The Strange Lizard
In the Jurassic period, almost a hundred million years before Tyrannosaurus, the western plains of North America were terrorised by something stranger. This was Allosaurus, the "strange lizard," one of the top predators of the Late Jurassic Earth, 150 million years ago. In its day, it was a successful carnivore, and now that I’ve finally illustrated this dinosaur, it is time we learn about it in this blogpost.
Allosaurus was one of the larger dinosaurs of the theropod clade, approximately measuring nine metres long and weighing two tonnes, but it was no tyrannosaur. It had plenty of differences in its anatomy. For example, it had larger arms with three claws instead of two, it was much lighter in build, making it much more mobile, and it had narrow jaws with serrated, shark-like teeth made to tear the flesh of dinosaurs rather than crush their bones. In fact, it was part of its own specific group of flesh-tearing theropods called the carnosaurs, which were the dominant predators in continents from the Late Jurassic before their extinction at the end of the Middle Cretaceous, which after that, tyrannosaurs would take their place. Allosaurus also had two horn-like crests over its eyes, giving it a somewhat angry appearance, and it was also a real bigmouth, able to open its mouth to a gape wider than a right-angle, mind-blowingly terrifying.
An Allosaurus skeleton mounted at the American Museum of Natural History, though in an outdated fashion of the 1900s. Photo by Scott Robert Anselmo.
Fossils were discovered into the 1870s in the United States, thought by the locals to be petrified horse hooves. It would soon be described in 1877 (almost thirty years before Tyrannosaurus) by Othniel Charles Marsh as a species of dinosaur named Allosaurus fragilis (the typal species depicted here), Greek for "fragile, strange lizard" after its hollow, hourglass-shaped bones unique from any dinosaur known at the time. Marsh was a rival to another palaeontologist, Edward Drinker Cope, in the infamous Bone Wars of Gilded Age America, a competitive palaeontological feud ought to discover and describe prehistoric animals the quickest, resulting in the growing discoveries of other famous dinosaurs like Stegosaurus and Brontosaurus, which were some of the allosaur’s principal prey. To the modern day, many exquisite fossils of Allosaurus have been discovered, making it one of the best-known dinosaurs and the second most famous large theropod (though not to the general public), and we’re continuing to learn more about it as we keep on digging. Bones have now been found outside of the United States in Portugal and possibly Tanzania, a successfully wide range, and two more species have been described.
3D reconstruction (for promotion of the documentary Walking With Dinosaurs) of Allosaurus in its fern prairie habitat.
Allosaurus, as a genus, roamed the Earth for ten million years, and its generally semi-arid habitat was influenced by location, geologic time and the drastically shifting wet and dry seasons, from fern savannahs and conifer forests to flooded swampland and desert. It lived with and hunted a menagerie of other celebrity dinosaurs, such as the colossal sauropods Diplodocus, Brontosaurus and Brachiosaurus, the armoured Stegosaurus and Mymoorapelta and the fleet-footed Dryosaurus and Camptosaurus. Some animals would come and go as the habitats changed around Allosaurus, and it too would evolve throughout the species within. The allosaur was the most abundant theropod of its setting, outnumbering all the others combined three to one, but it wasn’t alone in the top of the food chain. There was the smaller Ceratosaurus, named for the horns sprouting from its face, and the even larger Torvosaurus, a megalosaur close to Tyrannosaurus in size. Scientists think these predators were able to coexist with each other by niche partitioning, in a similar way to the the African savannah’s predators, thanks to evidence by their locations and indicating adaptations. Allosaurus itself had longer legs built for speed, great for the open floodplains, whilst Ceratosaurus and Torvosaurus had lower, sinuous builds suitable for traversing forested waterways.
Ceratosaurus, a smaller crested theropod that lived with Allosaurus.
The colours of the animal kingdom are influenced by many factors for survival, including the surroundings, where an animal can camouflage to remain inconspicuous from their predators or prey, from beige deer in woodlands, to golden lions among dry grass, and white polar bears on ice. It can give us a hint about the colours of extinct animals we don’t know the colours of. In Allosaurus’s case, it’s probable that, as reconstructed here, it was fern green to camouflage among the ferns of its habitat while lying in ambush. The green protofeathers depicted could’ve also made it look a bit leafier. I also speculate that, being a reptile, the allosaur could’ve even shifted the hue of its scales and feathers seasonally through habitat change as ferns come and go, from a lush green in the wet season to an arid brown in the dry season, like some species of lizards.
As a carnivore, Allosaurus was no picky eater, it would go for even the riskiest quarry, whether it’d be a live kill or a scavenged corpse. There are fossils of Stegosaurus and Allosaurus with injuries from each other, and some sauropod bones have been found with scrapes fitting allosaur teeth, that have also been found present among the bones. These long-necked dinosaurs were still titanic, and so the allosaurs would’ve "worked together" in a mob to bring them down, while also maybe stripping their flesh off, but unlike wolves and more like Komodo dragons, they probably weren’t so cooperative but rather territorially aggressive with each other, and would try intimidate or even cannibalise each other from the banquet. What primitive grumps.
Two allosaurs mob a barosaur in work to bring it down. By Fred Wierum.
Why Allosaurus is so well-studied is partly thanks to the Cleveland-Lloyd Dinosaur Quarry in Utah, also known as the Jurassic National Monument. Discovered in the 19th century by ranchers leading their sheep and cattle, it would soon become a treasure trove for palaeontologists, and that was to no disappointment. Discovered were thousands of bones belonging to 44-60 allosaurs of all ages. It had traditionally been suggested that all these allosaurs died here because it was a predator trap, a bit like the Pleistocene epoch’s La Brea Tar Pits, where prey would become stuck in deep mud and lure in predators to their doom too, but it’s more likely that extreme drought was to blame. Whatever really happened, the bones had given us more insight into Allosaurus than ever before.
One of the litters of allosaur remains at the Cleveland-Lloyd Dinosaur Quarry. Photo from www.gjhikes.com.
There is also "Big Al," an adolescent Madsen’s allosaur (Allosaurus jimmadseni, not the species depicted here) discovered in Wyoming in 1991, whose beautifully articulated skeleton is 95% complete and preserves many leftovers of injuries. One infection found on its middle toe was probably responsible for the dinosaur’s death. A medullary bone, a structure elsewhere found in mother birds (broadly the only dinosaurs left today) when laying their eggs, was also found in Big Al, suggesting that Al might’ve been a brooding female too. Al is now a bit of a celebrity for a dinosaur, thanks to its great preservation, and even had its life reconstructed in a special of the documentary series Walking With Dinosaurs in 2000; The Ballad of Big Al, which is available to watch. In 1996, another exceptional allosaur was discovered by the same team that discovered Big Al, nicknamed "Big Al II."
Big Al at the Wyoming Museum of Geology. Photo by Laura Vietta.
And that’s all about Allosaurus, hope you’ve enjoyed reading this blogpost of the Palaeodigon, which I’m finding the new style I’ve reworked for it so beautiful, and I really loved working out on those camouflaged fern-green skin patterns, very natural, and likely quite like what Allosaurus would’ve looked like. Sorry if this took a whole month, I had gone to Tenerife for a week-long holiday to and from Halloween, and coming back from all the fun I had to warm up again, but I’ve completed it well in the end. Up next is Anatosaurus… or Edmontosaurus? Whatever you’d like! Cheers!
