The Other Tyrants
The fame and glory of Tyrannosaurus rex has been repeatedly hyped endlessly ever since it was first described in 1905. Once T. rex stormed into the public consciousness, no other predatory dinosaur found before or since could measure up to its legacy, though a few came close. It might be news to many people, though, that Tyrannosaurus was not the only tyrannosaurid, a family which contains a menagerie of fascinating tyrant reptiles. These beasts were often the lords of their domain, millions of years before old ‘rex’ appeared on the scene, so let’s take a little survey of them now.
The tyrannosaurids all have a few things in common: they were large, bipedal theropods with fused nasal bones, incisor-like front teeth, and a prominent crest across the back of the skull (Holtz, 2004). The basal tyrannosauroids had three fingers, like most dinosaurs, but the proper tyrannosaurs had them reduced to two (not the ‘index’ and ‘middle’ though, the ‘index’ and ‘thumb’- something to remember when doing your best tyrannosaur impression). They also tend to have gnarly bone growth on the head, especially around the eyes and along the snout, and some had short horns in front of the eyes. The overlying tissue would have given many tyrannosaurs an extra-fearsome appearance.
The basal tyrannosauroids, the distant cousins to the tyrannosaur family, weren’t always as theatrical as their more advanced descendants. Arising in the mid-Jurassic during a time when the big megalosaurs were the top land predators, the tyrannosauroids started on the supercontinent of Laurasia, spreading all over the world from there. Some, like Dilong and Stokesaurus, were small feathered predators roughly the size of a large dog. Others like Eotyrannus and the crested Guanlong got to about bear-sized, and giants like Sinotyrannus, Yutyrannus, Alectrosaurus, and Dryptosaurus (the first predatory dinosaur known from skeletal material from North America) rivalled their tyrannosaurid relatives in size.
The family Tyrannosauridae (the ‘true’ tyrannosaurs’) are only known from the late Cretaceous of North America and Asia. While generally large predators, they could range anywhere from around 6 meters long to double that or more. The first tyrannosaurid fossils recognized by early paleontologists were shed teeth from western Canada and United States, which led to new taxa of dinosaurs being named from teeth alone. This led to confusion down the road, since tyrannosaur teeth are essentially impossible identify down to the genus level, and there’s at least as much variation in tooth shape and size in the mouth of one individual as there is between two different animals. As we learned in the Troodon article, naming new animals from teeth alone often comes back to bite science.
The tyrannosaurids are split between two different subfamilies: the slimmer, lower-skulled, longer-shinned albertosaurines and the bulkier, stouter tyrannosaurines. The albertosaurines are a small family, consisting of only two species- Albertosaurus sarcophagus and Gorgosaurus libratus. Albertosaurus is one of the first tyrannosaurs known from decent skeletal material, and the story of J. B. Tyrrell’s discovery of this dinosaur in the badlands near present day Drumheller has gone down in Canadian folklore. The holotype of Albertosaurus was a partial skull that the young Tyrrell found in an exposure of the Horseshoe Canyon Formation, but lacking the proper supplies and transportation to safely get it back to the museum in Ottawa, the specimen suffered a great deal from the bumpy journey by wagon and train back east. What was left of the skull upon arrival was looked at by the famously brilliant but contentious Edward Drinker Cope, who conveniently identified it as a species his eastern taxon Laelaps (which Marsh had already synonymized into Dryptosaurus). In 1905, Cope’s protégé Henry Fairfield Osborne re-described the Albertan skull as a separate genus which he named after the newly established province it hailed from (Osborne, 1905). Albertosaurus appears as little more than a footnote in the paper, which is largely dedicated to establishing another new Osborne taxa, Tyrannosaurus. Today, several more complete specimens of Albertosaurus have been found in central and southern Alberta, and it’s fairly likely that tyrannosaur remains from the Wapiti Formation of the northwest could be from Albertosaurus as well.
There’s a famous Albertosaurus bonebed at the beautiful Dry Island Buffalo Jump Provincial Park in southern Alberta. Discovered by famous fossil hunter Barnum Brown in 1910, Dr. Phil Currie, one of the world’s leading experts in predatory dinosaurs, re-discovered the sight in the late 90’s (Currie, 1998). More than a dozen individuals are thought to be buried at the site, an amazingly high estimate for a big carnivorous dinosaur assemblage (Eberth & Currie, 2010). While some paleontologists have taken this a good evidence that this species may have lived in packs like wolves, others have been skeptical of this interpretation, viewing Albertosaurus more like the modern Komodo dragon- living solo lives but gathering together during instances of major food availability. The bonebed is thought to have formed when a collection of these dinosaurs were caught up in a violent tropical storm that knocked down trees and flooded the landscape (Eberth & Currie, 2010), but who knows exactly why they were together in the first place.
Gorgosaurus libratus is found in the slightly older rocks of the Dinosaur Park Formation, and is known from dozens of wonderfully complete specimens ranging from juveniles up to fully grown adults. This makes it one of the best-studied tyrannosaurs and a common sight in museums all over the world. It was found in Dinosaur Provincial Park by the great Charlie Sternberg, and sent east to be described by Canada’s original paleontologist Lawrence Lambe (Lambe, 1914). Sternberg’s father, the long-time fossil hunter and author Charles Sr., imagined Gorgosaurus as something like a Mesozoic tiger, prowling the swamps in the undergrowth and lunging from close cover at its prey (Sternberg, 1917). While certainly vivid, this lifestyle isn’t quite how paleontologists imagine Gorgosaurus behaving today. Long shin bones, a pinched-in middle foot bone (which you can call an arctometatarsalian condition if you want to impress your friends), and big leg muscles made Gorgosaurus a very capable runner. One could imagine it dashing across the Cretaceous floodplains like a giant carnivorous emu after its prey. Alberta’s Dinosaur Park Formation was teeming with large herbivores, and some have argued that Gorgosaurus, with its slim build and long legs, may have specialized in chasing down unarmored hadrosaurs, leaving the more dangerous ceratopsids and ankylosaurs to its larger cousin that we’ll discuss later (Russell, 1970).
