Lecture 28 – The End Cretaceous Event(s)

Q: What is the K-Pg boundary, and what was it formerly called?

The boundary between the Cretaceous and Paleogene periods (~66 Ma). It was formerly called the K-T (Cretaceous-Tertiary) boundary.

Q: Where is a key K-Pg boundary outcrop located?

Hell Creek Formation in Montana — dinosaurs are found below the boundary and none above it.

Q: What happened to pterosaurs at the K-Pg extinction?

All remaining pterosaurs went extinct. They had been declining in diversity during the Cretaceous, possibly due to competition from birds.

Q: Did birds survive the K-Pg extinction?

Most birds survived, though the majority of individuals likely died — species persisted as long as small populations remained. Hesperornithiforms (diving, toothed, flightless birds of the Western Interior Seaway) are an exception and went extinct.

Q: Which marine invertebrates went extinct at the K-Pg boundary?

All large ammonoids and belemnites, over 60% of scleractinian corals, innumerable marine microorganisms, and many primary producers.

Q: Which marine vertebrates went extinct at the K-Pg boundary?

All plesiosaurs and mosasaurs. Relatively few bony fish or sharks went extinct — large taxa were preferentially selected against. All six turtle families survived with relatively stable diversity.

Q: What happened to plants during the K-Pg extinction?

Major extinction at the species level, but no major plant families went extinct — larger taxonomic levels made it through.

Q: What happened to crocodylians at the K-Pg boundary?

Five of ten crocodylian families went extinct. Survivors were either semi-aquatic or possibly burrowers; strictly terrestrial crocodylians went extinct.

Q: What happened to non-avian dinosaurs at the K-Pg event?

All taxa went extinct, regardless of size.

Q: What happened to mammals at the K-Pg boundary?

All major groups survived and took over as apex land animals after the Cretaceous, but they experienced high mortality at the generic and species level.

Q: What are the two broad extinction hypotheses for the K-Pg event?

Sudden extinction (caused by a rapid/catastrophic event) and gradual extinction (response to environmental change).

Q: Why doesn't the fossil record convincingly support gradual dinosaur extinction?

Although the last occurrences of dinosaur taxa in Hell Creek appear staggered, there are large gaps in the fossil record with no fossils for a quarter of a million years. The gap before the extinction boundary is statistically insignificant, so no gradual decline is supported by the data.

Q: What are the Deccan Traps, and how do they relate to the K-Pg extinction?

Massive volcanic deposits in India. The Deccan Traps continued extruding volcanism before and after the extinction event, so they alone do not explain the sudden extinction.

Q: Who proposed the impact hypothesis, and what was the key evidence?

Alvarez et al. (1980). They found a thin clay layer at the K-Pg boundary enriched in iridium — a rare element on Earth but common in solar system rocks — suggesting a ~10 km diameter bolide impact.

Q: What did the Alvarez impact hypothesis predict?

The presence of shocked quartz (formed under extreme temperatures and pressures), balls of molten rock, tektites associated with the K-Pg layer, and an impactor ~10 km in diameter.

Q: What is the Chicxulub crater?

The ~180 km diameter impact crater in Mexico, formed by a ~10 km impactor that struck at a ~30° angle. It was not published until 1991 (Hildebrand et al.).

Q: How fast was the K-Pg impactor traveling, and how quickly did it cross the atmosphere?

About 40 km/s (Mach 116, or 144,000 km/hr). Since the atmosphere is ~40 km thick, it crossed in roughly 2 seconds at an angle.

Q: How does the sound energy of the K-Pg impact compare to Krakatoa (1883)?

An estimated 1 billion times greater than Krakatoa, which was itself the loudest noise recorded by humans (heard 3,000 km away, pressure wave circled the globe 7 times). At that magnitude, it was no longer audible sound but a wall of sonic energy that tears things apart.

Q: What seismic effects did the K-Pg impact cause?

Earthquakes greater than 11 on the Richter scale (the largest ever recorded by humans is 9.5), with aftershocks continuing for months or years. The impact site vaporized the ocean and exposed the mantle.

Q: What was the ejecta-driven heat pulse?

Billions of tonnes of rock were blasted into sub-orbital trajectories, then burned up on re-entry, releasing intense infrared radiation. The IR pulse lasted about 30 minutes at any given location and was sufficient to kill any exposed organism — but it did NOT ignite global fires.

Q: What is a survival bottleneck in the context of the K-Pg event?

A test or threshold a species must pass to survive. The K-Pg event involved three sequential bottlenecks: the initial impact, the IR pulse/debris fallout, and the subsequent ecosystem collapse.

Q: How could organisms survive the IR radiation pulse?

Only sheltering organisms survived — those that could burrow, swim, or find other shelter from the sky within minutes. This matches the pattern of K-Pg survivors.

Q: What happened to terrestrial ecosystems after the initial K-Pg impact event?

Remaining ecosystems collapsed. A decreased light regime and lower temperatures (still above freezing in most places) persisted for approximately 30 years. Decreased light inhibited photosynthesis. Organisms with low energy requirements survived — small body size and generalist habits were advantageous (disaster species).

Q: What happened to marine ecosystems after the initial K-Pg impact event?

Shallow oceans acidified (sulfur from impacted rocks combined with water to form sulfuric acid), primary productivity died, and lack of sunlight inhibited photosynthesis. Disaster species (small generalists) survived.

Q: How long did the K-Pg mass extinction take?

The initial extinction pulse may have taken seconds to days depending on location. Surviving groups then faced collapsing ecosystems over years to decades. It was geologically instantaneous — not gradual.

Q: Why did birds (avian dinosaurs) survive the K-Pg extinction?

Surviving early birds were small, ground-dwelling, beak-mouthed omnivores that ate seeds from the soil, so they were not reliant on trees and forest ecosystems that were wiped out.