The Cretaceous Period was the final and longest part of the Mesozoic Era, lasting from about 145 million to 66 million years ago. It matters because many famous dinosaurs, including Tyrannosaurus rex, Triceratops, and hadrosaurs, lived during this time. The period also saw major changes in plants, oceans, climates, and continents.
By studying Cretaceous fossils, scientists connect ancient life with Earth systems such as climate, sea level, and extinction.
Key Facts
- The Cretaceous Period lasted from about 145 Ma to 66 Ma, where Ma means million years ago.
- Tyrannosaurus rex lived near the end of the Cretaceous, about 68 Ma to 66 Ma.
- The end-Cretaceous extinction occurred at 66 Ma and eliminated all non-avian dinosaurs.
- Relative age rule: in undisturbed rock layers, lower layers are older than higher layers.
- Half-life dating formula: remaining parent isotope fraction = (1/2)^n, where n is the number of half-lives.
- Impact energy can be estimated with KE = 1/2 mv^2, linking asteroid mass and speed to destructive power.
Vocabulary
- Cretaceous Period
- The Cretaceous Period was the geologic time interval from about 145 million to 66 million years ago, ending with a major mass extinction.
- Fossil
- A fossil is preserved evidence of ancient life, such as bones, teeth, shells, footprints, or plant remains.
- Theropod
- A theropod is a mostly meat-eating dinosaur group that included Tyrannosaurus rex and the ancestors of modern birds.
- K-Pg Boundary
- The K-Pg Boundary is the global rock layer marking the end of the Cretaceous Period and the beginning of the Paleogene Period.
- Radiometric Dating
- Radiometric dating is a method for finding the age of rocks by measuring the decay of radioactive isotopes.
Common Mistakes to Avoid
- Thinking all dinosaurs lived at the same time is wrong because the dinosaur era lasted over 160 million years and different species appeared in different periods.
- Calling T. rex a Jurassic dinosaur is wrong because T. rex lived in the Late Cretaceous, tens of millions of years after the Jurassic ended.
- Assuming every fossil is a complete skeleton is wrong because most fossils are fragments, traces, or partial remains that scientists interpret using comparison and context.
- Saying the asteroid impact was the only possible factor in the extinction is too simple because evidence also points to climate stress, volcanic activity, sea-level change, and ecosystem disruption.
Practice Questions
- 1 The Cretaceous Period lasted from about 145 Ma to 66 Ma. How many million years long was it?
- 2 A rock sample has 1/8 of its original parent isotope remaining. How many half-lives have passed?
- 3 A fossil tooth, a footprint trackway, and a layer of ash are found near each other. Explain how each type of evidence could help a paleontologist reconstruct a Late Cretaceous ecosystem.