Editing Echinoderm (section)

==Taxonomy and evolution==
{{see also|List of echinoderm orders}}

The characteristics of adult echinoderms are the possession of a [[water vascular system]] with external [[tube feet]] and a [[stereom]] endoskeleton. 
Stereom is a [[calcareous]] material consisting of [[ossicle (echinoderm)|ossicles]] connected by a mesh of [[collagen]] fibres, which is unique to this phylum.<ref name=Tree>{{harvnb|Wray|1999}}</ref>

===Phylogeny===

Echinoderm phylogeny has long been a contentious subject. While the relationships among extant taxa are well-understood, there is no broadly accepted consensus regarding the phylum's origins or the relationships among its extinct groups.<ref>{{harvnb|Ubaghs|1978}}</ref><ref>{{harvnb|Smith|1984}}</ref><ref name=twoclades>{{harvnb|Rahman|Zamora|2024|pp=308–309}}</ref>  Echinoderm evolution shows a high degree of [[homoplasy]], meaning that many features have evolved multiple times independently.  This means that many features initially assumed to indicate a genetic connection do not, in fact, do so, which has obscured the true relationships of various groups.<ref>{{harvnb|Nanglu|Cole|Wright|Souto|2023}}</ref>

====External phylogeny====

Echinoderms are [[bilateria]]ns, meaning that their ancestors were mirror-symmetric. Among the bilaterians, they belong to the [[deuterostome]] division, meaning that the [[blastopore]], the first opening to form during embryo development, becomes the [[anus]] instead of the mouth.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=777–779}}</ref><ref name="Lander">{{harvnb|Fox|2007}}</ref>

Echinoderms are the sister group of the [[Hemichordata]], with which they form the crown group [[Ambulacraria]].<ref>{{harvnb|Edgecombe|Giribet|Dunn|Hejnol|2011}}</ref>  Two taxa of uncertain placement, [[Vetulocystida]] and ''[[Yanjiahella]]'', have each been proposed as either stem-group echinoderms<ref>{{harvnb|Shu|Conway Morris|Han|Zhang|2004|pp=426–427}}</ref><ref>{{harvnb|Topper|Guo|Clausen|Skovsted|Zhang|2019|pp=5–7}}</ref> or stem-group ambulacrarians.<ref name=vetulocystids>{{harvnb|Conway Morris|Halgedahl|Selden|Jarrard|2015|pp=4–6}}</ref><ref name=yanjiahella-doubts>{{harvnb|Zamora|Wright|Mooi|Lefebvre|2020|p=2}}</ref>  Vetulocystids have also been proposed as stem-group [[chordates]],<ref>{{harvnb|Mussini|Smith|Vinther|Rahman|Murdock|Harper|Dunn|2024|loc=supplemental data figure S4}}</ref> while ''Yanjiahella'' has also been proposed to be a stem-group hemichordate.<ref name=yanjiahella-doubts />

The Ambulacrarian context of the echinoderms is shown below, simplified from Li et al. 2023,<ref>{{harvnb|Li|Dunn|Murdock|Guo|Rahman|Cong|2023|p=159}}</ref> with the possible ambulacrarian placements of the uncertain taxa shown with dashed lines and question marks:


