Ambulacraria

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))

{{cite journal

|last1=Mitchell |first1=C. E.

|last2=Melchin |first2=M. J.

|last3=Cameron |first3=C. B.

|last4=Maletz |first4=J.

|title=Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite

|journal=Lethaia

|year=2013

|volume=46 |number=1 |pages=34–56

|doi=10.1111/j.1502-3931.2012.00319.x

}}

(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,{{cite journal

|first1=Gregory D. |last1=Edgecombe

|first2=Gonzalo |last2=Giribet

|first3=Casey W. |last3=Dunn

|first4=Andreas |last4=Hejnol

|first5=Reinhardt M.|last5=Kristensen

|first6=Ricardo C. |last6=Neves

|first7=Greg W. |last7=Rouse

|first8=Katrine |last8=Worsaae

|first9=Martin V. |last9=Sørensen

|doi=10.1007/s13127-011-0044-4

|title=Higher-level metazoan relationships: recent progress and remaining questions

|date=June 2011

|volume=11 |issue=2| pages=151–172

|journal=Organisms, Diversity & Evolution

|s2cid=32169826

|url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:27755241

}}{{cite journal

|first1=Greg W. |last1=Rouse

|first2=Nerida G. |last2=Wilson

|first3=Jose I. |last3=Carvajal

|first4=Robert C. |last4=Vriejenhoek

|doi=10.1038/nature16545

|title=New deep-sea species of Xenoturbella and the position of Xenacoelomorpha

|date=4 February 2016

|volume=530 |issue=2| pages=94–97

|journal=Nature

|pmid=26842060

|bibcode=2016Natur.530...94R

|s2cid=3870574

|url=https://www.nature.com/articles/nature16545

}} and other authors asserting that the best choices of phylogenetic methods support the position of Xenacoelomorpha as the sister group to Ambulacraria.

{{cite journal

|author1=Herve Philippe

|display-authors=etal

|title=Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria

|journal=Current Biology

|doi=10.1016/j.cub.2019.04.009

|date=3 June 2019

|volume=29 |issue=11 |pages=1818–1826

|pmid=31104936

|doi-access=free

|hdl=21.11116/0000-0004-DC4B-1

|hdl-access=free

}}

{{cite journal

|last1=Kapli |first1=Paschalia

|last2=Telford |first2=Maximilian J.

|title=Topology-dependent asymmetry in systematic errors affects phylogenetic placement of Ctenophora and Xenacoelomorpha

|journal=Science Advances

|doi= 10.1126/sciadv.abc5162

|volume=6 |number=50 |pages=eabc5162

|date=11 December 2020

|pmid=33310849

|pmc=7732190

|bibcode=2020SciA....6.5162K

|doi-access=free

}}

Two extinct taxa of uncertain placement, Vetulocystida and Yanjiahella, have each been proposed as either stem-group echinoderms

{{cite journal

|last1=Shu |first1=D.-G.

|last2=Conway Morris |first2=S.

|last3=Han |first3=J.

|last4=Zhang |first4=Z.-F.

|last5=Liu |first5=J.-N.

|title=Ancestral echinoderms from the Chengjiang deposits of China|date=2004

|journal=Nature

|volume=430 |number=6998 |pages=422–428

|doi=10.1038/nature02648

|pmid=15269760

|bibcode=2004Natur.430..422S

|s2cid=4421182

|url=https://www.nature.com/articles/nature02648

}}

{{Cite journal

|last1=Topper |first1=Timothy P.

|last2=Guo |first2=Junfeng

|last3=Clausen |first3=Sébastien

|last4=Skovsted |first4=Christian B.

|last5=Zhang |first5=Zhifei

|date=2019-03-25

|title=A stem group echinoderm from the basal Cambrian of China and the origins of Ambulacraria |url= |journal=Nature Communications

|language=en

|volume=10

|issue=1

|pages=1366

|doi=10.1038/s41467-019-09059-3

|issn=2041-1723

|pmc=6433856

|pmid=30911013|bibcode=2019NatCo..10.1366T

}}

or stem-group ambulacrarians.

{{cite journal

|last1=Conway Morris

|first1=Simon

|last2=Halgedahl

|first2=Susan L.

|last3=Selden

|first3=Paul

|last4=Jarrard

|first4=Richard D.

|title=Rare primitive deuterostomes from the Cambrian (Series 3) of Utah

|journal=Journal of Paleontology

|date=2015

|volume=89

|number=4

|pages=631–636

|doi=10.1017/jpa.2015.40

|bibcode=2015JPal...89..631C

|url=https://core.ac.uk/download/pdf/51378622.pdf

}}

{{Cite journal

|last1=Zamora |first1=Samuel

|last2=Wright |first2=David F.

|last3=Mooi |first3=Rich

|last4=Lefebvre |first4=Bertrand

|last5=Guensburg |first5=Thomas E.

