''Pisidium'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of very small or minute freshwater clams known as pill clams or pea clams, aquatic bivalve molluscs in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Sphaeriidae Sphaeriidae is a family of small to minute freshwater bivalve molluscs in the order Sphaeriida. In the US, they are commonly known as pea clams or fingernail clams. Heard, William H. 1977. Reproduction of fingernail clams (Sphaeriidae: ''Sphae ...

, 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 Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientist Ivan Efremov t ...

). For example, in the

Dinosaur Park Formation The Dinosaur Park Formation is the uppermost member of the Belly River Group (also known as the Judith River Group), a major geologic unit in southern Alberta. It was deposited during the Campanian stage of the Late Cretaceous, between about 76. ...

, the fossil remains of

hadrosaur Hadrosaurids (), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod family, which incl ...

eggshells are rare. This is because the breakdown of

tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanner'' ...

s 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 A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

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. Image:Pisidium moitessierianum.svg, Drawing of the right valve external view of ''

Pisidium moitessierianum ''Pisidium moitessierianum'' is a species of minute freshwater clam. It is an aquatic bivalve mollusc in the family Sphaeriidae. Description This species is the smallest of the pea clams or ''Pisidium'' species, and the smallest of any ...

'' Image:Pisidium moitessierianum 3.svg, Drawing of the right valve internal view of ''Pisidium moitessierianum'' Image:Pisidium moitessierianum 2.svg, Drawing of the right valve lateral view of ''Pisidium moitessierianum'' Image:Pisidium sp.jpg, SEM image of ''Pisidium sp.''

Extant subgenera and species

Extant subgenera and species within the genus ''Pisidium'' include: Subgenus ''Euglesa'' Jenyns, 1832 *''

Pisidium casertanum ''Pisidium casertanum'', the pea cockle or pea clam, is a minute freshwater bivalve mollusc of the family Sphaeriidae. Description The shell is broad, sub-triangular or oval and is ornamented with sculpture of faint concentric striations. ...

'' (Poli, 1791) *''

Pisidium globulare ''Pisidium globulare'' is a species of freshwater bivalve from the family Sphaeriidae. Distribution * Czech Republic – in Bohemia, in Moravia Horsák M., Juřičková L., Beran L., Čejka T. & Dvořák L. (2010). "Komentovaný seznam měkkýš ...

'' Clessin, 1873 *''

Pisidium personatum ''Pisidium personatum'' is a species of freshwater bivalve from the family Sphaeriidae. Description The 2.5-3.5 mm shell is a characteristic round-regular oval shape. It has centrally placed low, rounded umbos. The surface (periostracum) is ...

'' Malm, 1855 Subgenus ''Pisidium'' Pfeiffer, 1821 *''

Pisidium amnicum ''Pisidium amnicum'' is a species of very small freshwater clam, sometimes known as the greater European peaclam or the River pea shell. It is an aquatic bivalve in the family Sphaeriidae. Description Although only 9 mm (range 7–11& ...

'' (O. F. Müller, 1774) *''

Pisidium dilatatum ''Pisidium dilatatum'' is a species of freshwater bivalve belonging to the family Sphaeriidae Sphaeriidae is a family of small to minute freshwater bivalve molluscs in the order Sphaeriida. In the US, they are commonly known as pea clams o ...

'' Westerlund, 1897 Subgenus ''Cyclocalyx'' Dall, 1903 *'' Pisidium hinzi'' Kuiper, 1975 *''

Pisidium obtusale ''Pisidium obtusale'' is a species of freshwater bivalve from family Sphaeriidae. Description The 2.5–3.5mm. shell is very tumid (swollen). It is an angular-oval shape with broad, rounded umbos which are behind the midpoint. The surface (perio ...

'' (

Lamarck Jean-Baptiste Pierre Antoine de Monet, chevalier de Lamarck (1 August 1744 – 18 December 1829), often known simply as Lamarck (; ), was a French naturalist, biologist, academic, and soldier. He was an early proponent of the idea that biolo ...

, 1818) Subgenus ''Tropidocyclas'' Dall, 1903 *''

Pisidium henslowanum ''Pisidium henslowanum'' is a species of very small freshwater clam or pea clam, an aquatic bivalve mollusc in the family Sphaeriidae. It is commonly referred to as the Henslow peaclam. Etymology The specific name honours John Stevens Henslo ...

'' (Sheppard, 1823) *''

Pisidium lilljeborgii ''Pisidium lilljeborgii'' is a species of freshwater bivalve from the family Sphaeriidae. Description A moderately swollen (tumid), trapezoidal shell. The umbos are just behind the mid point and described as fairly narrow. The surface (periostra ...

'' Clessin, 1886 *''

Pisidium supinum ''Pisidium supinum'' is a species of minute freshwater clam, a pea clam, an aquatic bivalve mollusc in the family Sphaeriidae (the fingernail clams and pea clams.) The shell of this species is 3 to 5 mm in size, and is roughly triangula ...

'' A. Schmidt, 1851 *'' Pisidium waldeni'' Kuiper, 1975 Subgenus ''Hiberneuglesa'' Starobogatov, 1983 *''

Pisidium hibernicum ''Pisidium hibernicum'' is a species of freshwater bivalve from the family Sphaeriidae. Description The 2.5-3.5mm. shell is very inflated (convex) hence the common name Globular Pea Shell. The central umbos is prominent. The periostracum (surfa ...

'' Westerlund, 1894 Subgenus ''Cingulipisidium'' Pirogov & Starobogatov, 1974 *'' Pisidium crassum'' Stelfox, 1918 *'' Pisidium milium'' Held, 1836 *'' Pisidium nitidum'' Jenyns, 1832 *''

Pisidium pseudosphaerium ''Pisidium pseudosphaerium'' is a species of freshwater bivalve from family Sphaeriidae. Description The 2.5-3.2mm shell is a flattened, regular, slightly angulated oval shape with low central umbos. The surface is silky with very fine concentri ...

'' Favre, 1927 Subgenus ''Pseudeupera'' Germain, 1909 *'' Pisidium pulchellum'' Jenyns, 1832 *'' Pisidium subtruncatum'' Malm, 1855 Subgenus ''Neopisidium'' Odhner, 1921 *'' Pisidium conventus'' Clessin, 1867 Subgenus ''Odhneripisidium'' Kuiper, 1962 *''

'' Paladilhe, 1866 *'' Pisidium tenuilineatum'' Stelfox, 1918 Subgenus ''Afropisidium'' Kuiper, 1962 *'' Pisidium giraudi'' Bourguignat, 1885 *'' Pisidium hodgkini'' ( Suter, 1905) *'' Pisidium pirothi'' Jickeli, 1881 Subgenus unknown *'' Pisidium artifex'' Kuiper, 1960 *'' Pisidium stewarti'' Preston, 1909 Subgenus ' *'' Pisidium annandalei'' Prashad, 1925 *'' Pisidium edlaueri'' Kuiper, 1960 *'' Pisidium javanum'' van Benthem Jutting, 1931 *'' Pisidium maasseni'' Kuiper, 1987 *'' Pisidium punctiferum'' (Guppy, 1867) *'' Pisidium raddei'' Dybowski, 1902 * 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
.

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 The Consortium for the Barcode of Life (CBOL) was an international initiative dedicated to supporting the development of DNA barcoding as a global standard for species identification. CBOL's Secretariat Office is hosted by the National Museum of ...

* https://web.archive.org/web/20141128091542/http://sunsite.ualberta.ca/Projects/Aquatic_Invertebrates/?Page=20 {{Taxonbar, from=Q600521 Bivalve genera