{{Clade 
|label1=[[Ambulacraria]]
|sublabel1={{small|([[total group]])}}
|1={{clade
      |1=? [[Vetulocystida]]† [[File:Vetulocystis catenata cropped.png|20px]] |state1=dashed
      |2=[[Cambroernida]]† [[File:Herpetogaster collinsi reconstruction.png|50px]]
      |3=? ''[[Yanjiahella]]''† [[File:Yanjiahella concept extract.png|50px]] |state3=dashed
      |label4=Ambulacraria
      |sublabel4={{small|([[crown group]])}}
      |4={{Clade
         |label1='''Echinodermata'''
         |sublabel1={{small|([[total group]])}}
         |1={{Clade
            |label1=
            |1= ? Vetulocystida† [[File:Vetulocystis catenata cropped.png|20px]] |state1=dashed
            |2=? ''Yanjiahella''† [[File:Yanjiahella concept extract.png|50px]] |state2=dashed
            |3='''Echinodermata''' {{small|([[crown group|crownward]])}} [[File:Echinaster serpentarius (USNM E28192) 001.png|50 px]]
         }}
         |label2=[[Hemichordata]]
         |sublabel2={{small|([[total group]])}}
         |2={{Clade
            |label1=
            |1=? ''Yanjiahella''† [[File:Yanjiahella concept extract.png|50px]] |state1=dashed
            |2=Hemichordata {{small|([[crown group|crownward]])}} [[File:Balanoglossus by Spengel 1893.png|45px]]
         }}
      }}
   }}
}}

====Internal phylogeny: extant classes====

The extant echinoderms consist of the [[Crinoidea]] and the [[Eleutherozoa]], the latter of which is divided into the [[Asterozoa]] and the [[Echinozoa]].<ref name="TelfordLowe2014">{{harvnb|Telford|Lowe|Cameron|Ortega-Martinez|2014}}</ref><ref name="Escriva Reich 2015">{{harvnb|Escriva|Reich|Dunn|Akasaka|2015}}</ref>

{{clade
|label1='''Echinodermata'''
|1={{clade
   |1=[[Crinoidea]] (feather stars) [[File:202012 Metacrinus rotundus.svg|60 px]]
   |2={{clade
      |label1=[[Echinozoa]]
      |1={{clade
         |label1=[[Holothuroidea]] 
         |sublabel1=sea cucumbers
         |1=[[File:Holothuria parvula.png|80 px]]
         |label2=[[Echinoidea]] 
         |sublabel2=sea urchins, etc
         |2=[[File:Echinometra viridis.png|70 px]]
         }}
      |label2=[[Asterozoa]]
      |2={{clade
         |label1=[[Ophiuroidea]] 
         |sublabel1=brittle and basket stars 
         |1=[[File:Ophiocoma (26150611418).jpg|80 px]]
         |label2=[[Asteroidea]] 
         |sublabel2=starfish 
         |2=[[File:Echinaster serpentarius (USNM E28192) 001.png|70 px]]
         }}
      }}
   }}
}}


====Internal phylogeny: total group====

The lack of a consensus [[cladistic]] phylogeny incorporating extinct echinoderm groups has resulted in the continued use of terms from [[Linnaean taxonomy|Linnaean taxonomies]], even when the named taxa are known to be [[paraphyletic]] and/or [[polyphyletic]].

=====Linnaean taxonomies=====

Three taxonomies introduced nearly all of the traditional subphyla and class divisions that continue to be referenced in cladistic work:

* [[Francis Arthur Bather|F. A. Bather]] produced the earliest widely referenced classification of both fossil and extant echinoderms in 1900, using a two-subphylum system.<ref>{{harvnb|Smith|1984|pp=433–434}}</ref>
* In 1966, the ''[[Treatise on Invertebrate Paleontology]]'', rejected Bather's classification, replacing it with a new four-subphylum scheme<ref>{{harvnb|Moore|1966|pp=U2–U3}}</ref> that had been previously proposed by [[Barry Fell|H. B. Fell]].<ref>{{harvnb|Sprinkle|1973|p=3}}</ref>
* James Sprinkle which added a fifth subphylum to the ''Treatise'' taxonomy in 1973.<ref>{{harvnb|Sprinkle|1973}}</ref>  His later class-level taxonomy of the five subphyla was the most recent approach cited in an early cladistic re-assessment of the phylum.<ref>{{harvnb|Smith|1984|p=436}}</ref>

