维基百科 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. Drawing of the right valve external view of Pisidium moitessierianum Drawing of the right valve internal view ofPisidium moitessierianum Drawing of the right valve lateral view of Pisidium moitessierianum Contents [hide] 1 Pisidium and taphonomy 2 Extant subgenera and species 3 Footnotes 4 References 5 External links Pisidium and taphonomy[edit] In large enough quantities, the minute shells of these bivalves can have an impact on 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]