Week 13 · Unit 4 Dating Planet Earth, opener; Lesson 1 The Fossil Record: how fossils form and their types, the fossil record, mass extinction, evolutionary relationships, past environments, why life forms differ, comparative anatomy · 3 lessons
1
How fossils form, and the kinds of fossil
Science Prep 3 Term 1, Unit 4: pp. 84–94
GoalExplain how fossils form and what the fossil record tells us about life in the past.
New words
fossil · the preserved remains or traces of a living thing from long agomould and cast · a mould is a hollow print left in rock; a cast forms when the hollow fills with mineralsfossil record · all the fossils found, arranged by age; it shows how life changed over time
Explanation
Most dead living things rot away. A fossil forms only in special conditions, usually when a body is buried quickly in mud or sand, away from air, and the hard parts such as bones and shells are slowly replaced by minerals. Kinds of fossils: petrified fossils (hard parts turned to stone), moulds and casts (a hollow print, and the mineral that fills it), trace fossils (footprints, burrows) and whole organisms preserved in ice or amber, such as a mammoth in permafrost. Fossils are found in sedimentary rock. The fossil record is all the fossils found, arranged by age. In layers that have not been disturbed, the older ones lie deeper, and their fossils show the kinds of life that existed then, including many species that no longer exist. The record is incomplete, because most living things never became fossils, and many fossils are still buried. The best way to say it: the record is a set of clues, and scientists build the picture from them.
organism dies→quickly buried in mud or sand→layers pile up above→minerals replace hard parts→fossil found in rock
Worked examples
Making a mould and a cast at home: press a shell into a flat piece of clay. Remove the shell: the hollow print is the mould. Fill the hollow with salt dough and let it dry: the dough is the cast. No heat, plaster or chemicals are needed.
A footprint in hardened mud is a trace fossil. It shows how an animal moved and how big it was, though no bone is left.
Why do few living things become fossils? They usually rot, are eaten or are broken before they are buried. So the fossil record is a set of clues, not a complete diary.
Class activity
Make a mould and cast, with a grown-up. Use a leaf, a shell or a small toy. Press it into clay, remove it, fill the print with salt dough and let it dry for a day. Label the mould and the cast, and draw an arrow from each to the words “hollow print” and “filled shape”. Write what a scientist could learn from only the cast.
Practice
1
Where is a fossil usually found?
In sedimentary rock.
2
What is the difference between a mould and a cast?
A mould is a hollow print; a cast is the shape that fills it.
3
Give one example of a trace fossil.
A footprint or a burrow.
4
Why is the fossil record incomplete?
Most living things never became fossils, and many fossils are not found yet.
5
How does a mammoth in permafrost differ from a petrified fossil?
The whole body is preserved in ice; in a petrified fossil the hard parts became stone.
