''Imperobator'' ("powerful warrior") is a genus of

paravian Paraves are a widespread group of theropod dinosaurs that originated in the Middle Jurassic period. In addition to the extinct dromaeosaurids, troodontids, anchiornithids, and possibly the scansoriopterygids, the group also contains the aviala ...

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 ...

that lived during the

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 ...

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 ...

period in what is now

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

. It contains a single species, ''I. antarcticus'', recovered from the

Snow Hill Island Formation The Snow Hill Island Formation is an Maastrichtian, Early Maastrichtian geologic Formation (geology), formation found on James Ross Island, James Ross Island group, Antarctica. Remains of a Paraves, paravian Theropoda, theropod ''Imperobator anta ...

. Before its description, it was informally nicknamed the "Naze

dromaeosaur 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 ...

", despite the lack of a "sickle claw" characteristic of the group. In 2019, the describing authors suggested that ''Imperobator'' was about the same size as ''

Utahraptor ''Utahraptor'' (meaning "Utah's thief") is a genus of large dromaeosaurid dinosaur that lived in North America during the Early Cretaceous period. It was a heavy-built, ground-dwelling, bipedal carnivore. It contains a single species, ''Utahrap ...

''.

Discovery and naming

The only described

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

is known from a single specimen, UCMP 276000, which was first uncovered in 2003 at the early Maastrichtian-aged Cape Lamb Member of the

James Ross Island James Ross Island is a large island off the southeast side and near the northeastern extremity of the Antarctic Peninsula, from which it is separated by Prince Gustav Channel. Rising to , it is irregularly shaped and extends in a north–south ...

, Antarctica. The Cape Lamb Member has been dated to the early Maastrichtian, about 71 million years ago. UCMP 276000 consists of an incomplete isolated left pes including a portion of the

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

, an incomplete

astragalus ''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...

, a partial

calcaneus In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock. S ...

and

fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity is ...

, as well as an

ungual An ungual (from Latin ''unguis'', i.e. ''nail'') is a highly modified distal toe bone which ends in a hoof, claw, or nail. Elephants and ungulates have ungual phalanges, as did the sauropods and horned dinosaurs. A claw is a highly modified ungual ...

, partial

phalanges The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...

and

metacarpals In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the skeleton, skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. The metacarpa ...

. The specimen was formally described as 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 ...

of a new genus and species, ''Imperobator antarcticus'', by Ely and Case in 2019. The generic name derives from 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 ...

for “powerful warrior“. The specific name refers to the continent in which the specimen was discovered. Before ''Imperobator'' was officially described, a paper published in 2007 announced the specimen and assigned it to the

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

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 ...

; as such it was nicknamed the "Naze dromaeosaur". This was problematic as UCMP 276000 lacked multiple characteristics of dromaeosaurids, including a prominent sickle claw, and as such the paper naming and describing ''Imperobator'' assigned it only to the clade

Paraves Paraves are a widespread group of theropod dinosaurs that originated in the Middle Jurassic period. In addition to the extinct dromaeosaurids, troodontids, anchiornithids, and possibly the scansoriopterygids, the group also contains the avial ...

Description

The holotype measures approximately 45 cm in

length Length is a measure of distance. In the International System of Quantities, length is a quantity with dimension distance. In most systems of measurement a base unit for length is chosen, from which all other units are derived. In the Interna ...

. From this, an estimated length of 3-4 meters for the animal has been obtained using measurements from related

such as ''

''. This shows that ''Imperobator'' exhibited

gigantism Gigantism ( el, γίγας, ''gígas'', "giant", plural γίγαντες, ''gígantes''), also known as giantism, is a condition characterized by excessive growth and height significantly above average. In humans, this condition is caused by ove ...

, a trait not often seen among pariavians and is best documented in

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

such as ''Utahraptor'', ''

Austroraptor ''Austroraptor'' ( ) is a genus of large dromaeosaurid dinosaur that lived during the Late Cretaceous Period in what is now Argentina. ''Austroraptor'' was a large-sized, moderately-built, ground-dwelling, bipedal carnivore, estimated at in len ...

'', ''

Deinonychus ''Deinonychus'' ( ; ) is a genus of dromaeosaurid theropod dinosaur with one described species, ''Deinonychus antirrhopus''. This species, which could grow up to long, lived during the early Cretaceous Period, about 115–108 million ye ...

'' and ''

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 ...

