Seed shrimp rule the arthropod fossil shelves
Ostracods, nicknamed seed shrimp, are crustaceans with roughly 33,000 described species—only about 13,000 still living. Greek ostrakon means shell or tile. They are by far the most common arthropods fossilized, spanning early Ordovician beds to modern lakes and seas.
Most measure about a millimetre, but sizes run from 0.2 mm to 32 mm in the marine giant Gigantocypris; the biggest freshwater kind, Megalocypris princeps, reaches 8 mm. A hinged pair of valves made of chitin, usually stiffened with calcium carbonate, encloses a body flattened side to side. They are classed in seven orders by shell and soft-part anatomy, though molecular studies have not firmly confirmed they form one natural group. Most eat sediment, while others hunt, graze, scavenge or filter.
The head is the largest part and carries two pairs of swimming antennae plus mandibles and maxillae; the trunk has largely lost its segments. Podocopa, the largest subclass, have no gills, heart or blood vessels and exchange gases across the whole body surface, whereas Myodocopa have a heart and some bear gills. Touch is probably the main sense; only one myodocopan group has compound eyes, and many others have just a small three-part larval eye.
Freshwater species have turned up in Eocene Baltic amber, presumably splashed onto trees by floods. At Riversleigh in Queensland, a cave find announced in 2014 preserved soft tissue so well that it set the Guinness record for the oldest penis, around 100 million years old; phosphorus from bat droppings fossilised the animals within days, and their sperm proved the largest relative to body size of any animal known.
Oxygen isotope and magnesium-to-calcium ratios in their calcite shells reveal past water temperatures, ice volume and hydrology, and a new temperature-range method borrows an approach used on beetles. Most marine species live on or in the seabed, one genus down to 9,307 m. Freshwater swimmers belong only to the Cypridoidea, and a few, such as Mesocypris, live in damp forest soils of South Africa, Australia and New Zealand.
Source: Ostracod
