Ambulacraria Explained

Ambulacraria, or Coelomopora, 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.

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, Pterobranchia, and possibly graptolites)

(These together sometimes are called the lower deuterostomes.)

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,^{[3] [4] [5]} and other authors asserting that the best choices of phylogenetic methods support the position of Xenacoelomorpha as the sister group to Ambulacraria.^{[6] [7]}

Fossil ambulacrarians

Fossil taxa that may lie on the stem lineage:

• Superphylum Ambulacraria (total group)
  • Cambroernida (unranked clade)^{[8] [9]}
    • Herpetogaster
    • Phlogites
    • Superfamily Eldonioidea
      • Family Eldoniidae
      • Family Rotadiscidae
      • Paropsonema
  • Yanjiahella^[10]

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.^{[11] [12]} 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.^[13]

Notes and References

1. Cannon. Johanna Taylor. Vellutini. Bruno Cossermelli. Smith. Julian. Ronquist. Fredrik. Jondelius. Ulf. Hejnol. Andreas. Xenacoelomorpha is the sister group to Nephrozoa. Nature. 530. 7588. 89–93. 10.1038/nature16520. 26842059. 2016. 2016Natur.530...89C. 205247296.
2. https://www.genomeweb.com/sequencing/sea-cucumber-genome-imparts-insight-genes-linked-organ-regeneration Sea Cucumber Genome Imparts Insight on Genes Linked to Organ Regeneration
3. Gregory D. . Edgecombe. Gonzalo . Giribet. Casey W. . Dunn. Andreas . Hejnol. Reinhardt M.. Kristensen. Ricardo C. . Neves. Greg W. . Rouse. Katrine . Worsaae. Martin V. . Sørensen. 10.1007/s13127-011-0044-4. Higher-level metazoan relationships: recent progress and remaining questions. June 2011. 11 . 2. 151–172. Organisms, Diversity & Evolution. 32169826.
4. Greg W. . Rouse. Nerida G. . Wilson. Jose I. . Carvajal. Robert C. . Vriejenhoek. 10.1038/nature16545. New deep-sea species of Xenoturbella and the position of Xenacoelomorpha. 4 February 2016. 530 . 2. 94–97. Nature. 26842060. 2016Natur.530...94R. 3870574.
5. Johanna Taylor . Cannon. Bruno Cossermelli . Vellutini. Julian . Smith III. Frederik . Ronquist. Ulf . Jondelius. Andreas . Hejnol. 10.1038/nature16520. Xenacoelomorpha is the sister group to Nephrozoa. 4 February 2016. 530 . 2. 89–93. Nature. 26842059. 2016Natur.530...89C. 205247296.
6. Herve Philippe. etal. Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria. Current Biology. 10.1016/j.cub.2019.04.009. 29. 3 June 2019. 11. 1818–1826. 31104936. free. 21.11116/0000-0004-DC4B-1. free.
7. Paschalia Kapli. Maximilian J. Telford. Topology-dependent asymmetry in systematic errors affects phylogenetic placement of Ctenophora and Xenacoelomorpha. Science Advances. 10.1126/sciadv.abc5162. 6. 50. 11 December 2020. eabc5162. 33310849. 7732190. 2020SciA....6.5162K. free.
8. Caron . J. . Conway Morris . S. . Shu . D. . 2010 . Tentaculate fossils from the Cambrian of Canada (British Columbia) and China (Yunnan) interpreted as primitive deuterostomes . PLOS ONE . 5 . 3 . e9586 . 2010PLoSO...5.9586C . 10.1371/journal.pone.0009586 . 2833208 . 20221405 . free.
9. Li . Yujing . Dunn . Frances S. . Murdock . Duncan J.E. . Guo . Jin . Rahman . Imran A. . Cong . Peiyun . Cambrian stem-group ambulacrarians and the nature of the ancestral deuterostome . Current Biology . May 10, 2023 . 10.1016/j.cub.2023.04.048 . 37167976 . 258592223 . 11 May 2023. free .
10. 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. en. 11. 1. 1286. 10.1038/s41467-020-14920-x. 2041-1723. 7063041. 32152310. 2020NatCo..11.1286Z.
11. Web site: Tutorial. Lacalli. Thurston Castle. Marine Invertebrate larvae: A study in morphological diversity. University of Saskatchewan. 2020-01-13.
12. 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 . 10.1111/j.1525-142X.2007.00189.x.
13. Web site: Dipleurula . 2020-01-13 . 1999 . Lexikon der Biologie . Spektrum Akademischer Verlag. Heidelberg . de.