Top Jurassic Carnivore Had an Amazing Gape

Picture of an allosaurus skeleton, lit with blue and red lights at a museum in New Zealand
Allosaurus dinosaur skeleton in the Canterbury Museum. Christchurch, Canterbury, South Island, New Zealand
© Travelscape Images / Alamy Stock Photo

Tyrannosaurus rex is the ultimate in Mesozoic clickbait. You could even say that it’s a real tyrant, casting a large shadow over the hundreds of other dinosaurs that could benefit from some time in the spotlight. So even though paleontologist Stephan Lautenschlager included T. rex in his new study on dinosaur bites, I don’t want to talk about the tyrant lizard king much. I want to focus on a different lizard.

Long time readers know that I have a soft spot for Allosaurus. The 150 million year old dinosaur was the top carnivore of the Late Jurassic, reaching sizes to rival that of the later T. rex and was much more common than its toothy neighbors Ceratosaurus and Torvosaurus. And as estimated by Lautenschlager, Allosaurus also had one of the most impressive bites of all time.

Working from virtual models of Tyrannosaurus, Allosaurus, and the herbivorous theropod Erlikosaurus fitted with digital jaw muscles based on the anatomy of buzzards and alligators, Lautenschlager estimated the optimal and maximum gapes for the three dinosaurs. Contrary to what you might expect from the hordes of cgi dinosaurs that stomp across basic cable science channels, these dinosaurs were able to deliver their best bites when their jaws were open about 28 degrees, not flung to their maximum extent.   (Erlikosaurus, which was more on the vegetarian side of the spectrum, had an optimal gape of about 20.5 degrees.)

When it came to just how wide the dinosaurs could drop their jaws without tearing up their own jaw muscles, though, Lautenschlager found that Allosaurus significantly surpassed ol’ T. rex. While Tyrannosaurus could open its mouth an impressive 63.5 degrees, the maximum Allosaurus gape came out to 79 degrees.

Bite angles for Allosaurus, Tyrannosaurus, and Erlikosaurus. From
Bite angles for Allosaurus, Tyrannosaurus, and Erlikosaurus. From Lautenschlager, 2015.

The figures of Lautenschlager’s yawning dinosaurs reminded me of an idea Bob Bakker mooted in the late 90’s. Allosaurus, in Bakker’s view, was the dinosaurian equivalent of a saber-toothed cat, using a wide gape and strong neck muscles to slash at prey rather than deliver a devastating chomp like T. rex. While the paper was more qualitative and speculative than Lautenschlager’s work, Bakker was nonetheless right that Allosaurus was capable of an astonishingly wide gape.

But why? That’s a little more difficult to suss out, especially since it’s easy to be misled by our love of extreme dinosaur traits and habits.

First off, most of the recent work on Allosaurus feeding has focused on a particular specimen and species. That’s MOR 693 – “Big Al” to you and me – and while it’s often called Allosaurus fragilis in papers, the dinosaur is actually an older, more slender-skulled species whose true name has yet to be officially published. This is important because, as Mark Loewen showed in his dissertation on the carnivore, the skull of true Allosaurus fragilis flared out more towards the back, giving them more space for powerful jaw muscles. How this would have affected bite performance hasn’t been given the same attention Big Al has enjoyed.

It’s also worth noting that Allosaurus might have had two killing techniques. Maximum gape and optimal gape, as Lautenschlager found, are not the same. Exceptionally wide bites would have actually been weaker than when the dinosaur held its jaws at optimal tension. In short, Allosaurus would have delivered its best bites with a smaller gape. (And the same would have been true for Tyrannosaurus, which could throw open its jaws impressively wide, too.) Why the dinosaurs was able to really open wide, then, might support something Bakker previously suggested. If Allosaurus really did attack with a wide gape, the dinosaur could have used its strong neck muscles to drive its toothy upper jaw at prey like some kind of biological “war club.” More of a slash than a bite.

So why was Allosaurus capable such an unusual bite? Could it be true that Allosaurus had such wide gapes because they were going after larger prey? That was Bakker’s contention. The big gape made Allosaurus a specialized “brontosaur killer” capable of taking down the surplus of giant sauropods that plodded around the floodplains of the Jurassic west. As dramatic as such visions are, though, there are a few problems with envisioning Allosaurus jumping onto the back of an adult Diplodocus and slicing massive chunks out of flesh.

Not as fragile as they look. From Taylor and Wedel, 2013.
Not as fragile as they look. From Taylor and Wedel, 2013.

Even if Allosaurus really did slice at sauropod hides in the way Bakker and others have suggested, though, taking down a full-grown sauropod would have been no simple task. Sauropod necks, for example, were not noodles just begging to be bitten through, but, as Mike Taylor and colleagues recently pointed out, were “constructed from tough elements including the often robust cervical ribs, bony laminae, ligaments, and tendons.” Decapitation is never as easy as the movies make it look.

And while “Clash of the Titans” dominates paleoimagery, such confrontations were probably rare. Like many modern carnivores, Dave Hone and Oliver Rauhut wrote, carnivorous dinosaurs likely targeted hatchlings and juveniles. (And this might explain why finding baby dinosaurs is so difficult!)

A young Allosaurus nips at a Barosaurus at the Natural History Museum of Utah. Photo by Brian Switek.
A young Allosaurus nips at a Barosaurus at the Natural History Museum of Utah. Photo by Brian Switek.

