Variable noding and palaeoecology of a Middle Jurassic limnocytherid ostracod: implications for modern brackish water taxa

No less than seven junior synonyms exist of Theriosynoecum kirtlingtonense Bate, 1965. This taxonomic confusion has been brought about by variations in the degree and patterns of noding within a single species, which was widespread both geographically — from the south of England to the Hebrides — and in the palaeoenvironments it occupied — from freshwater to polyhaline. A critical analysis is undertaken of the taxonomy of Theriosynoecum kirtlingtonense within a framework of palaeoenvironmental constraint, using only those bio-characters which are invariable at the intraspecific level. The distribution of the species in a section of the upper Bathonian Forest Marble Formation at Tarlton in Gloucestershire, is investigated. The population age structure was used to determine those levels in which the species occurs as an autochthonous component, and its variable noding investigated with respect to salinity variations. All aspects of noding in this limnocytherid and in the Neogene to Recent brackish water species Cyprideis torosa (Jones, 1850) are compared and contrasted and both are shown to be related fundamentally to salinity.