Beipiaosaurus: Beast of the Week

 Beipiaosaurus inexpectus was a theropod dinosaur that lived in what is now Liaoning, China, during the early Cretaceous period, 125 million years ago.  From snout to tail it measured about 7 feet (2.2 meters) long and would have eaten plants when alive.  The genus name means "Beipiao Reptile" which is in reference to the city, Beipiao, near where its fossils were discovered.  

Watercolor reconstruction of Beipiaosaurus by Christopher DiPiazza.  Note the combination of shorter, shaggy feathers and long, quill-like feathers.

Beipiaosaurus was an early member of the therizinosaur group of theropods, which are famous for being herbivores in an otherwise mostly meat-eating group.  They are also known for having proportionally short legs and long arms, equipped with three extremely long claws on each hand.  Later therizinosaurs, like the more famous, Therizinosaurus, are known for having long, slender necks with proportionally tiny heads, but Beipiaosaurus actually had a very large head, with a skull the same length as its femur.  Beipiaosaurus also had three weight-bearing toes on each foot, while its later relatives had four. 

Beipaosaurus had a long, narrow skull, with a small beak at the tip that was also lined with small leaf-shaped teeth, ideal for shredding plants.  It had long, powerful arms, each equipped with three extremely large, hooked claws.  Since Beipiaosaurus appears to have been a plant-eater, these claws could have been for manipulating branches as it ate, or possibly even for defense against predators.  Thanks to beautifully preserved remains, we know that Beipiaosaurus was mostly covered in shaggy, fur-like feathers that could have been an adaptation to keep the dinosaur warm. (the climate where it lived during the early Cretaceous could get cold at times)  Beipiaosaurus also had a second kind of unusual long, quill-like feather that appear to have been growing out of its body amongst the shorter, shaggier feathers.  We don't know exactly what these quill-like feathers were for. Perhaps they helped keep the animal dry by wicking off rain, or possibly they were for some kind of display among members of the same species? It's difficult to tell without seeing the animal alive. Beipiaosaurus also had a bone on the tip of its tail, called a pygostyle, which modern birds also have where their tail feathers attach.  Beipiaosaurus doesn't show any evidence of those kinds of feathers, however, so the bone, itself, must have evolved first.

Fossil Beipiaosaurus.  Note the feathers that preserved around the neck.

Perhaps the most amazing discovery about Beipiaosaurus, however, is that scientists were able to figure out what colors its feathers were in life.  Be examining the fossilized feathers from Beipiaosaurus' neck under a special kind of microscope, they were able to see cell structures, called melanosomes, which can determine the color of a feather according to the their shape.  Then they compared the shape of Beipiaosaurus' melanosomes to those of living birds and looked for matches.  The modern bird feathers that matched Beipiaosaurus' melanosomes the most were brown, so we can confidently assume that at least some Beipiaosaurus had brown feathers on their necks in life.  

Scientists also found patches of Beipiaosurus' skin!  By looking at its skin under a microscope and then comparing it to other animals, they were able to determine by the amount of keratin (material that feathers, hair, and nails are made of) in the cells, that Beipiaosaurus' body wouldn't have radiated as much heat as many modern birds.  They also were able to determine that Beipiaosaurus would have shed its skin more like modern birds, as dandruff, rather than other kinds of reptiles, like lizards, which shed skin in large pieces or flakes.  (fun fact: modern crocodilians also shed their skin as dandruff!)

Close up of a patch of Beipaosaurus skin from McNamara's 2018 paper.

Before Beipiaosaurus was discovered in 1996, the exact placement of therizinosaurs on the dinosaur family tree was more debated.  Originally, some paleontologists thought they were late-surviving descendants of the early sauropodomorphs from the Triassic and early Jurassic, like Plateosaurus, based on similarities with their small leaf-shaped teeth, long necks, wide bodies, and robust legs.  The discovery of Beipiaosaurus, however, which has more obvious theropod traits while clearly showing a direct connection to the more outlandish, later therizinosaurs, confirms therizinosaurs were indeed theropods.  

That's all or this week!  As always feel free to comment below!

References

Li, Q.; Clarke, J. A.; Gao, K.-Q.; Zhou, C.-F.; Meng, Q.; Li, D.; D’Alba, L.; Shawkey, M. D. (2014). "Melanosome evolution indicates a key physiological shift within feathered dinosaurs". Nature. 507 (7492): 350–353.

McNamara, M. E.; Zhang, F.; Kearns, S. L.; Orr, P. J.; Toulouse, A.; Foley, T.; Hone, D. W. E; Rogers, C. S.; Benton, M. J.; Johnson, D.; Xu, X.; Zhou, Z. (2018). "Fossilized skin reveals coevolution with feathers and metabolism in feathered dinosaurs and early birds". Nature Communications. 9 (2072): 2072.

