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Pikaia
''Pikaia gracilens'' is an extinct genus of chordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Sixteen specimens are known from the Greater Phyllopod bed, where they comprised 0.03% of the community.〔("''Pikaia gracilens''" ) ''Burgess Shale Fossil Gallery''. Virtual Museum of Canada. 2011.〕 It resembles the lancelet and perhaps swam much like an eel.
==Discovery==
''P. gracilens'' was discovered by Charles Walcott and first described by him in 1911. It was named after Pika Peak, a mountain in Alberta, Canada. Based on the obvious and regular segmentation of the body, Walcott classified it as a polychaete worm.
During his re-examination of the Burgess Shale fauna in 1979, paleontologist Simon Conway Morris placed ''P. gracilens'' among the chordates, making it perhaps the oldest known ancestor of modern vertebrates. He did this because it seemed to have a very primitive, proto-notochord, however, the status of ''Pikaia'' as a chordate is not universally accepted; its preservational mode suggests that it had cuticle, which is uncharacteristic of the vertebrates (although characteristic of other cephalochordates); further, its tentacles are unknown from other vertebrate lineages.〔 The presence of earlier chordates among the Chengjiang, including ''Haikouichthys'' and ''Myllokunmingia'', appears to show that cuticle is not necessary for preservation, overruling the taphonomic argument, but the presence of tentacles remains intriguing, and the organism cannot be assigned conclusively, even to the vertebrate stem group. Its anatomy closely resembles the modern creature ''Branchiostoma''.
Averaging about in length, ''Pikaia'' swam above the sea floor using its body and an expanded tail fin. ''Pikaia'' may have filtered particles from the water as it swam along. Its "tentacles" may be comparable to those in the present-day hagfish, a jawless chordate. Only 60 specimens have been found to date.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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