The extinction of Tyrannosaurus rex marks one of the most dramatic events in Earth’s history. Around 66 million years ago, at the end of the Cretaceous period, a mass extinction event wiped out about 75% of all species, including all non-avian dinosaurs. The leading cause of this extinction is believed to be a massive asteroid impact near the present-day Yucatán Peninsula, which triggered environmental disasters such as wildfires, a drop in global temperatures due to sunlight-blocking dust, acid rain, and a breakdown of ecosystems (Brusatte et al., 2023).
For top predators like T. rex, the sudden collapse of food chains proved devastating. With plant life decimated, herbivorous dinosaurs dwindled, leaving carnivores with no stable food supply. Large animals like T. rex, which needed significant energy to survive and reproduced slowly, were especially vulnerable to the rapid changes (Dalmsn et al., 2024).
Despite the loss of T. rex and other large theropods, their evolutionary legacy lived on through a branch of small, feathered theropods, the avian dinosaurs that managed to survive. Modern birds are considered the direct descendants of these theropods. Several key adaptations contributed to their survival. Many avian dinosaurs were small, had high reproductive rates, and could exploit a variety of ecological niches. Traits like endothermy (warm-bloodedness) and feathers for insulation, already present in some non-avian dinosaurs like Dilong and Guanlong (Brusatte & Carr, 2016; Brusatte et al., 2023), allowed these survivors to adapt more readily to a drastically changing world.
The evolutionary traits that helped birds survive, such as light, hollow bones, beaks instead of heavy jaws, and efficient lungs, were already beginning to develop in small theropod dinosaurs long before the extinction event (Brusatte et al., 2023). This evolutionary flexibility gave avian dinosaurs a better chance at adapting quickly to new environmental conditions compared to their larger relatives like T. rex.
Though T. rex did not survive, it belonged to a larger group of theropods that eventually gave rise to birds. Traits like sharp vision, quick reflexes, and advanced behaviors were passed down and can still be seen and traced in modern bird species. The extinction of T. rex marks both an end and a continuation, as its relatives took to the skies and evolved into the thousands of bird species we see around us today.
It may be surprising to learn that birds are not just descendants of dinosaurs—they are dinosaurs. Specifically, birds evolved from a group of small, feathered theropods and represent the only lineage of dinosaurs to survive the mass extinction event at the end of the Cretaceous period (Brusatte & Carr, 2016). Modern birds, or avian dinosaurs, carry forward physical and biological traits inherited from their Mesozoic ancestors.
Many of the traits that define birds today first appeared in their dinosaur ancestors. These include, feathers which originally was used for insulation or display in early theropods like Dilong, later adapted for flight by avian species (Brusatte & Carr, 2016). Endothermy or warm-bloodedness was another trait that birds inherited from their ancestors, which helped them maintain body temperature and survive rapid climate change after the asteroid impact (Brusatte et al., 2023). Additionally, lightweight Skeletal stystems like hollow bones evolved in some non-avian theropods and later became more pronounced in birds, enabling flight and efficient respiration. Sharp vision, a keen sense of smell, and strong cognitive abilities, all traits present in predators like T. rex, also developed further in birds, aiding in navigation, foraging, and communication (Millar, 2019).
When the asteroid struck Earth 66 million years ago, it triggered global devastations, blocking sunlight, lowering temperatures, and collapsing food chains. While most non-avian dinosaurs, including iconic giants like T. rex, went extinct, birds managed to survive. Their survival likely hinged on several evolutionary advantages. Their small body size allowed them to require fewer resources, and their ability to fly made it possible to escape harsh environments and reach new habitats. Furthermore, birds had relatively fast reproductive cycles and could adapt to different food sources, making them more resilient in a rapidly changing world (Brusatte et al., 2023).
The transition from non-avian dinosaurs to birds wasn’t sudden—it was gradual and supported by fossil evidence. Species like Archaeopteryx and other maniraptorans show a blend of dinosaurian and birdlike traits, including teeth, claws, long tails, and feathers. Over time, these features adapted toward flight, perching, and more complex behavior.
Endothermy also played a vital role in bird evolution. Research suggests a slow rise in metabolic rate across theropod lineages, culminating in the fully endothermic physiology of modern birds (Brusatte et al., 2023). This biological change may have helped early birds cope with colder climates and energy-intensive lifestyles.
Today, birds are one of the most diverse groups of vertebrates, with over 10,000 species occupying nearly every habitat on Earth. From penguins in Antarctica to parrots in tropical rainforests, birds reflect the continued success of their dinosaurian heritage. They sing, migrate, build nests, and raise young—behaviors rooted in the deep evolutionary past. The age of dinosaurs never truly ended; it continues every time a bird takes flight.