| Nanotyrannus | |
|---|---|
 | |
| Mounted skeleton of theN. lethaeusholotype (BMRP 2002.4.1, nicknamed "Jane") at theBurpee Museum | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Tyrannosauroidea |
| Clade: | †Eutyrannosauria |
| Clade: | †Nanotyrannidae Zanno & Napoli, 2025 |
| Genus: | †Nanotyrannus Bakker, Williams &Currie, 1988 |
| Type species | |
| †Gorgosaurus lancensis Gilmore, 1946 | |
| Species | |
| Synonyms | |
Genus synonymy
Species synonymy
| |
Nanotyrannus (/ˈnænoʊˌtaɪˌrænəs, -tɪ- /,NAN-o-ti-RAN-us)[2] is agenus oftyrannosauroid dinosaur that lived in what is now western North America during theMaastrichtian age of theLate Cretaceous,67 to 66 million years ago. Its fossils are known from theHell Creek Formation. The first named species,N. lancensis, wasdescribed as a new species ofGorgosaurus in 1946 byCharles W. Gilmore based on a single skull. Re-examination of the specimen in 1988 byRobert T. Bakker, Michael Williams andPhilip J. Currie moved the species to a new genus oftyrannosaurid, namedNanotyrannus in reference to its small body size compared to other tyrannosaurids.
Subsequent research indicated that the skull belonged to an immature animal, leading many researchers to favor its identification as a juvenileTyrannosaurus rex. Its taxonomic status has since been a subject of intense scientific debate. In 2025,Lindsay Zanno and James Napoli published an exhaustive revision ofNanotyrannus. Therein, they described a complete tyrannosauroid skeleton from theHell Creek Formation, nicknamed "Bloody Mary" (part of theDueling Dinosaurs specimen), which they demonstrated to be a matureN. lancensis individual. These researchers named a second species,N. lethaeus, based on the"Jane" specimen.
Nanotyrannus was a mid-sized tyrannosauroid. The adultN. lancensis may have weighed nearly 704 kg (1,552 lb) based on the "Bloody Mary" specimen.N. lethaeus was larger, and mature individuals may have reached a body mass of about 1,200 kg (2,600 lb). The morphology ofNanotyrannus combines characteristics from more basal tyrannosauroids, such asMoros andXiongguanlong, with more derived tyrannosaurids. UnlikeTyrannosaurus,Nanotyrannus had a slender, low skull, and small crests or horns immediately in front of its eyes. The muscle attachment sites at the back of its skull were large, suggesting that it could strongly flex its head from side to side. Its exact tooth count was variable. The teeth at the front of its upper jaw were notserrated, unlike many other tyrannosaurs. The rest of the teeth were serrated and compressed from side to side, a condition known asziphodonty, which is not seen in tyrannosaurids.Nanotyrannus was longer-legged than contemporary taxa, suggesting that it was apursuit predator, and had forelimbs which were both absolutely and proportionally larger than its relatives.
Nanotyrannus is the only definite representative of thecladeNanotyrannidae, though onephylogenetic hypothesis suggests close relations toAppalachiosaurus andDryptosaurus within this clade, and thatNanotyrannus descended from a lineage ofAppalachian tyrannosaurs which migrated to the other North American island continent,Laramidia, with the recession of theWestern Interior Seaway.
