Wednesday, May 21, 2008

Gastropoda - Heterostropha (Heterobranchia)

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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

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