''. Despite prior assignment to Dromaeosauridae, ''Imperobator'' has since been assigned to the

Paraves due to certain characteristics that differ from those of dromaeosaurids, including the lack of a sickle

claw A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or tarsus ...

, the smooth surface of the distal

metatarsal The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the med ...

II and the lack of an

of the second pedal digit. Based on evidence from other paravians, ''Imperobator'' most likely had a sort of feathery covering on most of its body. It would be also safe to assume that ''Imperobator'' was

carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other sof ...

, however, whether it scavenged or hunted its

prey Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...

at this stage is unclear.

Classification

When first reported in 2007, ''Imperobator'' was largely regarded as a dromaeosaurid dinosaur, mainly because of the purpoted pedal similarities with members of this group. During the formal description of ''Imperobator'', the authors recovered this taxon as a paravian theropod, related to smaller members of the group. Later in 2019 Scott Hartman and team in their description of ''

Hesperornithoides ''Hesperornithoides'' (meaning "western bird form"; nicknamed "Lori") is a genus of troodontid theropod dinosaur that lived in North America during the Late Jurassic period. Discovery Although several Troodontid teeth were found, with the trood ...

'' conducted a large phylogenetic analysis of

Theropoda 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 ca ...

, where they recovered ''Imperobator'' (left unnamed at the time the analysis was created) as a basal (primitive) ornithomimosaur closely allied to deinocheirid ornithomimosaurs; however, an updated version of that analysis made by one of the authors (including information from its formal description) recovered it as a basal member of the

Deinonychosauria Deinonychosauria is a clade of paravian dinosaurs which lived from the Late Jurassic to the Late Cretaceous periods. Fossils have been found across the globe in North America, Europe, Africa, Asia, South America, and Antarctica,Case, J.A., Mar ...

.https://www.theropoddatabase.com/Troodontidae.htm#Imperobatorantarcticus

Paleoecology

During the time in which ''Imperobator'' lived,

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...

was much warmer and humid than it is today and as a result

was without

ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...

. The environment was mainly dominated by large dense

conifer Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single ...

forests,

cycads Cycads are seed plants that typically have a stout and woody (ligneous) trunk with a crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants of a species are either male or ...

and gingkos. Despite the warmer climate, the animals inhabiting Antarctica at this time would have to endure long periods of darkness during the

winter Winter is the coldest season of the year in polar and temperate climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Different cultures ...

, much like what happens in modern-day Antarctica.Lamanna, Matthew C.; Case, Judd A.; Roberts, Eric M.; Arbour, Victoria M.; Ely, Ricardo C.; Salisbury, Steven W.; Clarke, Julia A.; Malinzak, D. Edward; West, Abagael R.; O'connor, Patrick M. (2019).
Late Cretaceous non-avian dinosaurs from the James Ross Basin, Antarctica: description of new material, updated synthesis, biostratigraphy, and paleobiogeography
. ''Advances in Polar Science''. 30 (3): 228–250. doi:10.13679/j.advps.2019.0007. Currently a plethora of other

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 ...

organisms In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fungi; ...

from the Snow Hill Island Formation have been discovered, which are likely to have interacted with ''Imperobator'' during the Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

period. This includes the

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 ...

'' Trinisaura santamartaensis'' and ''

Morrosaurus ''Morrosaurus'' is an extinct genus of herbivorous elasmarian dinosaur that lived in the late Cretaceous in Antarctica. The only known species is the type ''Morrosaurus antarcticus''. Discovery In 2002, the Argentine paleontologist Fernando ...

antarcticus'', the Parankylosaur ''

Antarctopelta ''Antarctopelta'' (; meaning 'Antarctic shield') is a genus of ankylosaurian dinosaur, a group of large, quadrupedal herbivores, that lived during the Campanian stage of the Late Cretaceous period on what is now James Ross Island, Antarctica. '' ...

oliveroi'', the

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimo ...

'' Notidanodon'' sp. as well as some intermediate

elasmosaurids Elasmosauridae is an extinct family of plesiosaurs, often called elasmosaurs. They had the longest necks of the plesiosaurs and existed from the Hauterivian to the Maastrichtian stages of the Cretaceous, and represented one of the two groups of p ...

titanosaurs Titanosaurs (or titanosaurians; members of the group Titanosauria) were a diverse group of sauropod dinosaurs, including genera from all seven continents. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still thr ...

and a

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...

References

{{Taxonbar, from=Q63135067 Prehistoric paravians Maastrichtian life Cretaceous Antarctica Dinosaurs of Antarctica Fossil taxa described in 2019