{| class="wikitable"
|+Notable Linnaean taxonomies of the phylum Echinodermata
! Bather, 1900<ref>{{harvnb|Bather|1900}}</ref>
! Moore (ed.), 1966–7<ref>{{harvnb|Moore|1966}}</ref><ref>{{harvnb|Ubaghs|1967}}</ref>
! Sprinkle, 1980<ref>{{harvnb|Sprinkle|1980a}}</ref><ref>{{harvnb|Sprinkle|1980b}}</ref>
|- style="vertical-align:top;"
|
* ''Phylum'' '''Echinoderma''' {{small|Latreille, 1825<ref>{{harvnb|Ubaghs|1967|p=S5|loc=footnote 1}}</ref>}}
** ''Subphylum'' [[Pelmatozoa]] {{small|Leuckart,&nbsp;1848}}
*** ''Class'' Cystidea† {{small|von&nbsp;Buch,&nbsp;1844}}
*** ''Class'' [[Blastoidea]]† {{small|Say,&nbsp;1825 ''sensu extenso''}}
*** ''Class'' [[Crinoidea]] {{small|Miller,&nbsp;1821}}
*** ''Class'' [[Edrioasteroidea]]† {{small|E.&nbsp;Billings,&nbsp;1854–58; Huxley,&nbsp;1877; Bather,&nbsp;1899}}
** ''Subphylum'' [[Eleutherozoa]] {{small|Bell, 1891}}
*** ''Class'' [[Holothuroidea]] {{small|C.&nbsp;T.&nbsp;v.&nbsp;Siebold,&nbsp;1848}}
*** ''Class'' [[Stelleroidea]] {{small|Gregory, 1900}}
*** ''Class'' [[Echinoidea]] {{small|[[Leske]], 1778}}
|
* ''Phylum'' '''Echinodermata''' {{small|Klein,&nbsp;1734; Brugière, 1789}}
** ''Subphylum'' [[Homalozoa]]† {{small|Whitehouse,&nbsp;1941}}
*** ''Class'' [[Stylophora]]† {{small|Gill&nbsp;&&nbsp;Caster,&nbsp;1960}}
*** ''Class'' [[Homostelea]]† {{small|Gill&nbsp;&&nbsp;Caster,&nbsp;1960}}
*** ''Class'' [[Homoiostelea]]† {{small|Gill&nbsp;&&nbsp;Caster,&nbsp;1960}}
** ''Subphylum'' [[Crinozoa]] {{small|Matsumoto,&nbsp;1929}}
*** ''Class'' [[Lepidocystis|Lepidocystoidea]]† {{small|Durham,&nbsp;1967}}
*** ''Class'' [[Eocrinoidea]]† {{small|Jaekel,&nbsp;1918}}
*** ''Class'' [[Cystoidea]]† {{small|von&nbsp;Buch,&nbsp;1846}}
*** ''Class'' Edrioblastoidea† {{small|Fay,&nbsp;1962}}
*** ''Class'' Parablastoidea† {{small|Hudson,&nbsp;1907}}
*** ''Class'' [[Blastoidea]]† {{small|Say,&nbsp;1825}}
*** ''Class'' [[Paracrinoidea]]† {{small|Regnéll,&nbsp;1945}}
*** ''Class'' [[Crinoidea]] {{small|Miller,&nbsp;1821}}
** ''Subphylum'' [[Asterozoa]] {{small|Zittel&nbsp;1895}}
*** ''Class'' [[Stelleroidea]] {{small|Lamarck&nbsp;1816}}
** ''Subphylum'' [[Echinozoa]] {{small|Haekel&nbsp; Zittel&nbsp;1895}}
