Pikaia: Difference between revisions

Pikaia; Temporal range: Early Cambrian to Middle Cambrian (Stage 3 to Wuliuan), 513–505 Ma PreꞒ Ꞓ O S D C P T J K Pg N
	Life reconstruction of Pikaia gracilens
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Family:	†Pikaiidae
Genus:	†Pikaia; Walcott, 1911
Species:	†P. gracilens
Binomial name
	†Pikaia gracilens; Walcott, 1911

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Pikaia gracilens is an extinct, primitive chordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway Morris as a chordate, it "became famously known as the earliest" of Cambrian chordates and is estimated to have lived during the latter period of the Cambrian explosion.^[1] Since it initial discovery, more than a hundred specimens have been recovered.^[2]

The body structure resembles that of the lancelet and perhaps swam much like an eel. A notochord and segmented blocks of skeletal muscles (myomeres) span the entire length of the body, and are considered as the defining signatures of chordate characters. Its primitive feature is indicated by the body covering, a cuticle, which is characteristic of invertebrates and some protochordates.

The exact phylogenetic position is unclear. Proposed affinities include those of cephalochordata, craniata, or a stem-chordate not closely related to any extant lineage.^[3] Popularly but falsely^[4] attributed to as ancestor of all vertebrates,^[5] or the oldest fish,^[6] or the oldest ancestor of humans,^[7]^[8] it is generally viewed as a basal chordate.^[1]^[9]

Discovery

The fossils of Pikaia gracilens was discovered by Charles Walcott from the Burgess shale member of the Stephen formation in British Columbia, and described it in 1911. He named it after Pika Peak, a mountain in Alberta, Canada. Based on the obvious and regular segmentation of the body, as is the feature of annelids,^[10] Walcott classified it as a polychaete worm and created a new family Pikaidae for it. (The family named was changed to Pikaiidae in 1962 by Princeton palaeontologist Benjamin Franklin Howell.^[11]) Walcott was aware of the limitation of his classification, as he noted: "I am unable to place it within any of the families of the Polychaeta, owing to the absence of parapodia [paired protrusions on the sides of polychaete worms] on the body segments back of the fifth."^[12]

In 1979, University of Cambridge palaeontologist Harry B. Whittington and his student Simon Conway Morrisre-examined the Burgess Shale fauna and noted the anatomical details of Pikaia for the first time. The fossil specimens bears features of notochord and muscle blocks that are fundamental structures of chordates, and not of annelids. The animal was obviously a chordate.^[13] As they wrote in the Scientific American:

Finally, we find among the Burgess Shale fauna one of the earliest-known invertebrate representatives of our own conspicuous corner of the animal kingdom: the chordate phylum... The chordates are represented in the Burgess Shale by the genus Pikaia and the single species P. gracilens.^[14]

Conway Morris formally placed P. gracilens among the chordates in a paper in the Annual Review of Ecology and Systematics in 1979.^[15] However, there were no structural analyses such as using microscopes to confirm the chordate features. The comparative description only earned a "putative" chordate status.^[16] As late as the the 1990s, there were sceptical remarks on the chordate nature.^[17]^[18]

Description

Pikaia was a primitive chordate that lacked a well-defined head and averaged about 1+1⁄2 inches (38 mm) in length. Walcott recorded the longest individuals as 5 cm in length.^[12] Once thought to be closely related to the ancestor of all vertebrates, it has for that reason received particular attention among the multitude of animal fossils found in the famous Burgess Shale in the mountains of British Columbia, Canada. Pikaia had a pair of large, antenna-like tentacles on its head, and a series of short appendages, which may be linked to gill slits, on either side of its head. In these ways, it differs from the modern lancelet. The "tentacles" on its head may be comparable to those in the present-day hagfish, a jawless chordate.^[19]

Although primitive, Pikaia shows the essential prerequisites for vertebrates. When alive, Pikaia was a compressed, leaf-shaped animal with an expanded tail fin; the flattened body is divided into pairs of segmented muscle blocks, seen as faint vertical lines. The muscles lie on either side of a flexible structure resembling a rod that runs from the tip of the head to the tip of the tail.^[20] It likely swam by throwing its body into a series of S-shaped, zigzag curves, similar to the movement of eels; fish inherited the same swimming movement, but they generally have stiffer backbones. These adaptations may have allowed Pikaia to filter particles from the water as it swam along.^[19] Pikaia was probably a slow swimmer, since it lacked the fast-twitch fibers that are associated with rapid swimming in modern chordates.^[19]

Reinterpretations

Walcott's original summary of the description of Pikaia reads:

