If you listened to the end of the Cambrian, you know that it was marked by a major mass extinction event dated to 488 to 485 million years ago, so that’s the official start of the Ordovician Period. You will see other times given, depending on the source. Many say 490 million years, reflecting a time when the date of the mass extinction wasn’t so accurately known, and others may round it off to about 500 million years which is a convenient number. But officially it’s 488 to 485 million years, at least for now.
Like the Cambrian the Ordovician is subdivided into three major sections, or epochs. They’re simply the early, middle, and late Ordovician, but each of those epochs is further broken down into two or three ages. Ages of time correspond to stages when we talk about the rocks themselves, just as early time equates with lower rocks. When we say Lower Ordovician, we’re talking about the rocks dating to Early Ordovician time.
Ages and stages tend to be regional in extent, so they don’t necessarily have the same names all over the globe. There’s a real attempt at standardization, and the International Commission on Stratigraphy approved a global system in 2008. Some of the names, like the Tremadocian, the oldest part of the early Ordovician, have a long history. That name, from Wales, was first used by Adam Sedgwick in 1846 when he thought it was part of the Cambrian. But the Sandbian age of the Late Ordovician, named for a place in Sweden, wasn’t introduced until 2006.
The Ordovician lasted about 45 million years, and ended with another major extinction event about 443 million years ago.
—Richard I. Gibson
Time scale from Wikipedia.