Allosaurus fragilis – When Your Bark is Bigger Than Your Bite

Article by: Adam Manning
Edited by: J. D. Dixon and Jack Wood

Allosaurus fragilis illustration by Stephanie Dziezyk.

Name: Allosaurus fragilis
Age: Kimmeridgian and Tithonian, Late Jurassic (155.7 – 150.8 million years ago)
Diet: Carnivore
Size: 12m long
Weight: 1500kg
Location: North America

Allosaurus fragilis is probably the most famous predator of the Late Jurassic, first discovered in 1877 by Othniel Charles Marsh in the Morrison Formation of Colorado. Its name means “different fragile lizard” due to the hour-glass shape of some of its vertebrae, which helped make the bones lighter, at the cost of strength. Allosaurus was a huge and successful carnivore that could contend with even the largest prey, like the sauropod Apatosaurus, and the most well-defended, like Stegosaurus. And yet, thanks to modern technology we now know this impressive dinosaur had a not-so-impressive bite. So how did it do so well?

Saying its bite force was weak is relative of course. Allosaurus had a bite force of about 3,572.56 Newtons, on par with modern mammals like wolves, which is nothing to laugh at, but Allosaurus was about ten times as big as an African Lion. Furthermore, Tyrannosaurus rex had a bite force of 13,400 N, while the modern Alligator mississippiensis has a bite force of 13,000 N. In contrast to this, the skull of Allosaurus was able to handle a force between 8.7 and 23.3 times its bite force before yielding. Furthermore, Allosaurus had 80 or more knife-like teeth, serrated front and back, adapted for slicing through flesh, rather than for crushing bone like the teeth of Tyrannosaurus. Data has been taken from Rayfield, et al (2001), and sadly the differences in bite force analyses and issues with estimating the bite force of extinct taxa will not be discussed in this article.

A graph comparing the bite force estimates of various living and extinct vertebrates. Data was taken from Rayfield, et al, 2001. Graph produced by Adam Manning.

Putting these clues together, it is thought that Allosaurus used a “slash and tear” attack style rather than a powerful muscle-driven bite. Using its head as a weapon, it drove its jaws into its prey like an axe, letting its serrated teeth slash through, creating catastrophic wounds. The victim ended up dying of shock and blood loss, a brutally effective killing strategy.  

Allosaurus on display in the Lapworth Museum of Geology. Image by J. D. Dixon.

Allosaurus’ neurobiology most likely resembled modern crocodilian species, but there is little conclusive evidence to suggest either way if Allosaurus hunted in packs or not. Many Allosaurus have been found in predator traps, but it is inconclusive if the Allosaurus scavenged there opportunistically and became stuck, or if they hunted in packs and forced their prey to become stuck, the pack then going after these as easy targets but becoming immobilised themselves. Like all big animals, Allosaurus likely both hunted and scavenged, so both possibilities could have occurred.

Overall, Allosaurus was an extremely successful dinosaur, and we have learnt a lot because of the many amazingly well-preserved fossils of its remains that have been discovered. Its kill method goes to show a different way for a predator to be adapted for its role as a ferocious hunter.

Skeletal reconstruction of Allosaurus fragilis by Scott Hartman. Available at: https://www.skeletaldrawing.com/theropods/allosaurus-fragilis.

Image References:
[1] Allosaurus fragilis illustration by Stephanie Dziezyk.
[2] A graph comparing the bite force estimates of various living and extinct vertebrates. Data was taken from Rayfield, et al, 2001. Graph produced by Adam Manning.
[3] Allosaurus on display in the Lapworth Museum of Geology. Image by J. D. Dixon.
[4] Skeletal reconstruction of Allosaurus fragilis by Scott Hartman. Available at: https://www.skeletaldrawing.com/theropods/allosaurus-fragilis.

Information References and Further Sources:
[1] Bybee, P. J., Lee, A. H., and Lamm, E-T. (2005). Sizing the Jurassic theropod dinosaur Allosaurus: Assessing growth strategy and evolution of ontogenetic scaling of limbs, Journal of Morphology, 267 (3), pp. 347-359. doi:  https://doi.org/10.1002/jmor.10406 
Available at: https://onlinelibrary.wiley.com/doi/abs/10.1002/jmor.10406. Accessed 24th August 2019.
[2] Castro, J. (2016). ‘Allosaurus: Facts About the ‘Different Lizard’’, LiveScience, published 15/03/2016 Available at: https://www.livescience.com/24815-allosaurus.html Accessed: 24th August 2019.
[3] Erickson, G. M. (2001). The bite of Allosaurus, Nature, 409, pp. 987-988. Available at: https://www.nature.com/articles/35059184. Accessed 24th August 2019.
[4] Fiorillo, A. R., and Eberth, D. A. (2004). ‘Dinosaur Taphonomy’, in Weishampel, D. B., Dodson, P., and Osmólska, H (2nd ed.) The Dinosauria. University of California Press. pp. 607-613. Available at: https://www.researchgate.net/publication/289846112_Dinosaur_Taphonomy. Accessed 24th August 2019.
[5] Rayfield, E. J., Norman, D. B., Horner, C. C., Horner, J. R., Smith, P. M., Thomason, J. J., and Upchurch, P. (2001). Cranial design and function in a large theropod dinosaur, Nature, 409, pp. 1033 – 1037. Available at:
[6] Richmond, D. R., and Morris, T. H. (1996) The dinosaur death-trap of the Cleveland Lloyd Quarry, Emery County, Utah, The Continental Jurassic, pp. 533 – 545. Available at: https://www.researchgate.net/publication/292224996_The_dinosaur_death-trap_of_the_Cleveland_Lloyd_Quarry_Emery_County_Utah. Accessed 24th August 2019.
https://www.nature.com/articles/35059070. Accessed 24th August 2019.
[7] Rogers, S. W. (1998). Exploring Dinosaur Neuropaleobiology: Computed Tomography Scanning and Analysis of an Allosaurus fragilis Endocast, Neuron, 21 (4), pp. 673-679. 
doi: https://doi.org/10.1016/S0896-6273(00)80585-1 
Available at: https://www.sciencedirect.com/science/article/pii/S0896627300805851. Accessed 24th August 2019.