Extinction Outline Fossils continued: radioisotope dating & calibration Cambrian explosion Patterns in the fossil record Extinction mass extinctions Cretaceous extinction Permian extinction Next week: human evolution, evolution of disease

The story so far... Earth forms First life soon thereafter (ideas, little good evidence) Multicellular organisms much later – needed atmospheric oxygen? Tremendous increase in animal diversity in Cambrian Fossils give us a biased, incomplete picture of the history of life

Extinctions through time: percent of families lost End Permian K/T boundary background extinction

The Cretaceous-Triassic Extinction (K/T) Iridium: rare on earth, more common in comets and asteroids Also, more common in magma Or, from nearby supernova Source: massive volcanic eruptions? Or impact? Initial evidence in favor of asteroid impact Iridium anomaly 77 Ir

If impact, what consequences (and what evidence might be found?) Evidence of impact? Why would this cause extinctions?

Evidence for impact Iridium anomaly found worldwide at K-T boundary Shocked quartz in Caribbean and North America Evidence for tsunami in North America Crater in Yucatan Cr isotopes found not volcanic

Evidence for impact: Ir anomaly Hallam Fig 8.3

Evidence for impact: tsunami deposits Fastovksy and Weishampel Fig 18.6

How does an asteroid cause extinction? Initial effects (first hour to day): Middle-term effects Long-term effects

Were the dinosaurs already dying out? (Or did an asteroid kill off a dying group?) Difficulties of answering!

No loss of diversity until boundary: instaneous death (at least by geological time) Fastovksy and Weishampel Fig 18.12

Who survived? Who went extinct? MarineContinental 83% planktonic foraminifera100% non-bird dinosaurs 100% ammonites100% pterosaurs 93% marine reptiles56% other reptiles 65% corals10% flowering plants All animals > 25 kg Percent of species extinct at K/T boundary Ammonite: extinct Nautiloid: survived

One other great extinction: the end of the Permian Cause?

Flood basalts

How would volcanic eruptions lead to mass extinction?

... yet? –methane (CH 4 ): very potent greenhouse gas [23x as potent as carbon dioxide]

Oligo Methane and Eocene Clathrates: frozen water surrounding methane Estimate 500 – 2,500 gigatons (=10 12 kg) If released, could lead to substantial global warming 55 mya: a thaw leading to methane release? Paleocene Eocene Oligocene Pliocene Pleistocene 65 myapresent

Evidence of warming:  18 O and  13 C

Possible Permian Scenario

Carbon and extinction?

The next mass extinction? Habitat destruction Global warming, ocean acidification Increase in diversity after K/T extinction

References Fastovsky, David E. and Weishampel, David B The evolution and extinction of the dinosaurs. Cambridge University Press. Hallam, Tony Catastrophes and lesser calamities: the causes of mass extinctions. Oxford University Press. Kolbert, Elizabeth The darkening sea: what carbon emissions are doing to the ocean. New Yorker November 20, Excellent article: something more to worry about even if carbon dioxides doesn’t heat up the earth. Taylor, Paul W. (ed) Extinctions in the history of life. Cambridge University Press.

Study questions 1.If the last fossil is dated to 70 million years ago, can we conclude that it went extinct at that time? Why or why not? 2.Figure shows the pattern of extinctions for all organisms. How might the data presented here be misleading? (Think about the biases discussed in topic 13). 3.How would an asteroid impact lead to extinctions of marine organisms? Of terrestrial organisms? 4.How could global warming have led to a mass extinction at the end of the Permian? 5.Ammonites and nautiloids are both molluscs with chambered shells. Ammonites went extinct at the end of the Cretaceous while nautiloids survived. Ammonites produce free-swimming young that feed at the surface and grow rapidly while nautiloids produce a few large eggs that may sit for up to a year in the deep before growing slowly. How might these differences explain the nautiloids survival while the ammonistes went extinct?

Study questions 6. Why are duplications of Hox genes thought to be crucial for the development of bilaterally symmetric animals?