Nanotyrannus isbased onCMNH 7541, a skull collected in 1942 byDavid Hosbrook Dunkle. It wasdescribed in 1946 byCharles W. Gilmore, who classified it as a new species in the tyrannosaurid genusGorgosaurus,G. lancensis.[3] The specific name of the species,lancensis, references theLance Formation, since Gilmore initially considered theHell Creek Formation—where the holotype was found—to be a member of the Lance Formation, hence noted as "Hell Creek member",[3] but the type locality has since been corrected as the Hell Creek Formation.[4]
In 1988, the specimen was re-described byRobert T. Bakker, Michael Williams, then the curator ofpaleontology at theCleveland Museum of Natural History, andPhil Currie, where the original specimen was housed and is now on display. They named the new genusNanotyrannus for the specimen, stating that the name derives from theLatin wordsnano, meaning'small', andtyrannus, meaning'tyrant'.[4] The etymology has subsequently been clarified to be derived from theGreek wordsνάνος (nanos), meaning'dwarf' or'pygmy', andτύραννος (tyrannos), meaning'king'.[2] The initial research by Bakker and colleagues indicated that the skull bones were fused, therefore implying it represents an adult specimen.[4] The possibility that the holotype represents a juvenileTyrannosaurus rex was first suggested byAnatoly Konstantinovich Rozhdestvensky in 1965.[5] Subsequent analysis of the specimen byThomas Carr in 1999 indicated that it was immature, leading Carr and many other paleontologists to consider it a juvenile specimen ofT. rex.[6][7]
In 2001, a more complete juvenile tyrannosaur (nicknamed "Jane",accession number BMRP 2002.4.1), suggested to belong to the same species as the originalNanotyrannus specimen, was uncovered. This discovery prompted a conference on tyrannosaurs focused on the issues of the validity ofNanotyrannus, held at theBurpee Museum of Natural History in 2005. Several paleontologists who had previously published opinions thatN. lancensis was a valid species, including Currie and Williams, saw the discovery of "Jane" as a confirmation thatNanotyrannus was, in fact, a juvenileT. rex.[8][9][10]Peter Larson continued to support the hypothesis thatNanotyrannus lancensis was a separate but closely related species, and also argued thatStygivenator (LACM 28471), which is generally considered to be a juvenile ofTyrannosaurus rex, could be a younger specimen ofNanotyrannus.[11][12]
In late 2011, news reports about a 2006 discovery of a new, virtually complete theropod specimen found alongside aceratopsid were made. The specimens were studied by Robert Bakker and Peter Larson on-site, who identified the theropod asNanotyrannus and the ceratopsian as a taxon possibly distinct fromTriceratops.[13] It was impossible to determine whether the theropod specimen, nicknamed "Bloody Mary", was distinct fromT. rex, as the specimen remained in private hands until 2020, when the ownership of the specimen was decided by theMontana Supreme Court to be given to the land-owners Mary Anne and Lige Murray, who agreed to sell the paired fossils to a U.S.-based museum.[14] The fossil was acquired by theNorth Carolina Museum of Natural Sciences in 2020, after which the museum built an additional research lab space in which toprepare, study, and display the specimens.[15][16] In 2025, paleontologistsLindsay Zanno and James Napoli published an initial description of the "Bloody Mary" specimen as part of an extensive revision of the genusNanotyrannus. They identified this specimen as a skeletally mature individual ofNanotyrannus and provided several lines of evidence supporting the validity of this genus. They also described a second species,N. lethaeus, based on the Jane specimen due to notable differences between it andN. lancensis.[1]
The specific name for the second species,lethaeus, comes from the Latin, and is the adjective ofLethe, referring to theriverLethe fromGreco-Roman mythology, which flows throughthe underworld (often equated tohell). The name references both the Hell Creek Formation, where the specimen was found, and the mythological function of drinking from the Lethe: to forget a prior life and be reincarnated.[1]
