Sunday, June 28, 2015

Pachycephalosaurus: Beast of the Week

This week we shall be checking out a well-known dinosaur with an iconic skull.  This dinosaur is one of my personal all time favorites.  I will never forget seeing it's awesome representation in The Lost World: Jurassic Park in the movie theater when I was only eight years old.  The way it smashed that changed me.  Say hello to Pachycephalosaurus!

Pachycephalosaurus was the largest known member of the pachycephalosaurid family at about fifteen feet long from beak to tail.  In life, it was most likely a plant-eater, but some have suggested it may have supplemented its diet with meat in the form of insects and possibly carrion.  The genus name, Pachycephalosaurus, translates to "Thick Head Lizard" in reference to the dinosaur's skull, which is almost ten inches thick in the biggest specimens.  Pachycephalosaurus lived during the very end of the Cretaceous period, between 70 and 66 million years ago, in what is now the Western United States, specifically Montana, South Dakota, and Wyoming.  In fact, its full genus and species name is Pachycephalosaurus wyomingensis, in reference to this.  When alive Pachycephalosaurus would have shared its environment with Tyrannosaurus, Anzu, Quetzalcoatlus, Anatotitan, Ankylosaurus, Triceratops, Troodon, Stygimiloch, and Dracorex.

Life reconstruction of a male Pachycephalosaurus by Christopher DiPiazza.

Sadly, none of Pachycephalosaurus' body beyond the skull has ever been discovered.  However, we have a good idea of what the whole animal looked like thanks to more complete skeletons of other kinds of Pachycephalosaurids it was closely related to.  It was most likely a biped, with short front arms, each ending in a five-fingered hand.  The hips were probably relatively wide with a thick tail base.  The end of the tail would have been stiffened with ossified tendons. (bony rod-like structures found in the tails of many dinosaurs)

Pachycephalosaurus' head was its trademark feature, however.  Like I said earlier, its skull was extremely thick, and made of solid bone, encasing a relatively small brain, forming a round, dome-shaped cranium.  Around the back of the head head, over the eyes, and on the top of the snout, were a series of short, knobby horns.  The mouth was tipped with a blunt beak and the teeth were small and leaf-shaped, most likely for shredding tough plants.  The eye sockets of Pachycephalosaurus skulls are relatively large, and face forward.  This indicates that this dinosaur would have had good vision and was able to see depth quite well in life.  The heavy skull would have been held up by a short, muscular neck in life.

Pachycephalosaurus skull on display at the American Museum of Natural History in New York.

The exact purpose of Pachycephalosaurus' thick skull has been debated much over the years.  The most common idea, especially in the beginning, was that rival Pachycephalosaurus would have ran into each other, head first, like modern day rams do, to establish dominance.  This idea became so popular, that you would be hard pressed to find paleo-art of Pachycephalosaurus not running with its head down, about to ram something in just about every toy, book, or other visual representation of the dinosaur.  More recently, however, there have been paleontologists who have challenged this, saying the shape of Pachycephalosaurus' skull would not have allowed it to have sustained crashing into other hard objects at high speeds without causing serious injury.  Many believed that the large domes of Pachycephalosaurus were mostly for display within the species.  Was it really possible that those thick skulls were just for show, though?  It seems unlikely considering how thick they were.  (Something that isn't entirely visual.)  Finally in 2013, a study on the skulls of many pachycephalosaurids discovered evidence of injuries that had healed over on the skulls of many individual specimens.  In fact, the kind of bone that the skulls of pachycephalosaurids were made of seems to have been especially good at healing and repairing itself after damage was inflicted on them.  This supported the idea, once again, that Pachycephalosaurus and its kin were indeed using their skulls as weapons.  They may have more likely swung their heads into each other at close quarters, instead of charging into each other from a running distance.  You can observe modern giraffes (like in the video below), and many other animals that sport weapons on their heads, combating each other in the same way.  If this indeed was the method used for Pachycephalosaurus fighting, the small horns on the sides of the head could have been utilized as weapons, enforced by the weight of the thick skull. Some still argue that using their skulls as weapons would have injured Pachycephalosaurus in life too much and was thus, is still an unrealistic idea or real life.  These folks need to remember, however, that many animals do get injured quite a lot when fighting rivals.  In fact, many species of animals become permanently maimed, or even die fighting peers for dominance.  If a Pachycephalosaurus did hurt itself or die in combat with another back in the Cretaceous, it would have just been another example of natural selection in the works. Evolution is never perfect.  If it was nothing would ever go extinct!

