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Pikaia gracilens
Pikaia gracilens (after Conway Morris and Caron 2012, pl.1a).
Pikaia gracilens was the first known fossil notochord, found in the Burgess Shale in British Columbia (Walcott 1911), dating from the Middle Cambrian period at 520 mya. The genus name Pikaia derives from Pika Peak, a mountain in Alberta, Canada, and the species name gracilens for its slender or gracile form. It belongs to the phylum Chordata, and the subphylum Cephalacordata.
The discoverer, Charles Walcott (1911), had originally classed the type specimen of P. gracilens in the U.S. National Museum as a polychaete or sea worm. Simon Conway Morris (1979) eventually identified Pikaia as an early fish-like chordate. Pikaia is now represented by a relatively large sample of 114 fossil specimens, many with excellent preservation, found in three quarries of the Burgess Shale. This very fine-grained shale of a type called a Lagerstatte has revealed a multitude of previously unknown marine organisms. Pikaia is compared by Morris and Caron (2012) to other Cephalochordata, including both the Early Cambrian Yunnanozoon (one of several early chordates found in a comparable Chinese Lagerstatte shale deposit), and the extant lancelet Branchiostoma.
The average body length of P. gracilens is 3.8 cm. It has a thin, flattened body with a notochord and regular segmentation, composed of paired muscle segments or myomeres. It lacked lateral fins, but had a dorsal fin, enabling it to swim in S-shaped movements, like an eel. Its head has a pair of antenna-like tentacles. A series of short appendages, which may be linked to gill slits, are on either side of its head. By comparison, Yunnanozoon, and three other Early Cambrian Chinese chordates grouped in the Myllokunminigidae family, also lacked lateral fins, and swam using their tail fins fin in S-shaped movements, much like an eel.
References:
Conway Morris 1979
Conway Morris and Caron 2012
Walcott 1911
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