Body elongate, slender, and tapering at each end. It is formed of many segments that are defined by strong annular shiny lines. Head small with two large eyes and two tentacles... Back of the head the first five segments carry short parapodia that appear to be divided into two parts. The enteric canal extends from end to end without change in character... This was one of the active, free-swimming annelids that suggest the Nephthydidae of the Polychaeta.^[12]

making it perhaps the oldest known ancestor of modern vertebrates. He did this because it seemed to have a very primitive, proto-notochord, however, the status of Pikaia as a chordate is not universally accepted; its preservational mode suggests that it had cuticle, which is uncharacteristic of the vertebrates ^[21] (although characteristic of other cephalochordates); further, its tentacles are unknown from other vertebrate lineages.^[21] The presence of earlier chordates among the Chengjiang, including Haikouichthys and Myllokunmingia, appears to show that cuticle is not necessary for preservation, overruling the taphonomic argument,^[22] but the presence of tentacles remains intriguing, and the organism cannot be assigned conclusively, even to the vertebrate stem group. Its anatomy closely resembles the modern creature Branchiostoma.^[23]

Conway Morris and Caron (2012) published an exhaustive description based on all 114 of the known fossil specimens; they discovered new and unexpected characteristics ^[which?] that they recognized as primitive features of the first chordate animals. On the basis of these findings, they constructed a new scenario for chordate evolution.^[24] Subsequently, Mallatt and Holland reconsidered Conway Morris and Caron's description, and concluded that many of the newly recognized characters are unique, already-divergent specializations that would not be helpful for establishing Pikaia as a basal chordate.^[9]

Evolution

Much debate on whether Pikaia is a vertebrate ancestor, its worm-like appearance notwithstanding, exists in scientific circles. It looks like a worm that has been flattened sideways (lateral compression). The fossils compressed within the Burgess Shale show chordate features such as traces of an elongate notochord, dorsal nerve cord, and blocks of muscles (myotomes) down either side of the body – all critical features for the evolution of the vertebrates.

The notochord, a flexible rod-like structure that runs along the back of the animal, lengthens and stiffens the body so that it can be flexed from side to side by the muscle blocks for swimming. In the fish and all subsequent vertebrates, the notochord forms the backbone (or vertebral column). The backbone strengthens the body, supports strut-like limbs, and protects the vital dorsal nerve cord, while at the same time allowing the body to bend.

A Pikaia lookalike, the lancelet Branchiostoma, still exists today. With a notochord and paired muscle blocks, the lancelet and Pikaia belong to the chordate group of animals from which the vertebrates descended. Molecular studies have refuted earlier hypotheses that lancelets might be the closest living relative to the vertebrates, instead favoring tunicates in this position;^[25] other extant and fossil groups, such as acorn worms and graptolites, are more primitive.^[a]

The presence of a creature as complex as Pikaia some 530 million years ago reinforces the controversial view that the diversification of life must have extended back well before Cambrian times – perhaps deep into the Precambrian.^[20] The discovery of Metaspriggina, a primitive fish of similar age which seems to have already started to develop a jaw and the presence of conodonts, the teeth of an extinct type of fish belonging to the Agnatha some 20 MA before Pikaia, does provide support for that view.

Development of the head

The first sign of head development, cephalization, is seen in chordates such as Pikaia and Branchiostoma. It is thought that development of a head structure resulted from a long body shape, a swimming habit, and a mouth at the end that came into contact with the environment first, as the animal swam forward. The search for food required ways of continually testing what lay ahead so it is thought that anatomical structures for seeing, feeling, and smelling developed around the mouth. The information these structures gathered was processed by a swelling of the nerve cord (efflorescence) – the precursor of the brain. Altogether, these front-end structures formed the distinct part of the vertebrate body known as the head.^[20]

Notes

^ Acorn worms are classified as hemichordates, which have a notochord-like structure only at the early stage of their lives.

