The Ordovician to Silurian period started 485 million years ago and finished 419 million years ago. This period in history had a huge increase in new species and the amount of animals fluctuated in the sea. Some of the sea creatures were:
Orthoceras, which was 15cm long, is related to squid, its descendant. It belongs to 2 scientific families; the Nautiloid and Mollusc family.
Guiyu, which was 30cm long, was a bony, jawed and finned fish which is from the Sarcopterygii family.
Fish like Guiyu developed there armor to defend themselves from Eurypterids like Pterygotus.
Websites:
en.wikipedia.org/wiki/Guiyu_oneiros
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Book:
A Brief Illustrated History of Life On Earth by Steve Parker and David West