The Cretaceous period, lasting from approximately 145 to 66 million years ago, was the final and longest chapter of the Mesozoic Era. This 79-million-year span was a time of significant geological and biological transformation, witnessing the emergence of diverse dinosaur species. During the Cretaceous, some of the most recognizable dinosaurs appeared, evolving in a world undergoing dramatic changes to its continents and plant life. The period culminated in a catastrophic event that would forever alter the course of life on Earth.
The World of the Cretaceous Period
The geography of the Cretaceous was different from the preceding Jurassic period. The supercontinent Pangaea continued its gradual separation, leading to smaller, more isolated landmasses which fostered regional differences in life. A defining feature of this era was high sea levels, which created vast, shallow inland seas. The most significant of these was the Western Interior Seaway, which split North America into two landmasses for a time.
The world was characterized by a warm, humid climate. The early part of the period experienced a cooling trend that continued from the late Jurassic, but temperatures rose again throughout the Late Cretaceous. This warmth meant that for much of the period, there were no polar ice caps, allowing dinosaurs to inhabit regions like Antarctica. The climate and increased coastlines created by the inland seas contributed to a globally wetter environment.
A profound biological development was the evolution and diversification of flowering plants, known as angiosperms. First appearing around 125 million years ago, these plants spread rapidly across the globe, altering terrestrial ecosystems. This introduced new food sources and habitats, directly impacting the evolution of herbivorous dinosaurs that adapted to browse this vegetation. The familiar forests of ferns and conifers were now increasingly interspersed with landscapes dominated by flowering plants.
Iconic Herbivores of the Cretaceous
Among the most recognizable herbivores of the Late Cretaceous were the ceratopsians, represented by Triceratops. This dinosaur roamed what is now North America, possessing a massive skull that could reach up to a third of its total body length. It featured three prominent facial horns and a large, solid bone frill. Scientists suggest this headgear was used for defense, mating displays, or in contests for dominance. Its mouth was equipped with a powerful beak for shearing tough, low-growing vegetation.
Ankylosaurus is a dinosaur often described as a living tank. This creature was covered in thick, bony plates, known as osteoderms, which provided extensive protection. Its body was low to the ground and broad, weighing several tons. The most distinctive feature of Ankylosaurus was its massive tail club, a formidable defensive weapon capable of delivering powerful blows to an attacking predator.
The hadrosaurs, or “duck-billed” dinosaurs, were also common during the Cretaceous. These dinosaurs, such as Edmontosaurus and Parasaurolophus, are noted for their flattened, beak-like snouts and often traveled in large herds. Some hadrosaurs, like Parasaurolophus, sported elaborate, hollow crests on their heads. These crests were connected to their nasal passages and were likely used for communication, allowing them to produce distinctive sounds to signal others.
Apex Predators and Carnivores
Tyrannosaurus rex was one of the largest land predators ever to have lived. This bipedal carnivore had a massive skull and powerful hind legs, with a long, heavy tail acting as a counterbalance. Its jaws were capable of delivering the most powerful bite of any terrestrial animal, strong enough to crush bone. While its arms were famously small, its senses were keen, with a highly developed sense of smell and forward-facing eyes that provided excellent depth perception. Most paleontologists now agree it likely engaged in both active hunting and scavenging.
The dromaeosaurs, popularly known as “raptors,” were another group of carnivores. These dinosaurs, such as Velociraptor and the larger Utahraptor, were intelligent and agile predators. Contrary to their depiction in popular media, Velociraptor was much smaller, about the size of a turkey, and covered in feathers. A signature feature of this group was a large, sickle-shaped claw on the second toe of each foot, which was likely used to pin down prey.
The Cretaceous period featured a diverse array of other carnivores. In the earlier part of the period, giant predators like Spinosaurus hunted in river systems, while in the Late Cretaceous, carnivores like Carnotaurus roamed South America. Carnotaurus was notable for its two prominent horns above its eyes and its extremely reduced forelimbs, even smaller in proportion than those of T. rex.
The Cretaceous–Paleogene Extinction Event
The age of dinosaurs ended approximately 66 million years ago in a mass extinction event known as the Cretaceous-Paleogene (K-Pg) extinction. The most widely accepted cause is the impact of a massive asteroid, estimated to be 10 to 15 kilometers wide, which struck the Earth in the Yucatán Peninsula. This impact created the Chicxulub crater, a scar over 180 kilometers in diameter. The immediate effects triggered widespread earthquakes, volcanic eruptions, and mega-tsunamis.
The collision ejected enormous quantities of vaporized rock and dust into the atmosphere. This material, combined with soot from global firestorms, shrouded the planet in darkness. This “impact winter” blocked sunlight for months or years, causing global temperatures to plummet and halting photosynthesis. The collapse of plant life had a cascading effect up the food chain, leading to widespread starvation.
This environmental disruption led to the extinction of an estimated 75% of all species on Earth. All non-avian dinosaurs, along with other large reptiles like pterosaurs and marine plesiosaurs, vanished. The event marked a turning point in the history of life, paving the way for the rise of mammals in the subsequent Cenozoic Era.