DYNAMICS OF DIVERSITY AND EVOLUTIONARY TRENDS OF JURASSIC FORAMINIFERS

Abstract

Composition of the Jurassic (Hettangian-Tithonian) foraminiferal communities in three paleozoochores: Tethyan, Boreal-Atlantic, and Arctic, is considered. Foraminiferal taxons of all ranks (from species to orders) are analyzed with an emphasis given to the generic level, which objectively reflects the macroevolutionary process in the geological record. The Triassic-Jurassic boundary is marked by the mass extinction of foraminifers (about 55 genera) that was particularly pronounced among miliolids (22), involutinids (11), rotaliids (7), and lagenids (4).1 Five major stages of foraminiferal evolution in the Jurassic are outlined: (I) Rhaetian-Lias (including the Hettangian Planorbis Zone) stage marked by the crisis in the foraminiferal development; (II) initial Early Jurassic stage characterized by the low diversity in the Hettangian-initial Sinemurian and by a sharp diversity increase at the end of the Sinemurian, in the Pliensbachian (particularly, in the late substage), and Toarcian; (III) Middle Jurassic (pre-Callovian) stage displaying the crisis trend at the Toarcian-Aalenian boundary and the taxonomic diversity increase in the Bajocian-Bathonian assemblages, particularly in the Tethys; (IV) Callovian-Oxfordian bloom universally accompanied by an intense radiation; and (V) Kimmeridgian-Tithonian stabilization period. The most important event in the evolution of Jurassic foraminifers was the appearance of first planktonic genera in the Bajocian. The foraminiferal diversity dynamics was fluctuating against the general background of about two-fold 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).