Editing Echinoderm (section)

====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]]
         }}
      }}
   }}
}}