External view of the left valve of Pisidium pseudosphaerium
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Sphaeriida
Superfamily: Sphaerioidea
Family: Sphaeriidae
Genus: Pisidium
C. Pfeiffer, 1821

See text


Tetragonocyclas Pirogov & Starobogatov, 1974

Pisidium is a genus of very small or minute freshwater clams known as pill clams or pea clams, aquatic bivalve molluscs in the family Sphaeriidae, the pea clams and fingernail clams.

In some bivalve classification systems, the family Sphaeriidae is referred to as Pisidiidae, and occasionally Pisidium species are grouped in a subfamily known as Pisidiinae.

Pisidium and taphonomy

In large enough quantities, the minute shells of these bivalves can affect environmental conditions, and this change in conditions can positively affect the ability of organic remains in the immediate environment to fossilize (one aspect of taphonomy).[1] For example, in the Dinosaur Park Formation, the fossil remains of hadrosaur eggshells are rare.[2] This is because the breakdown of tannins from the local coniferous vegetation caused the ancient waters to be acidic, and therefore usually eggshell fragments dissolved in the water before they had a chance to be fossilized.[3]

Hadrosaur eggshell fragments are however present in two microfossil sites in the area. Both of these sites are dominated by preserved shells of invertebrate life, primarily shells of pisidiids.[2] The slow dissolution of these minute bivalve shells released calcium carbonate into the water, raising the water's pH high enough that it prevented the hadrosaur eggshell fragments from dissolving before they could be fossilized.[4]

Extant subgenera and species

Extant subgenera and species within the genus Pisidium include:

Subgenus Euglesa Jenyns, 1832

Subgenus Pisidium Pfeiffer, 1821

Subgenus Cyclocalyx Dall, 1903

Subgenus Tropidocyclas Dall, 1903

Subgenus Hiberneuglesa Starobogatov, 1983

Subgenus Cingulipisidium Pirogov & Starobogatov, 1974

Subgenus Pseudeupera Germain, 1909

Subgenus Neopisidium Odhner, 1921

Subgenus Odhneripisidium Kuiper, 1962

Subgenus Afropisidium Kuiper, 1962

Subgenus unknown

Subgenus incertae sedis


  1. ^ Tanke and Brett-Surman (2001).
  2. ^ a b "Abstract," Tanke and Brett-Surman (2001). Page 206.
  3. ^ "Discussion," Tanke and Brett-Surman (2001). Page 212.
  4. ^ "Eggshell," Tanke and Brett-Surman (2001). Page 209.
  5. ^ Appleton C., Ghamizi M., Jørgensen A., Kristensen T. K., Lange C., Stensgaard A-S. & Van Damme D. (2009). Pisidium pirothi. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4. <www.iucnredlist.org>. Downloaded on 3 December 2010.
  6. ^ Lange C. N. & Ngereza C. (2004). Pisidium artifex. 2006 IUCN Red List of Threatened Species. Downloaded on 7 August 2007.
  7. ^ Bogan A. (2011). Pisidium stewarti. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. <www.iucnredlist.org>. Downloaded on 1 November 2012.
  8. ^ Kuiper J.G.J. (2009). "Fossil records of Palaearctic Pisidium species in tropical Africa". Zoologische Mededelingen 83(10): 593-594. HTM Archived 4 March 2016 at the Wayback Machine.