*** ''Class'' [[Helicoplacoidea]]† {{small|Durham&nbsp;&&nbsp;Caster,&nbsp;1963}}
*** ''Class'' [[Camptostroma|Camptostromatoidea]]† {{small|Durham,&nbsp;1966}}
*** ''Class'' [[Edrioasteroidea]]† {{small|Billings,&nbsp;1858}}
*** ''Class'' Cyclocystoidea† {{small|Miller&nbsp;&&nbsp;Gurley,&nbsp;1895}}
*** ''Class'' [[Ophiocistioidea]]† {{small|Solas,&nbsp;1899}}
*** ''Class'' [[Echinoidea]] {{small|Leske,&nbsp;1778}}
*** ''Class'' [[Holothuroidea]] {{small|de&nbsp;Blainville,&nbsp;1834}}
|
* ''Phylum'' '''Echinodermata''' {{small|de Bruguière ''[sic]'',&nbsp;1791}}
** ''Subphylum'' [[Homalozoa]]† {{small|Whitehouse,&nbsp;1941}}
*** ''Class'' [[Ctenocystoidea]]† {{small|Robinson&nbsp;& Sprinkle,&nbsp;1969}}
*** ''Class'' [[Stylophora]]† {{small|Gill&nbsp;&&nbsp;Caster,&nbsp;1960}}
*** ''Class'' [[Homostelea]]† {{small|Gill&nbsp;&&nbsp;Caster,&nbsp;1960}}
*** ''Class'' [[Homoiostelea]]† {{small|Gill&nbsp;&&nbsp;Caster,&nbsp;1960}}
** ''Subphylum'' [[Blastozoa]]† {{small|Sprinkle,&nbsp;1973}}
*** ''Class'' [[Eocrinoidea]]† {{small|Jaekel,&nbsp;1918}}
*** ''Class'' [[Rhombifera]]† {{small|Zittel,&nbsp;1879}}
*** ''Class'' [[Diploporita]]† {{small|Müller,&nbsp;1854}}
*** ''Class'' Parablastoidea† {{small|Hudson,&nbsp;1907}}
*** ''Class'' [[Blastoidea]]† {{small|Say,&nbsp;1825}}
** ''Subphylum'' [[Crinozoa]] {{small|Matsumoto,&nbsp;1929, restricted&nbsp;in Sprinkle,&nbsp;1973}}
*** ''Class'' [[Paracrinoidea]]† {{small|Regnéll,&nbsp;1945}}
*** ''Class'' [[Crinoidea]] {{small|Miller,&nbsp;1821}}
** ''Subphylum'' [[Asterozoa]] {{small|Zittel&nbsp;1895}}
*** ''Class'' [[Asteroidea]] {{small|de&nbsp;Blainville,&nbsp;1830}}
*** ''Class'' [[Ophiuroidea]] {{small|Gray,&nbsp;1840}}
** ''Subphylum'' [[Echinozoa]] {{small|Haekel&nbsp; Zittel&nbsp;1895}}
*** ''Class'' [[Helicoplacoidea]]† {{small|Durham&nbsp;&&nbsp;Caster,&nbsp;1963}}
*** ''Class'' [[Edrioasteroidea]]† {{small|Billings,&nbsp;1858}}
*** ''Class'' Edrioblastoidea† {{small|Fay,&nbsp;1962}}
*** ''Class'' Cyclocystoidea† {{small|Miller&nbsp;&&nbsp;Gurley,&nbsp;1895}}
*** ''Class'' [[Ophiocistioidea]]† {{small|Solas,&nbsp;1899}}
*** ''Class'' [[Echinoidea]] {{small|Leske,&nbsp;1778}}
*** ''Class'' [[Holothuroidea]] {{small|de&nbsp;Blainville,&nbsp;1834}}
|}


