Nanotyrannus lancensi, believed by some to be a juvenile T. rex
One of the key features of science is the so-called 'controversy', where different opinions are discussed within the scientific specialty. They are invariably resolved with new evidence because, in science, the facts are neutral in any debate and so act as referees. This is why science is convergent onto single answers, in contrast to religions.
Creationists, who generally crave certainty at the expense of truth, find this baffling and cite controversies as examples of scientists not being able to make their minds up, not appreciating that this is the scientific method moving toward the truth, unlike religions which have no such built-in method of resolving disagreements and converging on universal truths. If they did, there would only be one religion; instead we have some 40,000 different Christian sects alone, all claiming, without any supporting evidence, to be the one true religion, and continuing to spawn new sects at an average of 20 new sects per year, or roughly one every three weeks.
In the world of religious opinion, controversies frequently lead to schisms and splits because neither side can provide any evidence for its claims and, more often than not, the ancient documents around which disagreements swirl are so ambiguous or impossible to translate accurately that there is no way to determine the intentions of the author, who was probably only expressing an opinion or reporting on evidence-free beliefs and superstitions anyway.
One such controversy that has now probably been resolved was whether the fossil of a relatively small theropod dinosaur was that of a distinct species, Nanotyrannus lancensis, or that of a juvenile Tyrannosaurus rex.
Two palaeontologists, Dr. Nicholas Longrich of the Department of Life Sciences, University of Bath, Bath, UK and Dr. Evan Thomas Saitta of the Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL, USA have now provided evidence that supports the opinion that the fossil is that a full-grown Nanotyrannus lancensis.
A University of Bath news release explains their research:
A new analysis of fossils believed to be juveniles of T. rex now shows they were adults of a small tyrannosaur, with narrower jaws, longer legs, and bigger arms than T. rex. The species, Nanotyrannus lancensis, was first named decades ago but later reinterpreted as a young T. rex.Reading that last sentence makes me wonder if any Christian theologian has ever looked at something implausible in the Bible, such as the claim that Paul was going to Damascus (in the Roman province of Phoenicia) with a letter of authority from the Jewish Temple authorities in Jerusalem (in the Roman province of Judea), to arrest Christians and bring them back to Jerusalem to be tried for heresy, and remarked "It makes you wonder what else we've gotten wrong."
The first skull of Nanotyrannus was found in Montana in 1942, but for decades, paleontologists have gone back and forth on whether it was a separate species, or simply a juvenile of the much larger T. rex.
Dr Nick Longrich, from the Milner Centre for Evolution in the Department of Life Sciences at the University of Bath (UK), and Dr Evan Saitta, from the University of Chicago (USA) re-analysed the fossils, looking at growth rings, the anatomy of Nanotyrannus, and a previously unrecognized fossil of a young T. rex.
Measuring the growth rings in Nanotyrannus bones, they showed that they became more closely packed towards the outside of the bone - its growth was slowing. It suggests these animals were nearly full size; not fast-growing juveniles.
Modelling the growth of the fossils showed the animals would have reached a maximum of around 900-1500 kilograms and five metres - about 15 per cent of the size of the giant T. rex, which grew to 8,000 kilograms and nine metres or more.
The researchers have published their findings in Fossil Studies.
“When I saw these results I was pretty blown away,” said Longrich. “I didn’t expect it to be quite so conclusive.
“If they were young T. rex they should be growing like crazy, putting on hundreds of kilograms a year, but we’re not seeing that.
“We tried modelling the data in a lot of different ways and we kept getting low growth rates. This is looking like the end for the hypothesis that these animals are young T. rex.”
Supporting the existence of distinct species, the researchers found no evidence of fossils combining features of both the Nanotyrannus and T. rex - which would exist if the one turned into the other. Every fossil they examined could be confidently identified as one species or the other.
Neither did the patterns of growth in other tyrannosaurs fit with the hypothesis that these were young T. rex.
Dr Longrich said: “If you look at juveniles of other tyrannosaurs, they show many of the distinctive features of the adults. A very young Tarbosaurus - a close relative of T. rex - shows distinctive features of the adults.
“In the same way that kittens look like cats and puppies look like dogs, the juveniles of different tyrannosaurs are distinctive. And Nanotyrannus just doesn’t look anything like a T. rex.
“It could be growing in a way that’s completely unlike any other tyrannosaur, or any other dinosaur- but it’s more likely it’s just not a T. rex.”
But that raises a mystery — if Nanotyrannus isn’t a juvenile Tyrannosaurus, then why hasn’t anyone ever found a young T. rex?