The primary difference that some scientists have used to argue for the validity ofNanotyrannus lancensis concerns the number of teeth. Specimens referred toN. lancensis have 15-16 teeth on each side of the maxilla (upper jaw) and 16-18 teeth on each side of the dentary (lower jaw), whereas adultT. rex specimens had 11–12 tooth positions in the upper jaw and 12–13 in the lower.[17] The exact implications of this difference in tooth count have not been universally accepted, however. In his 1999 study of tyrannosaurid growth patterns, Carr showed that, inGorgosaurus libratus, the number of teeth decreased as the animal grew, and he used this data to support the hypothesis thatN. lancensis is simply a juvenileT. rex.[6] Takanobu Tsujihi and colleagues, who studied growth in the relatedTarbosaurus bataar, found little to no decrease in tooth count during growth, even though they had juvenile specimens much younger than theNanotyrannus specimens. These researchers also noted, however, that bothTyrannosaurus andGorgosaurus show significant differences in tooth count between individuals of the same age group, and that tooth count may vary on an individual basis unrelated to growth.[7]
Larson has also contended that, along with skull features,Nanotyrannus can also be distinguished fromTyrannosaurus by proportionally larger hands with phalanges on the third metacarpal and in thefurcula morphology.[11] Another difference cited in support of the validity ofN. lancensis is the presence of a small foramen on thequadratojugal. The holotypes of bothNanotyrannus species have this feature, which is also seen in the adult specimen ofDaspletosaurus horneri and an isolated quadratojugal from Alberta, indicating that this feature is seen in other distantly-related tyrannosaurs.[18]
A limb proportion analysis published in 2016 suggested thatNanotyrannus specimens have differing levels of cursoriality, cited as a potential difference betweenN. lancensis andT. rex.[19] However, paleontologist Manabu Sakomoto commented that this conclusion may be impacted by low sample size, and the discrepancy does not necessarily reflect taxonomic distinction.[20] A 2020 study by Holly N. Woodward and colleagues indicated that the specimens referred toNanotyrannus are ontogenetically immature and found it probable that these specimens belonged toTyrannosaurus rex.[21] Yun (2015) claimed that all of the differences used to distinguishNanotyrannus were individually or ontogenetically variable features, or the products oftaphonomic distortion of the bones.[22][23]
In 2024, Nicholas Longrich and Evan T. Saitta re-examined the holotype and referred specimens ofNanotyrannus. Based on several factors, including differences in morphology, ontogeny, and phylogeny, Longrich and Saitta suggested thatNanotyrannus is a distinct taxon that may fall outside ofTyrannosauridae, based on some of their phylogenetic analyses.[24] Voris et al. (June 2025) suggested that the scoring ofNanotyrannus in a more basal position by Longrich & Saitta (2024) may have been caused by "scoring characteristics related to immaturity" and problematic data sets, supporting the previous interpretation thatNanotyrannus represents a juvenileT. rex.[25] Later in June 2025,Gregory S. Paul also supported a basaleutyrannosaurian (non-tyrannosaurid) position forNanotyrannus, emphasizing the validity and distinction of this genus as part of his preferred "multiple small taxa hypothesis" (MSTH) for theropod diversity in the Hell Creek Formation. Based on the MSTH, Paul also argued for the validity ofStygivenator, a Hell Creek theropod known from a fragmentary skull, and claimed thatN. lancensis has been treated as awastebasket taxon that likely comprises fossils of multiple taxa.[26]