Other proposed classes not included at that rank in any of the above taxonomies include:

* [[Eleutherozoa#Asterozoa vs Cryptosyringida|Cryptosyringida]] {{small|Smith, 1984}}<ref>{{harvnb|Smith|1984|pp=436–439, 453–454}}</ref>
* [[Somasteroidea]] {{small|Spencer, 1951}}<ref>{{harvnb|PBDB Somasteroidea}}</ref> 
* [[Asterozoa#Stenuroidea|Stenuroidea]] {{small|Spencer, 1951}}<ref>{{harvnb|PBDB Stenuroidea}}</ref>
* Coronoidea {{small|Brett et al., 1983}}<ref>{{harvnb|PBDB Coronoidea}}</ref>
* [[Concentricycloidea]] {{small|Baker, Rowe & Clark, 1986}}<ref>{{harvnb|Rowe|Baker|Clark|1988}}</ref>

There are also several common alternative names involving homalozoans:

* [[Carpoidea]] {{small|Jaekel, 1900}} for Homalozoa, giving rise to the term "carpoids"<ref>{{harvnb|Rahman|2009}}</ref>
* [[Cincta]] {{small|Jaekel, 1918}} as either the senior synonym of or sole order within Homostelea<ref name=Rozhnov2022>{{harvnb|Rozhnov|2022|p=1310}}</ref>
* [[Soluta (echinoderm)|Soluta]] {{small|Jaekel, 1901}} as either the senior synonym of or sole order within Homoiostelea<ref name=Rozhnov2022 />
* Calcichordata {{small|Jeffries, 1967}}, a subphylum effectively identical to Stylophora that was central<ref>{{harvnb|Gee|1996|p=223}}</ref> to the now-disproven<ref>{{harvnb|Lefebvre|Guensburg|Martin|Mooi|2019}}</ref> [[calcichordate hypothesis]]

=====Cladograms=====

According to 2024 review, there are two main schools of thought regarding echinoderm phylogeny:  One that sees pentaradiality as a [[plesiomorphic]] trait of the phylum, and another that considers it a derived trait ([[apomorphy]]).<ref>{{harvnb|Rahman|Zamora|2024|p=308}}</ref>

Note that neither cladogram shown below includes all of the traditional classes, or even all of the classes mentioned in accompanying text.

; Pentaradiality as a plesiomorphy

Supporters of pentaradiality as an initial condition of the phylum note that radial forms are the first uncontested echinoderms to appear in the fossil record. They also define homologies of echinoderm anatomy based on a division of the skeleton into two parts: those that are or are not associated with the water vascular system.<ref name=twoclades />

The following cladogram is based on David & Mooi (1999)<ref>{{harvnb|David|Mooi|1999|p=100}}</ref> and David, Lefebvre, Mooi, and Parsley (2000):<ref>{{harvnb|David|Lefebvre|Mooi|Parsley|2000|p=549}}</ref>

