Ambulacraria

Ambulacraria /ˌæmbjuːləˈkrɛəriə/, or Coelomopora /siːləˈmɒpərə/, is a clade of invertebrate phyla that includes echinoderms and hemichordates;^[1] a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 million years ago.^[2] The Ambulacraria are part of the deuterostomes, a clade that also includes the many Chordata, and the few extinct species belonging to the Vetulicolia.

Ambulacrarians Temporal range: Early Cambrian–Recent PreꞒ Ꞓ O S D C P T J K Pg N
Various sea stars and sea urchins among mussel shells in the rocky intertidal zone of Kachemak Bay, southern Alaska, United States
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
Superphylum:	Deuterostomia
Clade:	Ambulacraria Metschnikoff, 1881
Phyla
Echinodermata Hemichordata

Phylogeny

The two living clades with representative organisms are:

• Echinodermata (sea stars, sea urchins, brittle stars, sea cucumbers, feather stars, sea lilies, etc.)
• Hemichordata (acorn worms (Enteropnuesta) and Pterobranchia (including Graptolithina))^[3]

(These together sometimes are called the lower deuterostomes.^[4])

Whether the Xenacoelomorpha clade is the sister group to the Ambulacraria remains a contentious issue, with some authors arguing that the former should be placed more basally among metazoans,^[5][6][1] and other authors asserting that the best choices of phylogenetic methods support the position of Xenacoelomorpha as the sister group to Ambulacraria.^[7][8]

Two extinct taxa of uncertain placement, Vetulocystida and Yanjiahella, have each been proposed as either stem-group echinoderms^[9][10] or stem-group ambulacrarians.^[11][12] Vetulocystids have also been tentatively proposed as the basal-most stem-group chordates,^[13] while Yanjiahella has also been proposed to be a stem-group hemichordate.^[12]

The following cladogram is based on a simplification of Li et al. 2023,^[14] with the possible placements of uncertain potential ambulacrarians shown with dashed lines and question marks:

? Xenambulacraria	? Xenacoelomorpha Ambulacraria ? Vetulocystida†     Cambroernida†     ? Yanjiahella†     Ambulacraria Echinodermata ? Vetulocystida†     ? Yanjiahella†     Echinodermata     (crownward) (total group) Hemichordata ? Yanjiahella†     Hemichordata     (crownward) (total group) (crown group) (total group)
Chordata	? Vetulocystida†     Chordata     (crownward)
(total group)

Ontogeny

As for many animals, the egg cell of any extant ambulacrarian divides and develops into a blastula ("cell ball"), which develops into a triploblast ("three-layered") gastrula. The gastrula then develops into a dipleurula larva form in the Asteroidea, Holothuroidea, Crinoidea, and Hemichordata, and into a pluteus larva form in the Echinoidea and Ophiuroidea.^[4][15] This, in its turn, is developed in various different kinds of larvae for different taxa of ambulacrarians.

It has been suggested that the adult form of the last common ancestor of the ambulacrarians was anatomically similar to the dipleurula larva; this hypothetic ancestor sometimes also is called dipleurula.^[16]

