Pikaia

This article is about the extinct chordate. For the anime series, see Pikaia!

Pikaia
Temporal range: Early Cambrian to Middle Cambrian (Stage 3 to Wuliuan), 513–505 Ma
Anatomical reconstruction of Pikaia gracilens after Mussini et al. (2024)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Family: †Pikaiidae
Walcott, 1911
Genus: †Pikaia
Walcott, 1911
Species:
†P. gracilens
Binomial name
†Pikaia gracilens
Walcott, 1911

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 "the most famous early chordate fossil",[1] or "famously known as the earliest described Cambrian chordate".[2] It is estimated to have lived during the latter period of the Cambrian explosion. Since its initial discovery, more than a hundred specimens have been recovered.[3]

The body structure resembles that of the lancelet and it swam perhaps much like an eel. A notochord and myomeres (segmented blocks of skeletal muscles) span the entire length of the body, and are considered the defining signatures of chordate characters. Its primitive nature is indicated by the body covering, a cuticle, which is characteristic of invertebrates and some protochordates. A reinterpretation in 2024 found evidence of the gut canal, dorsal nerve cord and myomeres, and suggested that the taxon was previously interpreted upside down.[4]

The exact phylogenetic position is unclear, though recent studies suggest that it is likely a stem-chordate with crown group traits.[4] Previously proposed affinities include those of cephalochordata, craniata, or a stem-chordate not closely related to any extant lineage.[5] Popularly but falsely[6] attributed as an ancestor of all vertebrates,[7] or the oldest fish,[8] or the oldest ancestor of humans,[9][10] it is generally viewed as a basal chordate alongside other Cambrian chordates; it is a close relative of vertebrate ancestors[11][12] but it is not an ancestor itself.[2][13]

  1. ^ Gee, Henry (2018). Across the Bridge: Understanding the Origin of the Vertebrates. University of Chicago Press. p. 83. ISBN 978-0-226-40319-9.
  2. ^ a b McMenamin, Mark A. S. (2019). "Cambrian Chordates and Vetulicolians". Geosciences. 9 (8): 354. Bibcode:2019Geosc...9..354M. doi:10.3390/geosciences9080354. ISSN 2076-3263.
  3. ^ Conway Morris, Simon; 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. PMID 22385518. S2CID 27671780.
  4. ^ a b Mussini, G.; Smith, M. P.; Vinther, J.; Rahman, I. A.; Murdock, D. J. E.; Harper, D. A. T.; Dunn, F. S. (2024). "A new interpretation of Pikaia reveals the origins of the chordate body plan". Current Biology. 34 (13): 2980-2989.e2. doi:10.1016/j.cub.2024.05.026.
  5. ^ 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. PMID 15522451.
  6. ^ SciTechDaily (2012-03-06). "Pikaia Gracilens Is a Relative of but not the Genetic Ancestor to All Vertebrates". SciTechDaily. Retrieved 2022-09-23.
  7. ^ "An Early Chordate that Lived 513 Million Years Ago". Discover Magazine. Retrieved 2022-09-23.
  8. ^ "Worm-like creature could be humans' oldest ancestor". 2012-03-05. Retrieved 2022-09-23.
  9. ^ "Human's oldest ancestor found". University of Cambridge. 2012-03-06. Retrieved 2022-09-23.
  10. ^ "Worm-like creature could be a relative of the earliest vertebrate". 2012-03-05. Retrieved 2022-09-23.
  11. ^ "Early Vertibrate Relative found". University of Cambridge. 2012-03-06. Retrieved 2022-09-23.
  12. ^ 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.