Palaeos		Gastropoda: Pulmonata
Mollusca		Stylommatophora

Stylommatophora

GASTROPODA  
|--Caenogastropoda
`--Heterobranchia
   |--Opisthobranchia
   |
   `--PULMONATA
      |==Basommatophora
      `--+--Stylommatophora
         `--+--Systellommatophora 
            `==Acteophila  

Introduction

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]

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

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.

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.

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).
		Achatinoidea  (formerly included under Sigmurethra)
			Achatinidae Swainson, 1840
			Coeliaxidae Pilsbry, 1907
			Ferrussaciidae Bourguignat, 1883
			Glessulidae Schileyko, 1996
			Subulinidae Thiele, 1931
			Thyrophorellidae Thiele, 1926
		'non-achatinoid' taxa (unnamed clade)
	Suborder SIGMURETHRA 

		Buliminoidea
			Buliminidae Clessin, 1879 [?Pfeiffer, 1879] [=Enidae Woodward, 1903, in part]
			Cerastuidae Wenz, 1923 [=Enidae Woodward, 1903, in part]

		Clausilioidea
			Clausiliidae Mörch, 1864
			Triptychiidae ?author

		Orthalicoidea [=Bulimulacea]
			Orthalicidae Albers-Martens, 1860
			Amphibulimidae Crosse & Fischer, 1873
			Anadromidae Zilch, 1959
			Cerionidae Pilsbry, 1901 [=Ceriidae]
			Coelociidae Nordsieck, 1986
			Grangerellidae Russel, 1931
			Megaspiridae Pilsbry, 1904 [=Coelociontidae Iredale, 1937]
			Microceramidae Pilsbry, 1903
			Odontostomidae Pilsbry & Vanatta, 1898
			Urocoptidae Pilsbry & Vanatta, 1898 [?1896]

		Aillyoidea
			Aillyidae Baker, 1930
			Oleacinoidea
			Oleacinidiae Adams & Adams, 1855
			Spiraxidae Baker, 1939
			Testacellidae Gray in Turton, 1840

		Streptaxoidea
			Streptaxidae Gray, 1806

		Strophocheiloidea
			Strophocheilidae Thiele, 1926
			Dorcasiidae Thiele, 1926
			Megalobulimidae Leme, 1973

		Acavoidea
			Acavidae Pilsbry, 1895
			Caryodidae Thiele, 1926
			Macrocyclidae Thiele, 1926
			Megomphicidae Baker, 1930 [=Ammonitellidae ?author]

		Rhytidoidea
			Rhytididae Pilsbry, 1893 [?1895; =Paryphantidae ?author]
			Aperidae ?author [=Chlamydephoridae Cockerell, 1935]
			Haplotrematidae Baker, 1925
			Systrophiidae Thiele, 1926 [=Scolodontidae Baker, 1925]

		Plectopylidoidea
			Plectopylididae Moellendorf, 1900 [=Corillidae ?author]
			Sculptariidae ?Vaught, 1989

		Arionoidea
			Arionidae Gray in Turton, 1840
			Philomycidae Gray, 1847
		
		Punctoidea [=Endodontoidea]
			Punctidae Morse, 1864
			Charopidae Hutton, 1884
			Discidae Thiele, 1931
			Endodontidae Pilsbry, 1895
			Helicodiscidae Baker, 1927
			Oopeltidae Cockerell, 1891
			Oreohelicidae Pilsbry, 1939
			Otoconchidae Cockerell, 1893
			?Philiomycidae ?author

		Sagdidoidea
			Sagdidae Pilsbry, 1895 [=Sagdididae Tracey, Todd & Erwin, 1993]

		Gastrodontoidea
			Gastrodontidae Tryon, 1866

		Helixarionoidea [=Helicarionoidea]
			Helixarionidae Bourguignat, 1883 [=Helicarionidae Bourguignat, 1883 & Godwin & Austen, 1883/8]
			Ariophantidae Godwin-Austen, 1888 [?Germain, 1921]
			Cystopeltidae ?Vaught, 1989
			Dyakiidae ?author
			Euconulidae Baker, 1928 [?Strebel & Pfeffer]
			?Sesaridae ?author
			Trochomorphidae Moellendorf, 1890
			Urocyclidae Simroth, 1889

		Vitrinoidea [=Zonitoidea]
			Vitrinidae Fitzinger, 1833
			Daudebardiidae Kobelt, 1906
			Milacidae Ellis, 1926 [?Germain, 1930]
			Parmacellidae Gray, 1860
			Plutoniidae Wiktor & Backeljau, 1995 
			Plutoniinae Cockerell, 1893 
			Zonitidae Mörch, 1864

		Limacoidea
			Limacidae Rafinesque, 1815 
			Agriolimacidae Wagner, 1935
			Boetgerillidae ?author
			Thyphorellidae ?author

		Trigonochlamydoidea
			Trigonchlamydidae Hesse, 1882
			?Papillodermidae Wiktor, Martin & Castillejo, 1990
		
		Polygyroidea [=Mesodontoidea]
			Polygyridae Pilsbry, 1894/5 [?1940] [=Mesodontidae Tryon, 1866]
			Thysanophoridae Pilsbry, 1926

		Camaenoidea
			Camaenidae Pilsbry, 1895
			Solaropsidae Nordsieck, 1986

		Helicoidea [=Xanthonychoidea]
			Helicidae Rafinesque, 1815
			Cochlicellidae Schileyko, 1972 {from Cochlicellinae (Hygromiidae)}
			Eulotidae Moellendorf, 1898 [=Bradybaenidae Pilsbry, 1934]
			Hygromiidae Tryon, 1866 [=Helicellidae ?author]
			Monadeniidae Schileyko, 1996 
			Monadeniinae Nordsieck, 1987 
			Sphincterochilidae Zilch, 1960
			Xanthonychidae Strebel & Pfeffer, 1880 [?=Helminthoglyptidae Pilsbry, 1939]

	Suborder ORTHURETHRA 

		Achatinelloidea 
			Achatinellidae Gulick, 1873 [?=Pacificellidae Steenberg, 1925]
			Cochlicopoidea [=Cionelloidea]
			Cochlicopidae Pilsbry, 1900 [=Cionellidae ?author]
			Amastridae Pilsbry, 1911
			Pupilloidea
			Pupillidae Turton, 1831
			Chondrinidae Steenburg, 1925
			Orculidae Pilsbry, 1918
			Pleurodiscidae Wenz, 1923
			Pyramidulidae Kennard & Woodward, 1914
			Strobilopsidae Pilsbry, 1918
			Valloniidae Morse, 1864 [?Pilsbry, 1900]
			Vertiginidae Fitzinger, 1833 [?Stimpson, 1851]
			Partuloidea
			Partulidae Pilsbry, 1900

		?Succineoidea [=Heterurethra]
			Succineidae Beck, 1837

		?Athoracophoroidea [=Tracheopulmonata]
			Athoracophoridae Fischer, 1883