References

1. Cannon, Johanna Taylor; Vellutini, Bruno Cossermelli; Smith, Julian; Ronquist, Fredrik; Jondelius, Ulf; Hejnol, Andreas (2016). "Xenacoelomorpha is the sister group to Nephrozoa". Nature. 530 (7588): 89–93. Bibcode:2016Natur.530...89C. doi:10.1038/nature16520. PMID 26842059. S2CID 205247296.
2. "Sea Cucumber Genome Imparts Insight on Genes Linked to Organ Regeneration". GenomeWeb. 12 October 2017. Retrieved 25 November 2024.
3. Mitchell, C. E.; Melchin, M. J.; Cameron, C. B.; Maletz, J. (2013). "Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite". Lethaia. 46 (1): 34–56. doi:10.1111/j.1502-3931.2012.00319.x.
4. Lacalli, Thurston Castle. "Tutorial". Marine Invertebrate larvae: A study in morphological diversity. University of Saskatchewan. Retrieved 2020-01-13.
5. Edgecombe, Gregory D.; Giribet, Gonzalo; Dunn, Casey W.; Hejnol, Andreas; Kristensen, Reinhardt M.; Neves, Ricardo C.; Rouse, Greg W.; Worsaae, Katrine; Sørensen, Martin V. (June 2011). "Higher-level metazoan relationships: recent progress and remaining questions". Organisms, Diversity & Evolution. 11 (2): 151–172. doi:10.1007/s13127-011-0044-4. S2CID 32169826.
6. Rouse, Greg W.; Wilson, Nerida G.; Carvajal, Jose I.; Vriejenhoek, Robert C. (4 February 2016). "New deep-sea species of Xenoturbella and the position of Xenacoelomorpha". Nature. 530 (2): 94–97. Bibcode:2016Natur.530...94R. doi:10.1038/nature16545. PMID 26842060. S2CID 3870574.
7. Herve Philippe; et al. (3 June 2019). "Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria". Current Biology. 29 (11): 1818–1826. doi:10.1016/j.cub.2019.04.009. hdl:21.11116/0000-0004-DC4B-1. PMID 31104936.
8. Kapli, Paschalia; Telford, Maximilian J. (11 December 2020). "Topology-dependent asymmetry in systematic errors affects phylogenetic placement of Ctenophora and Xenacoelomorpha". Science Advances. 6 (50): eabc5162. Bibcode:2020SciA....6.5162K. doi:10.1126/sciadv.abc5162. PMC 7732190. PMID 33310849.
9. Shu, D.-G.; Conway Morris, S.; Han, J.; Zhang, Z.-F.; Liu, J.-N. (2004). "Ancestral echinoderms from the Chengjiang deposits of China". Nature. 430 (6998): 422–428. Bibcode:2004Natur.430..422S. doi:10.1038/nature02648. PMID 15269760. S2CID 4421182.
10. Topper, Timothy P.; Guo, Junfeng; Clausen, Sébastien; Skovsted, Christian B.; Zhang, Zhifei (2019-03-25). "A stem group echinoderm from the basal Cambrian of China and the origins of Ambulacraria". Nature Communications. 10 (1): 1366. Bibcode:2019NatCo..10.1366T. doi:10.1038/s41467-019-09059-3. ISSN 2041-1723. PMC 6433856. PMID 30911013.
11. Conway Morris, Simon; Halgedahl, Susan L.; Selden, Paul; Jarrard, Richard D. (2015). "Rare primitive deuterostomes from the Cambrian (Series 3) of Utah" (PDF). Journal of Paleontology. 89 (4): 631–636. Bibcode:2015JPal...89..631C. doi:10.1017/jpa.2015.40.
12. Zamora, Samuel; Wright, David F.; Mooi, Rich; Lefebvre, Bertrand; Guensburg, Thomas E.; Gorzelak, Przemysław; David, Bruno; Sumrall, Colin D.; Cole, Selina R.; Hunter, Aaron W.; Sprinkle, James (2020-03-09). "Re-evaluating the phylogenetic position of the enigmatic early Cambrian deuterostome Yanjiahella". Nature Communications. 11 (1): 1286. Bibcode:2020NatCo..11.1286Z. doi:10.1038/s41467-020-14920-x. ISSN 2041-1723. PMC 7063041. PMID 32152310.
13. 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. Bibcode:2024CBio...34.2980M. doi:10.1016/j.cub.2024.05.026. PMID 38866005.
14. Li, Yujing; Dunn, Frances S.; Murdock, Duncan J.E.; Guo, Jin; Rahman, Imran A.; Cong, Peiyun (May 10, 2023). "Cambrian stem-group ambulacrarians and the nature of the ancestral deuterostome". Current Biology. 33 (12): 2359–2366.e2. Bibcode:2023CBio...33E2359L. doi:10.1016/j.cub.2023.04.048. PMID 37167976. S2CID 258592223.
15. Byrne, Maria; Nakajima, Yoko; Chee, Francis C.; Burke, Robert D. (2007). "Apical organs in echinoderm larvae: insights into larval evolution in the Ambulacraria". Evolution & Development. 9: 434–435, 438–440. doi:10.1111/j.1525-142X.2007.00189.x.
16. "Dipleurula". Lexikon der Biologie (in German). Spektrum Akademischer Verlag. Heidelberg. 1999. Retrieved 2020-01-13.