What characterized the Ordovician in terms of invertebrate life, ...

[asan beg]

What characterized the Ordovician in terms of invertebrate life, and how does that relate to what was
  happening geologically? What will be an ideal response?

Question 2

What is the Cambrian explosion? Is their appearance sudden? Why is it sudden, or why does it seem
  so? What will be an ideal response?

[Anton]

Answer to Question 1

A major transgression in the Middle Ordovician brought to the continent a vast, shallow sea with a
warm climate, so shelly fauna increased in diversity and number. Extensive glaciation in Gondwana at
the end of the period caused a regression and a mass extinction of more than 100 families of marine
invertebrates.

Answer to Question 2

The Cambrian explosion refers to the sudden appearance of new animals in the fossil record, animals
with skeletons. So many fossils appear rapidly because of the development of skeletons, which
fossilize much better. Also, these advances helped life to proliferate.