In late 1905, newspaper reporters gushed over the bones of a prehistoric monster that palaeontologists had unearthed in the badlands of Montana. When The New York Times described the new 'Tyrant saurian', the paper declared it “the most formidable fighting animal of which there is any record whatsoever”. In the century since, Tyrannosaurus rex has not loosened its grip on the imaginations of the public or palaeontologists.
Stretching more than 12 meters from snout to tail and sporting dozens of serrated teeth the size of rail spikes, the 66-million-year-old T. rex remains the ultimate example of a prehistoric predator — so much so that a media frenzy erupted this year over a paper debating whether T. rex predominantly hunted or scavenged its meals. This infuriated many palaeontologists, who say the matter was resolved long ago by ample evidence showing that T. rex could take down prey and dismantle carrion. What particularly vexed researchers was that this non-issue overshadowed other, more important questions about T. rex.
The dinosaur's evolutionary origins, for example, are still a mystery. Researchers are eagerly trying to determine how these kings of the Cretaceous period (which spanned from 145 million to 66 million years ago) arose from a line of tiny dinosaurs during the Jurassic period (201 million to 145 million years ago). There is also considerable debate about what T. rex was like as a juvenile, and whether palaeontologists have spent decades mistaking its young for a separate species. Even the basic appearance of T. rex is in dispute: many researchers argue that the giant was covered in fluff or fuzz rather than scales. And then there is the vexing question of why T. rex had such a massive head and legs but relatively puny arms.
In the first few decades after palaeontologist Henry Fairfield Osborn named and described T. rex, researchers viewed this giant dinosaur as the culmination of a trend towards bigger predators. In this view, T. rex was seen as the descendent of Allosaurus, a 9-meter-long predator that lived more than 80 million years earlier. These and other massive carnivorous dinosaurs were lumped together in a categorical wastebasket called the Carnosauria, with T. rex as the last and biggest of the ferocious family. But palaeontologists tore up that evolutionary tree when they started using a more rigorous form of analysis called cladistics in the 1990s. They re-examined relationships between dinosaur groups and found that T. rex had its roots in a lineage of small, fuzzy creatures that lived in the shadow of Allosaurus and other predators during the Jurassic period.
The view that emerged placed T. rex and its close relatives — together known as tyrannosaurids — as the top twig on a broader evolutionary bush called the Tyrannosauroidea, which emerged around 165 million years ago. Among the earliest known members of this group was Stokesosaurus clevelandi, a bipedal carnivore 2–3 meters long that lived about 150 million years ago. Little is known about this creature, but evidence from other early tyrannosauroids suggests that Stokesosaurus had a long, low skull and slender arms. Early tyrannosauroids were small, agile predators, but their size placed them low in the pecking order during the Jurassic. “They were more lapdogs than top predators,” says Brusatte.