In 2012, one of the most important discoveries in the history of dinosaur science was announced.
Three nearly complete skeletons belonging to a new species of carnivorous theropod had
been found with feathers around their bodies.
Named Yutyrannus huali, the beautiful feathered tyrant, it was found in the Yixian Formation
in northeastern China.
This region is famous for its excellently preserved fossils of feathered dinosaurs,
most of which come from Yutyrannus’s branch of theropods, the coelurosaurs.
What set Yutyrannus apart from the other feathered coelurosaurs was its size.
All previously discovered feathered dinosaurs were small creatures, and it had been assumed
that larger dinosaurs lost most of their feathers, yet this unambiguously feather theropod was
nine meters long and weighed nearly one and a half metric tons.
The previously largest known feathered dinosaur, the small, basal therizinosaur Beipiaosaurus,
was forty times smaller than Yutyrannus.
Most of the other dinosaurs with preserved feathers, were dwarfed by even this modest
therizinosaur.
The size of this new theropod was surprising in more ways than one.
It belonged to the clade Tyrannosauroidea, making it a relative of the famous Tyrannosaurus
rex.
However, Yutyrannus lived between one hundred and thirty to one hundred and twenty million
years ago, during the Early Cretaceous.
Most other Early Cretaceous tyrannosauroids were small to medium sized predators, and
the ancestors of Tyrannosaurus only even began to approach the size of Yutyrannus about ninety
to eighty million years ago, during the Late Cretaceous.
The beautiful feathered tyrant seems to have become an apex predator independently from
the lineage of tyrannosauroids which led to the later tyrant king.
As a basal tyrannosauroid, Yutyrannus differed in many ways from Tyrannosaurus and its more
familiar relatives, the tyrannosaurids.
The most obvious difference is that while the tyrannosaurids had tiny arms with only
two fingers, Yutyrannus’s forelimbs were much longer and had three fingers tipped with
large, curved claws.
There is no doubt that it used them as effective weapons.
Tyrannosaurids were specialized bone crushers, which they accomplished with their wide, robust
skulls and wide, almost banana shaped teeth.
In sharp contrast to the later tyrants, Yutyrannus’s skull was narrower and its teeth were strongly
compressed on the sides.
They were built to slice through flesh rather than pulverize bone.
Although Yutyrannus had a much weaker bite than its Late Cretaceous relatives, tyrannosaurids
were exceptional among the theropods, and its jaw were still quite powerful.
In contrast to its more dagger-like teeth, those at the end of the snout were small and
D-shaped, which was typical of tyrannosauroids both large and small.
Still, Yutyrannus’s arsenal was more like that of carnosaurs such as Allosaurus and
Torvosaurus.
There is quite a bit of irony here.
Although it is now clear that they were gigantic coelurosaurs, tyrannosaurs were once considered
to be the last, and most extreme, of the carnosaurs.
Yutyrannus was a tyrannosauroid who was built like the contemporary carnosaurs.
Unlike most basal tyrannosauroids, but like both carnosaurs and the later tyrannosaurids,
Yutyrannus’s skull was deep and proportionally large.
This allowed for it to support larger jaw muscles, and shows a shift towards hunting
larger prey than the other basal tyrannosauroids.
Tyrannosauroids more closely related to the tyrannosaurids, still had smaller, low skulls,
so this was clearly the result of convergent evolution.
Due to the greater weight of a larger skull, it was necessary for Yutyrannus’s neck become
shorter to support it.
Yutyrannus’s nares, the openings in the skull which held the nostrils, were large
and elongated, matching those of its smaller cousins instead of the tyrannosaurids.
As in all other tyrannosauroids, the nasal bones themselves were fused.
Yutyrannus possessed a long, rugose crest that ran along its snout.
This crest bridges the gap between basal and derived tyrannosauroids.
Many of the basal species had larger, more elaborate crests.
The tops of tyrannosaurid snouts retained the rugose texture of Yutyrannus’s crest,
but the bumpy structures they supported were far more subtle.
Yutyrannus also had small horns just before its eyes, another derived trait shared with
the tyrannosaurids.
The purpose of these crests and horns was to help attract mates or scare off rivals,
and in life they would have made Yutyrannus all the more beautiful.
An additional attribute of the later tyrants foreshadowed in Yutyrannus was speed.
Tyrannosaurids were able to run much faster than most other large theropods, although
the bulkiest species, like Tyrannosaurus and Tarbosaurus, were only able to do so when
they were not yet fully grown.
The proportions of Yutyrannus’s legs were almost on par with those of tyrannosaurids,
which would have made it far faster than most of the contemporary carnosaurs.
Juvenile Yutyrannus had proportionally longer legs than adults, suggesting they may have
been even faster.
The most revolutionary trait found on Yutyrannus was its namesake feathers.
