Nanotyrannus

Nanotyrannus is one of the most debated dinosaurs in palaeontology. For many years, most researchers interpreted its fossils as belonging to juvenile Tyrannosaurus rex rather than a separate type of dinosaur. However, studies published in 2025 argued that some specimens - such as "Bloody Mary" and "Jane" - represented mature individuals distinct from T. rex, based on skeletal evidence that the animals were fully grown, along with differences in their anatomy. These studies strengthened the case for Nanotyrannus being a valid dinosaur in its own right, although some researchers still favour the juvenile T. rex interpretation.

Recent studies have recognised two species: Nanotyrannus lancensis and Nanotyrannus lethaeus. The fossils were originally described in 1946 as a species of Gorgosaurus, before later being placed into the new genus, Nanotyrannus, in 1988. The name Nanotyrannus means "small tyrant".

Compared to giant tyrannosaurs like T. rex, Nanotyrannus appears to have been more lightly built, with a slimmer skull, relatively long legs, and proportionally larger arms. Many researchers think these features suggest it was faster and more agile than larger tyrannosaurs. Estimates suggest N. lancensis weighed around 700 kg, while the larger N. lethaeus may have reached about 1,200 kg.

Nanotyrannus was first identified from a skull discovered in the 1940s. For decades, scientists debated whether it represented a distinct dinosaur or simply a young Tyrannosaurus rex.

The debate gained renewed attention after the discovery of a more complete skeleton nicknamed "Jane" in 2001. Some scientists saw it as further evidence of a young T. rex, while others argued it showed consistent differences that supported Nanotyrannus as its own species. Another major find came in 2006, when a remarkably complete skeleton nicknamed "Bloody Mary" (later "Manteo") was discovered alongside a horned dinosaur.

A major 2025 study argued that this specimen represented a fully grown Nanotyrannus, strengthening the idea that the dinosaur was distinct from T. rex. The same study also proposed a second species, N. lethaeus, based on the "Jane" skeleton. The species name references the River Lethe from Greek mythology, as well as the Hell Creek Formation where the fossils were found.

One of the long-standing arguments for Nanotyrannus being distinct is its higher number of teeth compared to adult T. rex. However, scientists disagree on how reliable this is, since tooth counts can change during growth and may also vary between individuals. Other proposed differences include relatively larger hands, certain skull features, and overall body proportions.

Studies of bone structure and growth have produced mixed results. Some suggested the fossils belonged to immature animals, while others - especially more recent research - indicate that at least some specimens were fully grown. The 2025 research added further detail, including growth rings in the bones showing that "Bloody Mary" had reached adulthood. The researchers argued its growth pattern does not match what would be expected from a young T. rex.

Additional studies published soon after, including microscopic analysis of bone tissue that can reveal growth and maturity, also suggested that the original skull belonged to a mature individual, supporting the idea that Nanotyrannus may represent a distinct dinosaur rather than a juvenile T. rex.

Nanotyrannus appears to have been a medium-sized predator, much smaller than giants like T. rex. Early estimates suggested a length of around 5 m and a weight of roughly 450 kg, but more recent research indicates it could have been heavier - around 700 kg for N. lancensis, and potentially up to 1,200 kg or more for N. lethaeus.

Its skull was relatively long and low, rather than deep and heavy like that of T. rex. Some specimens had skulls over 70 cm long, suggesting a lightly built but still powerful head. The skull combined features seen in both early and advanced tyrannosaurs, having several skull features that differed from more famous tyrannosaurs like T. rex (including differences in certain openings and grooves in the skull).

It likely had small horn-like structures in front of the eyes, similar to other early tyrannosaurs. Interestingly, some skull bones lacked features that are normally present even in young members of the T. rex group, which has been used as an argument that it was not simply a juvenile.

The back of the skull had large areas for muscle attachment, suggesting the head could be moved powerfully from side to side, possibly helping it grip and tear prey. The lower jaw was slimmer and less deep than that of T. rex, giving its head a lighter, more agile appearance.

The rest of the skeleton suggests a more lightly built and agile animal. Its arms were proportionally larger than those of T. rex, with well-developed bones and large claws, although, like other tyrannosaurs, it had only two functional fingers.

Its legs were especially long and slender relative to its body size, more so than in most large tyrannosaurs. This lighter build and relatively long-legged body shape has led many researchers to suggest Nanotyrannus was adapted for speed and agility, possibly chasing down prey more actively than the heavier-built T. rex.