Xu, X.; Tang, Z.-L.; Wang, X. L. (1999). "A therizinosauroid dinosaur with integumentary structures from China". Nature. 339 (6734): 350–354.

Xu, X.; Cheng, Y.; Wang, X.-L.; Chang, C. (2003). "Pygostyle‐like Structure from Beipiaosaurus (Theropoda, Therizinosauroidea) from the Lower Cretaceous Yixian Formation of Liaoning, China". Acta Geologica Sinica. 77 (3): 294–298.

Xu, X.; Zheng, X.; You, H. (2009). "A new feather type in a nonavian theropod and the early evolution of feathers". Proceedings of the National Academy of Sciences. 106 (3): 832–834.

Allosaurus: Beast of the Week

This week we will be looking at a well-known, well-loved, and well-understood dinosaur.  Check out Allosaurus!

Allosaurus was a meat-eating dinosaur that lived during the Late Jurassic Period, between 150 and 155 million years ago.  Its bones have been uncovered in the Western United States, Portugal, and in parts of Africa.  An adult Allosaurus, on average, measured about thirty feet long from snout to tail, but some have been found that were slightly smaller or larger.  The genus name translates to "Other Reptile" because at the time of its original discovery in the late 1800s, its vertebrae were what paleontologists used to differentiate Allosaurus from "other" fossil dinosaurs they were finding in the area. (Underwhelming...I know.). In life, Allosaurus would have shared its habitat with (and probably ate) many other known dinosaurs, including, but not limited to Apatosaurus, Brontosaurus, Barosaurus, Diplodocus, Camarasaurus, Stegosaurus, Gargoyleosaurus, Torvosaurus, and Ceratosaurus. 

There are actually three different known species of this Allosaurus that have been identified.  Allosaurus fragilis, from North Ameirca, is the most commonly found, and therefore the most extensively studied.  Allosaurus jummadseni, is also found in North America but was from a few million years before Allosaurus fragilis.  There is also Allosaurus europaeus, which was from what is now Europe.  

Allosaurus fragilis life reconstruction in watercolors by Christopher DiPiazza

Because there have been many specimens of Allosaurus that have been unearthed over the years, paleontologists know a lot more about it and often use Allosaurus as a model to compare to other, less complete dinosaurs for references.  Allosaurus' skull, in particular, is subject to a lot of attention.  The holes in its skull on either side of its eye sockets, called fenestrae, were large, and the bone walls of the skull, including those surrounding the brain case, were thin.  In addition, Allosaurus also had hollow chambers in its other bones, including its vertebrae and its leg bones, like birds.  This suggests Allosaurus was light for its size. It also suggests Allosaurus had an advanced, one-way respiratory system that birds, and some other kinds of reptiles have today, where air initially breathed in via the lungs, was more efficiently circulated around the body, including through some of the bones.  This would have allowed Allosaurus to remain more active for longer periods of time without needing to rest.  These same adaptations would have also helped to keep Allosaurus cool, with easy airflow within the body to shed excess warmth, and preventing the animal from overheating.  It makes sense since we have evidence that the the environment Allosaurus was living in could get rather hot and arid during the Jurassic.

Allosaurus fragilis mount on display at the American Museum of Natural History in New York.

Allosaurus' teeth were curved, flat, and serrated.  They were not extremely large compared to some of its contemporary meat-eaters, like Ceratosaurus or Torvosaurus, which means Allosaurus had a different feeding, and probably hunting style from them.  Allosaurus' teeth were probably best for slicing and cutting, rather than crushing or piercing.  These teeth were backed up by rather slender lower jaws, which means that there was less muscle attached to them in life, and therefore Allosaurus had a proportionally weaker bite when compared to many other meat-eating dinosaurs. (this does not mean Allosaurus had a weak bite!  It just wasn't crushing solid bones with its jaws like some other more specialized dinosaurs could.)  That being said, the jaws of Allosaurus were also able to open much wider than those of other dinosaurs, an impressive 79 degrees, to be exact. Also, the back of Allosaurus' skull and its neck bones suggest that there were very large muscle attachments there in life, and its skull, although having weak jaw muscles, was, as a structure, very strong when it came to sustaining impact.  So what does all of this mean?  Some paleontologists think that instead of using just bites to inflict damage or remove flesh from a carcass, Allosaurus likely would have used its strong neck to swing its open mouth to hack away at its target like an axe...with teeth.

Section of an Allosaurus leg bone on the left compared with that of a modern bird on the right.  Note how there is a different fossilized mineral inside the Allosaurus bone, showing how it was hollow in life.

