Geologic time constrains what a fossil can be, where it can occur, and which organisms could exist together.
Dates change with scientific refinement: Use current International Chronostratigraphic Chart or USGS references for precise boundaries. Commercial listings frequently use rounded ages.
Three major Phanerozoic eras
| Era | Approximate span | Collector landmarks |
|---|---|---|
| Paleozoic | 539–252 million years ago | Trilobites, brachiopods, crinoids, early fishes, coal forests |
| Mesozoic | 252–66 million years ago | Ammonites, dinosaurs, marine reptiles, early birds, flowering plants |
| Cenozoic | 66 million years ago–present | Mammals, modern fish and sharks, flowering plants, Ice Age faunas |
How collectors should use age
- Check whether the claimed organism existed during the stated period.
- Confirm that the formation actually spans the claimed age.
- Use associated fossils and matrix as supporting evidence.
- Do not treat a rounded “million years old” sales phrase as precise stratigraphic data.
Age is not value by itself
A common 450-million-year-old brachiopod may be inexpensive, while a much younger specimen can be valuable because of rarity, anatomy, preparation, provenance, or scientific importance. Age should support identification, not replace it.