References

^ ^a ^b McMenamin, Mark A. S. (2019). "Cambrian Chordates and Vetulicolians". Geosciences. 9 (8): 354. doi:10.3390/geosciences9080354. ISSN 2076-3263.{{cite journal}}: CS1 maint: unflagged free DOI (link)
^ Morris, Simon Conway; Caron, Jean-Bernard (2012). "Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia". Biological Reviews. 87 (2): 480–512. doi:10.1111/j.1469-185X.2012.00220.x.
^ "Pikaia gracilens" Burgess Shale Fossil Gallery. Virtual Museum of Canada. 2011.
^ Danchin, Etienne G.J.; Pontarotti, Pierre (2004). "Towards the reconstruction of the bilaterian ancestral pre-MHC region". Trends in Genetics. 20 (12): 587–591. doi:10.1016/j.tig.2004.09.009.
^ SciTechDaily (2012-03-06). "Pikaia Gracilens Revealed as Possible Genetic Ancestors to All Vertebrates". SciTechDaily. Retrieved 2022-09-23.
^ "The Oldest Fish in the World Lived 500 Million Years Ago". Discover Magazine. Retrieved 2022-09-23.
^ "Worm-like creature could be humans' oldest ancestor". 2012-03-05. Retrieved 2022-09-23.
^ "Human's oldest ancestor found". University of Cambridge. 2012-03-06. Retrieved 2022-09-23.
^ ^a ^b Mallatt, Jon; Holland, Nicholas (2013). "Pikaia gracilens Walcott: Stem Chordate, or Already Specialized in the Cambrian?". Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. 320 (4): 247–271. doi:10.1002/jez.b.22500. PMID 23606659.
^ Balavoine, Guillaume (2014). "Segment formation in Annelids: patterns, processes and evolution". The International Journal of Developmental Biology. 58 (6–8): 469–483. doi:10.1387/ijdb.140148gb. ISSN 1696-3547. PMID 25690963.
^ Howell, B. F. (1962). "Worms". In Moore (ed.). Treatise on Invertebrate Paleontology Part W. Miscellanea (R. C. ed.). New York: Geological Society of America and University of Kansas Press. pp. W144–W177. ISBN 978-0-8137-3024-0.
^ ^a ^b ^c Walcott, Charles D. (1911). "Cambrian Geology and Paleontology II: No.5--Middle Cambrian Annelids" (PDF). Smithsonian Miscellaneous Collections. 57 (5): 109–144.
^ Whittington, H.B. (1980). "The significance of the fauna of the Burgess Shale, Middle Cambrian, British Columbia". Proceedings of the Geologists' Association. 91 (3): 127–148. doi:10.1016/S0016-7878(80)80034-4.
^ Morris, Simon Conway; Whittington, H. B. (1979). "The Animals of the Burgess Shale". Scientific American. 241 (1): 122–135. doi:10.2307/24965247. ISSN 0036-8733. JSTOR 24965247.
^ Morris, Simon Conway (1979). "The Burgess Shale (Middle Cambrian) Fauna". Annual Review of Ecology and Systematics. 10: 327–349. ISSN 0066-4162.
^ Butterfield, Nicholas J. (1990). "Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale". Paleobiology. 16 (3): 272–286. doi:10.1017/S0094837300009994. ISSN 0094-8373.
^ Insom, Emilio; Pucci, Antonella; Simonetta, Alberto M. (1995). "Cambrian Protochordata, their origin and significance". Bolletino di zoologia. 62 (3): 243–252. doi:10.1080/11250009509356072. ISSN 0373-4137.
^ Hall, Brian K. (1992), Hall, Brian K. (ed.), "The Burgess Shale: different types of animals", Evolutionary Developmental Biology, Dordrecht: Springer Netherlands, pp. 13–37, doi:10.1007/978-94-015-7926-1_2, ISBN 978-94-015-7926-1, retrieved 2022-09-23
^ ^a ^b ^c Lacalli, T. (2012). "The Middle Cambrian fossil Pikaia and the evolution of chordate swimming". EvoDevo. 3 (1): 12. doi:10.1186/2041-9139-3-12. PMC 3390900. PMID 22695332.{{cite journal}}: CS1 maint: unflagged free DOI (link)
^ ^a ^b ^c Palmer, D., (2000). The Atlas of the Prehistoric World. London: Marshall Publishing Ltd. p66-67.
^ ^a ^b Butterfield, N. J. (1990), "Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale", Paleobiology, 16 (3): 272–286, doi:10.1017/S0094837300009994, JSTOR 2400788
^ Conway Morris, S. (2008), "A Redescription of a Rare Chordate, Metaspriggina walcotti Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada", Journal of Paleontology, 82 (2): 424–430, doi:10.1666/06-130.1, S2CID 85619898, retrieved 2009-04-28
^ Donoghue, P. C. J.; Purnell, M. A. (2005), "Genome duplication, extinction and vertebrate evolution" (PDF), Trends in Ecology & Evolution, 20 (6): 312–319, doi:10.1016/j.tree.2005.04.008, PMID 16701387, archived from the original (PDF) on 2008-12-17, retrieved 2008-11-06
^ Conway Morris, S.; Caron, J. B. (2012) "Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia". Biological Reviews 87: 480-512.
^ Delsuc; et al. (2008). "Additional molecular support for the new chordate phylogeny" (PDF). Genesis. 46 (11): 592–604. doi:10.1002/dvg.20450. PMID 19003928.

External links

[26] Acorn worms are classified as hemichordates, which have a notochord-like structure only at the early stage of their lives.

[:0-1] McMenamin, Mark A. S. (2019). "Cambrian Chordates and Vetulicolians". Geosciences. 9 (8): 354. doi:10.3390/geosciences9080354. ISSN 2076-3263.{{cite journal}}: CS1 maint: unflagged free DOI (link)

[:3-2] Morris, Simon Conway; Caron, Jean-Bernard (2012). "Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia". Biological Reviews. 87 (2): 480–512. doi:10.1111/j.1469-185X.2012.00220.x.

