The ostracod body is contained within a horny or calcerous shell, to which it is attached by muscles. The muscles close the hinged valves and operate several pairs of appendages that are used for sensors, locomotion, eating and respiration. Like all arthropods, ostracods molt the shell for growth; some do so eight times before they reach their final size and shape.
Ostracods appeared in the seas more than 500 million years ago and have been abundant ever since. They are common in Indiana rocks. Some have smooth shells, but others are marked by ridges, knobs, pits and furrows, all of which are used for identification by paleontolgists. Because many species spread widely but did not live very long, they make excellent index fossils. They are also useful for understanding ancient environments.
Our Hoosier State Beneath Us: Paleontology