Pea Clams, Sphaeriidae, are tiny freshwater bivalves, typically only a few millimeters across. They are hermaphroditic, with eggs that are fertilized and grow within the mothers shell; newborn clams resemble adults, and are ready to breed almost immediately. This enables Pea Clams to rapidly colonize bodies of water with significant ecological - and palaeontological - consequences. Because of their small size, short life cycle and rapid reproduction, they rapidly build up large numbers of small, easily dissolved, calcium carbonate shells within bodies of water that they colonize. The addition of calcium carbonate will lower the pH of the water, enabling the colonization of waters by organisms unable to tolerate acid conditions, and slowing the dissolution of other mineralized tissues that fall into the water (e.g. other shells, bones, teeth), while at the same time aiding the formation of limestones in sediments deposited in the waterway, which greatly increases the chances of any organisms in the ecosystem of entering the fossil record.
In a paper published in the journal Fossil Record on 20 February 2013, Thomas Neubauer, Oleg Mandic and Mathias Harzhauser of the Department of Geology & Paleontology at the Naturhistorisches Museum Wien describe a new species of Pea Clam, discovered during a review of fossil Molluscs from Lake Gacko, a Middle Miocene lake that existed from about 15.8 to about 15.2 million years ago.
The new Clam is placed in the genus Pisidium, and given the specific name vukovici, after Boško Vuković, a geologist at the Rudnik i Termoelektrana Gacko, who enabled the study to be carried out at the site. Pisidium vukovici is a ~3 mm triangular clam, known only from a single horizon at the Gračanica site. It is thought to have lived approximately 15.83 million years ago.
Pisidium vukovici. Scale bar is 1 mm. Neubauer et al. (2013).
See also The evolution of Galeommatoid Bivalves, Symbiosis and the success of Galeommatoid Bivalves and A new species of Scallop from Western Australia.
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