Pikaia
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Pikaia gracilens is an extinct, primitive chordate marine animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in
Fossil of Pikaia gracilens (Syntype USNM PAL 57628)
Plate 20 of Walcott Cambrian Geology and Paleontology II (1911), by Charles Doolittle Walcott, showing fossils of Pikaia and Oesia
Notochord
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The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus
Notochord regression anomaly: linear pattern of indentations in vertebral discs.
A dissected spotted African lungfish showing the notochord