or to the high northern and southern latitudes. The oldest fossil chordates are of Cambrian age. The fossil record for chordates is highly significant because it implies the contemporary existence of the tunicates and craniates in the Early Cambrian during the so called Cambrian Explosion of animal life. Two other extinct Cambrian taxa, the calcichordates and conodonts, are uncertainly related to other Chordata (Nelson, 1994).
Animals such as amphioxus are commonly considered possible vertebrate progenitors. However, it is likely only that such animals are in the vertebrate lineage, but they are not its beginning. Instead, they might be considered its ultimate development in that evolution proceeds by reduction and they are the most reduced chordates. They no longer have any evolutionary potential because they have lost the parts which could have been adapted to new functions. However, this does not mean that such animals were not among the earliest vertebrates, for their reductive evolution from the vertebrate prototype could have been immediate and extreme. Some early successful chordates may have been morphologically simple, drastic reductions from the protovertebrate form. Other branches, perhaps more conservative so they retained more parts, only later became dominant. Based on the fossil evidence, there does not seem to have been a preponderance of these "primitive" types among early chordates. Among the oldest known vertebrates are fishes at least as complex as modern fishes, with complex skeletons, highly developed organs of perception, electric organs, armor plates, and other structures:
The progenitorship of the simple chordates is an idea based upon two things: their simplicity and their resemblance to the vertebrate embryo. These facts imply their progenitorship only if one assumes that evolution always proceeds from the simple to the complex, and that embryonic development mirrors evolution. It is more p...