Week 7: K/Pg Mass Extinction

George Cuvier and Catastrophism (early 19th Century)

• Explained the patterns of extinction and faunal turnover he witnessed in the fossil record as new life forms moving in from other areas after local floods

• Lost popularity with the subsequent prevailing view of uniformitarianism and gradualism, especially following work by Charles Lyell (1830s onwards)

What are the 5 phanerozoic mass extinctions identified by Sepkoski

1. Late Ordovician

2. Late Devonian

3. End Permian

4. Late Triassic

5. End Cretaceous

When was the K/Pg boundary

~ 66 Ma

What was the Late Cretaceous World like?

• greenhouse world but long term global cooling trend from ~90 Ma

• Ice-free poles

• High sea levels

• dinosaurs in antarctica + high arctic

• epicontinental seaways

• Global sea level regression at the Campanian/Maastrichtian boundary

  Fluctuations in the middle Maastrichtian, and declines in the latest Maastrichtian that continued across the K/Pg boundary

How many species went extinct?

75% species loss

• paraphyletic extinction of the dinosaurs

• pterosaurs, marine reptiles, ammonoids, belemnites

(Incorrect) Extinction Theories

• High CO2 levels destroyed dinosaur embryos

• Climate too cold/wet/dry/hot

• Caterpillars ate all of the plants

• Cataract blindness

• Disease epidemic

• Over-predation

• Aliens

→ theory needs to explain the extinction of diverse life forms on land and in the oceans

The Iridium Layer

• 1cm−thin clay layer at the K/Pg boundary in Gubbio, northern Italy, studied in 1976

• Large Iridium anomaly discovered

• Only (some) meteorites have iridium levels orders of magnitude more abundant than terrestrial sediments

Where is the crater?

Chicxulub crater, Yucatan peninsula of Mexico

~180 to 200 km diameter crater

Evidence for the Chicxulub Impact: Shocked Quartz

• ‘Shocked’ quartz grains found at Chicxulub crater and in ejecta

• Under intense pressure e.g nuclear bomb testing→ crystalline structure of quartz deformed long planes

Evidence for Chicxulub Impact: Tektites

• found in the surrounding area

• formed when rocks are instantaneously melted and ejected out of impact sites in the form of molten glass

• Cooled while spinning through the air

Evidence for Chicxulub Impact: Mega-tsunamis

• Tsunami-deposits found in surrounding coastal regions- e.g in Hawaii

• Estimated to have been 100 m in height, and may have reached 300 km inland

Evidence for Chicxulub Impact: Ejecta layer

• Decreasing ejecta- layer thickness with increasing distance from the impact

What do we know about the asteroid?

• Asteroid was ~10 km in diameter

• Steep impact angle (45–60°)

• Impact velocity of ~20 km/sec

• increased understanding from drilling the impact crater

Short term effects of the impact

• Global heat pulse that perhaps ignited large wildfires near the impact site, producing vast quantities of soot

• Impact occurred in a sulphate rich region

• Massive quantities of soot, sulphur, and other aerosols released into the atmosphere

Long term effects of the impact

• Aerosols would have trapped much of the long-wave solar radiation

• Resulting in Earth cooling by several to tens of degrees Celsius for years following the initial heat pulse

• “Impact winter” scenario

• Alvarez’s “dust cloud” scenario, blocking sunlight for a year – depressing photosynthesis- destroy food chain in terrestrial + marine realm

What are the deccan traps?

• large igneous province

• volcanic deposits in present-day India

• Multiple layers of flood basalts

• Erupted during the latest Maastrichtian and into the Paleocene

• 1.1 million km3 of basalt erupted in ~750,000 years

• Estimated to have covered 1.5 million km², approximately half the size of India

• Several phases of eruption, with the largest phase well before the K/Pg

• Produced large amounts of SO2 (cooling), but particularly CO2 (warming)

Chicxulub vs. Deccan as the kill mechanism

• Some have suggested both contributed

• Could Chicxulub have triggered increased volcanism through via impact-induced seismic energy?!

• Recent studies in several disciplines argue strongly for the asteroid as the sole kill-mechanism

• Deccan-induced warming might actually have reduced severity of extinction!

Which surviving groups suffered heavy losses?

• Sharks

• bony fishes

• insects

• bivalves

• forams

• plants

What are disaster taxa?

• Opportunistic species that rapidly become abundant in a wider range of habitats after a biotic crisis than before

Examples of disaster taxa

• the foram Guembelitria

• Ferns → have a higher environmental tolerance than most other plants

• can see similar patterns in present day e.g after eruption of Mount St. Helens

Effect of extinction on food chains

• For marine phytoplankton, suppression of photosynthesis was likely the major killing mechanism

• Loss of diverse vegetation on land would have led to destruction of diverse forest communities

• Knock-on food-chain effects in both cases

• Detritus-based food chains (e.g. in lakes) were seemingly less affected

Extinction Selectivity

What survived?

• freshwater amphibians

• small-bodied, non-picky eaters (generalits)

• animals who can live in multiple environments e.g marine crocs

• wide distribution (50:50)

What died?

• fully terrestrial, large-bodied species

Why did most birds survive but dinosaurs didn’t? (Key to small-bodied dinosaur extinction selectivity)

• Small body size probably a key reason why birds, but not other dinosaurs survived but, many non-avian dinosaurs were also small and some feathered, and many birds did go extinct

• Analyses of bone growth-curves via histology indicate that nonavian dinosaurs & basal birds were likely endothermic (‘warmblooded’)

• But they took longer than modern birds to reach sexual maturity

• Likely required greater resources than ectotherms, modern birds, or small mammals