In October 2025, the validity ofNanotyrannus was affirmed in an extensive paper byLindsay Zanno and James Napoli, in which the "Bloody Mary" specimen (NCSM 40000) was re-examined. The authors found that NCSM 40000 was skeletally mature at the time of its death, based on 25 cyclical growth marks indicating at least 14 years of growth, and that it had died between the ages of 17–22. In the same paper, Zanno and Napoli concluded that anotherNanotyrannus specimen, Jane (BMRP 2002.4.1), which they assigned toN. lethaeus, died between the ages of 8–14.[17] For these specimens to fit within the ontogenetic range ofTyrannosaurus, that taxon would need to spend the first two decades of its life growing slowly before entering an abrupt plateau, and finally re-entering a lengthy growth phase—an ontogeny pattern not seen in anyarchosaur that contradicts skeletal fusion patterns observed in NCSM 40000. Furthermore, they concluded that many of the traits commonly cited as ontogenetically plastic (i.e. variations in tooth count) were subject instead to individual variation. Further, they noted that certain traits, such as the "obliteration and resorption of asinus" which would be necessary forNanotyrannus andTyrannosaurus to represent the same taxon, have no precedent among livingamniotes. Bone proportions were another difference cited by Zanno and Napoli, as some of the arm, wrist, and hand bones ofNanotyrannus are larger than equivalent bones inTyrannosaurus. There are no amniotes known to the authors that undergo any degree of absolute limb shrinkage, merely instances where limbs grew slower than the rest of the body and became proportionally smaller.[1]
Zanno and Napoli's paper has led some paleontologists who were previously skeptical ofNanotyrannus to accept it as a valid taxon. Authors who have authored papers questioningNanotyrannus' validity, such asThomas Carr,Steve Brusatte and David Hone,[27] accepted that the "Bloody Mary" specimen was a distinct species fromTyrannosaurus rex, with Brusatte commenting that the “overarching mic drop of this paper...is that Nanotyrannus is real.”[28] Nevertheless, other paleontologists likeHolly Woodward Ballard urged caution before accepting the findings,[29] and Carr and Brusatte objected to the classification of the juvenile tyrannosaurid BMRP 2002.4.1 "Jane" asNanotyrannus lethaeus, with Carr opining thatNanotyrannus should be considered a species within theTyrannosaurus genus.[30]
Nanotyrannus was a mid-sized tyrannosauroid. Although no body size estimates were given in the 1988 description of the holotype, the accompanyingpress release noted that the animal would have reached approximately 17 ft (5.2 m) long and weighed 1,000 lb (450 kg).[31] In their 2025 publication arguing for the validity of the genus, Zanno and Napoli provided body mass estimates of 703.78 kg (1,551.6 lb) forN. lancensis (specifically NCSM 40000) and over 833 kg (1,836 lb) forN. lethaeus (the holotype, BMRP 2002.4.1). BMRP 2002.4.1 was not mature at the time of its death, however, and adultN. lethaeus may have weighed around 1,200 kg (2,600 lb).[1] A paper from the previous year estimated that the maximum body mass of the genus may have been approximately 1,500 kg (3,300 lb).[24]
The holotype skull (CMNH 7541) ofNanotyrannus measures 57.2 cm (22.5 in) when measured from theanterior (front) tip of thepremaxilla to theposterior (rear) tip of thequadrate.[4] That of NCSM 40000 measures 71.3 cm (28.1 in) in length, and has a maximum width of 27.3 cm (10.7 in).[1]
In many tyrannosauroids, the point of contact between thepremaxilla andmaxilla bears a prominent subnarialforamen along itsventral (lower) extremity; this is absent in every specimen ofNanotyrannus.[1] Like most tyrannosaurids, thelateral (outer) surface of the subnarial process faces dorsolaterally (upwards and outwards) inN. lancensis. In contrast, this process faces dorsomedially (upwards and towards the midline) inDaspletosaurus andTyrannosaurus. The maxilla ofNanotyrannus is long and low, lacking deepfossae on its subcutaneous surface, which are present in tyrannosaurids such asAlbertosaurus,Tarbosaurus, andTyrannosaurus; that ofN. lethaeus bears a weak maxillary flange; a similar structure is seen inGorgosaurus, albeit far more pronounced. The maxillaryfenestra, the small skull opening in front of theantorbital fenestra, resembles that ofBistahieversor inN. lancensis, whereas that ofN. lethaeus is more like that of tyrannosaurids. Thenasal bones are partly fused, with large, well-developed processes on their posterolateral (rearward and to the side) surfaces.[17]
