Intervals of geologic time are bound by important events, like mass extinctions or the appearance of animals with hard parts. The first single-celled organisms appeared about 3.5 billion years ago. Many sea animals first show up in the Cambrian period of the Paleozoic era. That was about 545 million years. It is sometimes referred to as the Cambrian explosion because it happened relatively quickly. It lasted just over 50 million years. The next period, , the Ordovician, ended with mass extinctions of 60% of all marine invertebrates. Dinosaurs wouldn't appear for over 200 million years, during the Jurassic period in the Mesozoic era. Based on the passage, in geology, 50 million years is A an era. B not considered a very long time. C too short for any significant events to occur. D about how long dinosaurs were on the planet.​