Walking out my back door, I can look at rocks exposed in my garden that carry 450 million year-old fossils of an ocean community that predates the appearance of true fish. While the earlier Cambrian period saw an explosion of phyla, the Ordovician period saw an explosion of families in existing phyla. Most modern phyla were in existence by the end of the Cambrian, but they took on a more familiar appearance during the Ordovician. Missing are the fish, which dominate visible life in today’s oceans. It would appear that the top predators were mollusks (Cephalopods) and arthropods (eurypterids) instead of the fish that we see today.