This clade was established as a new taxon by Jörger et al. in October 2010.[1] Euopisthobranchia is a monophyletic portion of the Opisthobranchia as that taxon was traditionally defined but is not a replacement name for that group as several marine opisthobranch orders including Nudibranchia, Sacoglossa and Acochlidiacea are not included.[1]
Euopisthobranchia consist of the following taxa:[1]
Previous studies discussed the gizzard (i.e. a muscular oesophageal crop lined with cuticula) with gizzard plates as homologous apomorphic structures supporting a clade composed of Cephalaspidea s.s., Pteropoda and Anaspidea.[1] A gizzard with gizzard plates probably originated in herbivorous taxa in which it worked like a grinding mill, thus might be secondarily reduced in carnivorous groups within Cephalaspidea s.s. and Gymnosomata.[1] Klussmann-Kolb and Dinapoli considered the gizzard in Umbraculoidea as non-homologous with the one in the previous groups, on account of the absence of gizzard plates or spines.[1] This contradicted Salvini-Plawen and Steiner, who had proposed the gizzard to be a synapomorphy of the larger clade of Paratectibranchia (Pteropoda, Cephalaspidea and Anaspidea) and Eleutherobranchia, secondarily lost in Nudipleura but still present in Umbraculoidea.[1] As coded in Wägele and Klussmann-Kolb, phylogenetic hypothesis by Jörger et al. (2010)[1] supports homology of the gizzard in Umbraculoidea with the gizzard with gizzard plates and spines in the other euopisthobranchian taxa.[1] Thus, the structure is proposed as a synapomorphy of Euopisthobranchia.[1]
Cladogram
A cladogram showing phylogenic relations of the Heterobranchia as proposed by Jörger et al. (2010):[1]
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