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Like many tyrannosauroids, thelacrimal ofNanotyrannus has a convex cornual process, a structure which would have supported a keratinous structure in life. Conversely, a cornual process is completely absent inTyrannosaurus. Thepostorbital bone has no cornual process, which is unusual among eutyrannosaurs, as even young tyrannosaurids had one. UnlikeBistahieversor,Daspletosaurus,Tarbosaurus, andTyrannosaurus, thejugal ofNanotyrannus lacks a lateral fossa on its postorbital process. Theparietal bone has a thinsagittal crest, expanding posteriorly into a largenuchal crest; this crest lacks the parasagittal fossae present in the nuchal crests of bothAlbertosaurus andTyrannosaurus. Unlike both tyrannosaurids and more basal tyrannosauroids, but likeXiongguanlong, thequadratojugal andsquamosal bones form a long flange. As inDaspletosaurus horneri (at least theparatype specimen), and an isolated quadratojugal from theDinosaur Park Formation, the quadratojugal ofNanotyrannus is invaded by pneumatic structures, and thus bears a prominent foramen. Thevomer has a deep keel on its ventral portion, similar toDaspletosaurus, and is lance-shaped rather than diamond-shaped as in more derived tyrannosaurids.[1][17] Thebasitubera (muscle attachment sites at the back of the head) ofNanotyrannus were strongly laterally expanded, suggesting that the head could be strongly flexed sideways.[4]
Thedentary ofNanotyrannus (the tooth-bearing bone of the lower jaw), has a rounded anterior margin. The dentaries of both species lackchins, unlike tyrannosaurids such asTyrannosaurus. InN. lethaeus, the ventral region of themandibular symphysis is ridged.[1][17] The lateral edge of the dentary, at least in the holotype, shows a clear line of separation between the dentary at the front, and thesurangular andangular bones at the back.[4]
The premaxillary teeth ofNanotyrannus are distinct from those of other tyrannosauroids in that they were completely devoid of serrations, likeMoros,Timurlengia, andXiongguanlong;[1] whilst the presence or absence of serrations has been suggested to vary with ontogeny, they are likely a diagnostic trait in the case ofNanotyrannus. Each premaxilla bore four chisel-shaped teeth. The maxillae ofNanotyrannus have variable tooth counts: the holotype specimen preserves fifteen, NCSM 40000 has sixteen on the left maxilla and seventeen on the right maxilla, andN. lethaeus appears to have sixteen per maxilla. This is higher than the maxillary tooth count of any tyrannosaurid, save forAlioramus andDaspletosaurus. UnlikeDaspletosaurus,Tarbosaurus, andTyrannosaurus, but like many other tyrannosauroids, the most anterior tooth of each maxilla was small and resembled those of the premaxillae.[17] Unlike most tyrannosaurids, the maxillary teeth wereziphodont.[4][1][17]Nanotyrannus has between sixteen and eighteen teeth per dentary, a higher count than in any tyrannosaurid except forAlioramus andDaspletosaurus. InN. lancensis, the two most anterior alveoli (tooth sockets) of the dentary are far smaller than those behind them. InN. lethaeus, meanwhile, only the first is small, as inTyrannosaurus.[17]
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The vertebral column ofNanotyrannus appears to consist of twenty presacralvertebrae (those before thesacrum), and thirty-fivecaudal (tail) vertebrae. The morphology and count of many of these vertebrae is difficult to determine, since in the most complete specimen, much of the presacral column is obscured by preserved soft tissue in the well-preserved "Bloody Mary" specimen.[17] InNanotyrannus, the morphology of theaxis, the secondcervical (neck)vertebra, differs between species. The axis ofN. lancensis has a straightneural spine (the large projection on the dorsal surface), one lacking any projections on its anterior surface, whereas that ofN. lethaeus is crenulate, bearing small ridges. Additionally,N. lancensis has severalpleurocoels (hollows in the bone for pneumaticity) in the axis, whereasN. lethaeus has only one. The caudal vertebrae ofN. lancensis are pneumatised, whereas those ofN. lethaeus are not.[1]
