Which event is characteristic of the Paleozoic Era?

The Paleozoic Era, which spanned from about 541 to 252 million years ago, is known for significant biological developments, particularly the Cambrian Explosion. This event, which occurred early in the Paleozoic, around 541 million years ago, marked a rapid increase in the diversity of life forms. During this time, many major groups of animals first appeared in the fossil record, including ancestors of most modern animal phyla.

The Cambrian Explosion fundamentally transformed the Earth's biosphere, leading to the establishment of complex ecosystems. It laid down the evolutionary groundwork for subsequent life forms, including vertebrates, which would further evolve in later periods.

While the formation of modern continents does relate to geological events, it is more characteristic of the Mesozoic and Cenozoic eras. Similarly, the age of dinosaurs and the appearance of the first mammals are events that belong to the Mesozoic Era and the early Cenozoic Era, respectively. The principal hallmark of the Paleozoic is indeed this explosion of life, making it the most accurate answer regarding this era's defining characteristic.