Palaeontology: the truth about t. Rex

Even one of the best known dinosaurs has kept some secrets. Here is what palaeontologists most want to know about the famous tyrant. You have full access to this article via your

institution. Download PDF Credit: Andrew Howe/Getty Images In late 1905, newspaper reporters gushed over the bones of a prehistoric monster that palaeontologists had unearthed in the

badlands of Montana. When _The New York Times_ described the new 'Tyrant saurian', the paper declared it “the most formidable fighting animal of which there is any record

whatever”. In the century since, _Tyrannosaurus rex_ has not loosened its grip on the imaginations of the public or palaeontologists. Marmosets make polite conversation Lopsided planetary

system throws astronomers off-balance 1.8-million-year-old skull dramatically simplifies story of human evolution Stretching more than 12 metres from snout to tail and sporting dozens of

serrated teeth the size of rail spikes, the 66-million-year-old _T. rex_ remains the ultimate example of a prehistoric predator — so much so that a media frenzy erupted this year over a

paper debating whether _T. rex_ predominantly hunted or scavenged its meals1. This infuriated many palaeontologists, who say the matter was resolved long ago by ample evidence showing that

_T. rex_ could take down prey and dismantle carrion. What particularly vexed researchers was that this non-issue overshadowed other, more important questions about _T. rex_. The

dinosaur's evolutionary origins, for example, are still a mystery. Researchers are eagerly trying to determine how these kings of the Cretaceous period (which spanned from 145 million

to 66 million years ago) arose from a line of tiny dinosaurs during the Jurassic period (201 million to 145 million years ago). There is also considerable debate about what _T. rex_ was like

as a juvenile, and whether palaeontologists have spent decades mistaking its young for a separate species. Even the basic appearance of _T. rex_ is in dispute: many researchers argue that

the giant was covered in fluff or fuzz rather than scales. And then there is the vexing question of why _T. rex_ had such a massive head and legs but relatively puny arms. LISTEN Your

browser does not support the audio element. How much do we really know about the world’s most famous dinosaur? On the bright side, palaeontologists have material to work with. “We have lots

of fossils of _T. rex_,” says palaeontologist Stephen Brusatte of the University of Edinburgh, UK. “It's rare to have so many good fossils of one dinosaur, so we can actually ask

questions about _T. rex_ — such as how it grew, what it ate and how it moved — that we can't for other dinosaurs.” Here, _Nature_ examines how palaeontologists are investigating these

and other hot topics for the most charismatic of carnivores. FUZZY ORIGINS In the first few decades after palaeontologist Henry Fairfield Osborn named and described _T. rex_, researchers

viewed this giant dinosaur as the culmination of a trend towards bigger predators. In this view, _T. rex_ was seen as the descendent of _Allosaurus_, a 9-metre-long predator that lived more

than 80 million years earlier. These and other massive carnivorous dinosaurs were lumped together in a categorical wastebasket called the Carnosauria, with _T. rex_ as the last and biggest

of the ferocious family. But palaeontologists tore up that evolutionary tree when they started using a more rigorous form of analysis called cladistics in the 1990s. They re-examined

relationships between dinosaur groups and found that _T. rex_ had its roots in a lineage of small, fuzzy creatures that lived in the shadow of _Allosaurus_ and other predators during the

Jurassic period. The view that emerged placed _T. rex_ and its close relatives — together known as tyrannosaurids — as the top twig on a broader evolutionary bush called the

Tyrannosauroidea, which emerged around 165 million years ago (see 'In the flesh'). Among the earliest known members of this group was _Stokesosaurus clevelandi_, a bipedal

carnivore 2–3 metres long that lived about 150 million years ago. Little is known about this creature, but evidence from other early tyrannosauroids suggests that _Stokesosaurus_ had a long,

low skull and slender arms. Early tyrannosauroids were small, agile predators, but their size placed them low in the pecking order during the Jurassic. “They were more lapdogs than top

predators,” says Brusatte. Credit: T. rex illustration by Emily Cooper; family tree from ref. 3 The question for palaeontologists is how tyrannosaurs rose to power from such humble

beginnings and why they took over as the apex predators in North America and Asia. At present, the key parts of this story are missing. There are relatively few dinosaur-rich rock formations

from the period between 145 million and 90 million years ago, when tyrannosaurs apparently took over, so palaeontologists have yet to fully chart the communities that existed at the time.