|last6=Gorzelak |first6=Przemysław

|last7=David |first7=Bruno

|last8=Sumrall |first8=Colin D.

|last9=Cole |first9=Selina R.

|last10=Hunter |first10=Aaron W.

|last11=Sprinkle |first11=James

|date=2020-03-09

|title=Re-evaluating the phylogenetic position of the enigmatic early Cambrian deuterostome Yanjiahella

|journal=Nature Communications

|language=en

|volume=11 |issue=1 |pages=1286

|doi=10.1038/s41467-020-14920-x

|issn=2041-1723

|pmc=7063041

|pmid=32152310

|bibcode=2020NatCo..11.1286Z

}}

Vetulocystids have also been tentatively proposed as the basal-most stem-group chordates,

{{Cite journal

|last1=Mussini |first1=G.

|last2=Smith |first2=M. P.

|last3=Vinther |first3=J.

|last4=Rahman |first4=I. A.

|last5=Murdock |first5=D. J. E.

|last6=Harper |first6=D. A. T.

|last7=Dunn |first7=F. S.

|year=2024

|title=A new interpretation of Pikaia reveals the origins of the chordate body plan

|journal=Current Biology

|volume=34 |issue=13 |pages=2980–2989.e2

|doi=10.1016/j.cub.2024.05.026 |doi-access=free

|pmid=38866005

|bibcode=2024CBio...34.2980M

}} while Yanjiahella has also been proposed to be a stem-group hemichordate.

The following cladogram is based on a simplification of Li et al. 2023,

{{cite journal

|last1=Li |first1=Yujing

|last2=Dunn |first2=Frances S.

|last3=Murdock |first3=Duncan J.E.

|last4=Guo |first4=Jin

|last5=Rahman |first5=Imran A.

|last6=Cong |first6=Peiyun

|title=Cambrian stem-group ambulacrarians and the nature of the ancestral deuterostome

|journal=Current Biology

|date=May 10, 2023

|volume=33 |issue=12 |pages=2359–2366.e2

|doi=10.1016/j.cub.2023.04.048 |doi-access=free

|pmid=37167976

|s2cid=258592223

|bibcode=2023CBio...33E2359L

}}

with the possible placements of uncertain potential ambulacrarians shown with dashed lines and question marks:

{{Clade

|label1=? Xenambulacraria

|1={{Clade

|1=? Xenacoelomorpha 40px

|state1=dashed

|label2=Ambulacraria

|sublabel2={{small|(total group)}}

|2={{Clade

|label1=? Vetulocystida†

|1=   20px |state1=dashed

|label2=Cambroernida†

|2=   50px

|label3=? Yanjiahella†

|3=   50px |state3=dashed

|label4=Ambulacraria

|sublabel4={{small|(crown group)}}

|4={{Clade

|label1=Echinodermata

|sublabel1={{small|(total group)}}

|1={{Clade

|label1=? Vetulocystida†

|1=   20px |state1=dashed

|label2=? Yanjiahella†

|2=   50px |state2=dashed

|label3=Echinodermata

|sublabel3={{small|(crownward)}}

|3=   50 px

}}

|label2=Hemichordata

|sublabel2={{small|(total group)}}

|2={{Clade

|label1=? Yanjiahella†

|1=   50px |state1=dashed

|label2=Hemichordata

|sublabel2={{small|(crownward)}}

|2=   45px

}}

}}

}}

}}

|label2=Chordata

|sublabel2={{small|(total group)}}

|2={{Clade

|label1=? Vetulocystida†

|1=   20px |state1=dashed

|label2=Chordata

|sublabel2={{small|(crownward)}}

|2=   50px

}}

}}

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.{{cite web|url=http://scaa.usask.ca/gallery/lacalli/tutorial/tutorial_all.php|title=Tutorial|last=Lacalli|first=Thurston Castle|website=Marine Invertebrate larvae: A study in morphological diversity|publisher=University of Saskatchewan|access-date=2020-01-13}}{{Cite journal |last=Byrne |first=Maria |last2=Nakajima |first2=Yoko |last3=Chee |first3=Francis C. |last4=Burke |first4=Robert D. |date=2007 |title=Apical organs in echinoderm larvae: insights into larval evolution in the Ambulacraria |url=https://eds.s.ebscohost.com/eds/pdfviewer/pdfviewer?vid=2&sid=f505dbea-6289-44f3-8005-19035ca0927b%40redis |journal=Evolution & Development |volume=9 |pages=434–435, 438–440 |doi=10.1111/j.1525-142X.2007.00189.x}} 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.{{cite web | url=http://www.spektrum.de/lexikon/biologie/dipleurula/18496 | title=Dipleurula | access-date=2020-01-13 | year=1999 | work=Lexikon der Biologie | publisher=Spektrum Akademischer Verlag. Heidelberg | language=de}}

{{Reflist}}

{{Animalia}}

{{Taxonbar|from=Q136956}}

Category:Ediacaran first appearances

Category:Superphyla