Pikaia is considered to represent a primitive chordate (Conway Morris, 1979; Conway Morris et al., 1982) possibly close to craniates (Janvier, 1998); a stem-chordate (Smith et al., 2001); or a cephalochordate (Shu et al., 1999). Its exact position within the chordates is still uncertain and this animal awaits a full redescription.
Pikaia – from the pika, a small alpine mammal and cousin of the rabbits. Pikas live in the Rocky Mountains, including near the Burgess Shale.
gracilens – from the Latin gracilens, “thin, simple,” in reference to the shape of the body.
Burgess Shale and vicinity: none.
Other deposits: none.
The Walcott Quarry on Fossil Ridge.
Pikaia was first described by Walcott based on a couple of specimens in a 1911 monograph dealing with various Burgess Shale worms. Two additional specimens were figured in a posthumous publication (Walcott, 1931). Walcott placed Pikaia in a now defunct group called the Gephyrea with other vermiform fossils such as Banffia, Ottoia and Oesia. Pikaia was later considered to be a primitive chordate (Conway Morris, 1979; Conway Morris et al., 1982), an interpretation which has since been followed to some degree in most discussions about early chordate evolution (e.g., Janvier, 1998). Pikaia played a major part in Gould’s interpretations of the Burgess Shale fossils in Wonderful Life (Gould, 1989; see also Briggs and Fortey, 2005). A full redescription of this animal is currently under way (Conway Morris and Caron, in prep.).
Pikaia resembles Metaspriggina in outline, another chordate animal from the Burgess Shale, with an elongate body and a small anterior region bearing the head. The body is laterally flattened and there is evidence of a ventral fin towards the posterior. Numerous V-shaped or ziz-zag segments interpreted as myomeres or muscle bands are visible in all specimens. A narrow dorsal structure which runs down the length of the organism might represent a notochord, but this interpretation remains to be confirmed. The head bears two equal lobes and a pair of short and slender tentacle-like structures. There is no evidence of eyes. Just behind the head, on the ventral side of the body, there is a series of up to twelve pairs of small, short, pointed structures on either side of the midline. These are thought to be related to gill openings. The gut is narrow and the anus is terminal.
Pikaia is relatively rare, known from more than 60 specimens, all from the Walcott Quarry where it represents 0.03% of the specimens counted in the community (Caron and Jackson, 2008).
The eel-like morphology and musculature of the animal suggest that it was likely free-swimming, although it probably spent time on the sea floor. The tentacles may have had a sensory function, and the presence of mud in its gut suggests that Pikaia was potentially a deposit feeder.
BRIGGS, D. E. G. AND R. A. FORTEY. 2005. Wonderful strife: Systematics, stem groups, and the phylogenetic signal of the Cambrian radiation. Paleobiology, 31(SUPPL.2 ): 94-112.
CONWAY MORRIS, S. 1979. The Burgess Shale (Middle Cambrian) fauna. Annual Review of Ecology and Systematics, 10(1): 327-349.
CARON, J.-B. AND D. A. JACKSON. 2008. Paleoecology of the Greater Phyllopod Bed community, Burgess Shale. Palaeogeography, Palaeoclimatology, Palaeoecology, 258: 222-256.
CONWAY MORRIS, S. H. B. WHITTINGTON, D. E. G. BRIGGS, C. P. HUGHES AND D. L. BRUTON. 1982. Atlas of the Burgess Shale. Palaeontological Association, 31 p. + 23 pl.
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SHU, D.-G,. H. L. LUO, S. CONWAY MORRIS, X. L. ZHANG, S. X. HU, L. CHEN, J. HAN, M. ZHU, Y. LI AND L. Z. CHEN. 1999. Lower Cambrian vertebrates from south China. Nature, 402(4 November 1999): 42-46.
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WALCOTT, C. 1931. Addenda to descriptions of Burgess Shale fossils. Smithsonian Miscellaneous Collections, 85(3): 1-46.