- K-Pg
The prevailing consensus among scientists is that the Cretaceous‑Paleogene (K‑Pg) extinction was a compound disaster initiated by an extraterrestrial impact and amplified by terrestrial volcanism.
1. The Chicxulub Impact
The most compelling evidence points to a roughly 10‑kilometre asteroid striking the Yucatán Peninsula in present‑day Mexico.
- Iridium anomaly – Iridium is scarce in Earth’s crust but abundant in meteorites. Layers marking the K‑Pg boundary worldwide contain unusually high iridium concentrations, a classic fingerprint of an impact event.
- Shock‑produced quartz and microspherules – The same strata hold quartz grains that show planar deformation features, a hallmark of the extreme pressures generated during an impact, along with tiny glassy spheres formed from molten ejecta.
2. The Deccan Traps Volcanism
Simultaneous with the impact, the Deccan Traps in India erupted massive volumes of basaltic lava.
- Synergistic stress – Prolonged volcanic activity released vast amounts of CO₂ and SO₂, driving rapid climate swings and ocean acidification. Many organisms were already stressed before the asteroid strike, lowering their resilience to the subsequent shock.
Cascading Environmental Collapse
The K‑Pg event was not a single instant of death but a series of interlinked catastrophes that unfolded over months and years.
- Immediate thermal pulse – The impact vaporised rock and sent molten material skyward. As this material re‑entered the atmosphere, it produced intense infrared radiation that ignited widespread wildfires.
- Impact winter – Sulfate aerosols and dust lofted into the stratosphere blocked sunlight, plunging the planet into a prolonged cold spell that halted photosynthesis.
- Food‑web breakdown
- Primary producers – Terrestrial plants and marine phytoplankton suffered massive die‑offs from light deprivation.
- Herbivores – Large dinosaurs, reliant on vegetation, collapsed first, which in turn starved apex predators such as Tyrannosaurus.
- Ocean acidification – Sulfur compounds entered the seas, lowering pH and devastating calcifying organisms like ammonites and many reef builders.
Selective Pressures and the Survivors
The extinction demonstrates a classic case of selective filtering: survival depended less on sheer size or strength and more on physiological and ecological flexibility.
Groups that Perished
- Non‑avian dinosaurs – Their large body mass, high metabolic demands, and lack of protective refuges made them vulnerable.
- Pterosaurs – With a single aerial niche, they could not adapt to the loss of vegetation and the ensuing climatic changes.
- Ammonites – Their planktonic larvae were especially sensitive to ocean chemistry shifts.
Groups that Endured
- Small mammals – Their diminutive size, low metabolic rates, and burrowing habits shielded them from surface extremes.
- Birds – Flight and diverse diets (including seed‑eating) allowed many to survive the vegetative collapse.
- Crocodilians and turtles – Semi‑aquatic lifestyles and slow metabolisms provided a buffer against the harsh conditions.
Macro‑Evolutionary Consequences: Re‑allocation of Niches
By eliminating the dominant reptilian lineages, the K‑Pg event opened a vast array of ecological opportunities.
- Rise of mammals – With the apex predators gone, mammals diversified rapidly into herbivorous, carnivorous, and aquatic forms, setting the stage for later mammalian dominance.
- Bird diversification – Freed from dinosaur competition, birds expanded into new niches, eventually giving rise to the modern avian diversity.
- Accelerated speciation – The sudden release of ecological space accelerated evolutionary rates, leading to a swift rebound in biodiversity during the Paleogene.
Final Thoughts
The K‑Pg extinction is a textbook example of how a single, planet‑wide shock can reshape life’s trajectory. It underscores that opportunity, not just power, drives evolutionary success. By wiping out a 160‑million‑year‑old era, it paved the way for the rise of mammals and birds, ultimately shaping the world we inhabit today.