After falling temperatures brought the Cambrian period to an end, the Ordovician period started. During this time, life had diversified even more. The fish had gone from the simple, worm-like vertebrates of the Cambrian to more modern and complex aquatic organisms; however, they still stayed small. On the other hand, the invertebrates had changed a lot, especially the arthropods. Gone were the radiodonts of the Cambrian; the new arthropod apex predators were the eurypterids, aquatic chelicerates that would continue to rule the waters for a course of 66.7 million years. As well as this, the orthoconic cephalopod mollusks had diversified a lot, and had grown to extreme sizes, proving to be the largest animals of the time. Meanwhile, the trilobites were still relatively diverse. The oceans were bustling with wildlife, while the land was just getting prepared for colonization, as land plants and fungi evolved. Soon, an ice age would sweep this period, driving many genera to extinction and giving rise to the Silurian.
Lasted from 485.4-443.8 mya
Early Ordovician Animals (note that those are not all the ones that exist)Edit