Pikaia

Pikaia is an extinct genus of Cambrian marine animals known from the Burgess Shale. Its body features, including a notochord-like structure, have led many researchers to regard it as an early chordate.

Chordate: A chordate is an animal that has a notochord at some stage of its life. Pikaia is often placed among early chordates because of its axial support structure.
Pikaia gracilens: The sole named species of Pikaia, described from Cambrian fossils in the Burgess Shale. This species is the fossil basis for claims about the genus's anatomy and affinities.
Haikouichthys: An early Cambrian animal from China often interpreted as a primitive vertebrate. Its more fish-like anatomy offers a comparison with Pikaia's less clearly defined body plan.
Burgess Shale fauna: The diverse community of Cambrian marine organisms preserved in the Burgess Shale. Pikaia is one member of this fossil community, whose diversity reshaped views of early animal life.
Evolutionary contingency: The idea that historical chance can shape evolutionary outcomes and the forms that survive. Gould used Pikaia to illustrate how different histories might have produced different living lineages.
Notochord: A flexible, rod-like support structure found in chordates, beneath the nerve cord. A proposed notochord is central to interpreting Pikaia's anatomy.
Charles Doolittle Walcott: An American paleontologist who discovered and studied many fossils from the Burgess Shale. Walcott collected and named Pikaia in his work on the Burgess Shale fauna.
Myllokunmingia: An early Cambrian fossil animal from China commonly classified as a primitive vertebrate. Its proposed vertebrate features sharpen the distinction between Pikaia and early fish-like forms.
Smithsonian Institution: A United States museum and research institution that holds major natural history collections. Walcott's Pikaia specimens are part of the institution's Burgess Shale collections.
Common descent: The principle that organisms share ancestors through branching lineages over evolutionary time. Pikaia's proposed chordate status bears on how the ancestry of vertebrates is reconstructed.

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