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Lecture 16

Lecture 16 - Earth's Oldest Fossils (Includes Essay Pointers)

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University of Toronto St. George
Earth Sciences

GLG 105 – Lecture 16 - November 16 th Essay Pointers: specific questions – objective is to not take in all details, but specific information, the two mars papers are telling the same story, so combine them to summarize them. Comparing them: brainstorm what are obvious differences: who published, who read, discourse? General questions. Similarities also. What is a typical problem when you think about the specific studies? ** The last paragraph won’t be the same for all of them Last Class: How did Steno perceive fossils? – he thought shark teeth fossils were snake tongues – first testable hypothesis How does the definition of species differ between biologists and paleontologists? – Earth’s Oldest Fossils 1. Evolution of life in the Phanerozoic Palaeozoic: Beginning of Cambrian: Lifeforms suddenly appear – invertebrates Fishamphibiansreptiles - progression from ocean onto land Life conquers land (spore plants) Mesozoic: time of dinosaurs, first birds and mammals, first flowering plants – first time there were a lot of colours (birds, flowers, dinosaurs, insects) Cenozoic: time of mammals (mammoths, horses, etc.) rise of humans The landscape would look very different if you could go back in time Typical late 19 Century chart: Phanerozoic: evolution of life - start off as simpler bottom plants, to more complicated with ones that we are more familiar with - older than Cambrian had no life 2. Darwin’s Dilemma - he argued against the Cambrian had no life because evolution takes long time - can we find something that is earlier than the Cambrian? John William Dawson – Nova Scotia – 1864- rocks were brought to him that showed green and white layers gathered from Precambrian rocks along Ottawa river – Dawson identified them as huge foraminifera (single-celled organisms with shell) Scientific name Eozoon Canadense “dawn animal of Canada” Fossil that would put Canada on the map Criticism – 1866 – King & Rowney – mineral not biological structure 1879 – Moebius – did not detect features of foraminifera 1894 – Gregory & Johnston-Lavis – discovered in young limestone blocks ejected from Vesuvius – so Eozoon is actually metamorphic rock (mineral) Charles Dolittle Walcott – 1850-1927 – 1876 0 interepreted rocks in NY as stromatolites 1883 – named “Precambrian” 1899 – names mm-sized black coaly disks in Precambrian shales in Grand Canyon as Chuaria “reminas of compressed conical shells” (now known to be a single-celled planktonic algae) Pushed the age of the Precambrian Back 1931 – textbook “Plant life through the ages” Albert Seward 1946 – saucer-sized jellyfish like fossils found in Australia – - pushed multicelled animals earlier 100-200Mys into the Precambrian you see so many fossils in the Cambrian because of hard shells, Precambrian had soft shells Why is it so hard to find the oldest fossils? - usually life doesn’t fossilize (eaten or decayed) - most life doesn’t die of natural causes - microbes are tiny, lack hard parts, are easily crushed - old rocks are very rare (cycle of uplift, erosion, mountain building) What should we look for? - old and little metamorphosed rocks - age of rock well known - rocks that have not been disturbed (not involved in mountain building) - black (carbon-rich) stromatolites - numerous specimens to make us more confident - not just one species, but a range of species, to show that there was an ecosystem at the time Beware of: Contaminents (even in the lab) Pseudofossils (like Eozoon) Stanley Tytler (1906-1963) - 195
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