The Cretaceous-Paleogene (K-Pg) extinction event, commonly called the “meteor hit,” occurred approximately 66 million years ago. This catastrophic moment marked the end of the Cretaceous period and the Mesozoic Era. The extinction was triggered by the Chicxulub impactor, a large asteroid that struck the Yucatán Peninsula in Mexico. The dinosaurs alive then belonged to the Maastrichtian stage, the final age of the Late Cretaceous.
The Diversity of Non-Avian Dinosaurs
Contrary to older ideas suggesting a long, slow decline, a wide variety of non-avian dinosaur groups populated the planet right up to the impact boundary. These groups spanned the globe, occupying diverse ecological roles. The dominant plant-eaters were the Hadrosaurs, or duck-billed dinosaurs, which used specialized dental batteries to process tough vegetation.
The heavily armored Ankylosaurids, characterized by bony plates and a massive tail club, were present alongside their close relatives, the Nodosaurids. Another major group of herbivores was the Pachycephalosaurs, distinguished by their thick, dome-shaped skulls used for intraspecies competition.
Smaller, feathered Theropods formed a complex web of predators and omnivores. This included Dromaeosaurids, or “raptors,” known for their sickle-shaped claws, and Troodontids, which possessed large brains. In the Southern Hemisphere, large Sauropods, specifically the long-necked Titanosauria, continued to exist, although their diversity was lower than in previous epochs.
Key Players of the Final Era
The clearest picture of the final dinosaur ecosystem comes from the fossil-rich sedimentary rocks of western North America, particularly the Hell Creek Formation. This region was home to the most recognizable species that met the sudden end of the Cretaceous. The apex predator of this environment was the colossal Tyrannosaurus rex, which was active right up to the extinction event.
T. rex was a massive, bipedal carnivore, reaching lengths of over 40 feet and weighing several tons, making it a formidable hunter. Sharing this landscape was Triceratops, one of the last and most abundant Ceratopsians. Triceratops possessed an enormous bony frill and three distinct horns, likely used for both defense and display.
The armored Ankylosaurus was one of the largest members of its family, measuring up to 30 feet long. Covered in osteoderms and sporting a powerful tail club, Ankylosaurus was a difficult target for any predator. These specific genera provide direct evidence of a complex and thriving ecosystem that was abruptly terminated by the impact.
The Dinosaurs That Survived
The statement that all dinosaurs went extinct is inaccurate, as a single lineage survived the global catastrophe. This surviving group is the Avian Dinosaurs, the direct ancestors of modern birds. While the larger, non-avian forms perished, smaller, flying dinosaurs were able to cross the K-Pg boundary.
The characteristics of these survivors, such as small body size and the ability to fly, provided advantages in a world plunged into darkness and cold by the impact winter. Being small meant they required less food, and flight allowed them to travel farther to find scattered resources. Furthermore, the modern bird lineage (Neornithes) that survived was largely toothless and possessed beaks, enabling them to consume seeds and other hard, stored plant material.
Most of the archaic bird groups present in the late Cretaceous, such as the Enantiornithes, did not survive the K-Pg event. The mass extinction was highly selective, wiping out nearly all large terrestrial life but allowing the small, adaptable ancestors of today’s birds to endure.