Approximately 66 million years ago, the Cretaceous-Paleogene (K-Pg) extinction event caused the demise of about three-quarters of Earth’s plant and animal species, including all non-avian dinosaurs. This mass extinction created an unprecedented ecological opportunity for surviving life forms, allowing for rapid diversification and the filling of newly vacant ecological niches.
The Post-Extinction Landscape
The K-Pg extinction event left an “ecological vacuum” due to the absence of large, dominant species. This created abundant resources and significantly reduced competition and predation pressure for surviving lineages. These conditions set the stage for adaptive radiation, where organisms rapidly diversified into a multitude of new forms, exploiting newly available opportunities.
Mammalian Dominance
Mammals, which were generally small and nocturnal before the K-Pg extinction, underwent a rapid diversification following the event. They quickly occupied a vast array of ecological niches previously filled by dinosaurs. Fossil evidence indicates immediate increases in mammalian body sizes and ecological diversity in the Paleocene epoch, the 10 million years after the extinction.
Ancestors of hoofed mammals, known as ungulates, rapidly diversified into various herbivorous roles across different continents. Early primates emerged and began their diversification, with some lineages developing arboreal, or tree-dwelling, adaptations. Predatory mammals, including early carnivorans, evolved to fill apex predator roles, with the oldest known fossils appearing in the late Paleocene. Rodents and lagomorphs adapted to various terrestrial niches.
Bats developed flight, allowing them to exploit aerial insectivorous and fruit-eating niches. Molecular evidence suggests their ancestors diverged around 80 million years ago, well into the Cretaceous, with the first bat fossils appearing about 52 million years ago. Later in the Cenozoic, some mammalian lineages made a significant return to aquatic environments, giving rise to marine mammals such as whales and manatees. These groups began adapting to water during the Eocene epoch, around 56 million years ago, developing features like flippers and nostrils positioned higher on their skulls.
Avian Radiation
Birds, direct descendants of avian dinosaurs, survived the K-Pg extinction event. Their ability to fly provided an advantage, as large flying reptiles like pterosaurs had gone extinct. This, combined with the absence of many terrestrial predators, allowed birds to rapidly colonize new environments and access diverse food sources.
Modern bird orders underwent a rapid evolutionary burst after the K-Pg boundary. A small fraction of Cretaceous bird species survived to give rise to today’s birds, leading to the incredible variety of species seen today, filling ecological niches left empty by the extinction.
Other Vertebrate Recoveries
Beyond mammals and birds, other vertebrate groups recovered and diversified after the K-Pg extinction. Non-dinosaurian reptiles like snakes and lizards underwent significant diversification in the Cenozoic Era. Crocodilians, semi-aquatic before the event, survived and adapted to various aquatic and semi-aquatic niches. Their survival is partly attributed to their amphibious lifestyle, low metabolism, and ability to endure periods of food scarcity.
Amphibians persisted in moist environments. Fish populations in both marine and freshwater environments experienced recovery and diversification, filling ecological roles left vacant by marine reptiles like mosasaurs and plesiosaurs.
Invertebrate Adaptations
Invertebrates adapted significantly to the changing flora and fauna of the post-extinction world. Insects continued their evolutionary success, adapting to newly emerging plant species, particularly flowering plants. This led to new ecological relationships between insects and plants. Marine invertebrates also showed signs of recovery and adaptation, contributing to the rebuilding of aquatic ecosystems. For instance, radiolarians, a type of marine plankton, showed no evidence of mass extinction and high productivity in the early Paleocene.