Biological response of ammonites to changing environmental conditions: an example of Boreal Amoeboceras invasions into Submediterranean Province during Late Oxfordian

Abstract

Amoeboceras faunas constituting the typical element of the Boreal Province occur abundantly in two Amoeboceras layers in the Submediterranean Province ammonite succession of the Upper Oxfordian of Central Poland. When compared with those from the home-area of ammonites of the genus Amoeboceras, these invasive forms in the Submediterranean Province differ in their smaller shell-sizes, in the lack of differentiation of the shells into separate size-related morphs, and in the crowding of septa for up to one and a half whorls of the last part of the phragmocone. The densely-spaced septa appear at the beginning of the fifth whorl, after the “normally” septate inner whorls. This fits well with the beginning of the sexual cycle postulated for the phyletically related genus Quenstedtoceras. Such a long section of phragmocone with densely-spaced septa, indicating a prolonged period of low rate of shell growth, may be related to the attainment of a long period of sexual maturity due to environmental conditions untypical for Boreal forms.