mongoliensis'' were separated into these based on the holotype from the

Nemegt Formation The Nemegt Formation (also known as Nemegtskaya Svita) is a geological formation in the Gobi Desert of Mongolia, dating to the Late Cretaceous. The formation consists of river channel sediments and contains fossils of fish, turtles, crocodilian ...

, and these based on IGM 100/23 from the Bayanshiree Formation. Several characters were also rescored and modified, and two maxillary characters were added to the matrix from Turner ''et al.'' (2012). This analysis yielded a more resolved topology, placing ''Acheroraptor'' in a relatively basal position within the

Velociraptorinae Velociraptorinae is a subfamily of the theropod group Dromaeosauridae. The earliest velociraptorines are probably ''Nuthetes'' from the United Kingdom, and possibly '' Deinonychus'' from North America. However, several indeterminate velociraptor ...

, which was otherwise found to include only Asian dromaeosaurids. The

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...

below shows the phylogenetic position of ''Acheroraptor'' following this analysis. A 2022 study of eudromaeosauria reclassified ''Acheroraptor'' as a derived member of

Saurornitholestinae Saurornitholestinae is a subfamily of the theropod group Dromaeosauridae. The saurornitholestines currently include three monotypic genera: ''Atrociraptor marshalli'', ''Bambiraptor feinbergi'', and ''Saurornitholestes langstoni''. All are medium ...

, with ''

Atrociraptor ''Atrociraptor'' (meaning "savage thief") is a genus of saurornitholestine dromaeosaurid theropod dinosaur from the Late Cretaceous (Maastrichtian stage) of Alberta, Canada. The type (and only) specimen of ''Atrociraptor'', holotype RTMP 95.1 ...

'' as its sister taxon.

Paleoecology

''Acheroraptor'' is the youngest species of dromaeosaurid, and is from the Hell Creek Formation. The Hell Creek Formation is from the time of the Cretaceous-Paleogene Extinction Event, and has been dated to 66 ± 0.07 million years ago. Many animals and plants have been found in the Hell Creek Formation, the discovery site of ''Acheroraptor''. As it is known from the ''

Tyrannosaurus ''Tyrannosaurus'' is a genus of large theropoda, theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosa ...

''-''

Triceratops ''Triceratops'' ( ; ) is a genus of herbivore, herbivorous Chasmosaurinae, chasmosaurine Ceratopsidae, ceratopsid dinosaur that first appeared during the late Maastrichtian stage of the Late Cretaceous Period (geology), period, about 68 m ...

'' fauna, ''Acheroraptor'' is the youngest dromaeosaurid, along with the much larger ''

''. Evans ''et al.'' found that ''Acheroraptor'' was the only dromaeosaurid from the Hell Creek Formation. Common teeth previously referred to ''

Dromaeosaurus ''Dromaeosaurus'' (, "running lizard") is a genus of dromaeosaurid theropod dinosaur which lived during the Late Cretaceous period (middle late Campanian and Maastrichtian), sometime between 80 and 69 million years ago, in Alberta, Canada and the ...

'' and ''

Saurornitholestes ''Saurornitholestes'' ("lizard-bird thief") is a genus of carnivorous dromaeosaurid theropod dinosaur from the late Cretaceous of Canada (Alberta) and the United States (Montana, New Mexico, Alabama, North Carolina, and South Carolina). Two spec ...

'' would then be considered ''Acheroraptor''. Evans ''et al.'' also concluded that there was probably only one dromaeosaurid in the Hell Creek-Lance assemblage. In 2015, this view was disproven, with the description of ''

'', a far larger second dromaeosaur from the formation. Other non-dromaeosaurid theropods from the formation are

tyrannosaurids Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family (biology), family of coelurosaurian Theropoda, theropod dinosaurs that comprises two subfamilies containing up to thirteen genus, genera, including the eponymous ''Tyrannos ...

ornithomimids Ornithomimidae (meaning "bird-mimics") is a family of theropod dinosaurs which bore a superficial resemblance to modern ostriches. Ornithomimids were fast, omnivorous or herbivorous dinosaurs known mainly from the Late Cretaceous Period of Lauras ...

troodontids Troodontidae is a clade of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinosaurian lineages. More recent fossil discov ...

birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

, and

caenagnathids Caenagnathidae is a family of bird-like maniraptoran theropod dinosaurs from the Cretaceous of North America and Asia. They are a member of the Oviraptorosauria, and close relatives of the Oviraptoridae. Like other oviraptorosaurs, caenagnathi ...

. The tyrannosaurids from the formation are ''

Nanotyrannus ''Tyrannosaurus'' is a genus of large theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosaurus'' live ...

