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"What were the first fish like?." Prehistoric, LET'S EXPLORE Prehistoric life, Q-files Encyclopedia, 9 Oct. 2018.
Prehistoric, LET'S EXPLORE Prehistoric life, s.v. "What were the first fish like?," accessed April 26, 2019.
The first fish evolved later in the Cambrian Period, probably about 500 million years ago. The fish had neither fins nor jaws. They swam with their mouths held open all the time, sucking in things to eat. Many of these jawless fish had bony armour on their bodies. This would help to protect them from attack by other marine creatures.
Sea scorpions were fearsome animals. Some grew up to two metres (6.6 feet) long. They scuttled around on the sea floor hunting fish and other marine creatures. They caught them in their claws.
There were more than 20,000 different kinds of trilobite. Thet's more than any other known extinct creature.