Euthyneura is a taxonomic clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 of snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s and slugs, which includes species from the sea, the land and freshwater; marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

, aquatic

Aquatic animal

An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...

 and terrestrial

Terrestrial animal

Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...

 gastropod mollusks in the clade Heterobranchia

Heterobranchia

Heterobranchia, the heterobranchs or Euthyneura, is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks....

.

Euthyneura are considered the crown group

Crown group

A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...

 of Gastropoda. Euthyneura are characterised by several autapomorphies. The Euthyneura are considered to be the most successful and diverse group of Gastropoda. Within this taxon Gastropoda have reached their peak in species richness

Species richness

Species richness is the number of different species in a given area. It is represented in equation form as S.Species richness is the fundamental unit in which to assess the homogeneity of an environment. Typically, species richness is used in conservation studies to determine the sensitivity of...

 and ecological diversity. This obvious evolutionary success can probably be attributed to several factors. Marine Opisthobranchia, e.g., have evolved several clades specialised on less utilised food resources such as sponges or cnidarians. A key innovation in the evolution of Pulmonata was the colonization of freshwater and terrestrial habitats.

Various phylogenetic studies focused on Euthyneura: Dayrat et al. (2001), Dayrat & Tillier (2002) and Grande et al. (2004). Morphological analyses by Dayrat and Tillier (2002) demonstrated the need to explore new datasets in order to critically analyse the phylogeny of this controversial group of gastropods. Klussmann-Kolb et al. (2008) traced an evolutionary scenario regarding colonisation of different habitats based on phylogenetic hypothesis and they showed that traditional classification of Euthyneura needs to be reconsidered.

2010 taxonomy

Jörger et al. (2010) have redefined major groups within the Heterobranchia and a cladogram

Cladogram

A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...

 showing phylogenic relations of Euthyneura is as follows: