Mollusca Stylommatophora


Abbreviated Dendrogram



Helix aspersa

Helix aspersa, (Müller, 1774) the common European garden snail. Width and height of shell about 2.5 to 3 cm.
This successful and adaptable species is found throughout Europe, the Middle-East, North and South America and Australia, where it has been introduced and established itself in suburban areas. See Snail Species - the Family Helicidae for more.
(Sigmurethra - Helicoidea - Helicidae - Helicinae]

image © Chris Elliott

The higher pulmonates, or Stylommatophora, are a successful clade that far outnumber the Basommatophora in species. This order, the most advanced and specialized of all the pulmonates, includes most land snails and slugs represent one of the major metazoan invasions of the land, comprising some 30,000 to 35,000 species (with many more probably awaiting discovery) - almost equivalent to vertebrates as a whole. They include the familiar garden snail and slug, but also thousands of less known species. They Stylommatophora are major components of many terrestrial ecosystems, and have also become important models for studies on the mechanisms of evolution and biogeographic studies (Wade, iNet). They frequently occur beneath leaf litter, stones and bark, and are especially abundant on oceanic islands, such as the Pacific [Barnes 1980 p.340]. The name "Stylommatophora" means "slender eye-bearing stalk", and refers to the fact that these animals have two pairs of tentacles, with eyes located at the tips of the upper (posterior / dorsal), pair, rather than at the base, as is the case with the Basommatophora. All species are terrestrial, and many are locally abundant. Although most stylommatophores feed on plants and detritus, a few are carnivores that feed on other snails. Chase 2002 p.15

Agathylla biloba

Agathylla biloba (A. J. Wagner 1914)
(Sigmurethra - Clausilioidea - Clausiliidae - Alopiinae]

Photo © V. Wiese, Haus der Natur Cismar, from Albanian non-marine molluscs

Most species have thin to moderately thick calcareous conispiral shells, which range from very low-spired to steep-sided, high-spired forms. A majority have many whorls, generally but not always more than among basommatophorans. [Moore, et al 1952, p.332]. There are also includes shell-less slugs and semi-slugs with residual shells that are too small to accommodate the body. In fact, as with the Opisthobranchia, shell reduction has occurred independently a number of times in different lineages. Slugs seem to have evolved in environments of low calcium and high humidity [Barnes 1980 p.340].

The aperture usually has a rather evenly rounded outer lip, often with a thickened rim. Tooth-like projections of the inner lip, and in some shells of the outer lip also, serve to keep out predators such as insects but allow the soft snail to squeeze through [Barnes 1980 p.340]. . Such features especially characterize genera of the Pupidae, and largely because of the close resemblance of some Pennsylvanian non-marine gastropods to the modern Pupa, these Paleozoic fossils were at one time thought to belong among the stylommatophorans [Moore, et al 1952, p.332]. This is now no longer believed to be the case, and it is generally agreed that the Pulmonates, including the Stylommatophora, evolved much later, during the Jurassic period. [Clarkson, 1979 p.163]

Most Stylommatophora are fairly small, averaging less than a centimeter in shell length or width, although the giant African snail Achatina achatina, have shells that reach 27 cm [Barnes 1980 p.340]

In terms of diversity, they are the largest clade of gastropods, with some 35,000 species, and from 71 to 92 families, depending on the classification scheme used (Bieler, 1992; Wade et al., 2001). Part of this diversity may be due to the fact that local populations easily become isolated, as the animals move slowly, and have little need to migrate. This leads to both greater speciation and greater intraspecific genetic variation [Chase 2002 pp.13-14 ]. According to Thomaz et al., 1996, Cepaea nemoralis has the most extreme intraspecific variation of mitochondrial DNA of any animal.

Cepaea nemoralis

the European land snail Cepaea nemoralis
(Sigmurethra - Helicoidea - Helicidae - Helicinae]
Earliest Pleistocene to Recent
Width and height of shell about 2 cm.
This very variable European species is found throughout the world. See Snail Species - the Family Helicidae for more.

image Michigan State University

The monophyletic composition of the Stylommatophora has been confirmed by molecular analysis of ribosomal RNA (Wade & Mordan, 2000, Wade et al., 2001).

