Size Nanotyrannus was a mid-sized tyrannosauroid. Although no body size estimates were given in the 1988 description of the holotype, the accompanying
press release noted that the animal would have reached approximately long and weighed . In their 2025 publication arguing for the validity of the genus, Zanno and Napoli provided body mass estimates of for
N. lancensis (specifically NCSM 40000) and over for
N. lethaeus (the holotype, BMRP 2002.4.1). BMRP 2002.4.1 was not mature at the time of its death, however, and adult
N. lethaeus may have weighed around . Like many tyrannosauroids, the of
Nanotyrannus has a convex cornual process, a structure which would have supported a keratinous structure in life. Conversely, a cornual process is completely absent in
Tyrannosaurus. The
postorbital bone has no cornual process, which is unusual among eutyrannosaurs, as even young tyrannosaurids had one. Unlike
Bistahieversor,
Daspletosaurus,
Tarbosaurus, and
Tyrannosaurus, the of
Nanotyrannus lacks a lateral fossa on its postorbital process. The bone has a thin
sagittal crest, expanding posteriorly into a large
nuchal crest; this crest lacks the parasagittal fossae present in the nuchal crests of both
Albertosaurus and
Tyrannosaurus. Unlike both tyrannosaurids and more basal tyrannosauroids, but like
Xiongguanlong, the and bones form a long flange. As in
Daspletosaurus horneri (at least the
paratype specimen), and an isolated quadratojugal from the
Dinosaur Park Formation, the quadratojugal of
Nanotyrannus is invaded by pneumatic structures, and thus bears a prominent foramen. The has a deep keel on its ventral portion, similar to
Daspletosaurus, and is lance-shaped rather than diamond-shaped as in more derived tyrannosaurids. The
basitubera (muscle attachment sites at the back of the head) of
Nanotyrannus were strongly laterally expanded, suggesting that the head could be strongly flexed sideways. The of
Nanotyrannus (the tooth-bearing bone of the lower jaw), has a rounded anterior margin. The dentaries of both species lack
chins, unlike tyrannosaurids such as
Tyrannosaurus. In
N. lethaeus, the ventral region of the
mandibular symphysis is ridged. The lateral edge of the dentary, at least in the holotype, shows a clear line of separation between the dentary at the front, and the and bones at the back. The premaxillary teeth of
Nanotyrannus are distinct from those of other tyrannosauroids in that they were completely devoid of serrations, like
Moros,
Timurlengia, and
Xiongguanlong; whilst the presence or absence of serrations has been suggested to vary with ontogeny, they are likely a diagnostic trait in the case of
Nanotyrannus. Each premaxilla bore four chisel-shaped teeth. The maxillae of
Nanotyrannus have variable tooth counts: the holotype specimen preserves fifteen, NCSM 40000 has sixteen on the left maxilla and seventeen on the right maxilla, and
N. lethaeus appears to have sixteen per maxilla. This is higher than the maxillary tooth count of any tyrannosaurid, save for
Alioramus and
Daspletosaurus. Unlike
Daspletosaurus,
Tarbosaurus, and
Tyrannosaurus, but like many other tyrannosauroids, the most anterior tooth of each maxilla was small and resembled those of the premaxillae. Unlike most tyrannosaurids, the maxillary teeth were .
Nanotyrannus has between sixteen and eighteen teeth per dentary, a higher count than in any tyrannosaurid except for
Alioramus and
Daspletosaurus. In
N. lancensis, the two most anterior alveoli (tooth sockets) of the dentary are far smaller than those behind them. In
N. lethaeus, meanwhile, only the first is small, as in
Tyrannosaurus.
Postcranial skeleton The vertebral column of
Nanotyrannus appears to consist of twenty presacral
vertebrae (those before the
sacrum), and thirty-five
caudal (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. In
Nanotyrannus, the morphology of the
axis, the second
cervical (neck)
vertebra, differs between species. The axis of
N. lancensis has a straight
neural spine (the large projection on the dorsal surface), one lacking any projections on its anterior surface, whereas that of
N. lethaeus is crenulate, bearing small ridges. Additionally,
N. lancensis has several
pleurocoels (hollows in the bone for pneumaticity) in the axis, whereas
N. lethaeus has only one. The caudal vertebrae of
N. lancensis are pneumatised, whereas those of
N. lethaeus are not. The
glenoid fossa of
Nanotyrannus extends to the
scapula's lateral surface. The arms of
Nanotyrannus are proportionally very large, proportionally more similar to basal tyrannosauroids than to those of tyrannosaurids. The
distal (further from the body) end of the (upper arm bone) of
Nanotyrannus bears five
tubercles, unlike other eutyrannosaurs. The distal portion of the
ulna (one of the two lower arm bones), is convex, with a strong medial projection. The wrist consists of four
carpal bones. The
manus (hand) is similar in some ways to
albertosaurines, 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 second
metacarpal may have contacted the forearm during wrist flexion. The manus was functionally
didactyl (two-fingered), though it does retain the first
phalanx (finger bone) of a vestigial third digit, as in
Gorgosaurus;
Tyrannosaurus also retains such a structure, though unlike
Gorgosaurus and
Nanotyrannus, it is fused with the preceding bone. The first phalanx of the second metacarpal of
Nanotyrannus bears a deep extensor pit with a prominent proximal tubercle. The first phalanx of the first manual
digit is nearly twice the size of that of the equivalent bone in
Tyrannosaurus. The
ungual phalanges of
Nanotyrannus, the bones which would have supported claws in life, were large, and the second was laterally compressed. The hind limbs of
Nanotyrannus are proportionally very long compared to those of other eutyrannosaurs. Unlike tyrannosaurines, the (thigh bone) of
Nanotyrannus retains a fossa on its distal medial surface. As in
Moros and the
Teratophoneus specimen UMNH VP 16690, the
fourth trochanter, (structure to which the muscle attaches), is parallel to the
femoral shaft, and in
N. lethaeus does not form a peak. Like
Bistahieversor and
Tyrannosaurus, the crista tibiofibularis (ridge on the medial portion of the
fibula to which the
tibia articulated), is enormous, raised from the lateral condyle by a pedicle, and concave medioposteriorly. Despite its body size being far smaller, the
metatarsals of NCSM 40000 are almost as long as those of the very largest
Tyrannosaurus specimens. The extreme proportions of their "hypercursorial" hind limbs may correlate to a different lifestyle from tyrannosaurids, one adapted more toward
pursuit predation. The distal
condyle of the fourth metatarsal is longer in
N. lethaeus than in
N. lancensis. == Classification ==