Evolutionary History of T. rex 

Tyrannosaurus rex, often simply called T. rex, lived approximately 68 to 66 million years ago during the Late Cretaceous period. It is one of the last and largest members of the tyrannosaurid family and is perhaps the most famous dinosaur ever discovered. T. rex fossils have been found primarily in western North America, in areas such as Montana, South Dakota, and Alberta, Canada. Adult T. rex individuals could reach lengths of up to 12 to 13 meters (about 40 feet) and weigh between 8 to 9 tons. They were bipedal predators with large, powerful skulls, bone-crushing jaws, and relatively small arms equipped with two clawed fingers.

T. rex was a carnivore that fed on large herbivorous dinosaurs like hadrosaurs and ceratopsians. Its keen senses—especially its eyesight and sense of smell—along with its immense bite force, made it a dominant predator in its ecosystem. Despite its size, fossil evidence suggests that T. rex was capable of bursts of speed and possibly even scavenged in addition to active hunting.

Early Relatives: The Small Hunters

Tyrannosaurs evolved during the Middle Jurassic period, over 160 million years ago, and gradually diversified throughout the Jurassic and Cretaceous periods. The early members of the tyrannosauroid lineage were much smaller and more agile than T. rex. This is because they possessed long legs, and lightweight bodies, and some even had feathers. Considerable examples include:

Dilong – This is a small, feathered tyrannosauroid from the Early Cretaceous of China, measuring about 1.5 meters in length. They provide some of the first evidence that early tyrannosaurs had insulating feathers.

Guanlong – This is another early relative from the late Cretaceous of China, mostly known for its distinctive crest. They measure about 3-3.5m in length. Guanlongs are also feathered and display a more graciled and agile build. They likely served as quick predators, and their skeletal features connect them to later tyrannosaurs.

These small hunters occupied lower-tier predatory roles in their ecosystems, preying on smaller animals and avoiding competition with larger carnivores. By the time T. rex appeared about 68 million years ago, they had grown large and had developed many of the features that has been used to describe them until now.  The figure also shows them as they evolved as top predators; some of these are:

Gorgosaurarus – A close relative of T. rex that lived in North America around 76 million years ago. They are more advanced tyrannosaurs, reaching about 8-9m in length. Gorgosaurus has some significant skeletal similarities to the T. rex and shows a phase in their evolution to a larger body size.

Daspletosaurus – This was a more robust tyrannosaurus also from the late Cretaceous in North America, that shows many of the physical and skeletal features that define T. rex; these include strong legs and short arms. 

Thus, with time, they reached the peak of their evolutionary development. With their massive skull, incredible strong bite force, and thick bones, they dominated their ecosystem. But unlike their ancestors, they had raw power, speed, and intelligence, which made them the most structured predators of all time.   

Evolutionary Connection of Birds and T. Rex

The evolutionary connection between birds and non-avian theropods, including Tyrannosaurus rex, is now one of the most well-supported transitions in vertebrate history (Brusatte et al. 2015). Extensive fossil evidence demonstrates that T. rex shares numerous characteristics with the lineage that led to modern birds, including hollow pneumatic bones, a three-fingered hand, a furcula (wishbone), and possibly even primitive feather-like structures in its close relatives (Brusatte et al. 2015). These shared features place T. rex within the broader group Coelurosauria, the same branch of theropods that ultimately gave rise to true birds. As illustrated in Figure 1 from Brusatte et al. (2015) 

Birds are nested within the larger group of maniraptoran theropods, showing that they are not merely related to dinosaurs but are themselves surviving dinosaurs. Moreover, Figure 1b visually details the gradual acquisition of avian traits such as reduced body size, elongated forelimbs, and feathered integument across successive theropod lineages. These patterns emphasize that the evolution of birds was a stepwise process, building on traits already present in large-bodied, non-avian dinosaurs like T. rex (Brusatte et al. 2015).