Eras and periods are divisions of geologic time arranged in a hierarchy, where eras are the larger units and periods are smaller subdivisions within them. Think of it like a calendar: if an era is a year, periods are the months inside it. Both are defined by major changes in life on Earth, but eras represent broader, more dramatic shifts while periods mark finer distinctions in the fossil record.
How the Geologic Time Scale Is Organized
Earth’s 4.5-billion-year history is broken into a nested set of time units. From largest to smallest, the hierarchy runs: eons, eras, periods, epochs, and ages. Each level fits inside the one above it. The Phanerozoic Eon (the last 542 million years, covering all complex animal life) contains three eras. Each of those eras contains multiple periods, and each period contains multiple epochs.
This system was originally built entirely on relative dating, using principles like superposition (deeper rock layers are older) and faunal succession (fossil communities change in a predictable order through time). Scientists later added radiometric ages to pin specific dates to the boundaries, but the framework itself comes from observing how rock layers and their fossils change from bottom to top.
What Defines an Era
Eras represent the biggest chapters in the story of life. The three eras within the Phanerozoic Eon are the Paleozoic (roughly 542 to 252 million years ago), the Mesozoic (252 to 66 million years ago), and the Cenozoic (66 million years ago to the present). Each era is bookended by a catastrophic reshuffling of life on Earth, typically a mass extinction.
The boundary between the Paleozoic and Mesozoic, for example, is defined by the largest mass extinction in Earth’s history. Between 83% and 97% of all species went extinct in a geologically brief window of time. Volcanic activity from the Siberian Traps ignited massive coal deposits, releasing enormous amounts of carbon dioxide that acidified the oceans, depleted oxygen from deep water, and destabilized methane deposits on the seafloor. That devastation was so complete that it created a clear dividing line in the rock record: the fossils below the boundary look fundamentally different from the fossils above it.
The Mesozoic-Cenozoic boundary tells a similar story. The asteroid impact 66 million years ago wiped out the non-bird dinosaurs and roughly 75% of species, clearing the way for mammals to diversify. Era boundaries, in short, mark moments when life on Earth was so thoroughly disrupted that the planet’s biology started over in a recognizable way.
What Defines a Period
Periods are subdivisions within eras, and they mark smaller (but still significant) shifts in the types of organisms preserved in the fossil record. Geologists identify period boundaries using index fossils, which are fossils from species that existed for a relatively short, well-defined span of time and are found across wide geographic areas. When a particular index fossil disappears from the rock record and a new assemblage takes over, that transition can mark the boundary of a period.
The Paleozoic Era alone contains six periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Mesozoic has three: Triassic, Jurassic, and Cretaceous. The Cenozoic has three: Paleogene, Neogene, and Quaternary. Periods typically span tens of millions of years, while eras span hundreds of millions.
Many periods are named after the places where their characteristic rocks were first studied. The Jurassic comes from the Jura Mountains in Europe. The Devonian is named after Devon, England. The Cambrian takes its name from Cambria, the Latin name for Wales. The Cretaceous comes from the Latin word for chalk, referring to the chalk deposits common in rocks of that age.
How Boundaries Are Made Official
The International Commission on Stratigraphy maintains the official geologic time chart and designates physical reference points in rock outcrops around the world called Global Boundary Stratotype Sections and Points (GSSPs). These are sometimes called “golden spikes.” Each one marks the exact spot in a rock sequence where a particular time boundary is defined, using a combination of fossil markers, chemical signatures, and other measurable features. The most recent version of the international chart (2024) lists precise dates for each boundary, like the start of the Triassic at approximately 251.9 million years ago or the start of the Paleogene at 66 million years ago.
The Era and Period We Live In Now
We currently live in the Cenozoic Era, the Quaternary Period, and the Holocene Epoch. The Holocene began about 11,700 years ago, at the end of the last major ice age. The Cenozoic as a whole is sometimes called the “Age of Mammals” because mammals rapidly diversified to fill ecological roles left empty after the dinosaur extinction. The Quaternary Period, which started about 2.6 million years ago, is defined largely by repeated cycles of glaciation and the evolution of the genus Homo.
A Quick Way to Remember the Difference
The simplest distinction: eras capture the broad arc of life’s evolution (dominated by marine invertebrates, then reptiles, then mammals), while periods capture more specific chapters within each arc (the rise of fish, the spread of forests, the reign of particular dinosaur groups). Eras are separated by mass extinctions that wiped out most of life. Periods are separated by less dramatic but still recognizable turnovers in the fossil record. Both sit within the same hierarchy, with periods always nested inside eras, and eras always nested inside eons.