Shifts in sea level or climate could have triggered events that led to tyrannosaur dominance, Brusatte says, but he admits that such a connection is speculative. “We really need more fossils

from this middle Cretaceous gap to help untangle this mystery.” In the past few years, researchers have started making headway in China, where rock formations record some segments of this

key interval. In 2009, Peter Makovicky at the Field Museum in Chicago, Illinois, and his colleagues described a long-snouted tyrannosaur named _Xiongguanlong baimoensis_ from rocks in

western China dating to between 100 million and 125 million years ago2. That animal reached about four metres long, a step up in size from the Jurassic tyrannosaurs. And, in 2012, Xu Xing of

the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing and his colleagues described a 9-metre-long tyrannosaur by the name of _Yutyrannus huali_3 from a similar time

period (see _Nature_ 489, 22–25; 2012 ). This may be the crucial transition during which tyrannosaurs overlapped with allosaurs, before the latter faded out in the same habitats. In studies

of rocks from northern China, Brusatte and his co-workers have found an allosaur five to six metres long named _Shaochilong maortuensis_, which lived about 90 million years ago4. “So it

seems like both allosauroids and tyrannosauroids were around in Asia during this time, and had relatively similar sizes,” he says. He hopes that further fossil discoveries will help to flesh

out how and when tyrannosaurs took over as the top predator in their ecosystems. ADOLESCENT ANGST Just as the evolutionary origins of _T. rex_ remain murky, so does its youth. In this case,

the big debate centres on an creature called _Nanotyrannus lancensis_, a tyrannosaur found in the same North American deposits as _T. rex_ that may have reached more than 6 metres in

length. When it was first discovered, this creature was thought to be a separate species, but some researchers now argue that _Nanotyrannus_ is actually just a juvenile _T. rex_. According

to Thomas Holtz Jr, a palaeontologist at the University of Maryland in College Park, _Nanotyrannus_ specimens look remarkably like _T. rex_, and the differences between the two are similar

to the differences between immature and mature individuals of other tyrannosaur species. The fact that all of the _Nanotyrannus_ specimens seem to be juvenile animals and all of the

specimens recognized as _T. rex_ are subadults or adults, Holtz says, indicates that the two are truly one. Lawrence Witmer, a palaeobiologist at Ohio University in Athens, is not so sure.

In 2010, he and his colleague Ryan Ridgely studied computed-tomography scans of a skull from the Cleveland Museum of Natural History in Ohio that is the defining specimen, or holotype, of N.

lancensis. “We went into the project with the bias or assumption that the Cleveland skull was a juvenile _T. rex_,” Witmer says. But they found some unusual indentations in the brain case

and sinuses, where air sacs filled the back of the skull in life5. These features are very different from those of _T. rex_ and may identify the skull as belonging to a different species,

says Witmer. Team _Nanotyrannus_ has no more vocal an advocate than Peter Larson, president of the Black Hills Institute of Geological Research, a company in Hill City, South Dakota, that

collects, prepares and casts fossils. Larson argues that the teeth of _Nanotyrannus_ are too finely serrated and closely packed to be those of a young _T. rex_. He also points to differences

between the two species in the anatomy of the shoulder socket and the openings in the skull. But some of these conclusions were gleaned from fossils not yet described in any publication,

and scientists may never have a chance to study them. A skeleton that has been identified as a _Nanotyrannus_ that could offer clues will be auctioned off next month in New York City. The

hype generated by this specimen and its relevance to the _Nanotyrannus_ debate has helped to drive up its price; estimates suggest that it may fetch up to US$9 million. But most

palaeontologists refuse to study such specimens unless they are placed in a reputable museum. A private buyer could rob researchers of that opportunity. “The solution may reside in the tired

plea for more fossils,” Witmer says. For _Nanotyrannus_ to have a shot at being a separate species, palaeontologists would like to see one of two discoveries: a young tyrannosaur more

similar to adult _T. rex_ than any _Nanotyrannus_ specimen, or an animal that is clearly an adult _Nanotyrannus_ that is different from _T. rex_. But where an animal as charismatic as _T.

rex_ is concerned, it may be impossible for researchers to abandon long-held views and resolve decades of debate. “I'm not sure how much data it'll take to break us out of that,”

Witmer says. A FLAP OVER FEATHERS For generations, artists have depicted _T. rex_ covered in scales, much like the modern-day reptiles to which it is only distantly related. But in the past

two decades, researchers in China have found specimens from many dinosaur groups bearing feathers or a fuzzy coating. Some of these discoveries include species closely related to _T. rex_.

