Name Meaning:  Different Lizard

Geologic Era:  Late Jurassic

Location Discovered:  Colorado (probably from Morrison Formation)

Size:  8.5 meters long on average

Estimated Range:  USA, Portugal, Tanzania

Extinction Date:  150 million years ago

Allosaurus was discovered in the Bone Rush of the mid-1800’s.  The reason for Allosaurus’ multiple alternate names is that competing paleontologists of the time wanted to discover and describe as many species as possible.  This led to several cases of the same animal being described multiple times.  These names for Allosaurus included Antrodemus, Labrosaurus, and Creosaurus.  When an organism is named, the first name officially given takes priority.  This led to a great deal of confusion as to how many species had actually been discovered, and it wasn’t until decades later that the whole mess was sorted out. 

One of the most well represented carnivores of the Jurassic period, it is also one of the most famous and well known.  Allosaurus, like many American dinosaurs, fell victim to the confusion that was the Bone Wars in the 1800’s, hence all of the alternate names.  Our specimen is a cast of a hand claw.  Allosaurus would have had three of these on each forelimb.  Allosaurus would have been one of the top predators of its ecosystem, with only one other animal (Saurophaganax) possibly being larger.  

Predators often lived rough lifestyles, and Allosaurus was no exception.  Analysis of the skull and neck show that Allosaurus would have been able to open its jaws extremely wide, as shown in the illustration below.  The jaw muscles were reduced overall, while the neck muscles were more enlarged.  While Allosaurus did not have a terribly strong bite (less pounds per square inch of pressure than a modern leopard), it was able to drive its head downward with much more force.  

This has lead to the hypothesis that Allosaurus used a “hatchet” approach to prey.  It would use its extremely wide gape to drive forward into prey, and then pull its head downward.  The result would be not unlike using a circular saw, as Allosaurus teeth were not very large.  This would allow for the gradual weakening and death of prey without unnecessary energy expense on the Allosaurus’ part.  This approach would have been particularly effective in pack hunting scenarios, which it is also hypothesized that Allosaurus did.

However, getting up close with prey animals, especially very large ones, came at a price.  Several Allosaurus specimens show numerous bone pathologies, especially healed breaks.  One Allosaurus tail vertebra has a hole in which a Stegosaurus tail spike fits quite nicely.  An even more impressive set of pathologies can be seen below.  Those bones colored red indicate left side, blue right side, and cyan a pathology on an unpaired bone (such as a vertebra).  Green shows bones on the left side with a possible, but uncertain, pathology.  

As you can see, the pathologies appear to be concentrated on the left side of the Allosaurus.  The only ones that are not are C4 and C5 (cervical vertebrae 4-5), cervical rib 4, dorsal rib 4, and the iscium.  All of the injuries (with the exception of the ischium) were healed at the time of death.  A study done on this skeleton showed that the pathologies on the dorsal ribs, scapula, and humerus all had callus formations, meaning there were fractures present.  

If one looks at the skeleton as a whole, one will see that these injuries lie in the same line, and probably happened at the same time.  One hypothesis is that this series of injuries was caused by a defensive strike from a sauropod tail.  Sauropods could easily grow to be 60-80 feet in length, with up to half of that being tail.  A sauropod could have easily inflicted that set of injuries.  Indeed, this Allosaurus was lucky that he lived to hunt another day, as these injuries could have very well proved fatal.

Image Credits:
Full Skeleton:  “Allosaurus SDNHM (1)” by Allosaurus_SDNHM.jpg: Sheep81derivative work: Creoqueteamo – This file was derived from  Allosaurus SDNHM.jpg:. Licensed under CC BY-SA 3.0 via Commons – https://commons.wikimedia.org/wiki/File:Allosaurus_SDNHM_(1).jpg#/media/File:Allosaurus_SDNHM_(1).jpg

Bone Pathologies on Big Al II:  “Allosaurus “Big Al II”” by Christian Foth​1,2, Serjoscha Evers2,3, Ben Pabst4, Octávio Mateus5,6, Alexander Flisch7, Mike Patthey8, Oliver W. M. Rauhut1,2 – https://peerj.com/preprints/824/. Licensed under CC BY 4.0 via Commons – https://commons.wikimedia.org/wiki/File:Allosaurus_%22Big_Al_II%22.jpg#/media/File:Allosaurus_%22Big_Al_II%22.jpg

Life Restoration:  “Allosaurus BW” by Nobu Tamura (http://spinops.blogspot.com) – Own work. Licensed under CC BY 2.5 via Commons – https://commons.wikimedia.org/wiki/File:Allosaurus_BW.jpg#/media/File:Allosaurus_BW.jpg

Attack Mode Allosaurus:  “Allosaurus Jaws Steveoc86” by I, Steveoc 86. Licensed under CC BY 2.5 via Commons – https://commons.wikimedia.org/wiki/File:Allosaurus_Jaws_Steveoc86.jpg#/media/File:Allosaurus_Jaws_Steveoc86.jpg

Allosaurus Pathology Report:  https://peerj.com/articles/940/   (New Insights into the Lifestyle of Allosaurus (Dinosauria: Theropoda) based on another specimen with multiple pathologies)