Abstract
The biotic turnover in the Pliensbachian-Toarcian transition and changes in assemblages of bivalves, ostracodes, foraminifers, dinocysts, spores, and pollen are described. Only five of 24 bivalve genera and two of four ostracode genera cross the Pliensbachian-Toarcian boundary so that composition of genera and families to be entirely renewed at the base of the Harpoceras falciferum Zone. In the interval of three ammonite zones, diversity of foraminifers is reducing from 27 genera in the Amaltheus margaritatus Zone (upper Pliensbachian) to 17 and then to 15 genera in the Tiltoniceras antiquum (lower Toarcian) and Harpoceras falciferum zones, respectively. Single dinocysts of the Pliensbachian are replaced by their abundant specimens at the base of the Toarcian, and substantial changes in composition of palynological assemblages are simultaneously established. Factors responsible for “mass extinctions” of marine invertebrates are suggested to be the paleogeographic reorganization, anoxic events, eustatic sea-level changes, and climatic fluctuations. The biotic turnover in the Arctic region is interrelated mainly with thermal changes, which caused the southward displacements of taxa distribution areas during a rapid cooling and their gradual return to former habitat areas in the period of warming, rather than with extinction events.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
