Sunday, April 30, 2017

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.) There are actually a few currently valid species of this dinosaur, but Allosaurus fragilis is the most commonly found, and the most studied.  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.  Allosaurus stands out as a very successful meat-eater of its time based on the sheer number of specimens that have been found.  Thanks to this, we know a lot about its anatomy, how it developed and aged, and how it may have behaved, including possible hunting and feeding behavior.

Allosaurus fragilis pair life reconstruction by Christopher DiPiazza.

For starters, Allosaurus had a very interesting skull.  The holes in its skull on either side of its eye sockets (we call those finestra) were large, and the bone walls of the skull, including those surrounding the brain case, were thin.  In addition, it also had hollow chambers in its bones, including its vertebrae and its leg bones.  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.  This would be advantageous Allosaurus endothermic, thus leading an active lifestyle.  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.  That being said, the jaws of Allosaurus were also able to open much wider than those of other dinosaurs, an impressive 79 degrees wide, 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 it's strong neck to swing its mouth, open wide, to hack away at its target like an axe...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 also allowed for this dinosaur to move its neck in an up-and-down motion very quickly.  Scientists hypothesize that because of this, Allosaurus could also have used it's jaws and teeth like a saw, to hack away mouthfuls of meat off of bones.  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 together, 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 most famous for its relatively long, strong arms, and three large, hook-shaped hand 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, and complete 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, which were proportionally long, each ending with three hooked claws to grab onto...something (likely struggling prey but we may never know for sure)...and 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.  Also, an 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.

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, communicate within the species who was mature and who wasn't.  It is 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's 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 body covering elsewhere.  We also can't be sure the skin disproves the presence of feathers, since we know feathers can exist with scales on the same dinosaur.

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.

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