Unranked clade (stem group chordates)
Pikaia is considered to represent a very basal stem group chordate (Conway Morris, 1979; Conway Morris and Caron, 2012).
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.
Lectotype – USNM 57628 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: Pikaia cf. gracilens from the Collins Quarry on Fossil Ridge (Conway Morris and Caron, 2012).
Other deposits: none.
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott and Raymond Quarries on Fossil Ridge.
Brief history of research:
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 has also been interpreted as a stem-chordate (Smith et al., 2001) or a cephalochordate (Shu et al., 1999) and 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 – based on 114 specimens – supports a position of Pikaia as the most basal stem group chordate known to date (Conway Morris and Caron, 2012).
The body averages about 40 mm in length and 4.5 mm in height and is laterally flattened. There is evidence of a ventral keel and a thin dorsal fin. A posterior ventral area with faint segmental structures represents a possible ventral fin or an area of softer tissues preserving gonadal structures. The head is tiny (about 1.5% of the length of the body) and 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 nine pairs of small appendages on either side of the midline with potential pharyngeal pores at their bases. The function of these appendages is uncertain but possibly they had a respiratory role. The mouth is located just beneath the head and is followed by a large pharyngeal cavity often preserved in relief. The gut is not preserved but the anus is probably terminal. An anterior dorsal unit representing a semi-rigid dorsal structure just behind the head occupies about one fifth of the total length of the organism. Up to 100 myomeres or muscle bands are preserved along the entire length of the animal. A prominent dorsal structure called a dorsal organ, originally interpreted as a notochord (Conway Morris, 1979), runs down the entire length of the organism. The notochord and nerve chord are ventral to the dorsal organ. Parts of a vascular system are also preserved, including a ventral blood vessel.
Pikaia is relatively rare, with 114 specimens (the ROM has the largest collection with 60 specimens), most of them come from the Walcott Quarry (Conway Morris and Caron, 2012). Pikaia represents 0.03% of the specimens counted in the Walcott Quarry community (Caron and Jackson, 2008).
Nektonic, nektobenthic, mobile
Suspension or deposit feeder
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 suspension feeder or a deposit feeder.