The eridostracan ostracode genus Conchoprimitia Öpik, 1935 appears among the earliest ostracodes in the carbonate succession of the Ordovician Baltoscandian Palaeobasin. Over the past one hundred years, more than twenty species of Conchoprimitia have been recognized from early Middle Ordovician sediments. The microsculpture has been regarded as one of the most important features for distinguishing between different species of Conchoprimitia. However, four main types of microsculpture (smooth, punctate, reticulate and rugose/striate) are often transitional, occurring on different areas of the carapace and are not therefore discrete taxonomically important characters. As no clear distinction can be made between different carapace morphologies, the early Middle Ordovician Conchoprimitia species of Baltoscandia should be regarded as morphotypes of one polymorphic species -Conchoprimitia socialis (Brøgger, 1882). C. socialis was an opportunistic species, which developed different microsculpture on the carapace surface in different lithofacies in the Baltoscandian Palaeobasin.•