Pachycephalosaurus is believed to have changed its form drastically as it matured into an adult by certain paleontologists.  The other two, slightly smaller, pachycephalosaurids, that shared its environment, Dracorex (sported lots of horns but no dome) and Stygimoloch (sported horns and a small dome), are believed by some to have actually been juvenile and subadult forms, respectively, of adult Pachycephalosaurus.  If this is the case, Pachycephalosaurus would have had no dome at all when young, and its horns would have become shorter, being absorbed into the growing thickness of the skull as it matured.  It is important to note, however, that there is no known example of this in any kind of living vertebrate.  Since the number of pachycephalosaurid specimens on the fossil record is still very limited, this hypothesis still needs a lot more evidence to be verified and these three dinosaurs may have indeed been different species after all.  It is an interesting idea to consider, however.

From left to right: Dracorex, Stygimoloch, and Pachycephalosaurus.  Paintings by Christopher DiPiazza.

That is all for this week!  As always feel free to comment below or on our facebook page!


Carpenter, Kenneth (1 December 1997). "Agonistic behavior in pachycephalosaurs (Ornithischia: Dinosauria): a new look at head-butting behavior" (PDF). Contributions to Geology 32 (1): 19–25.

Horner J.R. and Goodwin, M.B. (2009). "Extreme cranial ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus." PLoS ONE, 4(10): e7626.

Peterson, J. E.; Vittore, C. P. (2012). Farke, Andrew A, ed. "Cranial Pathologies in a Specimen of Pachycephalosaurus". PLoS ONE 7 (4): e36227. doi:10.1371/journal.pone.0036227.

Peterson JE, Dischler C, Longrich NR (2013) Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae). PLoS ONE 8(7): e68620. doi:10.1371/journal.pone.0068620

Sullivan, Robert M. (2006). "A taxonomic review of the Pachycephalosauridae (Dinosauria:Ornithischia)" (PDF). Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin 35: 347–366.

Sunday, June 14, 2015

Velociraptor: Beast of the Week

Jurassic World Came out this past weekend!  In its honor we will be reviewing a dinosaur that has been requested over and over again but I insisted on holding off on especially when I knew Jurassic Park was going to get a fourth movie installment.  I vowed to wait until the week of its release to finally do it.  This is a dinosaur which became one of the most popular ever right next to Tyrannosaurus, thanks to the Jurassic Park franchise alone.  Despite this, this dinosaur is extremely misunderstood by most people because of Jurassic Park's inaccurate portrayal of it at the exact same time.  Let's get to know Velociraptor!

Velociraptor mongoliensis family getting ready to sleep at dawn.  Painting by Christopher DiPiazza.

Velociraptor is actually known from a few species, but the most well known is called Velociaptor mongoliensis.  The genus name, Velociraptor, translates to "Swift Thief" and the species name is in reference to Mongolia, where the dinosaur's fossils were originally discovered.  Velociraptor lived during the late Cretaceous Period, between 80 and 71 million years ago.  It was a meat-eater, and measured only a little over six feet long from snout to tail.

The environment that Velociraptor lived in during the late Cretaceous would have been a desert, with hot days, cold nights, and little water nor food for animals.  For these reasons, desert animals had to have some sort of evolutionary strategy that would allow them to survive, especially when it came to finding food.  In the case of Velociraptor, paleontologists can tell that it would have had good vision under low light conditions, and likely was active either ad dusk or at night, thanks to the size of its eye sockets and width of its sclerotic rings   Sclerotic Rings are thin, ring-shaped bones that are in the eyes of certain animals, including dinosaurs, that are used to support the eyes.  The size and shape of these rings, when compared to those of animals today and what we know about their lifestyles, helps scientists guess how extinct dinosaurs would have been able to see in different lights.  This would have helped it for a number of reasons.  First of all, the temperatures aren't sweltering at night in the desert.  In fact, deserts at night get quite cool.  This means it would have been more comfortable for Velociraptor to have been active during these times.  Second, some of the prey Velociraptor could have hunted may not have been as active at night, making it easier for the little predator to ambush.  Owls, today frequently attack other birds (including large hawks!) in their sleep using this strategy.