“That’s always been one of the big questions. Well, it turns out we actually had found one,” said Longrich. “But the fossil was collected years ago, stuck in a box of unidentified bones in a museum drawer, and then forgotten.”
The research led Longrich and co-author Evan Saitta to a previous fossil discovery, stored in a museum in San Francisco which they identified as a juvenile Tyrannosaurus.
That young T. rex is represented by a skull bone - the frontal bone - with distinctive features that ally it with Tyrannosaurus, but which aren’t seen in Nanotyrannus. It comes from a small animal, one with a skull about 45 cm long and a body length of around 5 metres.
Dr Longrich said: “Yes, it’s just one specimen, and just one bone, but it only takes one. T. rex skull bones are very distinctive, nothing else looks like it. Young T. rex exist, they’re just incredibly rare, like juveniles of most dinosaurs.”
The researchers argue these findings are strong evidence that Nanotyrannus is a separate species, one not closely related to Tyrannosaurus. It was more lightly-built and long-limbed than its thick-set relative. It also had larger arms, unlike the famously short-armed T. rex.
“The arms are actually longer than those of T. rex. Even the biggest T. rex, has shorter arms and smaller claws than in these little Nanotyrannus. This was an animal where the arms were actually pretty formidable weapons. It’s really just a completely different animal - small, fast, agile.
“T. rex relied on size and strength, but this animal relied on speed.”
The long arms and other features suggest it was only distantly related to T. rex - and may have sat outside the family Tyrannosauridae, which T. rex is part of, in its own family of predatory dinosaurs.
The new study is the latest in a series of publications on the problem, going back decades.
Longrich said: “Nanotyrannus is highly controversial in paleontology. Not long ago, it seemed like we’d finally settled this problem, and it was a young T. rex.
“I was very skeptical about Nanotyrannus myself until about six years ago when I took a close look at the fossils and was surprised to realise we’d gotten it wrong all these years.”
The authors suggest that, given how difficult it is to tell dinosaurs apart based on their often incomplete skeletons, we may be underestimating the diversity of dinosaurs, and other fossil species.
Longrich said: “It’s amazing to think how much we still don’t know about the most famous of all the dinosaurs. It makes you wonder what else we’ve gotten wrong.”
The problem here is that a letter from the Jewish Temple authorities in Jerusalem the Roman province of Judea, would have had no authority in Damascus in the Roman province of Phoenicia where there was no such crime as heresy, so the claim is a lie, almost certainly made up by someone who either didn't know the geography or the law at the time and place in which he set the story.
Unlike science, which strives to resolve these sorts of controversies, religions apologetics seeks to gloss over them and minimise their importance to their preferred narrative, claiming 'metaphors' and 'allegories' to dismiss obvious fallacies. Events such as the alleged conversion of Saul on the road to Damascus are given great importance in the Christian narrative, until shown to be based on a lie, then they become insignificant and have no bearing on the truth of the rest of the accounts of Paul’s teaching, and promulgation of the Jesus myth throughout the Eastern Roman Empire.
Now back to the contrast between the scientific method and the religious method. Consider how detailed this scrutiny of the evidence is, compared to the casual waving aside of the (unwanted) evidence by religious apologists:
More technical detail and the account of how this scientific controversy was resolved by the scientific method is given in the Abstract and Introduction to the paper in Fossil Studies:
AbstractThis degree of detail in their analysis contrasts markedly with the approach of religious apologetics which readily latches onto any excuse, no matter how irrelevant, to dismiss contradictory evidence and to continue to believe accepted dogmas despite that evidence, or elevates minor disagreements to the level of irreconcilable differences when it suits the founders of new cults. This of course is only possible because religions are not founded in verifiable evidence but on evidence-free opinions and inherited superstitions.
