Allosaurus and Tyrannosaurus rex are both recognized as formidable large, bipedal carnivorous dinosaurs, belonging to the high-level grouping known as theropods. Their similar appearance often leads to the misconception that they were closely related. However, a scientific comparison reveals they are separated by vast stretches of time and evolutionary distance. Distinct anatomical features, different eras, and geographic ranges establish them as separate branches on the dinosaur family tree.
Placing Them on the Family Tree: Taxonomic Distance
The relationship between Allosaurus and T. rex is that of distant cousins, despite their superficial resemblance as large, two-legged meat-eaters. Both belong to the Theropoda suborder, but their familial branches separated early in dinosaur evolution. Their taxonomic distance is revealed by the specific groups they belong to.
Allosaurus belongs to the family Allosauridae, which is part of the lineage Carnosauria. This group includes some of the largest predators of the Jurassic and Early Cretaceous periods. T. rex belongs to the family Tyrannosauridae, which is part of the lineage Coelurosauria. The evolutionary split between the Allosauroidea and Tyrannosauroidea occurred in the mid-Jurassic period, meaning their last common ancestor was a much smaller theropod.
Separated by Epochs and Continents
The two predators were completely separated by time and space, meaning they never coexisted. Allosaurus lived much earlier, dominating the Late Jurassic period from approximately 155 to 145 million years ago. Its fossils are found abundantly in the Morrison Formation of western North America, and also in regions like Portugal.
T. rex thrived much later, during the very end of the Late Cretaceous period, around 68 to 66 million years ago. This places an approximately 80-million-year gap between the two species. T. rex was geographically restricted to the island continent of Laramidia, which now makes up western North America.
Defining Features: Anatomical Distinctions
The most apparent distinctions are found in their physical anatomy, reflecting different hunting and feeding strategies. The skull of Allosaurus was lighter, longer, and narrower, built for a slicing style of attack. It also featured a distinctive pair of bony crests just above the eyes, potentially serving a function in display.
The skull of T. rex was massive and robust, characterized by a deep, broad structure designed to withstand enormous crushing forces. This reinforced skull supported a powerful bite strong enough to shatter bone. The differences extend to their forelimbs: Allosaurus had relatively long, functional arms equipped with three clawed fingers for grasping prey. T. rex had diminutive forelimbs with only two functional fingers.
Their dentition also reveals different feeding methods. Allosaurus teeth were thin, blade-like, and highly serrated, optimized for cutting flesh. T. rex possessed thick, robust, banana-shaped teeth designed to puncture and crush bone. Furthermore, Allosaurus was generally a more slender animal, averaging about 28 feet in length, while T. rex was significantly bulkier and could reach lengths exceeding 40 feet.
The Shared Heritage of Theropods
Despite the vast differences, people often group them together due to their shared basic body plan as theropods. Both evolved as apex predators, requiring a similar suite of traits to dominate their environments. These common characteristics include a bipedal stance, a large head, powerful hind legs, and a carnivorous diet.
This similarity is an example of convergent evolution, where distantly related organisms independently evolve similar traits to adapt to similar ecological niches. The large, bipedal body plan proved an effective solution for dominant terrestrial predators across millions of years. Their shared ancestry in the Theropoda suborder explains their overall similarities, while their massive evolutionary separation accounts for their distinct classifications and specialized anatomy.