Pikaia gracilens is an extinct, primitive chordate marine animal known from the Middle Cambrian Burgess Shale of British Columbia.

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.

A reinterpretation in 2024 found evidence of the gut canal, dorsal nerve cord and myomeres, and suggested that the taxon was previously interpreted upside down.

[2][13] The fossils of Pikaia gracilens was discovered by Charles Walcott from the Burgess shale member of the Stephen formation in British Columbia, and described it in 1911.

Based on the obvious and regular segmentation of the body, as is the feature of annelids,[14] Walcott classified it as a polychaete worm and created a new family Pikaidae for it.

[15]) Walcott was aware of the limitation of his classification, as he noted: "I am unable to place it within any of the families of the Polychaeta, owing to the absence of parapodia [paired protrusions on the sides of polychaete worms] on the body segments back of the fifth.

"[16] University of Cambridge palaeontologist Harry B. Whittington and his student Simon Conway Morris re-examined the Burgess Shale fauna and noted the anatomical details of Pikaia for the first time.

[19]Conway Morris formally placed P. gracilens among the chordates in a paper in the Annual Review of Ecology and Systematics that same year.

[29] In these ways, it differs from the modern lancelets, which have distinct pharyngeal gill slits on either sides of the pharynx and are used for filter feeding.

When alive, Pikaia was a compressed, leaf-shaped animal with an expanded tail fin; the flattened body is divided into pairs of segmented muscle blocks, seen as faint vertical lines.

The enteric canal as observed by Walcott was not an ordinary digestive tract, it runs along with a stiff rod that resembles a notochord.

"[19] Conway Morris was convinced that the longitudinal rod was a notochord and the segments were muscle blocks that he concluded that Pikaia "is a primitive chordate rather than a polychaete.

[35] That year, Harvard University palaeontologist Stephen Jay Gould wrote in his book Wonderful Life: The Burgess Shale and the Nature of History: "Pikaia is not an annelid worm.

[39] He also noticed that Pikaia is similar to Amphioxus in most general aspects, with major difference in its notochord not reaching the anterior end.

[22][23] In 2001, Nicholas D. Holland from the Scripps Institution of Oceanography and Junyuan Chen from the Chinese Academy of Sciences criticised the presentation in Wonderful Life, saying that the "reinterpretation [of Pikaia as a chordate] became almost universally accepted after its unqualified and forceful endorsement by Gould"; concluding that "the cephalochordate affinity of Pikaia is at best only weakly indicated by the characters visible in fossils discovered so far.

[41][42] Another component of Pikaia fossils that constrains the animal to be accepted as a chordate is its distinct invertebrate character; its preservational mode suggests that it had cuticle.

A fossil species Myoscolex ateles, discovered in 1979 from Cambrian Emu Bay shale of Kangaroo Island in South Australia, had been debated as among the oldest annelids, or at least other invertebrate groups.

Both were initially identified as polychaetes and this line of inference perhaps deserves confrontation with more recent evidence than that available to the authors who proposed these genera.

[3] However, the 2024 study suggested that Pikaia was previously interpreted upside down, indicating that the 'dorsal and ventral' sides of Pikiaia were actually inverted.

[4] The backside of Pikaia fossils show a hollow tubular structure that extends throughout most of the body length, but not the anterior region.

The true notochord, along with a nerve cord, is a fine lateral line that runs just beneath the thick dorsal organ.

The muscle segments are not simply "annular shiny lines" as Walcott described,[16] but are in concentric bends in the form of V-shaped chevron.

With a notochord and paired muscle blocks, the lancelet and Pikaia belong to the chordate group of animals from which the vertebrates descended.

Molecular studies have refuted earlier hypotheses that lancelets might be the closest living relative to the vertebrates, instead favoring tunicates in this position;[50] other extant and fossil groups, such as acorn worms and graptolites, are more primitive.

[51] Subsequently, Mallatt and Holland reconsidered Conway Morris and Caron's description, and concluded that many of the newly recognized characters are unique, already-divergent specializations that would not be helpful for establishing Pikaia as a basal chordate.

It is thought that development of a head structure resulted from a long body shape, a swimming habit, and a mouth at the end that came into contact with the environment first, as the animal swam forward.

The search for food required ways of continually testing what lay ahead so it is thought that anatomical structures for seeing, feeling, and smelling developed around the mouth.

[34] Once thought to be closely related to the ancestor of all vertebrates, Pikaia has received particular attention among the multitude of animal fossils found in the famous Burgess Shale and other Cambrian fauna.

Gould, in his presidential address of the Paleontological Society on 27 October 1988, cited Pikaia to explain the trends of evolutionary changes: Wind back life's tape to the Burgess (first erasing what actually came after), let it play again, and this time a quite different cast may emerge.

[54] He elaborated the same idea in "An epilogue on Pikaia" in his book Wonderful Life "to save the best for the last," in which he made a statement:Pikaia is the missing and final link in our story of contingency—the direct connection between Burgess decimation and eventual human evolution... Wind the tape of life back to Burgess times, and let it play again.