Cambrian chordates
The first Cambrian chordate known is Pikaia gracilens, a lancelet-like animal from the Burgess Shale in British Columbia, Canada.
It was discovered by Charles Doolittle Walcott from the Burgess Shale in the mountains of British Columbia, Canada.
Italian palaeontologist Alberto M. Simonetta, while working at the US National Museum of Natural History, where Walcott's collections are maintained, became the first to analyse the specimen in 1960.
[18] He and Emilio Insom at the University of Camerino published the classification in 1993, giving the name Metaspriggina walcotti, an animal with unknown identity.
In 1991, Hou Xian-guang (of the Chinese Academy of Sciences), Lars Ramsköld and Jan Bergström (of the Swedish Museum of Natural History) reported a series of discoveries of Cambrian fossils from the Maotianshan Shales in Chengjiang County, Yunnan Province, China.
It may be noted that a thick cuticle with repetitive pattern is characteristic of some aschelminth groups, but of virtually no other extant animals apart from arthropods.
[19] The Maotianshan Shales have yielded other Cambrian chordates and chordate-related animals including Cathaymyrus species (C. diadexus and C. haikoensis), Haikouella species (H. lanceolata and H. jianshanensis, but possibly a type, synonym, of Yunnanozoon[20]), Myllokunmingia fengjiaoa, Shankouclava anningense, Zhongjianichthys rostratus, Zhongxiniscus intermedius, and a group called vetulicolians of uncertain classification.
[6] Pikaia gracilens was a primitive chordate having a lancelet-like body that lacked a well-defined head components and averaged about 3.8 cm in length.
[21] There are a series of short appendages on either side of the underside of the head just after the mouth, and their exact nature or function is unknown.
[22] A hollow tubular structure running from its anterior part of the body to the tail was earlier believed to be an indication of the presence of a notochord, a defining feature of all chordates from protochordates to mammals.
The notochord was reinterpreted to run underneath the dorsal organ a thin rod-like filament.
The body segments are anatomically blocks of skeletal muscles, called the myomeres, which are found in vertebrates only.
[24] The muscle orientation and flat shaped body indicate that Pikaia was an active and free swimmer.
The first two pairs are larger than the others and do not support any gills, a characteristic that suggests a distant relationship to gnathostomatans (jawed vertebrates from fish to humans).
[18] It lacked fins and it had a weakly developed cranium, but possessed two well-developed upward-facing eyes with nostrils behind them.
[6][31] Zhongxiniscus intermedius had a small, broad and short, fish-like body that was roughly 10 mm in length.
