The kinds of fossils found in rocks of different ages differ because life on Earth has changed through time. Stratigraphic ranges and origins of some major groups of animals and plants. If we begin at the present and examine older and older layers of rock, we will come to a level where no fossils of humans are present. If we continue backwards in time, we will successively come to levels where no fossils of flowering plants are present, no birds, no mammals, no reptiles, no four-footed vertebrates, no land plants, no fishes, no shells, and no animals. When we find the same kinds of fossils in rocks from different places, we know that the rocks are the same age.
Scientists look for ancestors and descendants through geologic time.
We can see fossils of many other reptiles in rock of the same age and even older, but Archaeopteryx lithographica is the oldest known fossil to have feathers. We conclude that this animal is a link between reptiles and birds and that birds are descended from reptiles. The specimen is about 45 centimeters long. Photograph courtesy of the National Museum of Natural History, Smithsonian Institution. Early explanations were built around the idea of successive natural disasters or catastrophes that periodically destroyed life.