[gallery-3] "Pikaia gracilens" Burgess Shale Fossil Gallery. Virtual Museum of Canada. 2011.

[4] Danchin, Etienne G.J.; Pontarotti, Pierre (2004). "Towards the reconstruction of the bilaterian ancestral pre-MHC region". Trends in Genetics. 20 (12): 587–591. doi:10.1016/j.tig.2004.09.009.

[5] SciTechDaily (2012-03-06). "Pikaia Gracilens Revealed as Possible Genetic Ancestors to All Vertebrates". SciTechDaily. Retrieved 2022-09-23.

[6] "The Oldest Fish in the World Lived 500 Million Years Ago". Discover Magazine. Retrieved 2022-09-23.

[7] "Worm-like creature could be humans' oldest ancestor". 2012-03-05. Retrieved 2022-09-23.

[8] "Human's oldest ancestor found". University of Cambridge. 2012-03-06. Retrieved 2022-09-23.

[:1-9] Mallatt, Jon; Holland, Nicholas (2013). "Pikaia gracilens Walcott: Stem Chordate, or Already Specialized in the Cambrian?". Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. 320 (4): 247–271. doi:10.1002/jez.b.22500. PMID 23606659.

[10] Balavoine, Guillaume (2014). "Segment formation in Annelids: patterns, processes and evolution". The International Journal of Developmental Biology. 58 (6–8): 469–483. doi:10.1387/ijdb.140148gb. ISSN 1696-3547. PMID 25690963.

[11] Howell, B. F. (1962). "Worms". In Moore (ed.). Treatise on Invertebrate Paleontology Part W. Miscellanea (R. C. ed.). New York: Geological Society of America and University of Kansas Press. pp. W144–W177. ISBN 978-0-8137-3024-0.

[:2-12] Walcott, Charles D. (1911). "Cambrian Geology and Paleontology II: No.5--Middle Cambrian Annelids" (PDF). Smithsonian Miscellaneous Collections. 57 (5): 109–144.

[13] Whittington, H.B. (1980). "The significance of the fauna of the Burgess Shale, Middle Cambrian, British Columbia". Proceedings of the Geologists' Association. 91 (3): 127–148. doi:10.1016/S0016-7878(80)80034-4.

[14] Morris, Simon Conway; Whittington, H. B. (1979). "The Animals of the Burgess Shale". Scientific American. 241 (1): 122–135. doi:10.2307/24965247. ISSN 0036-8733. JSTOR 24965247.

[15] Morris, Simon Conway (1979). "The Burgess Shale (Middle Cambrian) Fauna". Annual Review of Ecology and Systematics. 10: 327–349. ISSN 0066-4162.

[16] Butterfield, Nicholas J. (1990). "Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale". Paleobiology. 16 (3): 272–286. doi:10.1017/S0094837300009994. ISSN 0094-8373.

[17] Insom, Emilio; Pucci, Antonella; Simonetta, Alberto M. (1995). "Cambrian Protochordata, their origin and significance". Bolletino di zoologia. 62 (3): 243–252. doi:10.1080/11250009509356072. ISSN 0373-4137.

[18] Hall, Brian K. (1992), Hall, Brian K. (ed.), "The Burgess Shale: different types of animals", Evolutionary Developmental Biology, Dordrecht: Springer Netherlands, pp. 13–37, doi:10.1007/978-94-015-7926-1_2, ISBN 978-94-015-7926-1, retrieved 2022-09-23

[Lacalli2012-19] Lacalli, T. (2012). "The Middle Cambrian fossil Pikaia and the evolution of chordate swimming". EvoDevo. 3 (1): 12. doi:10.1186/2041-9139-3-12. PMC 3390900. PMID 22695332.{{cite journal}}: CS1 maint: unflagged free DOI (link)

[autogenerated1-20] Palmer, D., (2000). The Atlas of the Prehistoric World. London: Marshall Publishing Ltd. p66-67.

[Butterfield1990-21] Butterfield, N. J. (1990), "Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale", Paleobiology, 16 (3): 272–286, doi:10.1017/S0094837300009994, JSTOR 2400788

[Morris2008-22] Conway Morris, S. (2008), "A Redescription of a Rare Chordate, Metaspriggina walcotti Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada", Journal of Paleontology, 82 (2): 424–430, doi:10.1666/06-130.1, S2CID 85619898, retrieved 2009-04-28

[Donoghue2005-23] Donoghue, P. C. J.; Purnell, M. A. (2005), "Genome duplication, extinction and vertebrate evolution" (PDF), Trends in Ecology & Evolution, 20 (6): 312–319, doi:10.1016/j.tree.2005.04.008, PMID 16701387, archived from the original (PDF) on 2008-12-17, retrieved 2008-11-06

[conway2012-24] Conway Morris, S.; Caron, J. B. (2012) "Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia". Biological Reviews 87: 480-512.