{{Clade
|label1='''Echinodermata'''
|sublabel1={{small|([[total group]])}}
|1={{Clade
   |1=? ''[[Arkarua]]''† [[File:Arkarua adami pennetta.png|60px]]
   |2={{Clade
      |1=[[Helicoplacoidea]]† [[File:Helicoplacus-cropped.png|20px]]
      |2=[[Edrioasteroidea]]† [[File:Foerstediscus-splendens.png|30px]]
      |3={{Clade
         |label1=[[Blastozoa]]†
         |1={{Clade
            |grouplabel1={{Clade labels |top1=30% |color1=indigo |label1="[[Eocrinoidea]]"† }}
            |1=[[Lepidocystis|Lepidocystiodea]]† [[File:Kinzercystis durhami.svg|20px]]|barbegin1=indigo
            |2={{Clade
               |1=some Eocrinoids† [[File:Gogia-cropped.png|40px]] |bar1=indigo
               |2={{Clade
                  |1={{Clade
                     |1=some Eocrinoids† |bar1=indigo
                     |2={{Clade
                        |1=some Eocrinoids† |barend1=indigo
                        |2={{Clade
                           |1=[[Homostelea]]† {{small|(Cincta)}} [[File:Protcinctus-cropped.png|35px]]
                           |2=[[Ctenocystoidea]]† [[File:Ctenocystis-cropped.png|35px]]
                        }}
                     }}
                     |3=[[Homoiostelea]]† {{small|(Soluta)}} [[File:Coleicarpus-cropped.png|15px]]
                     |4={{Clade
                        |grouplabel1={{Clade labels |top1=30% |color1=indigo |label1="[[Cystoidea]]"† }}
                        |1=[[Diploporita]]† |barbegin1=indigo
                        |2={{Clade
                           |1=[[Rhombifera]]† |barend1=indigo
                           |2=[[Blastoidea]]†
                        }}
                     }}
                  }}
               }}
            }}
         }}
         |label2='''Echinodermata'''
         |sublabel2={{small|([[crown group]])}}
         |2={{Clade
            |label1=[[Crinozoa]]
            |1={{Clade
               |1=[[Crinoidea]] [[File:202012 Metacrinus rotundus.svg|40px]]
               |2=[[Stylophora]]† [[File:Ceratocystis-cropped.png|50px|]]
            }}
            |label2=[[Eleutherozoa]]
            |2={{Clade
               |label1=[[Asterozoa]]
               |1={{Clade
                  |1=[[Ophiuroidea]] [[File:Ophiocoma (26150611418).jpg|60px]]
                  |2=[[Asteroidea]] [[File:Echinaster serpentarius (USNM E28192) 001.png|50px]]
               }}
               |label2=[[Echinozoa]]
               |2={{Clade
                  |1=[[Echinoidea]] [[File:Echinometra viridis.png|50px]]
                  |2={{Clade
                     |1=[[Ophiocistioidea]]† [[File:Sollasina-cthulu-cropped.png|50px]]
                     |2=[[Holothuroidea]] [[File:Holothuria parvula.png|60px]]
                  }}
               }}
            }}
         }}
      }}
   }}
}}
}}

In this theory, the controversial<ref name=arkarua>{{harvnb|Rahman|Zamora|2024|p=300}}</ref> [[Ediacaran]] fossil ''[[Arkarua]]'' is tentatively placed as the sister to all other echinoderms.  [[Helicoplacoidea]] and [[Edrioasteroidea]] join it in the stem group. [[Pelmatozoa]], [[Eocrinoidea]], and [[Cystoidea]] are shown to be paraphyletic while [[Homalozoa]] is polyphyletic.

; Pentaradiality as an apomorphy

Those who find pentaradiality to be derived incorporate the recently-discovered fossils ''[[Ctenoimbricata]]'' (seen as a possible sister to all other echinoderms) and ''[[Helicocystis]]'' (seen as bridging the triradial helicoplacoids and the pentaradial crown group). They cite research indicating that the early appearance of pentaradial forms is likely due to an incomplete fossil record, as well as multiple studies showing non-radial forms as an early stem group, to argue that this is phylogeny represents an emerging consensus.<ref name=twoclades />  They reject ''Arkarua'' as an echinoderm due to its lack of [[stereom]] and possession of true pentaradiality instead of the 2-1-2 pseudo-pentaradiality seen in all early forms.<ref name=arkarua />

The following cladogram is based on Rahman & Zamora (2024),<ref>{{harvnb|Rahman|Zamora|2024|p=310}}</ref> incorporating class<ref>{{harvnb|Rahman|Zamora|2024|p=311}}</ref> and subphylum<ref>{{harvnb|Rahman|Zamora|2024|p=298}}</ref> names from the text:

