For over 160 million years, dinosaurs dominated Earth, thriving across diverse environments. Their long reign established them as one of the most successful groups of creatures in our planet’s history. However, around 66 million years ago, this era abruptly ended, leading to their sudden disappearance and dramatically reshaping life on Earth.
The K-Pg Extinction Event
The mass extinction event that marked the end of the dinosaurs is formally known as the Cretaceous-Paleogene (K-Pg) extinction event, previously called the Cretaceous-Tertiary (K-T) event. This catastrophic period occurred approximately 66 million years ago, bringing the Mesozoic Era to a close and ushering in the Cenozoic Era. It led to the demise of roughly three-quarters of all plant and animal species. Terrestrial species weighing more than 25 kilograms largely perished, alongside a wide range of marine life.
This global catastrophe affected all continents, causing widespread devastation across various ecosystems. While non-avian dinosaurs are the most recognized victims, many other groups, including pterosaurs, large marine reptiles like mosasaurs and plesiosaurs, and numerous plant species, also vanished. Researchers continue to study the precise mechanisms behind such widespread biological collapse.
The Asteroid Impact Theory
The prevailing scientific consensus attributes the K-Pg extinction primarily to a massive asteroid impact. This theory, proposed by Luis and Walter Alvarez in 1980, gained support from geological evidence. A key piece is a thin, global layer of sediment rich in iridium, an element rare in Earth’s crust but abundant in asteroids. This iridium anomaly is found worldwide at the K-Pg boundary.
Further supporting this theory is the Chicxulub crater, located beneath the Yucatán Peninsula in Mexico. This immense impact structure measures approximately 150 to 200 kilometers in diameter. It was formed by an asteroid estimated to be 10 to 15 kilometers wide, striking Earth with an explosive force equivalent to millions of hydrogen bombs. Rock cores extracted from the crater confirm its extraterrestrial origin and precise dating to 66 million years ago.
The impact generated immediate and widespread effects. The collision unleashed immense energy, causing earthquakes of magnitude 10-11 and triggering megatsunamis hundreds of meters high. Vast amounts of pulverized rock and vaporized asteroid material were ejected into the atmosphere, along with widespread wildfires ignited by superheated debris. Evidence includes globally distributed shocked quartz, deformed by extreme pressures, and tektites, small glass beads from rapidly cooled molten rock.
The injection of dust, aerosols, and soot into the atmosphere created a global cloud. This cloud blocked sunlight for months, potentially years, leading to “impact winter.” Prolonged darkness and a drastic drop in global temperatures halted photosynthesis, collapsing plant life and dependent food chains. Acid rain further damaged ecosystems. These immediate and lingering environmental changes overwhelmed Earth’s ecosystems, leading to the rapid extinction of vulnerable species, including non-avian dinosaurs.
Volcanic Activity and Other Contributing Factors
While the asteroid impact is widely accepted as the primary cause, other geological events and environmental shifts may have played a role in the K-Pg extinction or exacerbated its effects. The Deccan Traps, a vast volcanic province in western India, experienced massive eruptions around the same time as the asteroid impact. These eruptions, which created layers of solidified lava kilometers thick, released large quantities of gases such as carbon dioxide and sulfur dioxide into the atmosphere.
Such prolonged volcanic activity could have altered the global climate. Initial eruptions may have caused greenhouse warming due to carbon dioxide, while later sulfur dioxide releases could have led to atmospheric cooling and acid rain. Some researchers suggest this intense volcanism may have already been stressing ecosystems prior to the asteroid impact, making life more susceptible to a sudden event. However, recent studies indicate the most intense volcanic activity in the Deccan Traps occurred somewhat before or after the K-Pg boundary, with climate modeling favoring the asteroid impact as the dominant factor for the extinction.
Other environmental changes during the Late Cretaceous period also contributed to ecosystem vulnerability. These include sea level fluctuations, such as the Maastrichtian Regression, which involved a global drop in sea level. Such changes reduced continental shelf areas, affecting marine biodiversity. Pre-existing climate shifts, including oceanic temperature variations, also occurred. While these factors alone may not have caused a mass extinction of this scale, they represent a backdrop of environmental instability that could have influenced the K-Pg event’s severity.
The Aftermath and Legacy
The K-Pg extinction event reshaped the trajectory of life on Earth. Following the catastrophe, the planet entered a period of ecological recovery, with new species diversifying into vacant niches. Life forms that survived shared common traits offering resilience to extreme environmental upheaval. These survivors included smaller animals like certain mammals, birds (direct descendants of avian dinosaurs), amphibians, and some reptiles such as crocodiles, snakes, and lizards. Fish and many marine invertebrates also largely persisted.
Their smaller size meant lower food requirements and easier shelter, often by burrowing or seeking aquatic refuge. Many had adaptable diets, allowing them to subsist on alternative food sources when primary food chains collapsed. Birds, with their ability to fly, could escape localized disasters and access new food sources. Some early mammals were semi-aquatic or burrowing, providing protection from immediate environmental stresses.
The absence of large dinosaurs created unprecedented ecological opportunities. Mammals, previously small and often nocturnal, underwent rapid diversification in the Paleogene Period following the extinction. They quickly filled ecological roles once occupied by dinosaurs, evolving into larger body sizes and diverse habitats. This led to the vast array of mammalian species seen today, including humans. The K-Pg event fundamentally altered Earth’s biosphere, setting the stage for the age of mammals.