ink between invertebrates and vertebrates.

Pikaia is a vertebrate ancestor, with a worm-like appearance that has been flattened sideways. The fossils compressed within the Burgess Shale show chordate features such as traces of an elongate notochord, dorsal nerve cord, and blocks of muscles (myotomes) down either side of the body – all critical features for the origin and evolution of the vertebrates (fish, amphibians, reptiles, birds, and mammals).

(See Wierd Life of the Cambrian)