A fossil is created when a plant or animal is buried beneath layers of sand and mud. Over time, the mud becomes sedimentary rock, encasing what remains of the organism—bones, teeth, shells, or stems. In turn, minerals seep in, turning this organic matter into stone.

Other fossil types include the impressions plants and animals leave in rocks as they slowly decay, and “trace” fossils of dung, tracks, or footprints (such as the Academy’s print of a theropod).

Occasionally, whole animals are discovered frozen in ice or buried in tar pits, but “soft” tissue preservation is rare.

For billions of years, single-celled bacteria dominated the Earth, living in the soupy beginnings of the world’s oceans. Then, in a burst of evolutionary activity 545 million years ago called the “Cambrian Explosion,” a deluge of multicellular marine creatures appeared in the fossil record, revealing the forebears of nearly all modern animals. In 1909, the most complete record of this biological Big Bang was discovered in the Burgess Shale, an extraordinary fossil deposit in the Canadian Rockies. Many of these fossils (some of which the Academy has in its collection) are so well-preserved that even their soft appendages are captured in stone.

When did ferns flourish? How do we distinguish between the Age of Reptiles and the Age of Mammals? When did most prehistoric life go extinct? Each individual fossil provides important clues. Equally important is where each specific fossil appears in the many layers of the Earth’s crust. These “strata” of rock and soil represent different geologic time periods in Earth’s past. Where fossils appear in these strata help scientists determine the time in which they lived.

This geologic timeline, explains Academy paleontologist Peter Roopnarine, is divided into Eons (the largest units of time), then subdivided into Eras. Scientists divide the world’s three major Eras (Paleozoic, Mesozoic, and Cenozoic) into Periods and Epochs based on the emergence or extinction of specific plants and animals.

Earth formed as a solid planet during the Hadean Eon, 4.5 to 3.9 billion years ago. While significant milestones took place during the Earth’s three earliest eons (Proterozoic, Archaean and Hadean), scientists call this entire time period “Precambrian Time” to signify the beginning of the Cambrian Period (540 million to 490 million years ago) when multicellular life first appeared. The Cambrian Period was the first of six periods within the Paleozoic Era, which spanned 540 million to 248 million years ago.

Dinosaurs and other large reptiles reach their peak as Earth’s dominant life form during the Mesozoic Era, 248 million to 65 million years ago. The Cretaceous Period (144 million to 65 million years ago), the last of three periods within the Mesozoic Era, was marked by the mass extinction of dinosaurs and other large reptiles, plus several types of marine invertebrates.