While the planet today would likely still be dominated by non-avian dinosaurs, they would look vastly different from the famous giants of the Late Cretaceous. Over the last 66 million years, Earth's climate underwent massive changes. The warm, greenhouse conditions of the Cretaceous gradually gave way to a much cooler, drier world, eventually leading to the formation of polar ice caps and the spread of vast grasslands. Dinosaurs would have been forced to adapt to these new environments.
- The Rise of Grazing Dinosaurs: As dense forests retreated and tough, silica-rich grasses took over during the Oligocene and Miocene epochs, herbivorous dinosaurs like ceratopsians (relatives of Triceratops) and hadrosaurs (duck-billed dinosaurs) would likely have evolved specialized teeth and digestive systems. Just as horses and cattle developed flat molars to grind grass, these dinosaurs might have adapted into the dominant grazers of the plains.
- Feathered Winter Survivors: Because many theropods already possessed feathers for insulation, they were perfectly pre-adapted for the cooling climate. In a modern ice age scenario, the world might be populated by large, thick-feathered predatory dinosaurs hunting woolly relatives of the ankylosaur in snowy landscapes.
- Mammalian Suppression: Mammals thrived alongside dinosaurs for over 100 million years before the asteroid hit, but they were kept from growing large. They occupied the roles of small, mostly nocturnal insectivores and scavengers. Without a sudden mass extinction to vacate the large herbivore and apex predator niches, mammals would likely have remained small. Consequently, the evolutionary radiation that produced elephants, whales, and primates—including humans—would never have occurred.
- Marine Dominance: The oceans would still be ruled by massive marine reptiles like mosasaurs and plesiosaurs. Because land mammals would never have had the ecological space to evolve into large aquatic forms, whales, dolphins, and seals would be entirely absent from the seas.
Perhaps the most fascinating question is how far their intelligence would have developed. By the end of the Cretaceous, small theropods like Troodon already possessed relatively large brains and grasping hands. Today, surviving avian dinosaurs like crows and parrots demonstrate complex problem-solving skills and tool use. An unbroken terrestrial lineage of these intelligent theropods could easily have produced the dominant, tool-wielding animals of our modern ecosystem.