[25] Delsuc; et al. (2008). "Additional molecular support for the new chordate phylogeny" (PDF). Genesis. 46 (11): 592–604. doi:10.1002/dvg.20450. PMID 19003928.

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 The body structure resembles that of the [[lancelet]] and perhaps swam much like an [[eel]]. A notochord and segmented blocks of skeletal muscles (myomeres) span the entire length of the body, and are considered as the defining signatures of chordate characters. Its primitive feature is indicated by the body covering, a cuticle, which is characteristic of invertebrates and some protochordates.
-The exact phylogenetic position is unclear. Proposed affinities include [[cephalochordata]], [[craniata]], or a stem-chordate not closely related to any extant lineage.<ref name="gallery">[http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=101 "''Pikaia gracilens''"] ''Burgess Shale Fossil Gallery''. Virtual Museum of Canada. 2011.</ref> It is generally viewed as a [[Basal (phylogenetics)|basal]] chordate.<ref name=":0" /><ref name=":1">{{Cite journal |last=Mallatt |first=Jon |last2=Holland |first2=Nicholas |date=2013 |title=Pikaia gracilens Walcott: Stem Chordate, or Already Specialized in the Cambrian? |url=https://onlinelibrary.wiley.com/doi/10.1002/jez.b.22500 |journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |language=en |volume=320 |issue=4 |pages=247–271 |doi=10.1002/jez.b.22500 |pmid=23606659}}</ref>
+The exact phylogenetic position is unclear. Proposed affinities include those of [[cephalochordata]], [[craniata]], or a stem-chordate not closely related to any extant lineage.<ref name="gallery">[http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=101 "''Pikaia gracilens''"] ''Burgess Shale Fossil Gallery''. Virtual Museum of Canada. 2011.</ref> Popularly but falsely<ref>{{Cite journal |last=Danchin |first=Etienne G.J. |last2=Pontarotti |first2=Pierre |date=2004 |title=Towards the reconstruction of the bilaterian ancestral pre-MHC region |url=https://linkinghub.elsevier.com/retrieve/pii/S0168952504002768 |journal=Trends in Genetics |language=en |volume=20 |issue=12 |pages=587–591 |doi=10.1016/j.tig.2004.09.009}}</ref> attributed to as ancestor of all vertebrates,<ref>{{Cite web |last=SciTechDaily |date=2012-03-06 |title=Pikaia Gracilens Revealed as Possible Genetic Ancestors to All Vertebrates |url=https://scitechdaily.com/pikaia-gracilens-revealed-as-possible-genetic-ancestors-to-all-vertebrates/ |access-date=2022-09-23 |website=SciTechDaily |language=en-us}}</ref> or the oldest fish,<ref>{{Cite web |title=The Oldest Fish in the World Lived 500 Million Years Ago |url=https://www.discovermagazine.com/planet-earth/the-oldest-fish-in-the-world-lived-500-million-years-ago |access-date=2022-09-23 |website=Discover Magazine |language=en}}</ref> or the oldest ancestor of humans,<ref>{{Cite news |date=2012-03-05 |title=Worm-like creature could be humans' oldest ancestor |language=en-GB |url=https://www.bbc.co.uk/newsround/17261566 |access-date=2022-09-23}}</ref><ref>{{Cite web |date=2012-03-06 |title=Human's oldest ancestor found |url=https://www.cam.ac.uk/research/news/humans-oldest-ancestor-found |access-date=2022-09-23 |website=University of Cambridge |language=en}}</ref> it is generally viewed as a [[Basal (phylogenetics)|basal]] chordate.<ref name=":0" /><ref name=":1">{{Cite journal |last=Mallatt |first=Jon |last2=Holland |first2=Nicholas |date=2013 |title=Pikaia gracilens Walcott: Stem Chordate, or Already Specialized in the Cambrian? |url=https://onlinelibrary.wiley.com/doi/10.1002/jez.b.22500 |journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |language=en |volume=320 |issue=4 |pages=247–271 |doi=10.1002/jez.b.22500 |pmid=23606659}}</ref>
 ==Discovery==
-The fossils of ''Pikaia gracilens'' was discovered by [[Charles Doolittle Walcott|Charles Walcott]] from the [[Burgess shale]] member of the [[Stephen formation]] in British Columbia, and described it in 1911. He named it after [[Pika Peak]], a mountain in [[Alberta]], Canada. Based on the obvious and regular segmentation of the body, as is the feature of annelids,<ref>{{Cite journal |last=Balavoine |first=Guillaume |date=2014 |title=Segment formation in Annelids: patterns, processes and evolution |url=https://pubmed.ncbi.nlm.nih.gov/25690963 |journal=The International Journal of Developmental Biology |volume=58 |issue=6-8 |pages=469–483 |doi=10.1387/ijdb.140148gb |issn=1696-3547 |pmid=25690963}}</ref> Walcott classified it as a [[Polychaeta|polychaete]] worm and created a new family Pikaidae for it.