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.

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). For example, in the Dinosaur Park Formation, the fossil remains of hadrosaur eggshells are rare. 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.

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. 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.

Extant subgenera and species Extant species within the genus Pisidium include:

Pisidium amnicum (O. F. Müller, 1774) Pisidium amurense Moskvicheva, 1985 Pisidium armillatum Kuiper, 1966 Pisidium artifex Kuiper, 1960 Pisidium baicalense W. Dybowski, 1902 Pisidium bejumae H. B. Baker, 1930 Pisidium betafoense Kuiper, 1953 Pisidium boliviense Sturany, 1900 Pisidium chicha Ituarte, 2005 Pisidium chiquitanum Ituarte, 2001 Pisidium cruciatum Sterki, 1895 Pisidium decurtatum Lindholm, 1909 Pisidium dilatatum Westerlund, 1897 Pisidium dorbignyi Clessin, 1877 Pisidium dubium (Say, 1817) Pisidium fistulosum Mandahl-Barth, 1954 Pisidium forense Meier-Brook, 1967 Pisidium globulus Clessin, 1888 Pisidium habbemae Odhner, 1940 Pisidium huillichum Ituarte, 1999 Pisidium idahoense E. W. Roper, 1890 Pisidium inacayali Ituarte, 1996 Pisidium inflexum Odhner, 1940 Pisidium invenustum Kuiper, 1966 Pisidium iquito Ituarte, 2004 Pisidium jamaicense Prime, 1865 Pisidium johnsoni E. A. Smith, 1882 Pisidium lebruni Mabille, 1884 Pisidium llanquihuense Ituarte, 1999 Pisidium magellanicum (Dall, 1908) Pisidium meierbrooki Kuiper & Hinz, 1984 Pisidium montigenum Kuiper, 1966 Pisidium observationis Pilsbry, 1911 Pisidium ocloya Ituarte, 2005 Pisidium omaguaca Ituarte, 2005 Pisidium orientale Prozorova, 1995 Pisidium patagonicum Pilsbry, 1911 Pisidium pipoense Ituarte, 2000 Pisidium plenilunium (Melvill & Standen, 1907) Pisidium punctatum Sterki, 1895 Pisidium punctiferum (Guppy, 1867) Pisidium reticulatum Kuiper, 1966 Pisidium sanguinichristi D. W. Taylor, 1987 Pisidium sterkianum Pilsbry, 1897 Pisidium taraguyense Ituarte, 2000 Pisidium vile Pilsbry, 1897 Pisidium volutabundum Vanatta, 1910 Pisidium wilhelminae Odhner, 1940

The following species require evaluation to confirm their placement in Pisidium; many of the following are taxa inquirenda and have not been used by recent authors:

Pisidium acuminatum Clessin, 1874 Pisidium acutum L. Pfeiffer, 1841 Pisidium alienum Clessin, 1879 Pisidium bartolomeum Clessin, 1873 Pisidium colbeuai Clessin, 1875 Pisidium costatum Sterki, 1903 Pisidium costulatum Westerlund, 1898 Pisidium demissum Clessin, 1876 Pisidium duplicatum L. Pfeiffer, 1841 Pisidium foreli Clessin, 1876 Pisidium herminii Clessin, 1877 Pisidium italicum Clessin, 1875 Pisidium marteli Pallary, 1928 Pisidium miliolum Clessin, 1907 Pisidium moreanum Clessin, 1874 Pisidium obliquum L. Pfeiffer, 1841 Pisidium obtusale (Lamarck, 1818) sensu C. Pfeiffer, 1821 Pisidium obtusatum Clessin, 1875 Pisidium occupatum Clessin, 1876 Pisidium ovatum Clessin, 1877 Pisidium persicum Prashad, 1933 Pisidium pileus Clessin, 1873 Pisidium profundum Clessin, 1876 Pisidium rotundatum Pallary, 1922 Pisidium schmidti Clessin, 1907 Pisidium shadini Johansen, 1937 Pisidium sphaeriiforme Clessin, 1874 Pisidium tumidum Clessin, 1875 Pisidium watsoni Paiva, 1866 Pisidium wolfii Clessin, 1879 undescribed Pisidium species from Africa (Kuiper, in prep., awaiting additional records)Kuiper J.G.J. (2009). "Fossil records of Palaearctic Pisidium species in tropical Africa". Zoologische Mededelingen 83(10): 593-594. HTM .

The genera Conventus, Euglesa, Hindupisidium, Odhneripisidium, and Sphaerium were split out from Pisidium after taxonomic revision.

Footnotes

References Tanke, D.H. and Brett-Surman, M.K. 2001. Evidence of Hatchling and Nestling-Size Hadrosaurs (Reptilia:Ornithischia) from Dinosaur Provincial Park (Dinosaur Park Formation: Campanian), Alberta, Canada. pp. 206–218. In: Mesozoic Vertebrate Life—New Research Inspired by the Paleontology of Philip J. Currie. Edited by D.H. Tanke and K. Carpenter. Indiana University Press: Bloomington. xviii + 577 pp.

External links

Pisidium keyed, described and illustrated in Danmarks Fauna (Georg Mandahl-Barth) Pisidium images at Consortium for the Barcode of Life https://web.archive.org/web/20141128091542/http://sunsite.ualberta.ca/Projects/Aquatic_Invertebrates/?Page=20