It was also discovered that the muscles that would have been in Allosaurus' neck in life would have allowed for this dinosaur to move its neck in an up-and-down motion very quickly, taking many bites in a smaller amount of time.  Scientists hypothesize that Allosaurus may have used its jaws and teeth like a saw, to hack away mouthfuls of meat off of bones as it fed.  Adding to this, there are numerous sauropod bones from the same habitat as Allosaurus that were found with scrape marks that match Allosaurus teeth on them.  We may never know if Allosaurus actually killed these plant-eaters first or if it was simply scavenging an already dead animal, but either way we can agree Allosaurus' neck, skull, jaws, and teeth were a great butchering adaptation!

Image from Stephen Lautenschlager's 2015 study comparing the gapes, from left to right, of Allosaurus, Tyrannosaurus, and the therizinosaurid, Erlikosaurus.  Note how Allosaurus was capable of the widest bite.

Allosaurus' mouth wasn't its only weapon.  This dinosaur is possibly most famous for its relatively long, strong arms, and three large, hook-shaped claws on each hand.  The first finger of each hand possessed the largest claw, but all were more than capable of dealing substantial damage together and keeping struggling prey in place as the jaws did work on removing flesh from the bone.

Thanks to numerous well-preserved specimens of Allosaurus, scientists have been able to tell a bit about its lifestyle...and that its lifestyle was rough!  Allosaurus specimens have been found with numerous stress fractures on both the front limbs and the hind limbs, that healed over.  This tells us that Allosaurus was using its arms for something getting injured in the process sometimes.  It is possible Allosaurus was attacking large prey, like sauropods, with its front limbs, holding on with its hook-like claws, and slashing with its teeth to inflict bleeding wounds until its prey was too weak to stand any longer.  In addition to these injuries, Allosaurus have also been found with bite wounds from other Allosaurus on their skulls, which tells us that there was some intraspecies violence going on.  Another Allosaurus specimen was discovered with a nasty puncture wound in its tail that had healed over.  This wound matches the spike of a Stegosaurus, one of Allosaurus' contemporaries, suggesting that the two famous dinosaurs may have fought on occasion.  Lastly, yet another Allosaurus was discovered with a puncture wound through its PELVIS, likely inflicted by yet another Stegosaurus tail spike which appears to have not healed fully and therefore was the death of the unlucky Allosaurus. 

We also have juvenile specimens from Allosaurus, showing that this dinosaur was more slender, with proportionally longer legs when it was young, and bulked up as it matured into adulthood.  Allosaurus also had two small bony crests, one in front of each eye.  In life these crests likely had a layer of bony material, called keratin, making them even larger.  These were probably display adaptations to communicate within the species who was mature and who wasn't.  It is even possible that these crests were different sizes, or even colors between males and females.  Some believe these crests may have also been weapons that Allosaurus would have used to shove each other with in life to establish dominance.  We may never know for sure!

Drawing of Allosaurus jaw from Darren Tanke's 1998 paper, showing bite wounds that were proposed to have been from another Allosaurus.

Lastly, paleontologists have discovered impressions of some of Allosaurus' skin!  The skin would have been from the dinosaur's side, and had small, bumpy scales.  It is unknown if this kind of skin would have covered the whole body, or if there were different kinds of scales or other kinds of body covering, like feathers, elsewhere.

References

Carpenter, Kenneth (2002). "Forelimb biomechanics of nonavian theropod dinosaurs in predation". Senckenbergiana Lethaea. 82 (1): 59–76.

Gilmore, Charles W. (1920). "Osteology of the carnivorous dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus". Bulletin of the United States National Museum. 110: 1–159.

Holtz, Thomas R., Jr.; Molnar, Ralph E.; Currie, Philip J. (2004). "Basal Tetanurae". In Weishampel David B.; Dodson, Peter; Osmólska, Halszka. The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 71–110.

Lautenschlager, Stephan (2015-11-04). "Estimating cranial musculoskeletal constraints in theropod dinosaurs". The Royal Society. Archived from the original on 2016-03-19.

Madsen, James H., Jr. (1993) [1976]. Allosaurus fragilis: A Revised Osteology. Utah Geological Survey Bulletin 109 (2nd ed.). Salt Lake City: Utah Geological Survey.

Rayfield, Emily J.; Norman, DB; Horner, CC; Horner, JR; Smith, PM; Thomason, JJ; Upchurch, P (2001). "Cranial design and function in a large theropod dinosaur". Nature. 409 (6823): 1033–1037.

Snively, Eric.; Cotton, John R.; Ridgely, Ryan; Witmer, Lawrence M. (2013). "Multibody dynamics model of head and neck function in Allosaurus (Dinosauria, Theropoda)". Palaeontologica Electronica. 16 (2).

Tanke, Darren H. (1998). "Head-biting behavior in theropod dinosaurs: Paleopathological evidence" (PDF). Gaia (15): 167–184.