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Course: Science — Third Preparatory
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Science — Third Preparatory

Science — Third Preparatory

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Science — Third Preparatory 2026/2027

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Unit 1 - Force, Motion, Acceleration and Physical Quantities

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Text lesson

Unit 4 — Dating Planet Earth

Science — Third Preparatory

Unit 4 — Dating Planet Earth

1.3 Types of fossils

1.3 Types of fossils

(a) Body fossils

These include parts of the organism’s body→bones, teeth, shells, tree trunks, leaves, or a whole organism, such as an insect trapped in amber or a mammoth preserved in ice.
  • These do not include the body itself, but signs of its activity: footprints, tracks, burrows, or fossil droppings (coprolites).
  • They show the organism’s behaviour and feeding habits (for example, how it moved or what it ate).
Comparison aspectMain ideaExampleKeywords
Moldan impression of the outside surface of an organism preserved in rock. When an organism (
  • a shell) is buried and later dissolves or decays away
  • it leaves a hollow in the hardened sediment with the shape of its outer surface
impressionoutsidesurfaceorganism
Castthe filling of that hollow (or of the inner cavity of a shell) with mineral solutions
  • silica or calcite
  • or with sand and clay that harden over time. The cast has the shape of the inside (or the original form) of the organism. Example: mold and cast of ammonites
fillinghollowinnercavity
  • You can make a model of this press a shell into soft clay and remove it; the impression is a mold (model A).
  • Pour plaster into the impression, let it harden, and lift it out: it is a cast (model B).
  • If an organism stays buried for millions of years, minerals such as silica or calcium carbonate gradually replace the original organic matter in its tissues, and the remains turn into stone.
  • Example the petrified tree trunks of the Petrified Forest east of Maadi, in Cairo Governorate.
Type What is preserved Example
Body fossil Real body parts (or whole body) Mammoth in ice, insect in amber, bones
Trace fossil Signs of activity Dinosaur footprints, burrows, coprolites
Mold Impression of outer surface Ammonite shell imprint
Cast Material that filled the mold Ammonite cast
Petrified Tissue replaced by minerals Petrified tree trunks
  • Geological Museum in Cairo.
  • It contains igneous, metamorphic and sedimentary rock samples and fossils of animals and plants from different periods, including a complete ancient whale skeleton, a Moon rock and a Martian meteorite.
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