Editing Echinoderm (section)

=====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.