Question

What type of organism appeared during the Ordovician Period?

Answer 1

The first true vertebrates evolved about this time.

Explanation:

The first vertebrates (our ancestors) evolved during the Ordovician period. This time period is also known as the first time arthropods (insects) and plants probably colonized the land - for the first 3.5 billion years or so of life's evolution, marine environments were the sole place where life thrived.

http://www.ucmp.berkeley.edu/ordovician/ordovician.php

The end of the Ordovician is marked by a mass extinction event that killed off many marine organisms. This was probably brought on by a massive volcanic eruption that pumped huge amounts of CO2 into the atmosphere and in so doing, tripped off a global warming event.