Velociraptor mongoliensis skull on display at the American Museum of Natural History in New York.  It was an expedition by the AMNH during the 1920s, in fact, that yielded the first ever found remains of Velociraptor.

In addition to good vision, Velociraptor had other adaptations that would have helped it hunt.  Its snout was extremely long and narrow, and was filled with blade-like, serrated teeth for slicing meat.  Its hands were each equipped with three long fingers, each tipped with a hooked claw, and its feet, like all members of its family, called dromaeosauridae, were each equipped with a retractable, crescent-shaped claw, on the second toe, for holding struggling prey in place.  The tail of Veloceraptor was stiffened by bony rods inside it, called ossified tendons, and would have enabled it to have made sharper turns while running.

Velociraptor skeleton on display at the Wyoming Dinosaur Center.

Contrary to what you may see in popular media, Velociraptor was covered in feathers in life, just like a bird.  Despite the fact that no actual feathers have been found attached to Velociraptor bones, themselves, (Feathers would have rotted away somewhere within the last 70 million years of the dinosaur being dead.) we do have strong evidence that they were there.  A specimen of Velociraptor that was published in 2007 shows small, evenly spaced bumps on the dinosaur's, ulna. (forearm bone)  The bumps, called quill knobs, are exactly the same as those found on the arm/wing bones of modern birds, which are the attachment sites for large primary(wing) feathers.  Also, many specimens of dinosaurs closely related to Velociraptor that died in more favorable conditions for soft tissue preservation, do have feathers completely preserved right there in the fossil.  Thanks to phylogenetic bracketing, it is safe to say that Velociraptor was definitely a feathered animal.

Arrows pointing to quill knobs on a Velociraptor ulna.

So why did Velociraptor have feathers, especially since it would have been too heavy to fly?  Well, remember I said before it was likely active at night, when it is actually pretty cold in the desert.  Feathers would have helped Velociraptor maintain a warm body temperature during those times.  Today many nocturnal desert animals also have thick fur or feathers for insulation.  Another reason for feathers could have been to help Velociraptor protect its eggs.  We know that other theropod dinosaurs, like Citipati, used their arm feathers to protect their eggs from the elements, so Velociraptor could have been the same.

The Fennec Fox, native to deserts of Northern Africa, has a thick coat of fur to help keep it warm since it is most active at night, when temperatures in its habitat can drop quite low.  Velociraptor may have had feathers for the same reason in it's desert habitat.

Velociraptor was no doubt a predator when it was alive, but it probably wasn't as ruthless and terrifying (at least to a human) as popular media makes it out to have been.  There is no evidence that suggests Velociraptor hunted in packs.  (There is evidence for it in its larger relative, Deinonychus, however.) It is likely Velociraptor was good at hunting small prey, using its wicked retractable talon on each of its second toes to pin down prey so it could rip off pieces of meat with jaws.  There is evidence that Velociraptor scavenged a lot of its meals too, like most meat-eaters will when given the chance, thanks to Velociraptor teeth marks found on the bones of Protoceratops, and even a large pterosaur.

There is one spectacular fossil that shows us that at least sometimes Velociraptor did have run-ins with large prey.  This fossil consists of a Protoceratops and Velociraptor together as they were most likely both killed by a sandstorm and buried immediately.  The Velociraptor's foot, appears to be extended in the neck region of the Protoceratops.  In turn, the Velociraptor's hand is firmly clamped in the ceratopsian's powerful beak.  It is unlikely Velociraptor would have regularly attacked such a much-larger, and potentially dangerous animal, as Protoceratops.  It is most likely that the plant-eater was sick or injured, and/or possibly even the Velociraptor was starving and desperate.  The desert is a harsh place to live and starvation was probably an ever looming possibility for many animals.  Other dinosaurs, in addition to Protoceratops, the ceratopsian, that would have shared this unforgiving habitat with Veloceraptor were the theropods, Oviraptor and Citipati

Dramatic "fighting dinosaurs" fossil featuring Protoceratops biting the hand of Velociraptor.

That is all for this week!  Hope you enjoyed getting to know the real Velociraptor!  Please comment below or on our facebook page!


