Journey through geological time and discover what ancient life forms await in rocks of different ages
Understanding Geological Time
Geological time is vast — 4.6 billion years of Earth history. Different time periods preserved different ecosystems, so knowing the age of rocks helps predict what fossils you might find. Ages are given in "Ma" (millions of years ago). Period colors below follow the International Chronostratigraphic Chart — the same colors the Bone Zone app uses on its maps.
"Age of Mammals" — 66 million years ago to present
The "Ice Age" period with multiple glaciations. Modern ecosystems develop, humans evolve.
Mammoths, saber-tooth cats, giant ground sloths
Bears, deer, bison in Pleistocene deposits
Modern shells, corals, marine mammals
Pollen, seeds, wood from modern species
Grasslands expand, mammals diversify rapidly, early human ancestors appear.
Early horses, camels, rhinoceros
Early apes and human ancestors
Grass pollen, seeds from expanding prairies
Megalodon teeth, whale bones
Recovery after dinosaur extinction. Mammals explode in diversity, early whales, tropical climates.
Primitive horses, early elephants, carnivores
Ambulocetus, early whale fossils
Terror birds, early raptors
Modern flower and fruit fossils
K–Pg Mass Extinction
66 million years ago — Asteroid impact ends the age of dinosaurs
"Age of Reptiles" — 252 to 66 million years ago
Peak dinosaur diversity, flowering plants evolve, warm global climate, high sea levels.
T. rex, Giganotosaurus, raptors
Titanosaurs, long-necked giants
Early flowering plant fossils
Mosasaurs, plesiosaurs, ammonites
Classic dinosaur period, first giant sauropods, early birds, warm and humid climate.
Brontosaurus, Diplodocus, Camarasaurus
Archaeopteryx with feather preservation
Ichthyosaurs, belemnites, coral reefs
Early pine and cypress relatives
First dinosaurs appear, supercontinent Pangaea, recovery from worst mass extinction.
Coelophysis, Plateosaurus, small theropods
Early crocodiles, phytosaurs
Tiny shrew-like mammals
Seed ferns, early conifers
Permian–Triassic Extinction
252 million years ago — "The Great Dying" — 90% of species extinct
"Ancient Life" — 541 to 252 million years ago
Supercontinent Pangaea forms, mammal-like reptiles dominant, great coal swamps decline.
Mammal-like reptiles, early ancestors
Seed fern leaves (Gondwana indicator)
Last great brachiopod diversity
Early marine reptile adaptations
"Coal Age" — vast swamp forests, first reptiles, giant insects, oxygen-rich atmosphere.
Lepidodendron, Sigillaria, seed ferns
Meganeura (giant dragonflies), trilobites
Eryops, temnospondyls
Crinoids, bryozoans, corals
"Age of Fishes" — fish diversity explodes, first forests, early amphibians venture onto land.
Dunkleosteus, placoderms, early sharks
Archaeopteris, early tree fossils
Acanthostega, Ichthyostega
Peak trilobite diversity and size
First vascular plants colonize land, reef recovery, early jawed fish evolve.
Cooksonia, primitive vascular plants
Tabulate and rugose corals
Eurypterids, giant aquatic predators
First sharks and bony fish
Great biodiversification, marine life flourishes, first vertebrates, massive reef systems.
Huge diversity, many bizarre forms
"Sea lily" gardens, limestone beds
Peak diversity, abundant fossils
Conodonts, early jawless fish
"Cambrian Explosion" — most animal groups appear, first hard shells, weird and wonderful forms.
First appearance, primitive forms
Hallucigenia, Opabinia, soft bodies
First mineralized skeletons
Burrows, tracks, first bioturbation
Age-Based Collecting Strategy
Reading Geological Maps
Geological maps use standardized colors and symbols for different time periods: