The geologic time scale is the calendar of Earth history, stretching from Earth’s formation about 4.54 billion years ago to the present. It helps scientists organize enormous spans of time into eons, eras, periods, epochs, and ages. For paleontology, this scale is essential because fossils only make sense when they are placed in time.
Dinosaurs are especially important because they dominated land ecosystems during much of the Mesozoic Era.
Key Facts
- Earth formed about 4.54 billion years ago.
- The Phanerozoic Eon began about 541 million years ago and contains most visible fossil life.
- The Mesozoic Era lasted from about 252 million years ago to 66 million years ago.
- The dinosaur periods are Triassic, Jurassic, and Cretaceous.
- Half-life dating uses N = N0(1/2)^(t/T), where T is the half-life.
- The Cretaceous-Paleogene extinction occurred about 66 million years ago and ended all non-avian dinosaurs.
Vocabulary
- Geologic time scale
- A system that divides Earth history into named time intervals based on rock layers, fossils, and major events.
- Stratigraphy
- The study of rock layers and their order, ages, and relationships.
- Fossil
- Preserved evidence of past life, such as bones, shells, footprints, or impressions in rock.
- Index fossil
- A fossil from a species that lived for a short time over a wide area and helps date rock layers.
- Radiometric dating
- A method for finding the age of rocks or fossils by measuring the decay of radioactive isotopes.
Common Mistakes to Avoid
- Thinking all dinosaurs lived at the same time, which is wrong because different dinosaur groups lived across the Triassic, Jurassic, and Cretaceous periods over about 186 million years.
- Calling every ancient reptile a dinosaur, which is wrong because pterosaurs, plesiosaurs, and mosasaurs were separate reptile groups with different body plans and evolutionary histories.
- Assuming deeper fossils are always older in every situation, which is wrong because folding, faulting, erosion, or overturned rock layers can disturb the original order.
- Confusing relative dating with absolute dating, which is wrong because relative dating places events in sequence while radiometric dating estimates numerical ages.
Practice Questions
- 1 The Mesozoic Era began about 252 million years ago and ended about 66 million years ago. How long did the Mesozoic Era last?
- 2 A rock sample contains 25 percent of its original radioactive parent isotope. If the isotope has a half-life of 100 million years, how old is the sample?
- 3 A fossil dinosaur bone is found in sedimentary rock between two volcanic ash layers dated to 150 million years ago and 148 million years ago. Explain what scientists can conclude about the fossil’s age and why.