Their impressions have been found on its tail, pelvis, upper arms, close to its feet, and
on its neck.
Given their distribution, it is thought they covered most of Yutyrannus’s body.
The longest feathers were those on the neck, which measure twenty centimeters long.
The feathers found on Yutyrannus are not the same as the complex, pennaceous feathers seen
in birds today, which are only found in the clade Maniraptoriformes.
They instead consisted of simple filaments more like the down of bird hatchlings.
Yutyrannus is thought to have primarily used these simple feathers, sometimes called “protofeathers”,
for thermal insulation.
Dinosaurs had higher metabolisms than most other reptiles, which helped them become more
active, grow quickly, and to thrive in colder climates.
Fueling this metabolism required burning vast quantities of energy, so keeping the heat
their bodies produced became a major evolutionary priority.
Additionally, these simple feathers may have also served a role as display structures.
Even though they were not as complex as bird wings, feathers that appear to have been used
for display are found in other early coelurosaurs and even the contemporary, and otherwise scaly,
ornithischian dinosaur Psittacosaurus.
There was already speculation that Tyrannosaurus and the other tyrannosaurids had feathers
due to the discovery of the small feathered tyrannosauroid Dilong in 2004, but Yutyrannus
proved that large dinosaurs could retain extensive feathery coats.
However, further discoveries have found small scale impressions on various parts of Tyrannosaurus’s
body, showing it was not so fluffy after all.
If a composite is made of all tyrannosaurid skin impressions, areas where feathers are
preserved in Yutyrannus turn out to be scaly, and there is no room for the large, fluffy
coat found in the beautiful feathered tyrant.
This doesn’t mean the tyrannosaurids lost all of their feathers.
Given their smaller size, juveniles may have been as feathery as Yutyrannus or Dilong,
and adults may have retained a few feathers, but it does seem they had an at least predominantly
scaly integument.
The tyrannosaurids are not unique in this regard.
Although primitive feathers seem to have been present in the very first dinosaurs, they
were lost multiple times as they grew in size.
When animals reach a certain size, thermal insulation is no longer necessary on most
of the body.
Indeed, many of the tyrannosaurids would have been in danger of overheating if they keep
the fluffy coats of their smaller ancestors.
The question then is why Yutyrannus remained so fluffy.
Some of the tyrannosaurid scale impressions come from species of roughly equal size as
the feathered tyrant, and more extensive scale impressions are preserved on the holotype
of the theropod Carnotaurus.
This horned theropod was an abelisaurid, meaning it was not closely related to the tyrannosauroids,
but it was about the same size as Yutyrannus.
The answer is the climate Yutyrannus inhabited.
Whereas Carnotaurus and most tyrannosaurids are found in tropical or subtropical habitats,
the environment preserved in the Yixian Formation had an average temperature of only 10 degrees
Celsius, or 50 degrees Fahrenheit.
The winters here would have been freezing, so Yutyrannus would have continued to benefit
from the warmth its feathers provided.
Besides the climate, the dense forests of the Yixian Formation would have more shade
than more open environments.
That is not to say heat would never have been a problem for Yutyrannus.
It may have shed its longer feathers during the summer months, much like how many of today’s
mammals from similar environments shed their winter coats.
There was one tyrannosaurid who lived in conditions similar to Yutyrannus: the Alaskan Nanuqsaurus.
Although temperatures were higher during the Late Cretaceous, the Arctic home of this polar
tyrannosaur was still slightly colder than the Yixian Formation.
Likewise, Nanuqsaurus was about the same size as Yutyrannus, although most reconstructions,
including its appearance in Prehistoric Planet, are based on the remains of juveniles.
Therefore, Nanuqsaurus likely had a feathery coat like that of Yutyrannus.
Still, it seems Yutyrannus was the exception among large dinosaurs.
As of right now, it remains the largest one unambiguously preserved with feathers.
However, the bizarre Deinocheirus had a pygostyle, a bone used to anchor large feathers, at the
end of its tail.
Given the warmer climate it inhabited and its greater body size, Deinocheirus likely
only retained large display feathers in places like its tail or arms.
Despite most reconstructions giving it a similar integument as other ornithomimosaurs, this
would have left it vulnerable to heat stroke.
At the very least, the gap between Yutyrannus and the second largest dinosaur with preserved
feathers has shrunk.
Feathers were found on Deinocheirus’s smaller relative, the North American Ornithomimus,
the same year as Yutyrannus.
Yutyrannus was once considered to belong to a branch within Tyrannosauroidea called Proceratosauridae.
These basal tyrannosauroids are notable for their massive crests.
Although most of them were small, the proceratosaurid Sinotyrannus was about the same size as Yutyrannus,
and it was once thought to be the closest relative of the beautiful feathered tyrant.
However, a recent monograph detailing the anatomy of the tyrannosauroid Eotyrannus concluded
that Yutyrannus was instead slightly more closely related to the tyrannosaurids than
the proceratosaurids were.
Yutyrannus’s cranial gear, which seems to be transitional between that of proceratosaurids
and tyrannosaurids, certainly supports this position.
It therefore seems that Sinotyrannus obtained its massive size independently of both Yutyrannus
and the tyrannosaurids.
The same monograph found that there may have been a fourth group of large tyrannosauroids,
the megaraptorans, whose massive arms made those of Yutyrannus look puny in comparison.
However, the classification of these theropods is controversial, and they may instead belong
elsewhere in Coelurosauria or within Carnosauria.
Regardless of just how closely related Sinotyrannus was to Yutyrannus, it lived in China not long
after the beautiful feathered tyrant.
Throughout the rest of the world, most of the top predators were carnosaurs like the
deep skulled carcharodontosaurids and the long snouted spinosaurids.
Ceratosaurs like the abelisaurids were also sizeable carnivores, although they were restricted
to the southern continents.
The giant dromaeosaurid Utahraptor was previously thought to have lived 125 million years ago,
but it has recently been determined that Utahraptor was ten million years older than previously
believed.
In any event, none of these dinosaurs were present in China during the reign of these
two basal tyrannosauroids.
Likewise, China is the only region with large tyrannosaur fossils dating to the Early Cretaceous.
Yutyrannus and Sinotyrannus seem to have managed to carved out their own little kingdom, rising
to the top of the food chain during a time when the other tyrant dinosaurs were still
underdogs to the carnosaurs.
There is evidence that Yutyrannus hunted in packs.
All three skeletons were found together.
They consist of an adult, subadult, and juvenile.
Similar mixed aged groups of large theropods, including the fellow tyrannosauroids Teratophoneus
and Daspletosaurus, have been interpreted as packs.
Yutyrannus coexisted with large sauropod dinosaurs, who would have been too dangerous to confront
alone, but working together would have given the beautiful feathered tyrant a chance.
Given the presence of sauropod bones at the same site as the Yutyrannus skeletons, it
has been hypothesized they died when one of these hunts went terribly wrong.
However, it is also possible they instead fell victim to a predator trap or even a Komodo
dragon-like feeding frenzy.
Besides allowing Yutyrannus to take down the local long necked titans, hunting in groups
would have also been useful against less daunting prey.
There is a hypothesis that tyrannosaurid juveniles helped to lure animals into the jaws of the
more powerful but slower adults.
Given how the younger Yutyrannus had proportionally longer legs, and adults had proportionally
larger skulls, the same may have been true of them.
It has been questioned whether tyrannosauroids possessed the intelligence to hunt in a coordinated
manner, but this was not strictly necessary.
The younger tyrants would have simply need to catch their victims, giving the adults
time to finish them off.
Today’s Cuban crocodiles are able to hunt cooperatively, and although their intelligence
is often underestimated, they are no more intelligent than Yutyrannus was.
Despite the Yixian Formation being one of the richest Mesozoic sites, Yutyrannus is
one of the few large dinosaurs found there.
The only other examples are the iguanodontians Bolong and Jinzhousaurus and a handful of
sauropods.
The small, possibly semi-aquatic ankylosaur Liaoningosaurus might also be the juvenile
form of a larger ankylosaur.
Yutyrannus could have certainly hunted the smaller species of dinosaurs, yet it would
be easy to get the impression that there wasn’t enough food for the feathered tyrant.
The seeming overabundance of small dinosaurs in Early Cretaceous China is the result of
preservation bias.
It is not just the fossils of small dinosaurs which are more common there, but also the
remains of other tiny creatures like mammals, lizards, and even insects.
Because their bones are much more fragile, animals of this size are less likely to be
preserved, so the studier bones of medium to large sized dinosaurs are usually overrepresented
in the fossil record.
In contrast, most fossils from the Yixian Formation were preserved by the ash created
from continuous volcanic eruptions, which created a less biased sample of the local
fauna, and in exquisite detail no less.
It was this volcanic activity that fossilized the three nearly complete skeletons of this
early tyrant, and its beautiful feathers.
Although most comparable dinosaurs were probably not as fluffy, Yutyrannus has certainty changed
how the large, theropod dinosaurs are perceived, showing they were much more bird-like than
once envisioned.
Yutyrannus has also helped to bridge some gaps in the evolution of Tyrannosauroidea,
and provided further evidence of pack hunting in the large theropods.
However, while most of the other large, predatory theropods belonged to a handful of groups,
Yutyrannus obtained its size independently of any other known species, setting it apart
from all other dinosaurs.
The beautiful feathered tyrant is without a doubt one of the most significant dinosaurs
ever discovered.
Thank you for watching, and a thank you to the themattalorian for narrating this video.
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