Tyrannosaurs are among the most intensively studied and best-known dinosaurs. Despite this, their relationships and systematics are highly controversial. An ongoing debate concerns the validity of Nanotyrannus lancensis, interpreted either as a distinct genus of small-bodied tyrannosaur or a juvenile of Tyrannosaurus rex. We examine multiple lines of evidence and show that the evidence strongly supports recognition of Nanotyrannus as a distinct species for the following reasons: 1. High diversity of tyrannosaurs and predatory dinosaurs supports the idea that multiple tyrannosaurids inhabited the late Maastrichtian of Laramidia; 2. Nanotyrannus lacks characters supporting referral to Tyrannosaurus or Tyrannosaurinae but differs from T. rex in >150 morphological characters, while intermediate forms combining the features of Nanotyrannus and T. rex are unknown; 3. Histology shows specimens of Nanotyrannus showing (i) skeletal fusions, (ii) mature skull bone textures, (iii) slow growth rates relative to T. rex, (iv) decelerating growth in their final years of life, and (v) growth curves predicting adult masses of ~1500 kg or less, showing these animals are subadults and young adults, not juvenile Tyrannosaurus; 4. growth series of other tyrannosaurids, including Tarbosaurus and Gorgosaurus, do not show morphological changes proposed for a Nanotyrannus–Tyrannosaurus growth series, and deriving Tyrannosaurus from Nanotyrannus requires several changes inconsistent with known patterns of dinosaur development; 5. Juvenile T. rex exist, showing diagnostic features of Tyrannosaurus; 6. Phylogenetic analysis suggests that Nanotyrannus may lie outside Tyrannosauridae. Tyrannosaur diversity before the K-Pg extinction is higher than previously appreciated. The challenges inherent in diagnosing species based on fossils mean paleontologists may be systematically underestimating the diversity of ancient ecosystems.
- Introduction
- The Tyrannosaurs, the Most Iconic of All Dinosaurs
The first small, primitive tyrannosaurs evolved in the Late Jurassic of Laurasia and then diversified in the early Cretaceous [1]. Initially, tyrannosaurs were largely subordinate to larger-bodied megalosauroid and allosauroid [2,3] predators. However, by the end of the Cretaceous, tyrannosaurs became larger and more specialized [1,4,5], evolving giant forms with massive skulls and reduced forelimbs. Tyrannosaur evolution culminated in the late Maastrichtian, with the appearance of the giant Tyrannosaurus rex [4,6,7]. T. rex was among the last of the tyrannosaurs and the largest tyrannosaur, perhaps the largest terrestrial predator of all time. Tyrannosaurs are among the best-known and most intensively studied groups of dinosaurs [1,4,5]. Dozens of skeletons are known, representing almost thirty species [1,4,5]. Of these, far and away the most well-known, intensively studied species is Tyrannosaurus rex. Multiple studies have examined the evolution [1,4,5], development [8,9,10], locomotion [11], feeding [12,13,14,15], and systematics [9,16,17,18,19] of T. rex. It is one of the most well-known fossil organisms, and probably more thoroughly studied than most living species.
Despite this, much remains unknown. Remarkably, one of the most fundamental problems—how many tyrannosaur species are represented by fossils assigned to Tyrannosaurus—remains highly controversial. Among the most persistent issues concerns whether the latest Maastrichtian tyrannosaurs of western North America represent one species, showing remarkable variation through development, or whether small specimens represent a distinct lineage of small-bodied tyrannosaurs. This issue is of interest because this basic problem—classifying fossils into species—underpins our efforts to understand the evolution and extinction of fossil species, their geographic ranges, their growth, and their biology. That such a well-known, intensively studied animal remains so controversial is remarkable, and raises fundamental questions about the reliability of the taxonomies forming the foundation of paleontology.
- Abbreviations
AMNH, American Museum of Natural History, New York, NY, USA; BHI, Black Hills Institute of Geological Research, Hill City, SD, USA; BMNH, British Museum of Natural History, London, UK; BMRP, Burpee Museum of Natural History, Rockford, IL, USA; CM, Carnegie Museum, Pittsburgh, PA, USA; CMNH, Cleveland Museum of Natural History, Cleveland, OH, USA; DDM, Dinosaur Discovery Museum, Kenosha, WI, USA; FMNH, Field Museum of Natural History, Chicago, IL, USA; LACM, Natural History Museum, Los Angeles, CA, USA; KU, University of Kansas, Lawrence, Kansas; MOR, Museum of the Rockies, Bozeman, MT, USA; NHMUK, York Natural History Museum, London, England; NMMNH New Mexico Museum of Natural History and Science, Albuquerque, NM, USA; PIN, Paleontological Institute, Russian Academy of Science, Moscow, Russia; RSM, Royal Saskatchewan Museum, Eastend, Saskatchewan, Canada; SDSM, South Dakota School of Mines and Technology, Rapid City, SD, USA; HRS, Hanson Research Station, Newcastle, WY, USA; TMM, Texas Memorial Museum, Austin, TX, USA; TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; UALVP, University of Alberta Laboratory for Vertebrate Paleontology, Edmonton, Alberta, Canada; UCMP, University of California Berkeley, Berkeley, CA, USA; UMNH, University of Utah Museum of Natural History; USNM, United States National Museum, Smithsonian Institution, Washington, DC, USA; UWBM, University of Washington Burke Museum, Seattle, Washington, DC, USA; UWGM, Geology Museum, Madison, WI, USA; ZPAL, Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland.
