"And because it's very early in the evolution of the vertebrates, they don't have bones yet—this is why these fossils are ...

Learn more about a time period marked by an intense burst of evolution ... primitive chordate is Pikaia gracilens, a wormlike creature that swam in middle Cambrian seas. Fossils found in the ...

From so simple a beginningPhylum Chordata ... fossil record as we know it. That’s why some scientists have suggested that the Ediacarans represent “failed experiments” in the early evolution ...