{{Clade
   |label1='''Echinodermata'''
   |sublabel1={{small|([[total group]])}}
   |1={{Clade
         |label1=
         |1=''[[Ctenoimbricata]]''† [[File:Ctenoimbricata-cropped.png|35px]]
         |grouplabel2={{Clade labels |top1=20% |bar1=indigo |label1="Homalozoa"†}}
         |2={{Clade
            |1=''Ctenocystis''† [[File:Ctenocystis-cropped.png|35px]] {{small|(a [[Ctenocystoidea|Ctenocystoid]]†)}}
            |2=''Courtessolea''† [[File:Courtessolea-cropped.png|35px]] {{small|(a [[Ctenocystoidea|Ctenocystoid]]†)}}
         }}
         |barbegin2=indigo
         |3={{Clade
            |label1=
            |1=''[[Protocinctus]]''† [[File:Protcinctus-cropped.png|35px]] {{small|(a [[Cinctan]]† a.k.a. Homostelean†)}}
            |bar1=indigo
            |2={{Clade
               |label1=
               |1={{Clade
                  |label1=
                  |1=''Coleicarpus''† [[File:Coleicarpus-cropped.png|15px]] {{small|(a [[Soluta (echinoderm)|Solute]]† a.k.a. Homoiostelean†)}}
                  |2=? ''Ceratocystis''† [[File:Ceratocystis-cropped.png|50px|]] {{small|(a [[Stylophoran]]†)}} |state2=dashed
               }}
               |barend1=indigo
               |2={{Clade
                  |label1=
                  |1=''[[Helicoplacus]]''† [[File:Helicoplacus-cropped.png|20px]] {{small|(a [[Helicoplacoid]]†)}}
                  |2={{Clade
                     |1=''[[Helicocystis]]''† [[File:Helicocystis-cropped.png|15px]]
                     |label2='''Echinodermata'''
                     |sublabel2={{small|([[crown group]])}}
                     |2={{Clade
                        |label1=
                        |1=''[[Gogia]]''† [[File:Gogia-cropped.png|40px]] {{small|(an&nbsp;[[Eocrinoid]]†)}}
                        |2=''[[Camptostroma]]''† [[File:Camptostroma-cropped.png|30px]] {{small|(an&nbsp;[[Edrioasteroid]]†)}}
                        |3=[[Crinoidea]] [[File:202012 Metacrinus rotundus.svg|40px]]
                        |label4=[[Eleutherozoa]]
                        |4={{Clade
                           |label1=[[Echinozoa]]
                           |1={{Clade
                              |1=[[Echinoidea]] [[File:Echinometra viridis.png|50px]]
                              |2=[[Holothuroidea]] [[File:Holothuria parvula.png|60px]]
                           }}
                           |label2=[[Asterozoa]]
                           |2={{Clade
                              |1=[[Asteroidea]] [[File:Echinaster serpentarius (USNM E28192) 001.png|50px]]
                              |2=[[Ophiuroidea]] [[File:Ophiocoma (26150611418).jpg|60px]]
}} }} }} }} }} }} }} }} }}

Here, [[Homalozoa]] (with uncertain placement of [[Stylophora]]) is shown to be a paraphyletic assemblage along the stem group, followed by [[Helicoplacoidea]] and then ''[[Helicocystis]]'' as the sister of the crown group.  The details of [[Blastozoa]] vs [[Crinozoa]] are not addressed, as they are represented only by the classes [[Eocrinoidea]] and [[Crinoidea]], respectively, and the overall nature of [[Pelmatozoa]] remains unresolved.  The four-way [[polytomy]] including the [[Eleutherozoa]] and [[Crinoidea]] shows either ''[[Camptostroma]]'' or ''[[Gogia]]'' or both could prove to be outside of the crown group.

===Fossil history===

Echinoderms have a rich fossil record due to their mineralized endoskeletons.<ref>{{harvnb|Rahman|Zamora|2024|p=296}}</ref><ref name="Waggoner">{{harvnb|Waggoner|1995}}</ref>

====Possible early echinoderms====
The three oldest known candidate echinoderms all lack [[stereom]] and other echinoderm [[apomorphy|apomorphies]], making their inclusion in the phylum controversial.<ref name=origins>{{harvnb|Rahman|Zamora|2024|pp=299–303}}</ref>

[[File:Arkarua adami pennetta.png|thumb|Arkarua adami illustration by Pennetta]]

