When the Paleozoic Era dawned, 600 million years ago (MYA), the Earth was already four billion years old and life had existed in the primordial sea for three billion years. Shelled marine life had evolved but, through the first period of the Paleozoic, the Cambrian (600-500 MYA), these creatures exploded in number and variety; among them were the trilobites, which survived throughout the Era. The Ordovician Period (500-440 MYA) witnessed the appearance of bryozoans, conodonts (the earliest vertebrates) and jawless fish. During the Silurian (440-400 MYA), vascular plants, ammonites, freshwater fish and jawed fish (spiny fish and placoderms) evolved; it was in the Silurian that plants and animals first colonized the land.
The Devonian Period (400-350 MYA) marked the appearance of many life forms that persist today; among these were sharks, bony fish, lungfish, primitive amphibians, ferns, primitive arachnids, wingless terrestrial insects and the first tree-like plants. Web-spinning spiders and flying insects first evolved during the Mississippian Period (350-310 MYA), followed by horseshoe crabs and primitive reptiles during the Pennsylvanian Period (310-270 MYA); these two Paleozoic divisions are often referred to as the Carboniferous Period, the peak of coal forest habitat. The final Period of the Paleozoic Era was the Permian (270-225 MYA), during which frogs, lizards, pelycosaurs (mammal-like reptiles), modern reef corals and modern conifers appeared.
This cascade of Paleozoic evolution set the stage for the Mesozoic Era, the Age of Dinosaurs, during which mammals, flowering plants, birds, social bees and snakes also made their debut. But that's the subject for a future blog.
