Gastropoda - Heterostropha (Heterobranchia)

While the May edition of the Accretionary Wedge (see my contribution) is reaching its climax at Julian's Blog I finally reached the last chapter of theory about the Gastropoda. Today I will write an overview of the Heterobranchia (Heterostropha). In these Gastropods the shell is coiled differently in the Proto- and Teleconch sometimes providing for some peculiar shapes. This phenomen is called Heterostrophism. Usually there is a change from sinistral to dextral coiling between the larva and the adult shell. The Protoconch is two-parted. Heterobranchia are known since the Carboniferous and exist even today. All land snails belong into this sub-class.

Allogastropoda

Architectonica

• feeds on Coelenterata
• Triassic - today
• heterostrophic
• Protoconch overgrown by Teleconch gives "hole-like" appearance
• soft substrate, sand flats in shallow water
  

Nerinea

• large, similiar to Cerithium but weaker sculpturing
• Jurassic + Cretaceous (today: Pyramidelloidea)
• extremely high-towering shell
• characteristic spiral folds on the newel and whorl walls
• lagoon environment and Rudist reefs
• Aperture opening constricted

Valvata

• marine and terrestrial (freshwater)
• Upper Jurassic - today
• northern hemisphere
• Heterostrophism unrecognisable in shell
• marshes and Ca-rich water
  

Opistobranchia

• shell partially or completely reduced
• certain since Triassic
• most primitive form: Cephalaspidea

Acteon

• typical primitive Opistobranch
• Carboniferous - today
• Protoconch at 90° angle to Teleconch
• carnivor
• sand-dwelling

Pteropods

• holoplanctonic
• planctotrophic
• foot transformed to fins
• migrating vertically in the water column
• hunting with slime filaments for microparticles
• Palaeocene - today
• two orders Thecosomata (shell, sinistral, Aragonite) and Gymnosomata (no shell, tentacles)

Sacoglossa

• bivalve shell
• bivalve shell develops after metamorphism
• feeds on Hydrozoans
• cleptoplastic! therefore often green colored

Pulmonata

• pallial lung
• many varied forms
• Carboniferous - today, radiating since Tertiary
• Stratigraphic relevance: Tertiary
• climatic indicators for Quarternary

Archaeopulmonata

• typical for the transition zone of land to ocean
• Antracopupa found in Carboniferous coal seams
• Carboniferous - Jurassic: aquatic

Basommatophora

• mostly freshwater to terrestrial
  
• two families have returned back to marine
• Lower Jurassic - today

Gyraulus

• planispiral
• partially decoils depending on water chemistry
• Protoconch typically striped
• small
• holoartic
• herbivor?
• freshwater
  

Lymaea

Planorbis

• sinistral
• Tertiary - today
• freshwater - terrestrial
• Ca-rich water
• low energy environment

Stylomatophora

• terrestrial
• stalk eyes
• herbivor
• cool and moist environment

Helix

• small to medium sized
• typical "snail" appearance
• Cenozoic?
  

Pupilla

• very tiny
• terrestrial
  

Clausilia

• terrestrial
• turreted shell
• herbivor
• Upper Cretaceous - today