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

• 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)
Initial evidence in favor of asteroid impact
Iridium anomaly
77
Ir
192.217
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?

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

• 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

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?
Marine
Percent of species extinct at K/T boundary
Continental
83% planktonic foraminifera
100% ammonites
93% marine reptiles
65% corals
100% non-bird dinosaurs
100% pterosaurs
56% other reptiles
10% flowering plants
All animals > 25 kg
Ammonite: extinct
Nautiloid: survived

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

How would volcanic eruptions lead to mass extinction?

How would volcanic eruptions lead to mass extinction?

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

65 mya Eocene
Paleocene
Oligo Pliocene
Oligocene present
Pleistocene
• 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?

Evidence of warming:

18 O and

13 C

Carbon and extinction?

• Habitat destruction
• Global warming, ocean acidification
Increase in diversity after K/T extinction

Fastovsky, David E. and Weishampel, David B. 2005. The evolution and extinction of the dinosaurs. Cambridge
University Press.
Hallam, Tony. 2005. Catastrophes and lesser calamities: the causes of mass extinctions. Oxford University Press.
Kolbert, Elizabeth. 2006. The darkening sea: what carbon emissions are doing to the ocean. New Yorker November 20,
2006. Excellent article: something more to worry about even if carbon dioxides doesn’t heat up the earth.
Taylor, Paul W. (ed). 2004. 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 17.21 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?