Marking the Kimmeridgian-Tithonian transition with a bivalve – Protocardia gigantea sp. nov. (Bivalvia: Cardiidae) and its relatives from the Lusitanian Basin (Portugal)

The cardiid Protocardia gigantea sp. nov., which is newly described herein, represents the largest member of this globally distributed genus of shallow-water Mesozoic bivalves. It is found throughout the central Lusitanian Basin (Central Portugal) and may be utilised as a stratigraphic marker for the Kimmeridgian-Tithonian transition in this area, which is confirmed by Sr-isotope data. Its frequent occurrence, large size, and characteristic features make Protocardia gigantea a valuable tool for field stratigraphy. Its taxonomy, systematic position, and ontogenetic as well as intra-population morphological variability are outlined. Possible ancestry in and outside the Lusitanian Basin is discussed, and four other species of Protocardia from the Upper Jurassic of Portugal are taxonomically treated.