Cambrian chordates

The Cambrian chordates are an extinct group of animals belonging to the phylum Chordata that lived during the Cambrian, between 538 and 485 million years ago. The first Cambrian chordate known is Pikaia gracilens, a lancelet-like animal from the Burgess Shale in British Columbia, Canada. The discoverer, Charles Doolittle Walcott, described it as a kind of worm (annelid) in 1911, but it was later identified as a chordate.^[1] Subsequent discoveries of other Cambrian fossils from the Burgess Shale in 1991,^[2] and from the Chengjiang biota of China in 1991,^[3] which were later found to be of chordates,^[4][5] several Cambrian chordates are known, with some fossils considered as putative chordates.^[6]

The Cambrian chordates are characterised by the presence of segmented muscle blocks called myomeres and notochord, the two defining features of chordates. Before the full understanding of Cambrian fossils, chordates as members the most advanced phylum were believed to appear on Earth much later than the Cambrian. However, the better picture of Cambrian explosion in the light of Cambrian chordates, according to Stephen Jay Gould, prompted "revised views of evolution, ecology and development," and remarked: "So much for chordate uniqueness marked by slightly later evolution."^[7]