'' and ''Tyrannosaurus'', although the former might be a junior synonym of the latter. Among ornithomimids are the genera ''

Struthiomimus ''Struthiomimus'' (meaning "ostrich mimic", from the Ancient Greek, Greek στρούθειος/''stroutheios'' meaning "of the ostrich" and μῖμος/''mimos'' meaning "mimic" or "imitator") is a genus of Ornithomimidae, ornithomimid dinosaurs ...

'' as well as ''

Ornithomimus ''Ornithomimus'' (; "bird mimic") is a genus of ornithomimid dinosaurs from the Late Cretaceous Period of what is now North America. ''Ornithomimus'' was a swift bipedal theropod which fossil evidence indicates was covered in feathers, equippe ...

'', and "

Orcomimus This list of informally named dinosaurs is a listing of dinosaurs (excluding Aves; birds and their extinct relatives) that have never been given formally published scientific names. This list only includes names that were not properly published ...

." The birds known from the formation are ''

Avisaurus ''Avisaurus'' (meaning "bird lizard") is a genus of enantiornithine bird from the Late Cretaceous of North America. Discovery ''Avisaurus archibaldi'' was discovered in the Late Cretaceous Hell Creek Formation of North America (Maastrichtian, fr ...

'', ''

Brodavis ''Brodavis'' is a genus of freshwater hesperornithiform birds known from the Late Cretaceous (possibly Campanian and Maastrichtian stage) of North America and Asia. It was first described and named by Larry D. Martin, Evgeny N. Kurochkin and Tim ...

baileyi'', and two unnamed hesperornithoforms, possibly ''

Potamornis ''Potamornis'' is a prehistoric bird genus that dated back to the late Maastrichtian age of the late Cretaceous period. Its scrappy remains were found in the Lance Formation at Buck Creek, USA, and additional possible remains were found in the u ...

''. Only three

oviraptorosaurs Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot-like skulls, with or ...

are from the Hell Creek Formation, ''

Anzu Anzu may refer to: *Anzû, a divine storm-bird in several Ancient Mesopotamian religion, Mesopotamian religions *Anzu (dinosaur), ''Anzu'' (dinosaur), a genus of theropod dinosaur containing the species ''Anzu wyliei'' As a given name is a Japan ...

'', '' Leptorhynchos'' and third and undescribed specimen, very similar to ''

Gigantoraptor ''Gigantoraptor'' () is a genus of large oviraptorosaur dinosaur that lived in Asia during the Late Cretaceous period. It is known from the Iren Dabasu Formation of Inner Mongolia, where the first remains were found in 2005. ''Gigantoraptor'' wa ...

'', from South Dakota. However, only fossilized foot prints were discovered as of 2016. The known troodontids from this formation include ''

Troodon ''Troodon'' ( ; ''Troödon'' in older sources) is a wastebasket taxon and a dubious genus of relatively small, bird-like dinosaurs known definitively from the Campanian age of the Late Cretaceous period (about 77 mya). It includes at least ...

'', ''

Pectinodon ''Pectinodon'' is a genus of troodontid dinosaurs from the Late Cretaceous period (66 mya). It currently contains a single valid species, ''Pectinodon bakkeri'' (sometimes classified as ''Troodon bakkeri''), known only from teeth. In 1982, ...

'', and ''

Paronychodon ''Paronychodon'' (meaning "beside claw tooth") was a theropod dinosaur genus. It is a tooth taxon, often considered dubious because of the fragmentary nature of the fossils, which include "buckets" of teeth from many disparate times and places b ...

''. A single species of

coelurosaur Coelurosauria (; from Greek, meaning "hollow tailed lizards") is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. Coelurosauria is a subgroup of theropod dinosaurs that includes compsognathids, tyran ...

is known from similar fossil formations includes ''

Richardoestesia ''Richardoestesia'' is a morphogenus of theropod dinosaur teeth, originally described from the Late Cretaceous of what is now North America. It currently contains two species, ''R. gilmorei'' and ''R. isosceles''. It has been used as a morphota ...

''.

Ornithischia Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek s ...

ns are abundant in the Hell Creek Formation. The main groups of ornithischians are

ankylosauria Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. ...

ns,

ornithopods Ornithopoda () is a clade of ornithischian dinosaurs, called ornithopods (), that started out as small, bipedal running grazers and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world ...

ceratopsia Ceratopsia or Ceratopia ( or ; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. ...

ns, and

pachycephalosauria Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, most pachycephal ...

ns. One ankylosaurian and two nodosaurians are known, ''

Ankylosaurus ''Ankylosaurus'' is a genus of armored dinosaur. Its fossils have been found in geological formations dating to the very end of the Cretaceous Period, about 68–66 million years ago, in western North America, making it among the last of the n ...