Theglenoid fossa ofNanotyrannus extends to thescapula's lateral surface. The arms ofNanotyrannus are proportionally very large, proportionally more similar to basal tyrannosauroids than to those of tyrannosaurids. Thedistal (further from the body) end of thehumerus (upper arm bone) ofNanotyrannus bears fivetubercles, unlike other eutyrannosaurs. The distal portion of theulna (one of the two lower arm bones), is convex, with a strong medial projection. The wrist consists of fourcarpal bones. Themanus (hand) is similar in some ways toalbertosaurines, though it grossly differs in some regards from tyrannosaurids. The first metacarpal is subtriangular and reduced, unlike tyrannosaurs. Unlike all other coelurosaurs, the proximal (inner, towards the body) articular surface of the secondmetacarpal may have contacted the forearm during wrist flexion. The manus was functionallydidactyl (two-fingered), though it does retain the firstphalanx (finger bone) of a vestigial third digit, as inGorgosaurus;Tyrannosaurus also retains such a structure, though unlikeGorgosaurus andNanotyrannus, it is fused with the preceding bone. The first phalanx of the second metacarpal ofNanotyrannus bears a deep extensor pit with a prominent proximal tubercle.[1] The first phalanx of the first manualdigit is nearly twice the size of that of the equivalent bone inTyrannosaurus. Theungual phalanges ofNanotyrannus, the bones which would have supported claws in life, were large, and the second was laterally compressed.[17]
The hind limbs ofNanotyrannus are proportionally very long compared to those of other eutyrannosaurs. Unlike tyrannosaurines, thefemur (thigh bone) ofNanotyrannus retains a fossa on its distal medial surface. As inMoros and theTeratophoneus specimen UMNH VP 16690, thefourth trochanter, (structure to which thecaudofemoralis muscle attaches), is parallel to thefemoral shaft, and inN. lethaeus does not form a peak. LikeBistahieversor andTyrannosaurus, the crista tibiofibularis (ridge on the medial portion of thefibula to which thetibia articulated), is enormous, raised from the lateral condyle by a pedicle, and concave medioposteriorly. Despite its body size being far smaller, themetatarsals of NCSM 40000 are almost as long as those of the very largestTyrannosaurus specimens. The extreme proportions of their "hypercursorial" hind limbs may correlate to a different lifestyle from tyrannosaurids, one adapted more towardpursuit predation.[1] The distalcondyle of the fourth metatarsal is longer inN. lethaeus than inN. lancensis.[17]
In their 2025 paper, Zanno and Napoli conducted twophylogenetic analyses using an extensive novel dataset focused on sampling tyrannosauroids. Both recoveredNanotyrannus as a member of theEutyrannosauria in a new clade deemed the Nanotyrannidae, which is thesister taxon to the Tyrannosauridae. As such, it is only distantly related to 'derived'tyrannosaurines likeTyrannosaurus. However, the exact resolution of taxa within and in relation to the Nanotyrannidae differs between both analyses. The first, amaximum parsimony analysis (shown below asTopology A) recovered the 'mid'-Cretaceous taxonMoros intrepidus as the sister taxon to theNanoyrannus lethaeus holotype ("Jane", BMRP 2002.4.1), while all other specimens (Nanotyrannus lancensis) were recovered in an unresolvedpolytomous clade. The second, aBayesian inference analysis implementing a fossilized birth–death (BI-FBD) model (shown below asTopology B), placedMoros outside as a non-eutyrannosaurian as the sister toTimurlengia. Meanwhile, theAppalachian tyrannosauroidsAppalachiosaurus andDryptosaurus were found as the successive earliest-divering branches within the Nanotyrannidae. Referencing this analysis, Zanno and Napoli suggested that themost recent common ancestor (MRCA) between Nanotyrannidae and Tyrannosauridae may have lived roughly 103 million years ago, around the time of the formation of theWestern Interior Seaway. The formation of the seaway may have resulted in their divergence, with tyrannosaurids inhabiting the western island continent ofLaramidia, and nanotyrannids inhabiting the eastern island continent of Appalachia. The authors noted that further testing and more data would be required to support the results of one analysis over the other.[1]
Topology A: Maximum parsimony tree (K = 12)
| Topology B: BI-FBD tree
|
