The Maastrichtian Age represents the final chapter of the Cretaceous Period. Spanning from approximately 72.1 to 66 million years ago, it was the last interval of geologic time when non-avian dinosaurs walked the Earth. This age gets its name from the Dutch city of Maastricht, where limestone deposits from this period were first studied. These rock layers, known as the Maastricht Formation, are famous for the fossils they contain, which offer a window into the world as it existed just before a catastrophic global event.
A World of Extremes
During the Maastrichtian, the planet’s geography was different from today as continents drifted toward their modern positions. North America and Europe were separated by nascent ocean basins, and shallow inland seas like the Western Interior Seaway covered large portions of the continents. The climate was a “greenhouse world,” considerably warmer than the present day, allowing temperate conditions to extend to high latitudes. Polar regions experienced mild winters, though evidence suggests a gradual cooling trend and an increase in seasonality. This created a dynamic environment of vast coastal plains, extensive forests, and fluctuating sea levels.
The Last Dinosaur Dynasty
The terrestrial ecosystems of the Maastrichtian were dominated by a diverse dinosaur fauna. In North America, this was the age of icons like Tyrannosaurus rex, the apex predator, and the heavily armored Ankylosaurus. Vast herds of herbivorous dinosaurs roamed the landscapes, including Triceratops and large hadrosaurs, or duck-billed dinosaurs, such as Edmontosaurus. The skies were commanded by giant pterosaurs, most notably Quetzalcoatlus, a creature with a wingspan comparable to that of a small airplane. Mammals, though generally small and often nocturnal, were diversifying, and flowering plants, or angiosperms, were becoming increasingly widespread.
Rulers of the Cretaceous Seas
The marine environments of the Maastrichtian were dominated by a group of colossal predatory reptiles called mosasaurs. Having first appeared earlier in the Cretaceous, mosasaurs reached their peak in size and diversity during this final age, with genera like Mosasaurus hoffmannii becoming the apex predators of the seas. Fossil evidence shows they had a global distribution, inhabiting oceans from tropical to subpolar climates where they preyed on fish, sharks, and even other marine reptiles. Sharing these waters were other marine reptiles, such as the long-necked plesiosaurs of the elasmosaurid family. The marine ecosystem was rich with a variety of sharks, large bony fish, and abundant ammonites, whose distinctive fossils serve as key markers for geologists.
The Day the World Changed
The Maastrichtian age came to an abrupt and catastrophic end approximately 66 million years ago with the Cretaceous-Paleogene (K-Pg) extinction event. Scientific consensus attributes this mass extinction to the impact of a massive asteroid, estimated to be between 10 and 15 kilometers wide. This object struck the Earth in the Yucatán Peninsula, creating the 180-kilometer-wide Chicxulub crater. The immediate effects of the impact triggered mega-tsunamis and sent a heat pulse around the globe that likely ignited widespread firestorms.
The long-term consequences were even more severe. The impact vaporized sulfur-rich rock, ejecting immense quantities of dust and aerosols into the atmosphere. This created an “impact winter,” blocking sunlight for years, which caused global temperatures to plummet and halted photosynthesis. The collapse of these food chains led to the extinction of approximately 75% of all species, including all non-avian dinosaurs, pterosaurs, mosasaurs, and ammonites, clearing the way for the rise of mammals.