> It is becoming increasingly difficult to reject a fuzz-less > _tyrannosaurus_ with a straight face. In 2004, Xu named _Dilong paradoxus_ — a small, early tyrannosaur6. The fossil of

this animal showed impressions of fibres around the tail, jaw and other body parts, suggesting the animal had a coat of 'dinofuzz'. The giant _Y. huali_ from China also bore

plumage3. The feathers on these tyrannosaurs were not like those of living birds, but simplified precursors. Xu suggests that the earliest feathered dinosaurs might have used their plumage

for visual display. Later animals that were cloaked entirely in feathers might have relied on them for insulation. Because of the close evolutionary link between tyrannosaurs, he suggests

that “_T. rex_ might have had some kind of protofeathers”. Other researchers also favour the idea of feathered tyrannosaurs. “It is becoming increasingly difficult to reject a fuzz-less

_Tyrannosaurus_ with a straight face,” Holtz says. That does not mean that _T. rex_ looked like a Cretaceous chicken. Brusatte says it may have been covered in fairly inconspicuous hair-like

fibres, like many other feathered dinosaurs. As yet, no skin impressions have been found for _T. rex_, so researchers cannot say with certainty what kind of body covering it had. And some

are not ready to abandon the more conventional view. Thomas Carr, a palaeontologist at Carthage College in Kenosha, Wisconsin, argues, for example, that unpublished fossils with skin

impressions from close relatives of _T. rex_ show scaly skin. These findings suggest that even though some earlier tyrannosauroids had feathers, the subgroup called tyrannosauridae (which

includes _T. rex_), seems to have undergone an evolutionary reversal from fuzz to scales. “There is no empirical evidence that tyrannosaurids had feathers,” Carr says, “and artists have no

business decking them out with plumage until the day comes when a tyrannosaurid is found with feathers.” This argument goes well beyond what the creatures looked like. Whether _T. rex_ had

feathers will influence how researchers reconstruct the life of this dinosaur, from possible courtship behaviours to how it controlled its body temperature. ARMS RACE One of the biggest

mysteries about _T. rex_ has nagged palaeontologists for more than a century: what use did the giant have for arms so stubby that they could not even have reached its mouth? Early ideas,

later discarded, suggested that the two-clawed arms helped _T. rex_ to grip a partner during mating or to rise from repose. Later palaeontologists argued that the arms were vestigial — an

idea beloved by cartoonists, who never tire of showing _T. rex_ embarrassed by its useless, puny guns. But research by palaeobiologist Sara Burch at Ohio University suggests that such jokes

are unfair. She has studied the musculature of crocodylians as well as that of the only living members of the dinosaur line — birds. If the arms of _T. rex_ had been vestigial, they would

have lost the various anatomical landmarks that indicate muscle attachments, but the fossils “retain evidence of substantial musculature,” she says. But knowing that _T. rex_ used its arms

doesn't reveal what they were used for. To Carr, the arms were part of the dinosaur's arsenal. “Tyrannosaurids used their arms in the same way all theropods used their arms, for

grasping and stabilizing objects” — namely prey, he says. Holtz visualizes a less rigorous role for the forelimbs. On the basis of previous estimates of muscle strength, he argues that _T.

rex_ had weak arms. And because many tyrannosaurs have arms with healed fractures, he says, “their life habits could not require constant use of these arms”. Holtz suggests that they were

used primarily for display, perhaps during mating or competition— a possibility that seems more likely if these limbs were cloaked in feathers. He and other palaeontologists plan to keep

digging into the secrets of this superlative animal, one of the strongest ambassadors of the past in all of science. “Many aspects of _T. rex_, especially behavioural ones or physiological

ones, are still unknown,” Holtz says. But perhaps not forever. “As new methods of investigation are developed, we will have new avenues about their biology to explore.” And as researchers do

so, their views on the tyrant king will continue to evolve. Credit: T. rex illustration by Emily Cooper; family tree from ref. 3 REFERENCES * DePalma, R. A., Burnham, D. A., Martin, L. D.,

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* Xu, X. et al. _Nature_ 431, 680–684 (2004). Article  ADS  CAS  Google Scholar  Download references AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Brian Switek is a freelance writer in Salt

Lake City, Utah., Brian Switek Authors * Brian Switek View author publications You can also search for this author inPubMed Google Scholar RELATED LINKS RELATED LINKS RELATED LINKS IN NATURE

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ARTICLE Switek, B. Palaeontology: The truth about _T. rex_. _Nature_ 502, 424–426 (2013). https://doi.org/10.1038/502424a Download citation * Published: 23 October 2013 * Issue Date: 24

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