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Evidence of intraspecific attack was found by Joseph Peterson and his colleagues in Jane, then thought to be a juvenileTyrannosaurus. Peterson and his team found that Jane's skull showed healed puncture wounds on the upper jaw and snout which they believe came from another juvenileTyrannosaurus. Subsequent CT scans of Jane's skull would further confirm the team's hypothesis, showing that the puncture wounds came from a traumatic injury and that there was subsequent healing.[32] The team would also state that Jane's injuries were structurally different from the parasite-induced lesions found in the adultTyrannosaurus Sue and that Jane's injuries were on its face whereas the parasite that infected Sue caused lesions to the lower jaw.[33] In 2021, Peterson and colleagues also used Jane's maxillary tooth to estimate the bite force of a tyrannosaur that produced the puncture marks on Jane, and concluded that Jane and similarly-sized juvenileTyrannosaurus had a bite force of 5,269-5,641 N,[34] higher than the previously estimated maximum bite force of Jane (2,400-3,850 N) by Bates and Falkingham in 2012.[35][36]
Nanotyrannus is thought to have been significantly faster than an adultT. rex. In 2016, Scott Persons and Philip Currie calculated the cursorial limb proportion (CLP) score of various theropod specimens including Jane and the "Bloody Mary" specimen (then accessioned as BHI 6437) as an indicator to identify which theropod species had higher or lower running speeds. The CLP scores of Jane and the "Bloody Mary" specimen were 35.8 and 32.7 respectively, which were well above the CLP score of 11.5 for adultT. rex specimens, indicating thatNanotyrannus was more cursorily adapted thanT. rex.[19] In 2020, Alexander Dececchi and colleagues estimated that Jane had a maximum running speed of 12.57–13.82 m/s (45.3–49.8 km/h; 28.1–30.9 mph), nearly twice higher than that of adultT. rex with an estimated maximum running speed of 5.81–8.07 m/s (20.9–29.1 km/h; 13.0–18.1 mph),[37] using the equation by Myriam Hirt and colleagues in 2017.[38]
Nanotyrannus was found in theHell Creek Formation, which spans parts of modern-dayMontana,South Dakota,North Dakota, andWyoming. Thedepositional environment of this region was warm, humid, and swampy environment, reflectingmaritime tosubtropical conditions.[39][40][41] The average temperatures ranged from 11.3–11.6 °C (52.3–52.9 °F).[41]
The landscape featured lush, low-lying vegetation dominated byferns, while the forest canopy consisted of a mixture of diverseconifers (redwoods,cypresses, andpine-like trees) andangiosperms (flowering plants includingsycamores, members of themagnolia family,laurels, variouspalms, and more).Cycads andGinkgo trees were also present.[42]
The wetlands and waterways contained a diverse faunal assemblage. Small insects are known from what would have been the swampy forests,[43] while the rivers and lagoons hosted invertebrates (bivalves andammonites), fish (sharks,hybodonts,gars, andsturgeons), amphibians (frogs andsalamanders), and various reptiles (turtles,crocodilians, thechoristodereChampsosaurus, andmosasaurs in the coastal, brackish, and freshwater zones). Small terrestrial mammals and lizards were diverse, living alongside earlysnakes.[44] The holotype ofInfernodrakon, anazhdarchidpterosaur, is known from the Hell Creek formation, and was collected in the same field jacket as BMRP 2002.4.1 ("Jane"), the holotype ofNanotyrannus lethaeus.[45]
The Hell Creek Formation is best known for its assemblage of charismatic dinosaurs. Non-avian theropods besideNanotyrannus include the largertyrannosauridTyrannosaurus,ornithomimosaurs (Struthiomimus andOrnithomimus), thealvarezsauridTrierarchuncus,[46]oviraptorosaurs (Anzu andEoneophron),[47] and theparaviansPectinodon (atroodontid) andAcheroraptor (adromaeosaurid).[48]Avialan dinosaurs includeAvisaurus,Brodavis andMagnusavis.[49]Ornithischian dinosaurs include theankylosaursAnkylosaurus (anankylosaurid) andDenversaurus (anodosaurid), theneornithischianThescelosaurus, thehadrosauridEdmontosaurus,pachycephalosaurs (Pachycephalosaurus,Platytholus, andSphaerotholus),[50] andceratopsians (Leptoceratops,Torosaurus, andTriceratops).[44]
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