Produced by the Population Genetics and Evolution class, Furman University

The Cambrian: Pikaia
One of the oldest known chordates, Pikaia gracilens was an organism much like a small eel or hagfish that swam above the ocean floor using its expanded tailfin, seen at right, and its serpentine body (Smithsonian Institution 2010). It was found in the Burgess Shale Formation, and along with two other chordate taxa, Haikouichthys and Zhangjianichthys, in the Lower Cambrian Chengjiang Lagerstätte of Yunnan, China (Morris 2008). It is considered to be related to the hagfish because of fleshy apomorphies such as its tentacles, but lacks certain vertebral characteristics that prevent it from being classified in that taxa and is instead considered to be a tentative cephalochordate (Donoghue and Purnell 2005). Some have argued that it should be placed with the Arthropoda, but P. gracilens lacks key characteristics of that phylum. It lacks an exoskeleton and jointed legs, and though its body appears segmented, the “segmentations” are more likely to be muscle bundles imprinted on the surface of the skin. It is these “segmentations,” which appear in a V-shaped pattern in the fossil record, which led to the classification of P. gracilens as a chordate; such a pattern is typical of chordate morphology (UCMP 2010).

Page by Will Towler
Pikaia gracilens. Picture from: The Cambrian Critter Project

Donoghue P, Purnell M. 2005. Genome duplication, extinction and chordate evolution. Trends in Ecology and Evolution 20 (20): 1-8..

Morris S. 2008. A Redescription of a rare Chordate, Metaspriggina walcotti Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada. Journal of Paleontology 82:424-430.

University of California Museum of Paleontology Understanding Evolution: Pikaia, a Chordate. Accessed February 1, 2010.

Smithsonian Institution. 2010. Burgess Shale: Pikaia gracilens (a primitive chordate). Accessed February 1, 2010.