The World's Foundations; or, Geology for Beginners · Ch. XII · Unit 2: Part II: Ancient seas and coal forests
13. The age of lower animals
Goal: Your child can describe the seas of the Age of Lower Animals, explain why the limestone shows a shallow, warm sea, and describe a trilobite.
You'll need
- The book, open to this chapter
- Modeling clay or paper and pencils
- A magnifying glass (if you have a real fossil)
- A notebook for a "rock journal" and a pencil
What's in the book
Chapter XII begins the Primary or Ancient-Life Period with the Age of Lower Animals, when limestone was being built in shallow seas over much of today's land. It describes ripple marks and worm-borings, the many-lobed trilobite with its large eyes, and the sudden appearance of the first fishes near the end of the age.
The lesson, step by step
A question to start
SayIf you found coral in a rock in Scotland, near the cold North Sea, what would that tell you?
Keep the question for later.
Read the chapter together
Read Chapter XII (pages 107 to 117). Two sentences to stop on:
Even in the heaviest storms, the bottom of the deep sea is always quite calm.
When a piece of rock has such marks, you may be sure that they were made upon the beach between high and low water-line.
Reading the clues
Help your child follow the author's reasoning: limestone made of ground-up shells needs waves to grind them, so the sea was shallow. Corals grow only in warm water, and coral is found far north, so the sea was warm. Ripple marks and worm-borings show old beaches.
The trilobite
Read:
The name ' Trilobite ' has been given to it, from the shape ; but no man has ever seen the living creature, for nothing like it is in the earth now.
Tri-lobe means three lobes. Draw or model one: a head shield, a jointed body and a tail, divided lengthwise into three lobes (left, middle, right), with two big eyes on the head. The book says some were a foot long.
What the eyes tell
Giberne makes a neat deduction: the trilobite had large eyes, so there must have been light in those early seas. Ask your child to explain her reasoning in their own words.
The fishes arrive
Near the end of this age, fishes appear. The book describes a "bone-bed" in Scotland, about a hundred miles long, crowded with fish bones, teeth and scales. Giberne sees in the fossils a sudden appearance of a new, higher kind of animal.
Tell it back
Ask your child to describe a day in the shallow sea of this age: what lived there, and what did not yet exist?
What we know now
This time is now dated to the Cambrian and the periods after it, beginning about 539 million years ago. Trilobites lived for about 270 million years before dying out. Their eyes were made of clear crystals of calcite, one of the earliest kinds of eye known. Fossils of small jawless fishes have since been found in older rocks, so fishes appeared earlier and more gradually than it seemed in 1881. And the coral in northern rocks has another explanation too: the land that is now Britain lay much farther south then, in warmer waters, because continents move.
Afterwards
If a museum near you has trilobites, go and find one. Notice it this week: look at stone walls, steps and building stones for tiny fossil shells, which are common in limestone.
Answers to the book's problems
The waves must have been able to reach the bottom to grind shells into limestone, so the water was shallow; coral grows only in warm, fairly shallow seas. A trilobite had a head shield, jointed body and tail, divided lengthwise into three lobes, and large eyes. Its eyes show that there was light in the sea.
Worked out for this site from the scan. If a number in your copy differs, trust the book.