Barsbold, Rinchen; Osmólska, Halszka (1999). "The skull of Velociraptor (Theropoda) from the Late Cretaceous of Mongolia". Acta Palaeontologica Polonica 44 (2): 189–219.
Hone, David; Choiniere, Jonah; Sullivan, Corwin; Xu, Xing; Pittman, Michael; Tan, Qingwei (2010). "New evidence for a trophic relationship between the dinosaurs Velociraptor and Protoceratops". Palaeogeography, Palaeoclimatology, Palaeoecology 291 (3–4): 488–492
Hone, D.; Tsuihiji, T.; Watabe, M.; Tsogtbaatr, K. (2012). "Pterosaurs as a food source for small dromaeosaurs". Palaeogeography, Palaeoclimatology, Palaeoecology. 331-332: 27.

Motani, R., & Schmitz, L. (2011). Phylogenetic Versus Functional Signals in the Evolution of Form-Function Relationships in Terrestrial Vision Evolution

Osborn, Henry F. (1924a). "Three new Theropoda, Protoceratops zone, central Mongolia". American Museum Novitates 144: 1–12.
Schmitz, L.; Motani, R. (2011). "Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology". Science 332 (6030): 705–8.
Turner, A.H.; Makovicky, P.J.; Norell, M.A. (2007). "Feather quill knobs in the dinosaur Velociraptor". Science 317 (5845): 1721.

Sunday, June 7, 2015

Tyrannosaurus: Beast of the Week

This week we will be looking at the most well-recognized dinosaur of all time!  Make way for Tyrannosaurus rex!  Tyrannosaurus lived during the Late Cretaceous Period, between 68 and 66 million years ago in what is now Western North America, including Colorado, Wyoming, Montana, South Dakota, Utah, Texas, and also parts of Southern Canada.  The genus and species name, Tyrannosaurus rex, translates to "Tyrant Lizard King".  The biggest adult specimen on the fossil record of a Tyrannosaurus measured about forty two feet long from snout to tail, making it the biggest known meat eater from its environment.

Tyrannosaurus rex attacking a young Alamosaurus.  Reconstruction by Christopher DiPiazza.  Yup, those are tiny feathers.  Given the most recent findings confirming Tyrannosaurus had fine scales, these feathers are sparse enough to have still been a possibility.

Tyrannosaurus is the most well-known and loved dinosaur of all time.  This is because for a very long time, starting in the early 1900s, Tyrannosaurus was by far the largest meat-eating dinosaur known to science.  Since then several other meat-eating dinosaurs have been discovered that rival or even surpass the tyrant king in length, but Tyrannosaurus was still the most robust and massive of all of them.

The head of Tyrannosaurus is iconic, and easily distinguishable from any other dinosaur.  It's general shape is very rectangular, and its face, especially near the rear of the jaw, was generally wider than what you would see in other dinosaurs, suggesting there were large muscles attached there in life.  Tyrannosaurus would have had good eyesight, complete with good depth perception, as well as an extremely good sense of smell.  Its nose would have had one of the most acute senses of smell of any animal known.  Scientists can tell this by looking at the negative space inside of the skull, where the brain used to be, getting an accurate shape of the brain.  Thanks to this, it can be observed that the parts of the brain associated with sight and smell were proportionally large and well-developed in Tyrannosaurus.

Cast of the inside of a Tyrannosaurus' braincase on display at the Sydney Museum.  The large part on the far left side is the olfactory bulbs, the part associated with sense of smell.

The teeth of Tyrannosaurus were larger than those of any other dinosaur (some were a foot long!) and totally unique in form, as well.  Most every other kind of meat-eating dinosaur tooth was either flattened and blade-like, for slicing meat, or pointed, and cone-shaped, for holding on.  The teeth of Tyrannosaurus, however were curved, serrated and really thick, comparable to the shape of bananas actually...pointy, serrated bananas.  This suggests that Tyrannosaurus teeth were adept at utterly pulverizing anything it bit into, including bone.  Scientists estimate, thanks to computer simulations of the skull of Tyrannosaurus, that the king of dinosaurs could bite down with over 12 thousand pounds of pressure per square inch, making it the owner of the strongest jaws of any known animal.  This makes sense when you consider the animals Tyrannosaurus was coexisting with and feeding on.  As the result of one of the most extreme evolutionary arms races in natural history, dinosaurs like Triceratops and Ankylosaurus, both of which coexisted with Tyrannosaurus, were the largest and most heavily-armored forms of each of their families.  Tyrannosaurus, in turn, was the largest and most powerful of its family, the pinnacle of tyrannosauridae.  In fact, there is direct evidence of Tyrannosaurus having fed on dinosaurs like Triceratops, despite the solid-bone frill, thanks Tyrannosaurus teeth found embedded in Triceratops bones, and chunks of bone found in fossilized Tyrannosaurus poop. 

