The Cretaceous-Paleogene (K-Pg) extinction event, occurring approximately 66 million years ago, represents a profound turning point in Earth’s history. This catastrophic episode saw the mass extinction of roughly three-quarters of the planet’s plant and animal species. The primary cause is widely accepted to be the impact of a massive asteroid, 10-15 kilometers wide, which struck the Yucatán Peninsula. This impact triggered an “impact winter,” plunging the Earth into prolonged darkness and freezing temperatures by blocking sunlight and halting photosynthesis. Such immense environmental upheaval dramatically reshaped ecosystems, leading to the demise of many dominant life forms, including all non-avian dinosaurs.
The Immediate Survivors
Despite widespread devastation, various life forms endured the K-Pg extinction. Most large land animals, specifically tetrapods over 25 kilograms, perished, with the exception of some ectothermic species like sea turtles and crocodilians.
Among mammals, all major Late Cretaceous lineages survived, though they experienced significant diversity losses. Only about 7% of mammal species, about 23, made it through this bottleneck.
Birds, direct descendants of avian dinosaurs, also survived. A small fraction of Cretaceous bird species navigated the catastrophe. These lineages gave rise to all modern birds, with four distinct groups—ostriches and their relatives, waterfowl, ground-living fowl, and Neoaves—persisting.
Reptiles such as crocodilians, turtles, snakes, and lizards continued their lineage. Amphibians like frogs and salamanders similarly pulled through.
Marine and freshwater environments also saw survivors. Many species of ray-finned fish, sharks, and coelacanths endured. Jawless fish like lampreys and hagfish persisted.
Invertebrates, including worms and insects, generally fared better than vertebrates. Flowering plants, or angiosperms, proved remarkably resilient. While some species went extinct, major lineages survived.
Traits of Resilience
Survival was not random; specific characteristics contributed to endurance. Small body size was a prominent factor; most tetrapods over 25 kilograms did not survive.
Smaller animals required less food and could more easily find shelter in the resource-scarce post-impact environment. Birds that survived were also relatively small, enabling them to navigate the altered landscape.
The capacity to seek shelter, particularly underground or underwater, proved crucial. Animals that could burrow or find refuge in protected environments were better shielded from immediate heat, fires, and atmospheric fallout.
Some early mammals may have had semi-aquatic or burrowing habits, offering additional protection. Many bird species also built burrows or nested in tree holes, providing important shelter.
A generalist or opportunistic diet was another trait. Organisms consuming varied food sources adapted better to drastic shifts in available resources.
Surviving mammals were often omnivorous or insectivorous, unlike specialized herbivores or carnivores that perished due to specific dietary needs. Beaked birds, with flexible diets, survived while toothed birds did not.
Freshwater habitats offered advantages, as freshwater fish species were less severely impacted than many marine or terrestrial counterparts. Some survivors, like sharks, possessed low metabolic rates, allowing them to endure extended periods without food.
The Path to Recovery
The K-Pg extinction event created an unprecedented ecological vacuum, allowing surviving lineages to diversify and flourish. The removal of dominant groups, particularly non-avian dinosaurs, opened vast ecological niches previously unavailable.
This absence set the stage for remarkable evolutionary radiation. Mammals, largely small and peripheral during the age of dinosaurs, underwent significant diversification.
They rapidly evolved to fill vacant roles, leading to a four-fold increase in species richness following the K-Pg boundary. Within approximately 700,000 years, some mammals grew substantially in body size, reaching up to 50 kilograms (110 pounds), a hundred-fold increase from their ancestors.
This increase was accompanied by an expansion of their ecological roles, leading to the rise of large mammals and diversification of groups like primates. Birds also experienced rapid evolutionary expansion.
Surviving ground-dwelling avian dinosaurs diversified into the immense variety of modern bird species. This radiation filled ecological niches left empty by extinct archaic birds, with many modern bird groups evolving as forests recovered.
The rapid proliferation of Neoaves coincides with the post-extinction period, highlighting adaptive opportunities. Flowering plants similarly capitalized on the altered environment.
Despite initial regional losses, major flowering plant lineages survived and diversified, ultimately becoming the dominant plant group on Earth. Their adaptability, including varied seed dispersal and pollination, contributed to their success in colonizing and reshaping terrestrial ecosystems.
The long-term recovery of biodiversity after the K-Pg event spanned several million years, with adaptive radiations filling newly available niches and fundamentally reshaping life on Earth into the forms we see today.