A 2025 paper by Lindsay Zanno and James Napoli concluded that some bones from the 'Dueling Dinosaurs' specimen represent a fully grown individual of Nanotyrannus lancensis rather than a juvenile Tyrannosaurus rex.
October 30, 2025
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temporal
Taxonomic reinterpretation of bones from the well-known 'Dueling Dinosaurs' fossil assemblage.
The taxon name Nanotyrannus lancensis was assigned to a medium-sized theropod dinosaur in 1988.
January 01, 1988
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taxonomy
Historical taxonomic designation for a medium-sized tyrannosaur specimen.
The Hell Creek Formation is a geologic layer from the Late Cretaceous that dates to about 67 million years ago.
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temporal
Geologic age and formation name used as a temporal and stratigraphic reference for Late Cretaceous fossils.
Specimens identified as Nanotyrannus have been reported to show, relative to Tyrannosaurus rex, proportionally longer forearms, fewer tail vertebrae, and a greater number of teeth.
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morphological
Reported anatomical differences cited in comparative studies of small-bodied tyrannosaur specimens versus T. rex.
Cross sections of dinosaur limb bones commonly preserve growth rings (lines of arrested growth) that can be used to estimate an individual animal's age.
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biological_process
Bone histology technique used to infer age and growth history in extinct vertebrates.
Growth rings preserved in dinosaur bones can be used to estimate the age of an individual and to distinguish adults from juveniles.
high
temporal
Skeletochronology is used in vertebrate paleontology to infer individual age and growth stage.
Tyrannosaurus rex existed approximately 67 million years ago during the late Cretaceous period.
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temporal
Tyrannosaurus rex is commonly placed near the end of the Cretaceous in biostratigraphic and geochronological frameworks.
Nanotyrannus lancensis is described as a small-bodied, agile tyrannosauroid with relatively long legs and proportionally larger forelimbs compared with Tyrannosaurus rex.
high
taxonomic
Morphological descriptions of small tyrannosauroid taxa emphasize adaptations for speed and agility versus the larger, more robust form of T. rex.
Skull characters such as bone structure, cranial nerve pathways, and sinus morphology can provide diagnostic evidence for distinguishing dinosaur species and are not necessarily explained by ontogenetic (age-related) change alone.
high
morphological
Comparative cranial anatomy is used to differentiate species-level versus growth-related variation in fossil theropods.