Micropalaeontological investigation of one of a number of black shale horizons within the carbonate platform sediments of the Calcari Grigi Formation (Trento Platform, NE Italy) has yielded the first recorded oligohaline, early Jurassic, ostracod assemblage. The shale is dated as late Sinemurian on the basis of large benthic foraminiferal biostratigraphy in the sediments above and below. The shale is devoid of foraminifera and ammonites, which supports a non-marine context. Three previously unknown ostracod species are identified, however, the assemblage is dominated (>95%) by a single taxon which is erected as a new genus and species (Phraterfabanella tridentinensis Whatley and Boomer gen. et sp. nov.) which further supports the interpretation of a "stressed" environment. The two remaining ostracod taxa are assigned to Klieana and Limnocythere both considered to represent non-marine or very low salinity conditions. This new genus includes probable members from the Rhaetian of Hungary and France as well as from the Liassic of France. The new genus is shown to be one of the earliest representatives of the Cytherideidae, a family which survives to the present and includes the modern pandemic, euryhaline species Cyprideis torosa. The discovery of such an assemblage within an otherwise marine sequence indicates temporary, physical isolation from marine influence. Sedimentological, faunal and geochemical evidence suggests that the organic rich shale may indicate high organic carbon supply via continental runoff in this tropical palaeogeographical setting. Resulting dysaerobia within the sediment would have created a particularly stressed environment.
- Early Jurassic
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