Rock Cycle - Past Life (6)
Post Lab 

  • Discovering how paleontologists observe evolution.
  • Exploring how changes in fossil morphology help geologists to age date rocks
  • evolution
  • molar
  • ungulate

Students use a worksheet to study extinct horses.

A bee preserved by carbonization


The "present is the key to the past," refers to an important principle in paleontology. Paleontologists infer that the way evolution works today is the same as it worked in the past history of the earth. Evolution, in paleontological terms, is the change in the morphology of fossils through time. The presence and extinction of small organisms, especially the invertebrates and protozoa, show many examples of evolution. The larger the organism, the harder it is to see evolution. For instance, complete invertebrate skeletons are frequently preserved, but complete dinosaur skeletons are virtually nonexistent. For this reason (among many others), human evolution is not as well documented as the many other organisms that lived on Earth.
One of the best known examples of change through time of a vertebrate is the evolution of the horse. The oldest known horse is found in both North American and Europe. According to the fossil record, the evolution to the modern horse consisted of a variety of changes, including an overall increase in size, development of a single hoof from several toes, development of large molar teeth for grazing, and an increase in brain size. Most of these changes were caused by changes in the environment where the ancestral horses lived. For example, the molar teeth grew larger in order to chew grass, which is a very tough plant food. Horse evolution was not a linear trend, but more like a branching tree; many types of horse evolved and went extinct before the appearance of modern horses.

  1. The worksheeton the next page shows the skeleton of a Mesohippus, an early form of horse that lived during Oligocene Epoch (about 38-26 million years ago). Have the students recreate what they think Mesohippus might have looked like, using the bones as a guide. Do not tell the students it is a horse, because they will not think about what the bones are telling them.
  2. Paleontologists have found these fossils. The Hyracotherium (commonly known as Eohippus, the dawn horse) is found below Mesohippus, below Merychippus, and so on. Point out to students the development of the brain and the hoofs. Paleontologists can only interpret the record of their fossils, which leads them to believe that horses have evolved through the last 50 million years.
  3. The pdf worksheet has more information on the evolution of the horse and has students determine which genus the bones represent.
  4. Explain to the students that horses are just one of many fossil examples that paleontologists have used to demonstrate a slow, but constant change of life on the Earth. This change is called evolution.

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