The Tyrannosaurus rex, also known as T. rex, is undoubtedly one of the most iconic dinosaurs of all time. Standing over 12 meters tall and weighing around 9 tons, this massive predator had huge, bone-crushing jaws filled with serrated teeth the size of bananas.

T. rex lived during the late Cretaceous period, around 67 to 65 million years ago, and fossils have been found across western North America. If you’re short on time, here’s a quick answer to your question: While T. rex itself is extinct and has no direct descendants alive today, some of its closest living relatives include modern birds and crocodilians.

T. rex Biological Classification

The Tyrannosaurus rex is one of the most iconic dinosaurs that ever lived. But where exactly does this giant carnivore fit into the dinosaur family tree? Let’s take a closer look at the biological classification of T. rex.

Dinosauria Clade

All dinosaurs, including T. rex, belong to a clade known as Dinosauria. A clade is a group of organisms that share a common ancestor. All dinosaurs evolved from early archosaurs (reptiles) during the Triassic period over 230 million years ago.

Dinosaurs ruled the Earth for over 160 million years until the Cretaceous-Paleogene extinction event around 66 million years ago. Birds are the only living descendants of dinosaurs today.

Tyrannosauroidea Superfamily

Within Dinosauria, T. rex belongs to the theropod subgroup. Theropods were bipedal, mostly carnivorous dinosaurs. T. rex is further classified into the superfamily Tyrannosauroidea.

Tyrannosauroids were large, predatory theropods that lived between the Jurassic and late Cretaceous periods. Well-known tyrannosauroids include T. rex and its close relatives Albertosaurus, Daspletosaurus, Gorgosaurus and Tarbosaurus.

Tyrannosauridae Family

The Tyrannosauridae family consists exclusively of the iconic late Cretaceous tyrannosaurs. T. rex is by far the largest and most well-known member of this family.

Other tyrannosaurids include Daspletosaurus, Tarbosaurus and the earlier tyrannosaur Lythronax which lived around 80 million years ago. Tyrannosaurids were apex predators during the late Cretaceous and likely hunted hadrosaurs, ceratopsians and ankylosaurs.

T. rex Extinction

Asteroid Impact Theory

The extinction of the mighty Tyrannosaurus rex has fascinated scientists and dinosaur enthusiasts for decades. The asteroid impact theory has emerged as the leading explanation for the demise of the T. rex approximately 65 million years ago.

This theory states that a massive asteroid, estimated to be around 6 miles (10 km) wide, collided with Earth and triggered catastrophic changes in the environment that made it impossible for large dinosaurs like T. rex to survive.

The asteroid impact caused huge wildfires, sent massive amounts of debris and ash into the atmosphere, and precipitated a period of extended darkness and cold.

This radically changed climate deprived T. rex and other dinosaur species of their food sources, leading to widespread starvation. The combined effects of the impact winter, global wildfires, and ecological collapse ultimately drove Tyrannosaurus rex and 75% of life on Earth to extinction.

The asteroid impact theory gained prominence after scientists discovered the Chicxulub crater buried underneath the Yucatan Peninsula in Mexico. Dating of the crater matched the timeframe of the mass extinction event at the end of the Cretaceous period when T. rex disappeared.

Further research and modeling have bolstered the link between the impact and dinosaur extinction.

While some scientists propose additional factors like intensive volcanic activity contributed to the demise of dinosaurs, the asteroid impact is widely considered the primary driver of the mass extinction.

The cosmic catastrophe abruptly erased Tyrannosaurus rex after over 2 million years of dominance.

Competition With Other Species

In addition to asteroid impacts, some paleontologists argue that competition with other species may have also played a role in the extinction of T. rex. As ecosystems changed near the end of the Cretaceous period, T. rex faced threats from newly evolved species competing for resources.

Some experts theorize that T. rex struggled to compete with other large theropod dinosaurs like Giganotosaurus and Carcharodontosaurus. These massive predators appeared later in the Cretaceous and rivaled T. rex in size and hunting abilities.

Direct competition for prey would have put survival pressure on Tyrannosaurus populations.

Mammal species were also diversifying close to the extinction timeline. Early mammal groups were evolving traits like larger body size and complex teeth. This allowed them to exploit food sources traditionally reserved for dinosaurs.

Increased competition from mammals for nutrients may have also pushed T. rex populations toward decline.

However, many paleontologists argue that T. rex was unlikely to have been overwhelmed by competition in the Late Cretaceous landscape. As the apex predator of North America, T. rex would have remained an effective hunter even with new challenges from other species in its environment.

Ultimately, the asteroid impact is considered the primary extinction driver due to its sudden and catastrophic effects across environments globally. But for isolated T. rex populations already struggling, competition from other species may have been the final factor leading to extinction.

The true cause still sparks lively debate among paleontologists today.

T. rex Evolutionary Relationships

Archosaurs Superorder

Tyrannosaurus rex belonged to a larger group of reptiles called archosaurs, which includes modern crocodilians and birds. Archosaurs first emerged around 250 million years ago during the Triassic period and quickly became the dominant land vertebrates.

This superorder evolved key features like upright limbs, enhanced breathing capabilities, and fast growth rates that helped them thrive and diversify (American Museum of Natural History).