Gorgosaurus and Albertosaurus are very similar animals, and for a while some paleontologists argued that they were actually the game genus. Since Albertosaurus had priority, Gorgosaurus was re-dubbed by some as Albertosaurus libratus (Russell, 1970). Currie undid this by showing that enough differences exist between the two for Gorgosaurus to be considered valid, and it remains so today (Currie, 2003). That said, from glancing at the skeleton alone, it’s nearly impossible to tell the difference between Gorgosaurus and Albertosaurus. A very detailed look at the bones is required to tell them apart.
The tyrannosaurine subfamily is a much more diverse one, and several new taxa have been added to this group in the past few years. It seems that, while Alberta was largely dominated by albertosaurines during the Campanian time (about 83-72 million years ago), the western United States was the land of the tyrannosaurines. Species like Lythronax, Teratophoneus, Bistahieversor, and Dynamoterror roamed from New Mexico to Utah preying on large herbivores along the Western Interior Seaway. These were generally medium to large predators, ranging from about 7 to 9 meters long. There’s also the small, enigmatic Nanuqsaurus from the Maastrichtian of northern Alaska. At only 5 meters long, this dinosaur was likely densely feathered to keep warm, and may have been some kind of dwarf species due to limited food availability where it lived (Fiorillo & Tykoski, 2014).
One species that’s been well-known to paleontologists for decades yet has somewhat flown under the public radar is the large and robust Daspletosaurus of Alberta and Montana. There’s a few different species of this tyrannosaur, but a currently undescribed one from the Dinosaur Park Formation of Alberta coexisted with Gorgosaurus, as mentioned earlier. The stoutness of Daspletosaurus might have made it more adept at dealing with hazardous prey like Centrosarus and Euoplocephalus. The presence of two big tyrannosaurs haunting the bayous of Cretaceous Alberta at the same time must have made it an exceedingly dangerous place.
China and Mongolia are also excellent places to find very interesting tyrannosaurs. Two unusual types, Alioramus and Qianzhousaurus, had extremely slender, pointed jaws for tyrannosaurs, and are sometimes grouped into their own tribe within the Tyrannosauridae.
From the vast Gobi Desert of Mongolia comes one of the largest tyrannosaurs known to science. Tarbosaurus baatar, discovered in the late 40’s by a Soviet-Mongolian expedition, was only slightly smaller than Tyrannosaurus rex (Maleev, 1955). The two are so similar that Tarbosaurus was initially described as an Asian species of Tyrannosaurus, but was soon after reclassified as a separate, but closely related genus (though some paleontologists still argue for their synonymy). While big, Tarbosaurus is a fair bit slimmer than Tyrannosaurus, which a noticeably narrower skull. This might be attributable to the lack of big horned dinosaurs in its ecosystem. At the same time that Albertosaurus was prowling western Canada, Tarbosaurus was living in what was then a lush, swampy woodland with various prey options like hadrosaurs, sauropods, big ornithomimosaurs, and oviraptorids. It must have done pretty well too, since Tarbosaurus is one of the most commonly found (and, sadly, poached) dinosaurs from Mongolia.
In the early 2010’s, the mysterious Zhuchengtyrannus was described from eastern China (Hone et al., 2011). While known from significantly less material than either Tarbosaurus or Tyrannosaurus, it seems to be intermediate in size between the two of them. Zhuchengtyrannus lived about 73.5 million years ago, and coexisted with dinosaurs similar to those found in North America at the time- hadrosaurs, ankylosaurs, and even Asia’s only known ceratopsid, Sinoceratops. A potential source of food may have been the colossal duckbill Shantungosaurus, but adults of this species would have likely made for difficult prey even for such a big predator.
Before our grand finale, it’s worth mentioning the hotly controversial dinosaur Nanotyrannus lancensis. This taxon is based on a small, slim tyrannosaur skull from the latest Cretaceous of Montana. The authors who described this skull believed, at the time, that it was from an adult animal, and so described it as a new genus of dwarf tyrannosaur (Bakker et al., 1988). Other features such as the high number of teeth in Nanotyrannus have been seen as support for its validity, but many scientists have argued that the holotype of this dinosaur is really a juvenile, and almost certainly one of Tyrannosaurus (Carr, 1999).
It was once thought that Tyrannosaurus rex evolved from earlier North American species like Daspletosaurus. However, with more similar dinosaurs like Tarbosaurus and Zhuchengtyrannus now known from Asia, it’s suspected that Tyrannosaurus might have arisen from an Asian lineage of tyrannosaurs that migrated into North America thanks to the fact the two continents were connected at the time (Brusatte & Carr, 2016). Having coexisted with large and hazardous prey such as Torosaurus, Ankylosaurus, and Triceratops, it seems like Tyrannosaurus invested a lot into being big, bulky, and powerful.
There’s little to say here about Tyrannosaurus that hasn’t already been said, and you can learn more about it in a previous entry here. The point here was to put the spotlight on the forbearers of Tyrannosaurus. The other tyrants might not be movie stars yet, but they deserve our attention and admiration.
By Nicholas Carter
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