'', ''

Denversaurus ''Denversaurus'' (meaning "Denver lizard") is a genus of panoplosaurin nodosaurid dinosaur from the Late Cretaceous (late Maastrichtian) of western North America. Although at one point treated as a junior synonym of ''Edmontonia'' by some t ...

'' and possibly ''

Edmontonia ''Edmontonia'' is a genus of panoplosaurin nodosaurid dinosaur from the Late Cretaceous Period. It is part of the Nodosauridae, a family within Ankylosauria. It is named after the Edmonton Formation (now the Horseshoe Canyon Formation in Canada) ...

''. Multiple genera of ceratopsians are known from the formation, the

leptoceratopsid Leptoceratopsidae is an extinct family of neoceratopsian dinosaurs from Asia, North America and Europe. Leptoceratopsids resembled, and were closely related to, other neoceratopsians, such as the families Protoceratopsidae and Ceratopsidae, bu ...

Leptoceratops ''Leptoceratops'' (meaning 'Thin-horned face' and derived from Greek ''lepto-/λεπτο-'' meaning 'small', 'insignificant', 'slender', 'meagre' or 'lean', ''kerat-/κερατ-'' meaning 'horn' and ''-ops/ωψ'' meaning face), is a genus of l ...

'' and the

chasmosaurines Chasmosaurinae is a subfamily of ceratopsid dinosaurs. They were one of the most successful groups of herbivores of their time. Chasmosaurines appeared in the early Campanian, and became extinct, along with all other non-avian dinosaurs, during t ...

Nedoceratops ''Nedoceratops'' (meaning "insufficient horned face") is a controversial genus of ceratopsid dinosaur from the Late Cretaceous period Lance Formation of North America. It is known only from a single skull discovered in Wyoming. Its status is the ...

'', ''

Torosaurus ''Torosaurus'' ("perforated lizard", in reference to the large openings in its frill) is a genus of herbivorous ceratopsid dinosaur that lived during the late Maastrichtian stage of the Cretaceous period, between 68 and 66 million years ago, th ...

'', ''Triceratops'', and ''

Tatankaceratops ''Tatankaceratops'' (meaning "Bison horn face") is a controversial genus of herbivorous ceratopsian dinosaur. It is a small chasmosaurine ceratopsian which lived during the Late Cretaceous period (latest Maastrichtian stage) in what is now South ...

''. Hadrosaurs are common in the Hell Creek Formation, and are known from multiple species of the ornithopod ''

Thescelosaurus ''Thescelosaurus'' ( ; ancient Greek - (''-'') meaning "godlike", "marvellous", or "wondrous" and (') "lizard") was a genus of small neornithischian dinosaur that appeared at the very end of the Late Cretaceous period in North America. It was ...

'', and the hadrosaurids ''

Edmontosaurus ''Edmontosaurus'' ( ) (meaning "lizard from Edmonton") is a genus of hadrosaurid (duck-billed) dinosaur. It contains two known species: ''Edmontosaurus regalis'' and ''Edmontosaurus annectens''. Fossils of ''E. regalis'' have been found in rocks ...

'', and an undescribed genus similar to ''

Parasaurolophus ''Parasaurolophus'' (; meaning "near crested lizard" in reference to '' Saurolophus)'' is a genus of herbivorous hadrosaurid ornithopod dinosaur that lived in what is now North America and possibly Asia during the Late Cretaceous Period, abou ...

''. Five pachycephalosaurians have been found in the Hell Creek Formation. Among them are the derived pachycephalosaurids ''

Sphaerotholus ''Sphaerotholus'' is a genus of pachycephalosaurid dinosaur from the Upper Cretaceous of the western United States and Canada. To date, three species have been described: the type species, ''S. goodwini'', from the Den-na-zin Member of the Kirtla ...

'', ''

Stygimoloch ''Pachycephalosaurus'' (; meaning "thick-headed lizard", from Greek ''pachys-/'' "thick", ''kephale/'' "head" and ''sauros/'' "lizard") is a genus of pachycephalosaurid dinosaurs. The type species, ''P. wyomingensis'', is the only known species ...

'', ''

Dracorex ''Pachycephalosaurus'' (; meaning "thick-headed lizard", from Greek ''pachys-/'' "thick", ''kephale/'' "head" and ''sauros/'' "lizard") is a genus of pachycephalosaurid dinosaurs. The type species, ''P. wyomingensis'', is the only known species, ...

'', ''

Pachycephalosaurus ''Pachycephalosaurus'' (; meaning "thick-headed lizard", from Greek ''pachys-/'' "thick", ''kephale/'' "head" and ''sauros/'' "lizard") is a genus of pachycephalosaurid dinosaurs. The type species, ''P. wyomingensis'', is the only known species, ...