<ref name=":3" /> (The family named was changed to Pikaiidae in 1962 by [[Princeton University|Princeton]] palaeontologist Benjamin Franklin Howell.<ref>{{Cite book |last=Howell |first=B. F. |url=http://paleo.ku.edu/treatise2/parts/w.html |title=Treatise on Invertebrate Paleontology Part W. Miscellanea |publisher=Geological Society of America and University of Kansas Press |year=1962 |isbn=978-0-8137-3024-0 |editor-last=Moore |edition=R. C. |location=New York |pages=W144–W177 |language=en |chapter=Worms}}</ref>) Walcott was aware of the limitation of his classification, as he noted: "I am unable to place it within any of the families of the Polychaeta, owing to the absence of [[parapodia]] [paired protrusions on the sides of polychaete worms] on the body segments back of the fifth."<ref name=":2">{{Cite journal |last=Walcott |first=Charles D. |date=1911 |title=Cambrian Geology and Paleontology II: No.5--Middle Cambrian Annelids |url=https://repository.si.edu/bitstream/handle/10088/34820/SMC_57_Walcott_1910_5_109-144.pdf?sequence=1&isAllowed=y |journal=Smithsonian Miscellaneous Collections |volume=57 |issue=5 |pages=109–144}}</ref>
+The fossils of ''Pikaia gracilens'' was discovered by [[Charles Doolittle Walcott|Charles Walcott]] from the [[Burgess shale]] member of the [[Stephen formation]] in British Columbia, and described it in 1911. He named it after [[Pika Peak]], a mountain in [[Alberta]], Canada. Based on the obvious and regular segmentation of the body, as is the feature of annelids,<ref>{{Cite journal |last=Balavoine |first=Guillaume |date=2014 |title=Segment formation in Annelids: patterns, processes and evolution |url=https://pubmed.ncbi.nlm.nih.gov/25690963 |journal=The International Journal of Developmental Biology |volume=58 |issue=6-8 |pages=469–483 |doi=10.1387/ijdb.140148gb |issn=1696-3547 |pmid=25690963}}</ref> Walcott classified it as a [[Polychaeta|polychaete]] worm and created a new family Pikaidae for it. (The family named was changed to Pikaiidae in 1962 by [[Princeton University|Princeton]] palaeontologist Benjamin Franklin Howell.<ref>{{Cite book |last=Howell |first=B. F. |url=http://paleo.ku.edu/treatise2/parts/w.html |title=Treatise on Invertebrate Paleontology Part W. Miscellanea |publisher=Geological Society of America and University of Kansas Press |year=1962 |isbn=978-0-8137-3024-0 |editor-last=Moore |edition=R. C. |location=New York |pages=W144–W177 |language=en |chapter=Worms}}</ref>) Walcott was aware of the limitation of his classification, as he noted: "I am unable to place it within any of the families of the Polychaeta, owing to the absence of [[parapodia]] [paired protrusions on the sides of polychaete worms] on the body segments back of the fifth."<ref name=":2">{{Cite journal |last=Walcott |first=Charles D. |date=1911 |title=Cambrian Geology and Paleontology II: No.5--Middle Cambrian Annelids |url=https://repository.si.edu/bitstream/handle/10088/34820/SMC_57_Walcott_1910_5_109-144.pdf?sequence=1&isAllowed=y |journal=Smithsonian Miscellaneous Collections |volume=57 |issue=5 |pages=109–144}}</ref>
+In 1979, [[University of Cambridge]] palaeontologist [[Harry B. Whittington]] and his student [[Simon Conway Morris]]<nowiki/>re-examined the Burgess Shale fauna and noted the anatomical details of ''Pikaia'' for the first time. The fossil specimens bears features of notochord and muscle blocks that are fundamental structures of chordates, and not of annelids. The animal was obviously a chordate.<ref>{{Cite journal |last=Whittington |first=H.B. |date=1980 |title=The significance of the fauna of the Burgess Shale, Middle Cambrian, British Columbia |url=https://linkinghub.elsevier.com/retrieve/pii/S0016787880800344 |journal=Proceedings of the Geologists' Association |language=en |volume=91 |issue=3 |pages=127–148 |doi=10.1016/S0016-7878(80)80034-4}}</ref> As they wrote in the ''[[Scientific American]]'':<blockquote>Finally, we find among the Burgess Shale fauna one of the earliest-known invertebrate representatives of our own conspicuous corner of the animal kingdom: the chordate phylum... The chordates are represented in the Burgess Shale by the genus Pikaia and the single species P. gracilens.<ref>{{Cite journal |last=Morris |first=Simon Conway |last2=Whittington |first2=H. B. |date=1979 |title=The Animals of the Burgess Shale |url=https://www.jstor.org/stable/24965247 |journal=Scientific American |volume=241 |issue=1 |pages=122–135 |doi=10.2307/24965247 |issn=0036-8733 |jstor=24965247}}</ref></blockquote>Conway Morris formally placed ''P. gracilens'' among the [[chordate]]s in a paper in the ''Annual Review of Ecology and Systematics'' in 1979.