Because bone is resistant to decay, the fossil record of vertebrates is extensive and has been studied for over 200 years. We can't present all of it on one page; visit our exhibits on specific vertebrate groups for more detailed information. But to give a very brief summary:
The first known vertebrate fossils, found at the Chengjiang locality in China, date back to the early Cambrian. These early vertebrates, such as Haikouichthys, are small, tapered, streamlined animals showing eyes, a brain, pharyngeal arches, a notochord, and rudimentary vertebrae. Vertebrates appear to have radiated in the late Ordovician, about 450 million years ago. However, most Ordovician fossil fossil vertebrates are rare and fragmentary, although available material suggests that ancestors of the sharks and jawed fish were present along with various lineages of armored jawless fish. By the middle Silurian, about 400 million years ago, the picture is clearer: the armored jawless fish were quite diverse, and the first definite jawed fish had appeared -- the Silurian is sometimes called the "Age of Fishes." By the late Devonian, 360 million years ago, early cartilaginous fish and bony fish were diversifying.
The late Devonian also marked the first tetrapods -- vertebrates with true legs that could walk on land. By about 330 million years ago, in the Mississippian, several groups of land-dwelling amphibians had appeared. The oldest known amniotes -- close to the ancestry of all reptiles, birds, and mammals -- appeared in the early Pennsylvanian, about 310 million years ago. Land amniotes continued to diversify, and by the middle Pennsylvanian had split into several taxa, two of which would go on to dominate the Mesozoic and Cenozoic: the diapsids and the synapsids.