Pikaia gracilens
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Pikaia gracilens was a primitive chordate known from the Mid-Cambrian in what is now Alberta, Canada. It resembled a modern-day lancelet fish with a small head and a 4 centimeter long, laterally compressed body showing signs of segmentation from bands of striated muscle tissue.
It was this superficial segmentation that led its discoverer, to classify it as a polychaete worm. In his reassessment of the Burgess Shale fossils, Simon Conway Morris reclassified P. gracilens as a chordate.
Pikaia gracilens appears to be a primitive chordate that evolved out of a segmented worm.