The oldest potential echinoderm [[fossil]] is ''[[Arkarua]]'' from the late [[Ediacaran]] of Australia ''circa'' 555 [[megaannum|Ma]]. These fossils are disc-like, with radial ridges on the rim and a five-pointed central depression marked with radial lines. However, the fossils have no [[stereom]] or internal structure indicating a water vascular system, so they cannot be conclusively identified.<ref>{{harvnb|Smith|2005}}</ref>  Additionally, all known early pentaradial echinoderms are pseudo-pentaradial in a 2-1-2 pattern, with true pentaradiality as seen in ''Arkarua'' not seen until the emergence of the [[Eleutherozoa]].<ref name=origins />

The next possible echinoderms are the [[Vetulocystida|vetulocystids]], which date to the early to mid [[Cambrian]], 541–501 Ma.  While the youngest vetulocystid, ''Thylacocercus'', displays some characteristics that could be interemediate between older vetulocystids and ''[[Yanjiahella]]'', its discoverers consider vetulocystids more likely to be stem ambulacrarians than stem echinoderms.<ref name=vetulocystids />

[[File:Yanjiahella concept.png|thumb|Artist's conception of ''Y. biscarpa'']]

''[[Yanjiahella]]'', from the [[Fortunian]] (''circa'' 539–529 Ma), is unlike the older fossils in that it has a plated theca, albeit one without evidence of stereom.  To some, this is a reason to place it as a stem ambulacrarian or stem hemichordate.<ref>{{harvnb|Zamora|Wright|Mooi|Lefebvre|2020}}</ref>  Others argue that absence of evidence for stereom is not evidence of absence, and consider a stem echinoderm position more likely.<ref>{{harvnb|Topper|Guo|Clausen|Skovsted|2020}}</ref>

====Echinoderms in the Cambrian and Ordovician====

The first universally accepted echinoderms appear in the [[Lower Cambrian]] period; asterozoans appeared in the [[Ordovician]], while the crinoids were a dominant group in the [[Paleozoic]].

It is hypothesised that the ancestor of all echinoderms was a simple, motile, bilaterally symmetrical animal with a mouth, gut and anus. This ancestral organism adopted an attached mode of life with suspension feeding, and developed radial symmetry. Even so, the larvae of all echinoderms are bilaterally symmetrical, and all develop radial symmetry at metamorphosis. Like their ancestor, the starfish and crinoids still attach themselves to the seabed while changing to their adult form.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=792–793}}</ref>

The first known echinoderms were non-motile,<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=792–793}}</ref><ref name="Dornbos Bottjer 2000">{{harvnb|Dornbos|Bottjer|2000}}</ref> but evolved into animals able to move freely. These soon developed endoskeletal plates with stereom structure, and external ciliary grooves for feeding.<ref>{{harvnb|UCMP Berkeley|loc=Echinodermata: Morphology}}</ref> The Paleozoic echinoderms were globular, attached to the [[Substrate (biology)|substrate]] and were orientated with their oral surfaces facing upwards. These early echinoderms had [[ambulacral groove]]s extending down the side of the body, fringed on either side by brachioles, like the pinnules of a modern crinoid. Eventually, the mobile [[eleutherozoa]]ns reversed their orientation to become mouth-downward. Before this happened, the podia probably had a feeding function, as they do in the crinoids today. The locomotor function of the podia came later, when the re-orientation of the mouth brought the podia into contact with the substrate for the first time.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=792–793}}</ref>

<gallery mode="packed">
File:EarlyEchinoderms NT.jpg|Early echinoderms ''[[Ctenoimbricata]]'', ''[[Ctenocystoidea|Ctenocystis]]'', ''[[Gogia]]'', ''[[Protocinctus]]'' and ''[[Rhenocystis]]''
File:Echinosphaerites.JPG|The Ordovician cystoid ''[[Echinosphaerites]]'' from northeastern Estonia
File:fossile-seelilie.jpg|Fossil [[crinoid]] crowns
File:Hyperoblastus.jpg|Calyx of ''Hyperoblastus'', a [[blastoid]] from the [[Devonian]] of [[Wisconsin]]
</gallery>