'', and an undescribed specimen from North Dakota. Mammals are plentiful in the Hell Creek Formation. Groups represented include

multituberculates Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, ...

metatheria Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well ...

ns, and

eutheria Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials. Eutherians are distinguished from noneutherians by various phenotypic tra ...

ns. The multituberculates represented include ''

Paracimexomys ''Paracimexomys'' is a genus of extinct mammals in the also extinct Multituberculata order. ''Paracimexomys'' lived during the Cretaceous period. The few fossils remains come from North America. Some Romanian fossils were also tentatively assig ...

'', the cimolomyids '' Paressonodon'', '' Meniscoessus'', ''

Essonodon ''Essonodon'' is a mammal genus from the Upper Cretaceous of North America. It was a member of the extinct order Multituberculata and lived towards the end of the "age of the dinosaurs." It is within the suborder Cimolodonta and perhaps the ...

'', ''

Cimolomys ''Cimolomys'' is a mammal genus from the Upper Cretaceous of North America. It was a member of the extinct order Multituberculata within the suborder Cimolodonta and family Cimolomyidae. The genus ''Cimolomys'' was named by Othniel Charles Mar ...

'', ''

Cimolodon ''Cimolodon'' is a genus of the extinct mammal order of Multituberculata within the suborder Cimolodonta and the family Cimolodontidae. Specimens are known from the Late Cretaceous of North America. Systematics The genus ''Cimolodon'' was nam ...

'', and ''

Cimexomys ''Cimexomys'' is an extinct North American mammal that lived from the Upper Cretaceous to the Paleocene. For a while, it shared the world with dinosaurs, but outlived them. It was a member of the extinct order Multituberculata and lies within t ...

''; and the neoplagiaulacids ''

Mesodma ''Mesodma'' is an extinct genus of mammal, a member of the extinct order Multituberculata within the suborder Cimolodonta, family Neoplagiaulacidae. It lived during the upper Cretaceous and Paleocene Periods of what is now North America. The e ...

'', and ''

Neoplagiaulax ''Neoplagiaulax'' is a mammal genus from the Paleocene of Europe and North America. In the case of the latter continent, there may possibly be some slightly earlier, Upper Cretaceous material too. It existed in the age immediately following the e ...

''. The alphadontids ''

Alphadon ''Alphadon'' (meaning "first tooth") is an extinct genus of small, primitive mammal that was a member of the metatherians, a group of mammals that includes modern-day marsupials. Its fossils were first discovered and named by George Gaylord Sim ...

'', '' Protalphodon'', and '' Turgidodon'', pediomyids '' Pediomys'', '' Protolambda'', and '' Leptalestes'', the stagodontid ''

Didelphodon ''Didelphodon'' (from ''is''/nowiki>.html" ;"title="/nowiki>''is''/nowiki>">/nowiki>''is''/nowiki> "opossum" plus "tooth") is a genus of stagodont metatherians from the Late Cretaceous of North America. Description Although perhaps little l ...

'', the deltatheridiid ''

Nanocuris ''Nanocuris'' is an extinct genus of Deltatheridiidae from the Cretaceous of Canada (Saskatchewan) and United States (Wyoming - Lance Formation).R. C. Fox, C. S. Scott, and H. N. Bryant. 2007. A new, unusual therian mammal from the Upper Cretac ...

'', the herpetotheriid '' Nortedelphys'', and the glasbiid '' Glasbius'' all represent metatherians of the Hell Creek Formation. A few eutherians are known, being represented by '' Alostera'', ''

Protungulatum ''Protungulatum'' ('first ungulate') is a extinct genus of pan-euungulate mammals within extinct family Protungulatidae, and also one of the earliest known placental mammals in the fossil record, that lived in North America from the Late Creta ...

'', the cimolestids ''

Cimolestes ''Cimolestes'' (from Ancient Greek , 'chalk robber') is a genus of early eutherians with a full complement of teeth adapted for eating insects and other small animals. Paleontologists have disagreed on its relationship to other mammals, in part b ...

'' and '' Batodon'', the gypsonictopsid ''

Gypsonictops ''Gypsonictops'' is an extinct genus of leptictidan mammals of the monotypic family Gypsonictopidae, which was described in 1927 by George Gaylord Simpson. Species in this genus were small mammals and the first representatives of the order Lepti ...

'', and the possible nyctitheriid '' Paranyctoides''.

References

{{Taxonbar, from=Q15215620 Eudromaeosaurs Late Cretaceous dinosaurs of North America Fossil taxa described in 2013 Maastrichtian life Taxa named by Philip J. Currie Hell Creek fauna Paleontology in Montana Maastrichtian genus first appearances Maastrichtian genus extinctions

Discovery and naming

Classification

Paleoecology

See also

References