<ref>{{Cite journal |last=Morris |first=Simon Conway |date=1979 |title=The Burgess Shale (Middle Cambrian) Fauna |url=https://www.jstor.org/stable/2096795 |journal=Annual Review of Ecology and Systematics |volume=10 |pages=327–349 |issn=0066-4162}}</ref> However, there were no structural analyses such as using microscopes to confirm the chordate features. The comparative description only earned a "putative" chordate status.<ref>{{Cite journal |last=Butterfield |first=Nicholas J. |date=1990 |title=Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale |url=https://www.cambridge.org/core/journals/paleobiology/article/abs/organic-preservation-of-nonmineralizing-organisms-and-the-taphonomy-of-the-burgess-shale/E050B821F3A4D42CAA0ACAA817F76C72 |journal=Paleobiology |language=en |volume=16 |issue=3 |pages=272–286 |doi=10.1017/S0094837300009994 |issn=0094-8373}}</ref> As late as the the 1990s, there were sceptical remarks on the chordate nature.<ref>{{Cite journal |last=Insom |first=Emilio |last2=Pucci |first2=Antonella |last3=Simonetta |first3=Alberto M. |date=1995 |title=Cambrian Protochordata, their origin and significance |url=http://www.tandfonline.com/doi/abs/10.1080/11250009509356072 |journal=Bolletino di zoologia |language=en |volume=62 |issue=3 |pages=243–252 |doi=10.1080/11250009509356072 |issn=0373-4137}}</ref><ref>{{Citation |last=Hall |first=Brian K. |title=The Burgess Shale: different types of animals |date=1992 |url=https://doi.org/10.1007/978-94-015-7926-1_2 |work=Evolutionary Developmental Biology |pages=13–37 |editor-last=Hall |editor-first=Brian K. |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-94-015-7926-1_2 |isbn=978-94-015-7926-1 |access-date=2022-09-23}}</ref>
-During his re-examination of the Burgess Shale fauna in 1979, [[Paleontology|paleontologist]] [[Simon Conway Morris]] placed ''P. gracilens'' among the [[chordate]]s, making it perhaps the oldest known ancestor of modern [[vertebrate]]s. He did this because it seemed to have a very primitive, proto-[[notochord]], however, the status of ''Pikaia'' as a chordate is not universally accepted; its preservational mode suggests that it had [[cuticle]], which is uncharacteristic of the vertebrates <ref name="Butterfield1990">{{citation |last=Butterfield |first=N. J. |title=Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale |journal=Paleobiology |volume=16 |issue=3 |pages=272–286 |year=1990 |doi=10.1017/S0094837300009994 |jstor=2400788}}</ref> (although characteristic of other [[cephalochordate]]s); further, its tentacles are unknown from other vertebrate lineages.<ref name="Butterfield1990" /> The presence of earlier chordates among the [[Chengjiang]], including ''[[Haikouichthys]]'' and ''[[Myllokunmingia]]'', appears to show that cuticle is not necessary for preservation, overruling the taphonomic argument,<ref name="Morris2008">{{citation |last=Conway Morris |first=S. |title=A Redescription of a Rare Chordate,'' Metaspriggina walcotti'' Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada |url=http://jpaleontol.geoscienceworld.org/cgi/content/extract/82/2/424 |journal=Journal of Paleontology |volume=82 |issue=2 |pages=424–430 |year=2008 |doi=10.1666/06-130.1 |access-date=2009-04-28 |s2cid=85619898}}</ref> but the presence of tentacles remains intriguing, and the organism cannot be assigned conclusively, even to the vertebrate [[stem group]]. Its anatomy closely resembles the modern creature ''[[Branchiostoma]]''.<ref name="Donoghue2005">{{citation |last1=Donoghue |first1=P. C. J. |title=Genome duplication, extinction and vertebrate evolution |url=http://palaeo.gly.bris.ac.uk/Publs/donoghue/Donoghue_and_Purnell_2005.pdf |journal=Trends in Ecology & Evolution |volume=20 |issue=6 |pages=312–319 |year=2005 |archive-url=https://web.archive.org/web/20081217052403/http://palaeo.gly.bris.ac.uk/Publs/donoghue/Donoghue_and_Purnell_2005.pdf |doi=10.1016/j.tree.2005.04.008 |pmid=16701387 |access-date=2008-11-06 |archive-date=2008-12-17 |last2=Purnell |first2=M. A. |url-status=dead}}</ref>
 == Description ==
@@ Line 34: / Line 34: @@
 [[File:Pikaia NT small.jpg|thumb|Life reconstruction of ''Pikaia gracilens'']]
