Rock Cycle - Past Life (2B)

  • Discovering the components of a fossiliferous rock.
  • Learning how fossils are made.
  • bivalves
  • fossil
  • gastropod
  • mollusk

Students examine rocks containing fossils.

An example of a Mesozoic sea floor community


Organisms have changed through time. Scientists call this documented change with time evolution. Evolution is a non-reversible process, for instance, we will never have dinosaurs again; they and many other organisms that roamed the earth eons ago are now extinct. The details of how evolution takes place are still under study, but the basic mechanism is well tested. Present day evolutionary theories are based not only on data from living organisms, but also from the remains of organisms in layers of rock.

The probability that an organism will be preserved as a fossil is low. The critters of long ago, could not go to a "Paleo-Photo Shop" to take a picture that we can use trace their ancestries. Also, consider that once an organism becomes a fossil, it just does not "hang around" for someone to find it. Geological processes such as erosion, weathering, sedimentation, leaching and many more also constantly "attack" the fossil, and may destroy the fossil before anyone sees it.

Children sometimes get the impression that fossils look exactly like the animal when it was alive. However, most fossils are not well preserved. In this lab the students will compare present day specimens with fossil specimens, and see if they can predict which present day organisms might be found in the fossil record.

  1. Have students point out some the key characteristics of the modern organism specimens. List their observations on the board, and record the information as a class. Note that gastropods or snails have one shell that spirals, bivalves (to which the clam and scallop both belong) have two equal shells. Have the students color the clam, scallop, and gastropod on their lab sheets. Have students use a magnifying glass to look at samples.
  2. After you discuss the characteristics of the living organisms (Mollusk: Turritella, Rapana, Moon snail, Abalone, Bivalve), have students look at the fossil sample. See if they can distinguish any characteristics that might help them decide if what they are looking at is a bivalve or gastropod. Make sure the students look at the samples very closely. Observational skills are very important in the study of fossils.

    The students will discover that the fossils do not hold as much information about the organisms as their modern day counterparts. The fossils show less detail, and maybe broken or partially enclosed in rock.

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