The rest of Tyrannosaurus' body was interesting, too.  It is well-known for its proportionally small arms, which are actually about the same length as adult human arms!  Despite being the butt of many jokes, Tyrannosaurus' arms were actually quite strong.   Paleontologists estimate, by closely studying the arm bones and predicting the amount of muscle that would have attached to them in life, that each one of Tyrannosaurus' arms could lift over four hundred pounds of weight!  The function of the tiny arms, which each were tipped with two fingers and claws, is still a mystery.  Some paleontologists guess that they would have helped the dinosaur get up from a resting position.  It is also possible that they could have aided in holding on while Tyrannosaurus mated. (But then again, both sexes had the same arms, so who knows?)

Tyrannosaurus rex skeletal mount on display at the American Museum of Natural History in New York.

The legs and tail of Tyrannosaurus would have been extremely muscular.  Because it was so large and heavy, Tyrannosaurus likely wasn't a very fast runner, probably not even being able to break twenty miles per hour at top speed, but then again, most of the dinosaurs it would have hunted were about that same speed or slower, anyway, so no real loss there.  At top speed, because of its immense weight, Tyrannosaurus would likely always would have one foot on the ground at any time, so it would have been more of a power walk than a run.  Interestingly enough, it is the feet of Tyrannosaurus that holds the answer to where Tyrannosaurus' group, the tyrannosaurids, fall on the dinosaur family tree.  The feet of all dinosaurs each contain three bones, called metatarsals.  (Humans have five metatarsals in each foot.)  Most large theropod dinosaurs, like Allosaurus, for instance, have all three metatarsals roughly the same length, neatly positioned next to each other in the foot.  In the smaller, more bird-like theropods (including birds), called the coelurosaurs, the middle metatarsal of each foot is a little shorter than the surrounding two, forming an upside down V shape when looking at the skeleton.  Well, Tyrannosaurus feet show this same shortened middle metatarsal.  At first this may seem odd that the gigantic Tyrannosaurus is more closely related to dinosaurs like Troodon, and modern birds, than it is to other giant carnivores, like Allosaurus or Spinosaurus, but remember that Tyrannosaurus' older relatives, are smaller and smaller the farther back in time you go.  Check out Eotyrannus or Guanlong for examples.  So even though Tyrannosaurus was a particularly gigantic dinosaur, it was more of an exception among the many more gracile tyrannosauroids that it was most closely related to.

Tyrannosaurus skeleton foot.  Notice the upside down V formed by the shorter middle foot bone.

Paleontologists have been unearthing and study specimens of what are believed to have been from juvenile Tyrannosaurus within recent years.  What is interesting about these specimens is that they don't look like just smaller versions of the adult Tyrannosaurus.  Their legs are longer proportionally and they even have different teeth, which are flatter and more blade-like.  The number of teeth in their jaws is even different.  This tells us that Tyrannosaurus may have been filling a different predatory ecological niche as a juvenile than it was as an older adult.  Perhaps the younger Tyranosaurus were better at chasing down and eating more fast moving prey like Ornithomimus or Pachycephalosaurus and then graduated to hunting more heavily armored, but slower Triceratops and Anatotitan, when they were bigger and stronger.  There is another theory that at least some of these smaller, seemingly Tyrannosaurus specimens are actually a different kind of dinosaur, which would be called Nanotyrannus.  There needs to be more specimens discovered and more research done to completely prove either one of these sides completely, however.  Paleontology is mysterious!

Juvenile Tyrannosaurus skeleton nicknamed "Jane" on display at the Burpee Museum of Natural History in Illinois.  Note how the skull is not as robust as an adult's and how the legs are proportionally much longer.  Some believe Jane may actually represent a different kind of dinosaur, called Nanotyrannus.