-Walcott's original summary of the description of ''Pikaia'' reads:<blockquote>Body elongate, slender, and tapering at each end. It is formed of many segments that are defined by strong annular shiny lines. Head small with two large eyes and two tentacles... Back of the head the first five segments carry short parapodia that appear to be divided into two parts. The enteric canal extends from end to end without change in character... This was one of the active, free-swimming annelids that suggest the Nephthydidae of the Polychaeta.<ref name=":2" /></blockquote>Conway Morris and Caron (2012) published an exhaustive description based on all 114 of the known fossil specimens; they discovered new and unexpected characteristics {{which|date=December 2016}} that they recognized as primitive features of the first chordate animals. On the basis of these findings, they constructed a new scenario for chordate evolution.<ref name="conway2012">Conway Morris, S.; Caron, J. B. (2012) "''Pikaia gracilens'' Walcott, a stem-group chordate from the Middle Cambrian of British Columbia". ''Biological Reviews'' 87: 480-512.</ref> Subsequently, Mallatt and Holland reconsidered Conway Morris and Caron's description, and concluded that many of the newly recognized characters are unique, already-divergent specializations that would not be helpful for establishing ''Pikaia'' as a basal chordate.<ref name=":1" />
+Walcott's original summary of the description of ''Pikaia'' reads:<blockquote>Body elongate, slender, and tapering at each end. It is formed of many segments that are defined by strong annular shiny lines. Head small with two large eyes and two tentacles... Back of the head the first five segments carry short parapodia that appear to be divided into two parts. The enteric canal extends from end to end without change in character... This was one of the active, free-swimming annelids that suggest the Nephthydidae of the Polychaeta.<ref name=":2" /></blockquote>making it perhaps the oldest known ancestor of modern [[vertebrate]]s. He did this because it seemed to have a very primitive, proto-[[notochord]], however, the status of ''Pikaia'' as a chordate is not universally accepted; its preservational mode suggests that it had [[cuticle]], which is uncharacteristic of the vertebrates <ref name="Butterfield1990">{{citation |last=Butterfield |first=N. J. |title=Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale |journal=Paleobiology |volume=16 |issue=3 |pages=272–286 |year=1990 |doi=10.1017/S0094837300009994 |jstor=2400788}}</ref> (although characteristic of other [[cephalochordate]]s); further, its tentacles are unknown from other vertebrate lineages.<ref name="Butterfield1990" /> The presence of earlier chordates among the [[Chengjiang]], including ''[[Haikouichthys]]'' and ''[[Myllokunmingia]]'', appears to show that cuticle is not necessary for preservation, overruling the taphonomic argument,<ref name="Morris2008">{{citation |last=Conway Morris |first=S. |title=A Redescription of a Rare Chordate,'' Metaspriggina walcotti'' Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada |url=http://jpaleontol.geoscienceworld.org/cgi/content/extract/82/2/424 |journal=Journal of Paleontology |volume=82 |issue=2 |pages=424–430 |year=2008 |doi=10.1666/06-130.1 |access-date=2009-04-28 |s2cid=85619898}}</ref> but the presence of tentacles remains intriguing, and the organism cannot be assigned conclusively, even to the vertebrate [[stem group]]. Its anatomy closely resembles the modern creature ''[[Branchiostoma]]''.<ref name="Donoghue2005">{{citation |last1=Donoghue |first1=P. C. J. |title=Genome duplication, extinction and vertebrate evolution |url=http://palaeo.gly.bris.ac.uk/Publs/donoghue/Donoghue_and_Purnell_2005.pdf |journal=Trends in Ecology & Evolution |volume=20 |issue=6 |pages=312–319 |year=2005 |archive-url=https://web.archive.org/web/20081217052403/http://palaeo.gly.bris.ac.uk/Publs/donoghue/Donoghue_and_Purnell_2005.pdf |doi=10.1016/j.tree.2005.04.008 |pmid=16701387 |access-date=2008-11-06 |archive-date=2008-12-17 |last2=Purnell |first2=M. A. |url-status=dead}}</ref>
+Conway Morris and Caron (2012) published an exhaustive description based on all 114 of the known fossil specimens; they discovered new and unexpected characteristics {{which|date=December 2016}} that they recognized as primitive features of the first chordate animals. On the basis of these findings, they constructed a new scenario for chordate evolution.<ref name="conway2012">Conway Morris, S.; Caron, J. B. (2012) "''Pikaia gracilens'' Walcott, a stem-group chordate from the Middle Cambrian of British Columbia". ''Biological Reviews'' 87: 480-512.</ref> Subsequently, Mallatt and Holland reconsidered Conway Morris and Caron's description, and concluded that many of the newly recognized characters are unique, already-divergent specializations that would not be helpful for establishing ''Pikaia'' as a basal chordate.<ref name=":1" />
 ==Evolution==