The origins and the deep-level evolutionary relationships of their major groupings within the Stylommatophora are controversial, with anatomical studies giving confusing and conflicting results. The following arrangement from Jeffery 2001 (contrast with older classification by Abbott & Dance, 2000) has been modified in the light of molecular research by Chris Wade involving ribosomal RNA gene sequencing. Dr Wade's research reveals a single assemblage including the families Achatinidae, Subulinidae and Streptaxidae near the base of the group, forming a sister group to all remaining stylommatophorans and dividing the order into 'achatinoid' and 'non-achatinoid' taxa. The Orthurethra appear to be relatively advanced within the 'non-achatinoid clade', and broadly equivalent to other super-familial clusters, indicating that supposedly primitive features such as the orthurethran kidney are in fact derived. (Wade, iNet)

I have also modified the Linnaean ranking in keeping with the standard ordinal grade normally given to the Stylommatophora

Although the molecular tree also suggests that the origin of the Stylommatophora is much earlier than the main period of their diversification (Wade, iNet), this may simply (as with the Cambrian explosion and other important events) be a result of evolutionary heterochrony.

Order Stylommatophora 'achatinoid' taxa (unnamed clade). Superfamily Achatinoidea (formerly included under Sigmurethra) Family Achatinidae Swainson, 1840 Family Coeliaxidae Pilsbry, 1907 Family Ferrussaciidae Bourguignat, 1883 Family Glessulidae Schileyko, 1996 Family Subulinidae Thiele, 1931 Family Thyrophorellidae Thiele, 1926 'non-achatinoid' taxa (unnamed clade) Suborder Sigmurethra Superfamily Buliminoidea Family Buliminidae Clessin, 1879 [?Pfeiffer, 1879] [=Enidae Woodward, 1903, in part] Family Cerastuidae Wenz, 1923 [=Enidae Woodward, 1903, in part] Superfamily Clausilioidea Family Clausiliidae Mörch, 1864 Family Triptychiidae ?author Superfamily Orthalicoidea [=Bulimulacea] Family Orthalicidae Albers-Martens, 1860 Family Amphibulimidae Crosse & Fischer, 1873 Family Anadromidae Zilch, 1959 Family Cerionidae Pilsbry, 1901 [=Ceriidae] Family Coelociidae Nordsieck, 1986 Family Grangerellidae Russel, 1931 Family Megaspiridae Pilsbry, 1904 [=Coelociontidae Iredale, 1937] Family Microceramidae Pilsbry, 1903 Family Odontostomidae Pilsbry & Vanatta, 1898 Family Urocoptidae Pilsbry & Vanatta, 1898 [?1896] Superfamily Aillyoidea Family Aillyidae Baker, 1930 Family Oleacinoidea Family Oleacinidiae Adams & Adams, 1855 Family Spiraxidae Baker, 1939 Family Testacellidae Gray in Turton, 1840 Superfamily Streptaxoidea Family Streptaxidae Gray, 1806 Superfamily Strophocheiloidea Family Strophocheilidae Thiele, 1926 Family Dorcasiidae Thiele, 1926 Family Megalobulimidae Leme, 1973 Superfamily Acavoidea Family Acavidae Pilsbry, 1895 Family Caryodidae Thiele, 1926 Family Macrocyclidae Thiele, 1926 Family Megomphicidae Baker, 1930 [=Ammonitellidae ?author] Superfamily Rhytidoidea Family Rhytididae Pilsbry, 1893 [?1895; =Paryphantidae ?author] Family Aperidae ?author [=Chlamydephoridae Cockerell, 1935] Family Haplotrematidae Baker, 1925 Family Systrophiidae Thiele, 1926 [=Scolodontidae Baker, 1925] Superfamily