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Pikaia gracilens

3D animation of Pikaia gracilens.

ANIMATION BY PHLESCH BUBBLE © ROYAL ONTARIO MUSEUM

Pikaia gracilens (USNM 57628b) – Syntype. Left, plate 20 of Walcott (1911), showing a retouched image of the original specimen described as a worm (figure 1 – figure 2 is the second syntype specimen, USNM 57629) together with Oesia disjuncta (figures 3-5). Right, images of the same specimen showing a contorted body, with head to the right. Specimen length = 53 mm. Specimen dry – direct light (top), dry – polarized light (bottom). Walcott Quarry.

Taxonomy:

Kingdom: Chordata

Phylum: Chordata

Higher Taxonomic assignment: Unranked clade (stem group chordates)

Species name: Pikaia gracilens

Remarks:

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.

Described by: Walcott

Description date: 1911

Etymology:

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.

Type Specimens: Syntypes –USNM57628b, 57629 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.

Other species:

Burgess Shale and vicinity: none.

Other deposits: none.

Age & Localities:

Age:

Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).

Principal localities:

The Walcott Quarry on Fossil Ridge.

History of Research:

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 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.).

Description:

Morphology:

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.

Abundance:

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).

Maximum Size:

55 mm

Ecology:

Life habits: Chordata

Feeding strategies: Chordata

Ecological Interpretations:

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.

References:

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.

GOULD, S. J. 1989. Wonderful Life. The Burgess Shale and the Nature of History. Norton, New York, 347 p.

JANVIER, P. 1998. Les vertébrés avant le Silurien. GeoBios, 30: 931-950.

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.

SMITH, M. P., I. J. SANSOM AND K. D. COCHRANE. 2001. The Cambrian origin of vertebrates, p. 67-84. In P. E. Ahlberg (ed.), Major Events in Early Vertebrate Evolution: Palaeontology, Phylogeny, Genetics and Development. Taylor and Francis, London.

WALCOTT, C. 1911. Cambrian Geology and Paleontology II. Middle Cambrian annelids. Smithsonian Miscellaneous Collections, 57(5): 109-145.

WALCOTT, C. 1931. Addenda to descriptions of Burgess Shale fossils. Smithsonian Miscellaneous Collections, 85(3): 1-46.

Metaspriggina walcotti

Metaspriggina walcotti (USNM 198611) – Holotype, part and counterpart. Lateral specimen showing clear myomeres; anterior to the right. Specimen length = 43 mm. Specimen dry – direct light (left column), dry – polarized light (right column). Walcott Quarry.

Metaspriggina walcotti (USNM 198612) – Lectotype, part and counterpart. Dorsoventral specimen showing the gut (left) in the narrow part of the body and cranium (to the right). Specimen length = 69 mm. Specimen dry – direct light (top left), dry – polarized light (right column), wet – polarized light (bottom left). Walcott Quarry.

Taxonomy:

Kingdom: Chordata

Phylum: Chordata

Higher Taxonomic assignment: Unranked clade (stem group chordates)

Species name: Metaspriggina walcotti

Remarks:

Metaspriggina is considered to represent a primitive chordate, possibly transitional between cephalochordates and the earliest vertebrates (Conway Morris, 2008).

Described by: Simonetta and Insom

Description date: 1993

Etymology:

Metaspriggina – from the Greek meta, “in company with, or later in time,” and the morphologically similar Ediacaran organism Spriggina (which is no longer thought to be related). Spriggina honours Reg Sprigg, discoverer of the Precambrian fossils of the Ediacara Hills in Australia.

walcotti – after Charles Walcott, discoverer of the Burgess Shale.

Type Specimens: Lectotype –USNM198612 and former holotype 198611 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.

Other species:

Burgess Shale and vicinity: none.

Other deposits: none.

Age & Localities:

Age:

Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).

Principal localities:

The Walcott Quarry on Fossil Ridge.

History of Research:

Brief history of research:

Set aside by Walcott for further study, the two known specimens of this species were briefly examined by Conway Morris (1979). Simonetta and Insom (1993) described one of the two specimens (the original holotype specimen) as a potential relative of the Ediacaran organism Spriggina, whereas the second specimen (now the lectotype) was interpreted as a potential chordate. A chordate interpretation for both specimens was proposed (Janvier, 1998; Smith et al., 2001) and a detailed redescription was eventually instigated by Conway Morris (2008) with both specimens being included in the same genus and species.

Description:

Morphology:

Metaspriggina is elongate in shape with a small anterior cranial region and a long triangular and laterally flattened trunk; there is no evidence of fins. The larger of the two known fossil specimens is around 7 cm in length. Both specimens possess numerous V-shaped or zig-zag segments interpreted as myomeres or muscle bands. A narrow central structure runs down the length of the organism and is interpreted as a gut. The front of one specimen appears to show a rudimentary cranium which is poorly preserved and seems to have lacked eyes.

Abundance:

M. walcotti is very rare in the Walcott Quarry, known from just two specimens.

Maximum Size:

69 mm

Ecology:

Life habits: Chordata

Feeding strategies: Chordata

Ecological Interpretations:

With only two specimens, and poor preservation of the head, the diet and feeding habits of Metaspriggina remain a mystery. The rarity of fossils suggests that the animal was likely free-swimming, which is consistent with its musculature, although it is possible that it also spent some time on the sea floor.

References:

CONWAY MORRIS, S. 1979. The Burgess Shale (Middle Cambrian) fauna. Annual Review of Ecology and Systematics, 10(1): 327-349.

CONWAY MORRIS, S. 2008. A redescription of a rare chordate, Metaspriggina walcottiSimonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada. Journal of Paleontology, 82(2): 424-430.

JANVIER, P. 1998. Les vertébrés avant le Silurien. GeoBios, 30: 931-950.

SIMONETTA, A. M. AND E. INSOM. 1993. New animals from the Burgess Shale (Middle Cambrian) and their possible significance for the understanding of the Bilateria. Bolletino di Zoologia, 60(1): 97 – 107.