There is much debate as to exactly how Tyrannosaurus would have behaved when it was alive.  There is this ongoing argument amongst fanboys as to whether or not Tyrannosaurus was indeed an active predator, killing other dinosaurs on a regular basis for food, or a scavenger, using its keen sense of smell and extremely powerful jaws track and eat the bones and leftover carcasses from prey of other predators.  Frankly, I think this is a really stupid debate, mostly egged on by the media more than anything else to get people talking.  If you want to know how Tyrannosaurus fed, just look at literally any large meat-eating animal alive today and you will get your answer.  It scavenged when it could, and killed when it had to.  Seriously, I don't need a time machine to be confident about this.  I'd actually be willing to bet large sums of money this was the case.  Go ahead, check out modern meat-eaters of all kinds.  Lions, bears, crocodiles, eagles, sharks...they all scavenge AND hunt.  Debate over.  That was easy!

Tyrannosaurus skeleton on display at the Field Museum of Natural History in Chicago.  This specimen, nicknamed "Sue" is the largest Tyrannosaurus known to date, at forty two feet long.

Several small patches of skin from Tyrannosaruus have been found.  The largest of these is about the size of a playing card and is from the bottom of the tail.  The others are from the neck and the hips.  All of these skin patches show small pebbly scales.  When I say small, I mean each scale is about a millimeter in diameter, comparable to the texture of a basketball.  That being said, keep in mind Tyrannosaurus was a coelurosaur, the same group of theropods that includes modern birds.  In fact, coelurosaurs are known on the fossil record to possess feathers.  At least one example of every major branch of this group of dinosaurs has been found with them, including Tyrannosaurus' branch.  So did Tyrannosaurus have feathers too?  Well, since these scales are so small and close together, it doesn't appear that there would have been room for feathers to have been growing from between them in life.  So if Tyrannosaurus did have feathers, they were either very sparse, or concentrated to the top of the neck and back, stopping before the hips. (That second possibility I find unlikely.) It is also possible that Tyrannosaurus simply didn't have feathers at all, making it an evolutionary anomaly, considering the rest of its known family tree had them. For now we still don't know for sure, and that is exciting!

Photographs and diagrams showcasing the known scales of Tyrannosaurus from Bell's 2017 paper.  To give you an idea as to how small these patches are, the black scale bars for the top two images is 5 centimeters. (neck and hips)  The scale bars for the bottom images (tail) are 10 centimeters.

That is all for this week!  As always feel free to comment below or on our facebook page


Bakker, R.T.; Williams, M.; Currie, P.J. (1988). "Nanotyrannus, a new genus of pygmy tyrannosaur, from the latest Cretaceous of Montana". Hunteria 1 (5): 1–30.

Bates, K.T & Falkingham P.L. (2012). Estimating maximum bite performance in Tyrannosaurus rex using multi-body dynamics. Biological Letters.

Bell, P. R., Campione, N. E., Persons, W. S., Currie, P. J., Larson, P. L., Tanke, D. H., & Bakker, R. T. (2017). Tyrannosauroid integument reveals conflicting patterns of gigantism and feather evolution. Biology Letters, 13(6), 20170092.

Carpenter, Kenneth; Smith, Matt (2001). "Forelimb Osteology and Biomechanics of Tyrannosaurus rex". In Tanke, Darren; Carpenter, Kenneth. Mesozoic vertebrate life. Bloomington: Indiana University Press. pp. 90–116.

Carr, T.D.; Williamson, T.E. (2004). "Diversity of late Maastrichtian Tyrannosauridae (Dinosauria: Theropoda) from western North America". Zoological Journal of the Linnean Society 142 (4): 479–523.

Hutchinson, J.R. (2004). "Biomechanical Modeling and Sensitivity Analysis of Bipedal Running Ability. II. Extinct Taxa" (PDF). Journal of Morphology 262 (1): 441–461.

Stevens, Kent A. (June 2006). "Binocular vision in theropod dinosaurs". Journal of Vertebrate Paleontology 26 (2): 321–330.

Meers, Mason B. (August 2003). "Maximum bite force and prey size of Tyrannosaurus rex and their relationships to the inference of feeding behavior". Historical Biology: A Journal of Paleobiology 16 (1): 1–12.

Xu, X.; Wang, K.; Zhang, K.; Ma, Q.; Xing, L.; Sullivan, C.; Hu, D.; Cheng, S.; Wang, S. et al. (2012). "A gigantic feathered dinosaur from the Lower Cretaceous of China" (PDF). Nature 484 (7392): 92–95.