Within archosaurs, T. rex belongs to a subgroup called theropods, bipedal dinosaurs specially adapted for an active predatory lifestyle. Theropods appeared about 230 million years ago and over millions of years evolved into various ferocious hunters like Velociraptor, Spinosaurus, Giganotosaurus, and of course, the mighty Tyrannosaurus rex.

So while long extinct, T. rex share a common archosaur ancestor with modern crocodilians and birds.


Recent research indicates crocodilians are the closest living relatives of dinosaurs like T. rex. Genetic studies show crocodilians share about 80% of their DNA with birds and dinosaurs. Like T. rex, crocodilians also have enhanced breathing capacities and rapid growth rates during youth that tapper off as adults (AMNH).

However, some key differences exist – crocodilians live amphibious lifestyles hunting near water, while T. rex were highly mobile land predators. And despite genetic similarities, some researchers argue convergent evolution led to anatomical similarities between the two groups, rather than direct descent.

T. rex Crocodilians
Strictly terrestrial Semi-aquatic lifestyle
Bipedal with upright posture Sprawled posture
Endothermic (warm-blooded) Ectothermic (cold-blooded)


Birds share the most direct evolutionary connection to T. rex as modern descendants of feathered theropods. In fact, birds are classified scientifically as avian dinosaurs. Recent fossils showed some dinosaurs like Velociraptor had feathers and wings nearing the end of the Cretaceous period.

After the mass extinction 66 million years ago wiped out T. rex and relatives, some feathered theropods survived and evolved into modern birds we see today (AMNH).

So while Tyrannosaurus rex is long gone, bits of its genetic code live on in today’s birds. Both groups share key traits like feathers, warm-blooded metabolism, enlarged brains, and fast growth rates. And some bird behaviors seem eerily similar to T. rex – just watch how a hawk soars through the sky searching for prey!

Shared Traits Between T. rex and Modern Relatives

Skeletal Similarities

When examining the skeletal structures of T. rex and its modern bird and reptile relatives, some fascinating commonalities emerge. For example, the wishbone, officially called the furcula, has been found in both T. rex and modern birds. This V-shaped bone likely helped T.


breathe and was integral to flight in today’s birds. Additionally, T. rex had hollow bones, a feature shared by birds that is thought to make flying easier via weight reduction. Could the hollow bones have aided the agility of the fearsome T. rex? We can only speculate!

Behavioral Similarities

Looking at current dinosaur cousins can also provide intriguing clues to T. rex behavior. Recent research suggests relatively small forelimbs in tyrannosaurs were likely used for grasping during hunting, much like raptors do today.

And the discovery of medullary bone, which in modern birds helps produce eggshells, in a T. rex fossil indicates that – like many modern reptiles – T. rex mothers laid eggs. Finding biochemical proof of complex nesting behaviors definitely makes these extinct creatures seem less alien!

Why T. rex Has No Direct Descendants

Genetic Bottleneck of Mass Extinction

The Tyrannosaurus rex species lived during the late Cretaceous period, approximately 65 million years ago. According to American Museum of Natural History, the Cretaceous-Paleogene extinction event 66 million years ago caused the extinction of all dinosaurs except some avian species.

This mass extinction led to a genetic bottleneck, eliminating over 75% of plant and animal species from the Earth (ScienceDaily, 2021). As a result, the direct ancestral line of T. rex was severed, leading to their eventual extinction.

Evidence shows that all non-avian dinosaurs, including T. rex, went completely extinct within a short period due to this catastrophic event. With no survivors to pass on genes to future generations, T. rex and other dinosaurs could not maintain a direct hereditary line.

Ultimately, the abrupt climate changes and environmental pressures proved too difficult for the recovering ecosystems to sustain giant theropod predators like T. rex. This explains why paleontologists have not discovered any direct descendants of T. rex existing today.

Differences in Environment and Ecology

Additionally, the prehistoric environment and ecology that T. rex inhabited no longer exists. The late Cretaceous climate was much warmer, with higher concentrations of carbon dioxide in the atmosphere (National Geographic, 2022).

Dense, lush forests covered the landscape alongside diverse dinosaurian fauna. T. rex and other large theropods occupied an ecological niche as apex predators at the top of the food chain.

In contrast, the modern world has significantly different climatic conditions, flora, and fauna. The biodiversity and environmental pressures that giant bipedal predators like T. rex adapted to changed considerably after the extinction event.

As ecosystems recovered, the niches dominated by tyrant dinosaurs likely could no longer provide enough sustenance. Even if T. rex descendants survived, competition from emerging mammalian predators like saber-toothed cats may have made their extinction inevitable.


In summary, while the iconic Tyrannosaurus rex has been extinct for around 65 million years and has no direct descendants still roaming the Earth today, some of its closest living relatives likely include modern crocodilians and birds.

T. rex shares common distant ancestors like archosaurs with these animals and has similarities in areas like skeletal structure and predatory behavior. However, the catastrophic mass extinction that wiped out dinosaurs created a genetic bottleneck, and major differences in environment and ecology also prevent modern crocodilians or birds from being considered true direct descendants of T. rex.

While we may never see the mighty T. rex itself again, looking at its modern relatives can give us a glimpse into its biology and connection to the present day.

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