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 truck...it 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!

References

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.

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!

References

Barsbold, Rinchen; Osmólska, Halszka (1999). "The skull of Velociraptor (Theropoda) from the Late Cretaceous of Mongolia". Acta Palaeontologica Polonica 44 (2): 189–219.

Carpenter, Kenneth (1998). "Evidence of predatory behavior by theropod dinosaurs" (PDF). Gaia 15: 135–144.

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.