Plectopylidoidea Family Plectopylididae Moellendorf, 1900 [=Corillidae ?author] Family Sculptariidae ?Vaught, 1989 Superfamily Arionoidea Family Arionidae Gray in Turton, 1840 Family Philomycidae Gray, 1847 Superfamily Punctoidea [=Endodontoidea] Family Punctidae Morse, 1864 Family Charopidae Hutton, 1884 Family Discidae Thiele, 1931 Family Endodontidae Pilsbry, 1895 Family Helicodiscidae Baker, 1927 Family Oopeltidae Cockerell, 1891 Family Oreohelicidae Pilsbry, 1939 Family Otoconchidae Cockerell, 1893 Family ?Philiomycidae ?author Superfamily Sagdidoidea Family Sagdidae Pilsbry, 1895 [=Sagdididae Tracey, Todd & Erwin, 1993] Superfamily Gastrodontoidea Family Gastrodontidae Tryon, 1866 Superfamily Helixarionoidea [=Helicarionoidea] Family Helixarionidae Bourguignat, 1883 [=Helicarionidae Bourguignat, 1883 & Godwin & Austen, 1883/8] Family Ariophantidae Godwin-Austen, 1888 [?Germain, 1921] Family Cystopeltidae ?Vaught, 1989 Family Dyakiidae ?author Family Euconulidae Baker, 1928 [?Strebel & Pfeffer] Family ?Sesaridae ?author Family Trochomorphidae Moellendorf, 1890 Family Urocyclidae Simroth, 1889 Superfamily Vitrinoidea [=Zonitoidea] Family Vitrinidae Fitzinger, 1833 Family Daudebardiidae Kobelt, 1906 Family Milacidae Ellis, 1926 [?Germain, 1930] Family Parmacellidae Gray, 1860 Family Plutoniidae Wiktor & Backeljau, 1995 Family Plutoniinae Cockerell, 1893 Family Zonitidae Mörch, 1864 Superfamily Limacoidea Family Limacidae Rafinesque, 1815 Family Agriolimacidae Wagner, 1935 Family Boetgerillidae ?author Family Thyphorellidae ?author Superfamily Trigonochlamydoidea Family Trigonchlamydidae Hesse, 1882 Family ?Papillodermidae Wiktor, Martin & Castillejo, 1990 Superfamily Polygyroidea [=Mesodontoidea] Family Polygyridae Pilsbry, 1894/5 [?1940] [=Mesodontidae Tryon, 1866] Family Thysanophoridae Pilsbry, 1926 Superfamily Camaenoidea Family Camaenidae Pilsbry, 1895 Family Solaropsidae Nordsieck, 1986 Superfamily Helicoidea [=Xanthonychoidea] Family Helicidae Rafinesque, 1815 Family Cochlicellidae Schileyko, 1972 {from Cochlicellinae (Hygromiidae)} Family Eulotidae Moellendorf, 1898 [=Bradybaenidae Pilsbry, 1934] Family Hygromiidae Tryon, 1866 [=Helicellidae ?author] Family Monadeniidae Schileyko, 1996 Family Monadeniinae Nordsieck, 1987 Family Sphincterochilidae Zilch, 1960 Family Xanthonychidae Strebel & Pfeffer, 1880 [?=Helminthoglyptidae Pilsbry, 1939] Suborder Orthurethra Superfamily Achatinelloidea Family Achatinellidae Gulick, 1873 [?=Pacificellidae Steenberg, 1925] Superfamily Cochlicopoidea [=Cionelloidea] Family Cochlicopidae Pilsbry, 1900 [=Cionellidae ?author] Family Amastridae Pilsbry, 1911 Superfamily Pupilloidea Family Pupillidae Turton, 1831 Family Chondrinidae Steenburg, 1925 Family Orculidae Pilsbry, 1918 Family Pleurodiscidae Wenz, 1923 Family Pyramidulidae Kennard & Woodward, 1914 Family Strobilopsidae Pilsbry, 1918 Family Valloniidae Morse, 1864 [?Pilsbry, 1900] Family Vertiginidae Fitzinger, 1833 [?Stimpson, 1851] Superfamily Partuloidea Family Partulidae Pilsbry, 1900 Superfamily ?Succineoidea [=Heterurethra] Family Succineidae Beck, 1837 Superfamily ?Athoracophoroidea [=Tracheopulmonata] Family Athoracophoridae Fischer, 1883

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