Planktonic Organism
Plankton helped play a role in the Ordovician period, these small organisms were the marine animals main food source. But even then it played a more significant role with producing carbon dioxide and helping with biochemical cycles in the water. An example would be the Conodont, a large toothlike microfossil which was like a filter. The plankton organisms back in this period were a large role of keeping water temperatures warm and supplying food for the larger organisms of the deep ocean. The Conodont had teeth like hard rock, probably for a small defense against smaller marine animals.
Trilobites
Some of the earliest arthropods, living for over 250 million years before dying out, these ocean dwellers had a big taxonomy either being predators or scavengers at the surface of the ocean, or swimming and feeding off of the plankton. Besides being one of the successful marine animals, this animal left behind many fossils since it was covered by an exo-skeleton. This animal was an expert on evolving and adapting to its environment, over the course of over 250 million years this arthropod constantly evolved as well as its relatives, things like eye placement, increasing or decreasing its thorax and its segments, even one of the taxa of this animal was believed to evolve into being a burrower.
Endocerida
A member of the cephalopods, also known as the superpredator of this time period due to its protection from its shell and its burrowing/abmush ability. From adapting from they created two main families, one increasing in size probably to seem more intimidating to the larger marine animals, and another family that had a more complex siphuncle.
Echinoderm
The easiest example of an echinoderm is a starfish, although there is much change between the modern day starfish and an echinoderm from the ordovician time period. They are free swimming organisms until they begin their bilateral symmetry, where the right side of the organism will grow larger than the left side, and eventually the left side grows symmetrical to the right side, forming the five point symmetry and as they evolved they began to form their protective layers.