Dinosaurs: origins and diversity
Dinosaurs first appeared in the Late Triassic, roughly 231 to 243 million years ago, and rapidly diversified into the dominant large-bodied terrestrial vertebrates on Earth, a position they held for approximately 165 million years.
Living fossils and evolutionary stasis
"Living fossils" are organisms whose body plan has remained visually similar to ancient relatives for tens or hundreds of millions of years, but molecular evidence demonstrates that genetic evolution never stopped in any of these lineages.
Mass extinctions
Five major mass extinctions have punctuated the history of animal life, each eliminating more than 75% of species on Earth and permanently redirecting the trajectory of evolution.
Rise of mammals
Mammals trace their origins to the synapsids—a lineage that diverged from reptiles more than 300 million years ago—and the transition from reptile-grade ancestors to true mammals is documented by hundreds of fossils showing gradual changes in skull, jaw, and ear anatomy.
The Cambrian explosion
Between roughly 538 and 520 million years ago, nearly all major animal body plans appeared in the fossil record within an interval of geological time representing less than 3 percent of the history of animal life—a diversification known as the Cambrian explosion.
The end-Cretaceous extinction
Roughly 66 million years ago, an asteroid roughly 10 kilometers in diameter struck what is now Mexico's Yucatán Peninsula, triggering a global catastrophe that killed approximately 76 percent of all species on Earth.
The fossil record and deep time
Fossils form through several preservation pathways—permineralization, molds and casts, amber entombment, and trace fossil formation—each biased toward organisms with hard parts in low-energy depositional environments.
Transitional fossils
Transitional fossils are organisms that display anatomical features intermediate between two major groups, providing direct physical evidence of the step-by-step nature of evolutionary change documented across hundreds of lineages.
What is Paleontology?
Paleontology is the study of ancient life, from dinosaurs to prehistoric plants, primarily through the examination of fossils.