1. Class 8 : Geological ages, life on Earth, and mass extinctions Geological ages and some important geological terminology. Development of life on Earth. Introduce idea of mass extinctions.

2. I: Geologic Time Divided into Eons, Eras, Periods, and Epochs. E.g. We are currently in the Holocene Epoch of the Quaternary Period of the Cenozoic Era of the Phanerozoic Eon!… Smaller (regional) subdivisions possible, i.e. Ages and Chrons. Based on appearance (or disappearance) of fossils in rock, and/or radiometric dating.

4. II : Life on Earth Before 3.8 billion years ago, heavy bombardment continuously vaporized surface of oceans… Any emergent life would be wiped out. First life appeared 3.5-3.8 billion years ago. Simple, single-celled organisms (like modern bacteria). First multi-cellular life: 1 billion years ago. After that, things went quite quickly...

5. Life moved onto the land 570 million years ago – start of Cambrian period. Microbes first, then plants, then animals. Spread rapidly across the lands, adapting to different environments. Dinosaurs were walking the Earth by about 225 million years ago…

6. III : Dinosaurs

9. Most successful species to inhabit Earth. Amongst biggest animals to ever live. Adapted to most environments. Top of food chain. Then, 65 Myrs ago, they all died!

12. IV : The geological time-machine The geological record. Digging deeper into the Earth’s surface corresponds to going back in time… Older rocks/fossils are deeper. 65 million years ago, an event occurred Ended the Cretaceous Period (and Mesozoic Era). Started the Tertiary Period (and the Cenozoic Era – the current era). Called the K-T transition. A lot of evidence suggests that a 10 km asteroid hit the Earth at this point in time…

13. V : The K-T extinctions At the K-T boundary, 70% of all species became extinct. Was extinction sudden or gradual? Many geologists believe that dinosaurs were already starting to become extinct even before any possible asteroid impact. If so, impact may have helped extinction but was not the primary cause. But it’s very difficult to say whether extinction was gradual or sudden.

15. Ideally, if extinction were sharp, there would be a depth above which fossils suddenly disappeared. But several things can disrupt/blur the ideal time-ordering of strata… Bioturbation (sedimentary rock disturbed by tunneling animals). Reworking (sedimentary rock disturbed by non- biological factors – waves etc.) Erosion (can result in missing strata). Debate rages on… but the hypothesis of sudden extinction at KT boundary has yet to be refuted.