- Tyrannosaurus rex
Historically, a single tyrannosaur species has been recognized from the latest Maastrichtian of western North America, Tyrannosaurus rex. Named by Henry Fairfield Osborn in 1905 [6], T. rex has a convoluted history (Table 1). Like many dinosaurs discovered in the ‘Dinosaur Rush’ of the late 19th and early 20th centuries, it was part of the ‘Bone Wars’, the scientific rivalry between Othniel Charles Marsh and Edward Drinker Cope [20], although it would not be well-known or studied in detail until many years later.
Table 1. Taxonomy of Tyrannosaurus and late Maastrichtian Tyrannosaurini.Among the first discoveries that can be referred to as Tyrannosaurus is an isolated fourth metatarsal, USNM 2110, collected by J. B. Hatcher in 1890 from the late Maastrichtian-aged Lance Formation of Wyoming [20]. Later that year, Marsh named the fossil as an ornithomimid, Ornithomimus grandis [21]. Two years later, in 1892, Cope collected a pair of huge vertebrae from the Hell Creek Formation of South Dakota, which he described and named Manospondylus gigas [22]. Both animals, being giant tyrannosaurs, probably belong to Tyrannosaurus but are not diagnostic to species level, so these names are considered invalid.
TaxonPublicationSpecimenStatus
A few years later, Barnum Brown collected a pair of tyrannosaur skeletons for the American Museum of Natural History in New York. The first, AMNH 5866 (sold to the British Museum, now BMNH R7995), was collected in 1900 from the Lance Formation of Wyoming. He collected the second, AMNH 973 (sold to the Carnegie Museum, now CM 9380), in 1902 from the uppermost Hell Creek Formation of Montana [20].
In 1905, Osborn described the first skeleton, AMNH 5866, as Dynamosaurus imperiosus, and the second, AMNH 973, as Tyrannosaurus rex [6]. Following further preparation and study, Osborn concluded in 1906 that the two animals were “generically if not specifically identical” [25]. Because Dynamosaurus and Tyrannosaurus were published in the same paper, ICZN rules let Osborn, as the first reviser, choose which name to retain. Unsurprisingly, he retained the now-iconic name Tyrannosaurus rex [25].
Since Osborn, Tyrannosaurus has generally been considered to contain a single species [4,5,9,17,19], T. rex. However, even setting aside the controversial specimens later assigned to Nanotyrannus, significant differences exist between specimens unambiguously assigned to Tyrannosaurus [16,18]. The possibility of multiple species has been raised several times [16,18].
Larson [16] cites a communication with Bakker as the source of this idea and, following Bakker, proposed that T. rex could be divided into T. rex and “T. x”, which differed in subtle details such as the shape of the second dentary tooth, tooth count, and the size of the lacrimal pneumatic foramen. Recently, Paul et al. [18] recognized three species of Tyrannosaurus: T. rex, T. imperator, and T. regina; these do not neatly conform to the T. rex and T. “x” of Larson, given that Larson [16] considered the holotype of T. imperator, FMNH PR 2081, the famed Sue specimen, to represent T. rex. The existence of multiple species is plausible, given the extensive variation seen in the genus [16,18] and the existence of species-level turnover within the Hell Creek Formation [26], but it remains controversial [19]. We consider this hypothesis viable but in need of further study.
- Nanotyrannus
The systematics of late Maastrichtian tyrannosaurs were further complicated by the naming of Gorgosaurus lancensis, later renamed Nanotyrannus lancensis, from the late Maastrichtian beds that produced T. rex. In 1942, a field party from the Cleveland Museum of Natural History discovered a small tyrannosaur skull (Figure 1) in the Hell Creek Formation of Montana [27]. The skull, CMNH 7541, was described and named by Charles Gilmore in a paper published posthumously in 1946 [27].The skull differs markedly from Tyrannosaurus in its proportions (Figure 2). Gilmore compared the animal to Gorgosaurus libratus from the Campanian of Canada and, noting similarities, argued that the skull was referable to Gorgosaurus [27]. He described the skull as a distinct species, Gorgosaurus lancensis, primarily based on the long period of time separating the two [27]. Since then, the status of Nanotyrannus has proven controversial.
