First appearance of Globorotalia truncatulinoides: cladogenesis and immigration
The first appearance datum of the planktic foraminifer Globorotalia truncatulinoides is widely used to identify the base of the Quaternary Period. However, its appearance is not globally isochronous. We have previously shown that G. truncatulinoides evolved gradually from its ancestor Globorotalia crassaformis via the intermediate species Globorotalia tosaensis. This cladogenesis was documented in the southwest Pacific during the Late Pliocene in sympatric or parapatric populations. Based on qualitative observations, similar but younger, gradual transitions have been reported from other areas of the world's oceans. Therefore, this gradual evolutionary branching might have occurred in response to changing environments at different times in different ocean areas. To evaluate the hypothesis of a repeated, environmentally driven gradual, sympatric cladogenesis, we studied the morphological transitions of the three taxa, using image analytical techniques, in several deep-sea sections from various areas. Our study confirms that G. truncatulinoides evolved between 2.8 and 2.3 Ma sympatrically in large populations from its ancestor G. crassaformis in the southwest Pacific. Differentiated morphotypes of G. truncatulinoides subsequently immigrated into the Indian and Atlantic oceans between 2.3 and 1.9 Ma. Our morphometric data show these younger appearances outside the southwest Pacific to be punctuated. We hypothesize that the global cooling of surface waters, coinciding with the northern hemisphere glaciation, led to the formation of oceanographic barriers that could have retarded the expansion of G. truncatulinoides up to 2.3 Ma. At this time, a relative warming and subsequent transgression could have spurred the migration, possibly through the Indonesian passage. A direct link between the speciation and surface water changes linked to the northern hemisphere glaciation has not been proven so far and seems unlikely. In fact, stable isotope data in this lineage indicate that the three species' depth habitat preferences remained unchanged through the speciation and migration of G. truncatulinoides and that all three species were dominantly deep-dwellers, in agreement with their present environmental preferences.