Earth 125 1st Edition Lecture 22 Outline of Last Lecture I. Human EvolutionII. The K/T eventOutline of Current Lecture II. Impacts:a. Other impacts/extinctionsIII. Biological causes (Examples)a. Great American InterchangeCurrent LectureI. Quiz Review:1. Why did we use the Moon rather than the Earth data?i. Impact-caused craters not eroded because a) no atmosphere, b) no plate tectonics2. Do you think the impact data from the Moon underestimate or overestimate the history of impacts on the Earth? Give reasons why.i. Underestimate: size: smaller cross-sectional area, lower gravity superposition: younger craters can cover smaller cratersii. Overestimate: no atmosphere: even small objects reach surfaceII. Impacts1. How long (on average) do impacts of different sizes affect the Earth?2. Could extraterrestrial bolides be cause of extinctions in general?i. Compelling story to tellii. Permian-Triassic1. Smaller crater (44km diameter)2. Could not have been sole cause of extinction3. Other impacts from Australia and Antarctica that hint toward extinctioniii. Manicouagan, Canada – 100km crater1. Impressive size, 214 Myr 2. Around time of Triassic-Jurassic extinction3.iv. Evidence around Chesapeake Bay1. Shocked quartz, tektites from core drilled from the eventv. Chicxulub: largest impact (K/T)1. Bedout, Manicouagan, Chesapeake Bay3. Question to think about: i. You’ve probably never heard about any person dying from impacts, but you’ve read about shark attacksii. Given the above, how can the odds of dying from impacts be greater than from shark attacks?1. Odds are 1 in 3,000 for death from asteroid/comet impact2. Odds are 1 in 8,000,000 for death from shark attack4. “Impacts are a general cause of extinctions?”i. Remains highly controversialii. Problems that must be addressed:1. Were extinctions sudden or drawn out?2. What is the size/age of impacts?III. Terrestrial causes of extinction:1. Double-wedge: competition as a cause of extinctioni. Case 1: Competition is not a compelling extinction in this case?1. But is there any evidence of competition as cause of extinction?IV. Case 2: The Great American Interchange (GAI)1. If competition is cause, expect a pattern of replacement 2. Back to time of Pangaea – all continents are connected (170 Ma)i. Continents (North and South America) get farther and farther away from each otherii. Deep ocean between continents1. Between 10 million years ago and today, the isthmus of Panama formsa. Land-bridge3. Two groups of organisms occupying two parts of globei. Bridge forms1. Allows for interchange of faunas2. S. American marsupials, N. American placentalsa. 50 my of evolution between themii. Was the exchange of these animals symmetrical?4. Look specifically at ungulatesi. Looking only what happens in South America as we go from 5 million years, from formation of bridge, to today.ii.1. Not a single ungulate from North America, 5 mya, was found inSouth America2. At 3 million years, the number of ungulates from S. America has decreased by 1 (from 13 to 12) and increased to 1 in N. America3. Number of ungulates native to S. America decreases, while number of immigrants from N. America increasesa. One group succeeds (N. American) even though ecological environments are strikingly similar5. Before GAI: S. American and N. America have 2 distinct and very different faunasi. After GAI: Two faunas became much more similar as the N. American fauna essentially replaced the S. American fauna in S. America while in N. America, the S. American fauna had little impact V. Extinction Earth-Bound Causes1. Predation or the human impact on extinctioni. Case 3: Megafaunal extinctions (Pleistocene – last 100ky)1. Major events:a. Spread of modern humansb. Dramatic climatic changes