Figure 1. Holotype of Nanotyrannus lancensis, CMNH 7541. In (A), left lateral view; (B), right lateral view; (C), anterior view; (D), dorsal view; (E), ventral view. Scale = 20 cm.Rozhdestvensky [28], based on a growth series of Tarbosaurus, suggested that Gorgosaurus lancensis might represent a juvenile T. rex [28], while Russell [29] regarded G. lancensis as an adult.
Figure 2. Comparison of Nanotyrannus lancensis CMNH 7541 (A,C) and Tyrannosaurus AMNH 5027 (B,D) (not to scale).
A paper by Bakker, Williams, and Currie in 1988 [30] would support Gilmore’s recognition of the animal as a distinct species. Bakker and colleagues, however, argued that Gorgosaurus lancensis was not only distinct from T. rex but distinct from Gorgosaurus and represented a deeper diverging lineage of tyrannosaur [30]. This would put it outside of Tyrannosauridae as currently defined (i.e., Tyrannosaurus rex + Albertosaurus sarcophagus). If so, CMNH 7541 would represent a non-tyrannosaurid member of Tyrannosauroidea. Accordingly, Bakker et al. created a new genus, renaming CMNH 7541 Nanotyrannus lancensis [30]. The taxonomy of Nanotyrannus is further complicated by the discovery of a small tyrannosaur near Jordan, Montana [23,31,32]. The “Jordan theropod” has unserrated premaxillary teeth with a chisel-shaped tip, similar to those described as Aublysodon by Leidy [33] from the Campanian of Montana. Molnar and Carpenter referred the Jordan theropod to Aublysodon [32]. Paul (1988) described the Jordan theropod as a new species of Aublysodon, Aublysodon molnari [23]. It was later named as a distinct genus, Stygivenator, by Olshevsky [24]. Stygivenator molnari is similar to the type of Nanotyrannus in overall morphology, raising the possibility that it is synonymous with Nanotyrannus (in which case it also could be a juvenile T. rex). However, there are subtle, potentially significant differences in the shape of the teeth, maxillae, and dentaries (see Discussion).
Subsequently, Carpenter [34] suggested that the holotype of Nanotyrannus lancensis might be immature and, following Rozhdestvensky, interpreted it as a juvenile T. rex. Further study by Carr [9] agreed with Rozhdestvensky and Carpenter’s identification of the animal as immature. Evidence for immaturity comes from the existence of striated texture of the surficial bone, [9] which is typical of young, rapidly growing dinosaurs, including extant birds [35,36] (but see below). In recent years, the interpretation of Nanotyrannus as a juvenile Tyrannosaurus has been widely adopted [4,5,7,37,38] but not universally accepted. Critically, we will argue, it is unclear that these animals are in fact juveniles, or that they show features referable to T. rex. Carr identified 13 characters in the Nanotyrannus type that supposedly support referral to T. rex [9]. These characters are problematic because many (if not all) are widely distributed in Tyrannosauridae [39] or appear to be absent from Nanotyrannus (see Discussion).
Meanwhile, others have contested the referral to Tyrannosaurus. Currie [39] suggested that the difference in tooth count between the animals is of taxonomic significance, arguing that theropods do not show large changes in tooth count over ontogeny. Larson [40] provided the most comprehensive case for recognizing Nanotyrannus as distinct, cataloging characters potentially differentiating the two [16]. He also argued that a new and larger animal showing the Nanotyrannus morphology, the “Jane” specimen [41], represented an adult of the species [40]. Larson [42] also reported a new skeleton, part of the “Dueling Dinosaurs”, noting features such as elongated forelimbs, suggesting Nanotyrannus was distinct from T. rex [42].
Witmer and Ridgeley [43,44] noted extensive differences between the holotypes of Nanotyrannus and Tyrannosaurus. They found many characters difficult to ascribe to ontogeny but remained agnostic about the taxon’s validity [44]. Schmerge and Rothschild [45] noted a lateral groove on the dentary as potentially supporting Nanotyrannus as distinct, a conclusion disputed by Brusatte et al. [43].
Woodward et al. [46] sectioned bones of putative Nanotyrannus, including “Jane” and “Petey” BMRP 2006.4.4, interpreting them as juveniles of Tyrannosaurus. They noted that the bones lack an external fundamental system, a characteristic of old, slow-growing adults, and argued the histology is consistent with the animals representing juvenile Tyrannosaurus. However, a more recent analysis suggested that the growth trajectory of the second animal, BMRP 2006.4.4, could not be linked to the growth curves of T. rex [47].
