Let us now see whether the several facts and laws relating to the geological succession of organic beings accord best with the common view of the immutability of species, or with that of their slow and gradual modification, through variation and natural selection. In this episode we explore Chapter XI that deals with the fossil record and how patterns in the fossil record fit with Darwin's model of descent with modification. We begin the discussion with Darwin's views on extinction and how this phenomenon is part of his model of evolution. Sarah introduced the idea that extinction occurs at two different rates - normal background rate of extinction where species themselves have a measured lifespan based on how long they exist in the fossil record. Current analysis of the fossil record indicates that many species persist for a period of time and then go extinct. From this  data scientists calculate an average lifespan for a species - That is, how long in the geological record does a species typically persist? Many authors have written about this idea and many of their findings have been summarized in Extinction Rates, a book edited by J. Lawton, and R. May (1995). Generally species lifespans are varied and biased by the size of the organisms. Small unicellular marine dinoflagellates have species "lifespans" in the 10's million years whereas large multicellular Cenozoic animals had a species 'lifespan" of 1-2 million years. These normal background rates of extinctions in the fossil record are punctuated by five massive extinction events where 75%-96% of all species went extinct. Sarah mentioned the dramatic "soot' line that demarcated the mass extinction event called the K-Pg (formally called the K-T) extinction event that occurred 66 million years ago. This extinction event was correlated with a thin layer of metal iridium that is visually and chemically distinct from rock material layers above and below the line. This layer was now thought to be have been created when a small asteroid impacted the earth near the Yucatan peninsula contributing to massive habitat destruction and climate change. These massive changes in climate are thought to have contributed to the extinction of all non-avian dinosaurs. image from http://waynesword.palomar.edu/images2/KT1c.jpg Sarah recommended a book on extinctions by Peter Ward called Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future (Harper 2008). Affinities of past to current organisms Let us now look to the mutual affinities of extinct and living species. All fall into a few grand classes; and this fact is at once explained on the principle of descent. We discussed how many of the organisms in the fossil record can be directly related to species we see today - the wide array of extinct elephants themselves are a dramatic example of extinct forms being related to extant forms today. image from http://img13.deviantart.net/1309/i/2015/120/9/7/elephants_by_fotostomias-d34pdf2.jpg James spent some time explaining the background story of the archeopteryx fossil that was found in 1861 in Germany just after Darwin published Origin of Species. Photograph: Jason Edwards/Getty Images On the photo above notice the fingers on the wing and the long bony tail that extends off of the pelvis region. Museums are now creating displays that highlight the feathered dinosaurs that have been found in the past 10 years.  In 2016 the American Museum of Natural History opened a display that highlighted many new feathered dinosaurs. http://www.nytimes.com/2016/03/18/arts/design/fine-feathered-fiends-dinosaurs-among-us-at-the-american-museum-of-natural-history.html The relationship between organisms we see today to extinct versions in the fossil record is one of the stronger pieces of evidence that supports Darwin's model of descent with modification. In the next episode we will discuss how this relationship between extinct and extant organi