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CHEMISTRY · Stage 9 · Chapter 13: Planet Earth and Continental Drift

Chemistry 13.2 Evidence from fossils — Mesosaurus, Glossopteris and Rock Strata

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13.2 Evidence from fossils — Mesosaurus, Glossopteris and Rock Strata — Alhussan International Academy — Chemistry (Stages 7–9) notes: Use fossil and rock evidence to support the idea that continents were once joined and that Antarctica once had a warmer climate.Fossils = the pre…

  • Importance
  • Scott's expedition
  • The fossils
  • Same species elsewhere
  • Lystrosaurus
  • Cynognathus

Part 1 of 6What fossils are

  • Fossils = the preserved remains or traces of organisms that lived long ago, usually found in sedimentary rock.
  • Importance — they show what lived where and when.
    • If the same kind of fossil is found on continents now separated by thousands of kilometres of ocean, scientists must explain how it got there.
  • Wegener — Alfred Wegener used fossil evidence in 1912 to support his hypothesis that the continents were once a single piece of land that broke up and drifted apart.

Part 2 of 6Evidence from trees: Glossopteris and Antarctica

1Scott's expedition1 point
— in 1912 Robert Falcon Scott's team, returning from the South Pole, collected heavy fossils from a rocky area and kept them.
  • A rescue team found the fossils nine months later next to the explorers' frozen bodies.
2The fossils1 point
— one was a piece of coal with beautifully traced leaves in layers and well-preserved impressions of thick stems.
  • They are remains of an extinct tree called Glossopteris, which may have grown 30 m tall.
3Climate1 point
— trees do not grow on Antarctica today because it is too cold around the South Pole.
  • The fossilised leaves are evidence that the land now called Antarctica once had a warmer climate, so the continent must have been further north, away from the South Pole.
4Same species elsewhere1 point
— Glossopteris fossils had already been found in Africa, Australia, India and South America, which suggests the continents were once joined together.
  • Its seeds were heavy and could not have been blown across oceans.

Part 3 of 6Evidence from animals

1Mesosaurus1 point
— a small freshwater reptile about one metre long, whose fossils are found in South America and southern Africa in rocks of the same age, around 270 million years old.
  • It could not have swum across the salty Atlantic Ocean, which is thousands of kilometres wide.
2Lystrosaurus1 point
— an animal with just two teeth that used its beak to tear leaves from plants.
  • Its fossils have been found in Africa, India, Antarctica, China and Mongolia.
3Cynognathus
— its fossils are found in South America and Africa.
4Joined land mass
— the simplest explanation is that the continents formed a single land mass when the organisms lived, and the land later moved apart.

Part 4 of 6Rock strata and other matching evidence

  • Rock layers (strata)layers of the same type, age and sequence are found on opposite sides of the Atlantic, as if a single layered rock had been cut.
  • Mountain chainsthe Appalachians in North America and the Caledonian mountains in Scotland and Norway line up when the continents are placed together.
  • Glaciersevidence of past glaciers on southern continents, now in warm climates, fits together when the continents are joined near the south pole.
  • CoastlinesSouth America and Africa fit like jigsaw pieces, especially when the edges of the continental shelves are used.

Part 5 of 6Interpreting a fossil map

  • Patternshaded areas where each species is found touch when the continents are joined, such as Mesosaurus in a band across southern Africa and eastern South America.
  • Explanationa freshwater animal could not swim the ocean, so a joined land mass is the best explanation.
  • Combining evidencefossils of two species can show two continents were joined, for example India and Africa.

Part 6 of 6Evaluating the evidence

  • Land bridges — critics of Wegener suggested land bridges once linked the continents and later sank, but there is no evidence for such bridges on the sea floor.
  • Not enough alone — fossil evidence supports continental drift but is not sufficient by itself.
  • Mechanism — Wegener could not explain what force moves continents, so many scientists rejected his idea at first.
    • Stronger evidence came later from the sea floor and from measurements of plate motion.
  • Wording — evidence supports a hypothesis rather than proves it.

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