Dynamics of diversity and evolutionary trends of Jurassic foraminifers

Composition of the Jurassic (Hettangjan-Tithonian) foraminiferal communitie s in three paleozoo-chores: Tethyan, Boreal-Atlantic, and Arctic, is consid ered. Foraminiferal taxons of all ranks (from species to orders) are analyz ed with an emphasis given to the generic lever, which objectively reflects the macroevolutionary process in the geological record. The Triassic-Jurass ic boundary is marked by the mass extinction of foraminifers (about 55 gene ra) that was particularly pronounced among miliolids (22), involutinids (11 ), rotaliids (7), and lagenids (4).(1) Five major stages of foraminiferal e volution in the Jurassic are outlined: (I) Rhaetian-Lias (including the Het tangian Planorbis Zone) stage marked by the crisis in the foraminiferal dev elopment; (II) initial Early Jurassic stage characterized by the low divers ity in the Hettangian-initial Sinemurian and by a sharp diversity increase at the end of the Sinemurian, in the Pliensbachian (particularly, in the la te substage), and Toarcian; (III) Middle Jurassic (pre-Callovian) stage dis playing the crisis trend at the Toarcian-Aalenian boundary and the taxonomi c diversity increase in the Bajocian-Bathonian assemblages, particularly in the Tethys; (TV) Callovian-Oxfordian bloom universally accompanied by an i ntense radiation; and (V) Kimmeridgian-Tithonian stabilization period. The most important event in the evolution of Jurassic foraminifers was the appe arance of first planktonic genera in the Bajocian. The foraminiferal divers ity dynamics was fluctuating against the general background of about two-fo ld increase in the taxa abundance during the period from the Hettangian to Tithonian (60 and 127 new genera, respectively, versus 243 known genera of Jurassic foraminifers).