Finally, Carr [10] attempted to synthesize the known data to assemble specimens assigned to Nanotyrannus and Tyrannosaurus into a hypothetical ‘growth curve’. The taxonomy of Nanotyrannus is, therefore, as complicated as that of T. rex (Table 2), and its status currently remains unresolved: was Nanotyrannus a juvenile of Tyrannosaurus or a distinct lineage of tyrannosaur (Figure 3)?
Figure 3. Competing hypotheses. Nanotyrannus as either juveniles of Tyrannosaurus or as a distinct lineage of small-bodied tyrannosaur. Scale = 10 cm.
Table 2. Current taxonomy of Nanotyrannus lancensis and N. lancensis-like fossils.
TaxonPublicationSpecimenStatus
Nanotyrannus lancensisBakker et al., 1988 [30]CMNH 7541Nanotyrannus lancensis or Tyrannosaurus (?)
Aublysodon molnariPaul, 1988 [23]LACM 28471Nanotyrannus lancensis, Stygivenator molnari, or Tyrannosaurus (?)
Stygivenator molnariOlshevsky, 1995 [24]LACM 28471Nanotyrannus lancensis, Stygivenator molnari, or Tyrannosaurus (?)
- Purpose and Approach of This Paper
The debate over Nanotyrannus has been considered resolved by some, but important questions remain. Our goal here is to synthesize all available evidence concerning the taxonomic status of Nanotyrannus. Rather than focusing on a single line of evidence, such as a growth series or histology, our approach is to synthesize multiple different lines of evidence to provide a robust inference.
A single line of argumentation and analysis, even one that appears conclusive, could be incorrect due to biases or errors in the data. It could also arrive at incorrect conclusions due to errors in the analysis of the data, choice of models used, or interpretation. It is less likely that multiple lines of evidence will independently point to the wrong conclusions, an approach referred to as triangulation of conclusions. As Galileo argued, ‘two truths can never contradict each other’, i.e., all lines of evidence are necessarily consilient with the correct hypothesis and must agree with each other.We consider the following lines of evidence:
- Patterns of diversity in Tyrannosauridae and other predators. The frequent coexistence of two species of tyrannosaur suggests that this pattern is the rule; the existence of a distinct taxon alongside Tyrannosaurus is expected from known patterns of dinosaur diversity.
- Morphological differences between Nanotyrannus and Tyrannosaurus and lack of intermediates. The hypothesis that the animals represent a growth series of a single species predicts intermediate forms linking the two. However, dozens of characters in almost every skull bone distinguish Nanotyrannus and Tyrannosaurus, while no intermediate forms are known.
- Developmental patterns seen in other tyrannosaurs. The hypothesized synonymy of Nanotyrannus and Tyrannosaurus suggests that Nanotyrannus will resemble the juveniles of other tyrannosaur species. However, the major changes in morphology proposed for the Nanotyrannus–Tyrannosaurus growth series are not seen in tyrannosaurs such as Tarbosaurus and Gorgosaurus, arguing against ontogeny as an explanation for morphological differences.
- Adult specimens referable to Nanotyrannus. The hypothesis that Nanotyrannus is a juvenile Tyrannosaurus predicts that all Nanotyrannus will show features associated with immaturity, including immature bone texture and lack of fusion between skeletal elements. Histology is also predicted to show Nanotyrannus specimens rapidly growing to reach adult sizes on the order of ~8000 kg. Nanotyrannus specimens instead show patterns of bone fusion and bone texture consistent with maturity, are slow growing compared to maximal growth rates of T. rex, and show a pattern of decelerating growth in their final years of life, consistent with fairly mature animals. Reconstructed growth curves predict small adult body mass (<2000 kg, more likely ~900–1500 kg).
- Juvenile tyrannosaurs showing diagnostic features of T. rex. The hypothesized Nanotyrannus–Tyrannosaurus growth series requires that small specimens showing features of Tyrannosaurus do not exist. A juvenile skull, slightly larger than Nanotyrannus, and a juvenile frontal, smaller than the holotype, represent juveniles of Tyrannosaurus.
- Phylogenetic analysis of Nanotyrannus. If Nanotyrannus is a juvenile Tyrannosaurus, then phylogenetic analysis with ontogenetically variable characters removed should cause Nanotyrannus to cluster with Tyrannosaurus. The morphology of Nanotyrannus instead places it outside of Tyrannosaurinae and Tyrannosauridae, even when restricting analysis to characters known to be stable during ontogeny.
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