Understanding Evolution: your one-stop source for information on evolution
Resource library Teaching materials Evolution 101 Support this project
print print
The Arthropod Story Table of Contents   

Pikaia: A chordate

Pikaia is not part of the arthropod lineage. It lacked segments, an exoskeleton, and jointed legs.

Pikaia was not an arthropod

When Pikaiaís fossils were first discovered in the early 1900s, scientists wrote it off as a worm. But in the 1970s, Pikaia was reexamined. Scientists noticed that the rod running along its back resembled a backbone-like structure. Furthermore, the markings on the sides of its body were unusually shaped: they formed V-shapes (pointing towards the head). This zigzag shape is not seen in arthropod segments, but is the typical shape of chordate muscle bundles. Pikaia was probably a chordate — the same group that includes fish, dinosaurs, and humans!

A fossil Pikaia has a visible notochord and myotomes.
Zebrafish embryo
A zebrafish embryo clearly shows the same structures.

Amphioxus images from BIODIDAC © Jon Houseman; Pikaia fossil photo by Chip Clark, Museum of Natural History, Smithsonian Institution; Zebrafish embryo photo reproduced with permission from: Amacher SL (2002) Zebrafish Embryo as a Developmental System. In: Nature Encyclopedia of Life Sciences. London: Nature Publishing Group. http://www.els.net/

previous | next  >