Not to mention that Bakker’s chosen analogues for Allosaurus – the sabercats – didn’t target the biggest prey on the landscape. Geochemical signatures in bones pulled from the La Brea asphalt seeps indicate Smilodon targeted bison and camels, not giant sloths or mammoths, and, in a recent review of Ice Age ecosystems, Blaire Van Valkenburgh and colleagues found that large herbivores are vulnerable to carnivores when they’re juveniles. A Homotherium den in Texas littered with the bones of young mastodons attests to the fact that these sabercats typically targeted juvenile giants rather than risking injury and death under the feet of the adults. The same was probably true for Allosaurus, especially since sauropod dinosaurs laid multiple eggs at a time and the landscape may have been flooded with naive young dinosaurs when hatching season rolled around.

So we’re left with the perpetual paleontological problem of what an animal could have done and what it actually did. Perhaps Allosaurus both bit at its prey and swung its skull like a tooth-studded hatchet, making it a more versatile predator and perhaps explaining why it’s apparently so much more common in Morrison Formation rocks than any of its competitors. And that makes me feel sorry for the baby sauropods that walked into the fern-covered floodplains and conifer stands of the Late Jurassic world. Imagine hearing the snap of a twig, turning at the sound to see the sky blocked out by a set of serrated teeth swinging down on you from above.

References:

Bakker, R. 1998. Brontosaur killers: Late Jurassic allosaurids as sabre-tooth cat analogues. Gaia. 15: 145-158

Coltrain, J., Harris, J., Cerling, T., Ehleringer, J., Dearing, M., Ward, J., Allen, J. 2004. Rancho La Brea stable isotope biogeochemistry and its implications for the palaeoecology of late Pleistocene, coastal southern California. Palaeo. doi: 10.1016/j.palaeo.2003.12.008

Hone, D., Rauhut, O. 2010. Feeding behavior and bone utilization by theropod dinosaurs. Lethaia. doi: 10.1111/j.1502-3931.2009.00187.x

Lautenschlager, S. 2015. Estimating cranial musculoskeletal constraints in theropod dinosaurs. Royal Society Open Science. doi: 10.1098/rsos.150495

Loewen, M. 2009. Variation in the Late Jurassic theropod dinosaur Allosaurus: ontogenetic, functional, and taxonomic implications. University of Utah dissertation.

Snively, E., Cotton, J., Ridgely, R., Witmer, L. 2013. Multibody dynamics model of head and neck function in Allosaurus (Dinosauria, Theropoda). Palaeontologia Electronica. 16 (2): 1-29

Taylor, M., Hone, D., Wedel, M., Naish. 2011. The long necks of sauropods did not evolve primarily through sexual selection. Journal of Zoology. doi: 10.1111/j.1469-7998.2011.00824.x

Van Valkenburgh, B., Hayward, M., Ripple, W., Meloro, C., Roth, V. 2015. The impact of large terrestrial carnivores on Pleistocene ecosystems. PNAS. doi: 10.1073/pnas.1502554112

8 thoughts on “Top Jurassic Carnivore Had an Amazing Gape

  1. A very interesting article, with well-pondered observations.

    An increased gape would have enabled Allosaurus to tear off and carry away larger chunks of his prey. Let us remember that even these ferocious beasts had helpless nestlings waiting for them: wee creatures hungry for meat. So the more Allosaurus could place in his jaws, the better for him and his family.

    The comparison with sabre-toothed cats seems sensible. These felines were apparently able to rip off sizeable portions of flesh. Both Allosaurs and Smilodons may well have slain their prey, eaten hasty meals on the spot, and then used the “take-out” service of their gaping jaws to bring home as much “beef” as was possible.

  2. It’s interesting that the incredibly wide gape is impressive-looking, but dubious in its actual functionality. That contrast brings to my mind an image of two theropods circling each other with mouths open wide at 70-or-so-degrees. Perhaps the extended gape had little to do with hunting, and more to do with social interaction. Allosaurus’ full gape would have been an effective threat display (especially if you throw some hypothetical bright coloration into that mouth).

    1. That reminds me of an earlier, and as far as I know also completely unsubstantiated hypothesis by Bakker, that allosaurs could partially unhinge their jaws, like modern pythons, in order to swallow larger chunks of prey, or smaller animals intact.

  3. For whatever it’s worth in the attempt to understand the function of a wide gape, look to the debatable “extinct” thylacine (Tasmanian tiger). The last captive individual passed on in a Hobart zoo in 1936, and there is film footage of it. It’s trademark gape was the widest of any known mammal, approximated to be between 90 and 120 degrees. Surely a species so recent can give clues as to what wide gapes are about for terrestrial predators. The thylacine was/is top dog in the world of modern marsupial carnivores.

  4. “[R]eaching sizes to rival that of the later T. rex …”

    Wait? Is this accurate? Allosaurus grew larger than the Tyrannosaurus Rex?

  5. The larger gape does sound like it would be more of an advantage in consuming food than in hunting (as suggested above). Using the jaw and neck as a “war club” seems like an unnecessarily risky behavior. A broken tooth can be a costly incident for many modern predators and I would assume that might hold true here. Very interesting article

  6. As for the size of Allosaurus: I have done a bit of research, and it appears that, although many Allosaurs were indeed lighter and somewhat smaller than Tyrannosaurus, some members of this species did arrive at a comparable length and width .

    From the article called “Allosaurus vs. Tyrannosaurus”, found at the diffen.com site, I gleaned the following comparative data:

    Allosaurus: 9-12 metres in length, approximately 5 metres in height
    Tyrannosaurus: 12 metres in length, 4-6 metres in height

    Female Allosaurs may have been smaller and more gracile than males.

  7. Allosaurus was probably hunting decent-sized prey-remember that even juvenile sauropods are huge (adults are too big, period). It probably used its jaw not for strength, but speed and friction. You don’t need to apply huge amounts of force to saw a tree. Likewise an allosaur didn’t need too much strength to saw into prey.

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