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Pikaia gracilens is an extinct, primitive chordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway Morris as a chordate, it became the "one of the most famous early chordate fossils," or "famously known as the earliest described Cambrian chordate". It is estimated to have lived during the latter period of the Cambrian explosion. Since its initial discovery, more than a hundred specimens have been recovered.
|Life reconstruction of Pikaia gracilens|
The body structure resembles that of the lancelet and it swam perhaps much like an eel. A notochord and myomeres (segmented blocks of skeletal muscles) span the entire length of the body, and are considered the defining signatures of chordate characters. Its primitive nature is indicated by the body covering, a cuticle, which is characteristic of invertebrates and some protochordates.
The exact phylogenetic position is unclear. Proposed affinities include those of cephalochordata, craniata, or a stem-chordate not closely related to any extant lineage. Popularly but falsely attributed as an ancestor of all vertebrates, or the oldest fish, a close relative of the ancestor of humans, it is generally viewed as a basal chordate alongside other Cambrian chordates; it is a close relative of vertebrate ancestors but it is not an ancestor itself.