The evolutionary origin and development of the Neogene planktonic foraminiferal Globorotalia (Truncorotalia) subgenus: The mode and tempo of speciation and the origin of coiling direction reversals and dominance.

摘要

The Globorotalia Truncorotalia lineage of the Neogene planktonic foraminifera was investigated to determine the relative timing of the acquisition of morphologic, isotopic, and coiling direction bimodality at three geographic locations in an attempt to determine the mode and tempo of speciation and the origin of coiling direction reversals and dominance. Sites investigated include: DSDP 72/516---western, temperate South Atlantic; DSDP 90/591---southwestern, temperate Pacific Ocean; and ODP 130/806B---Ontong-Java Plateau, equatorial western Pacific. Statistical analysis of measured morphometric variables was used to determine when in the lineage morphologic bimodality appears within the species. The stable light isotopes of oxygen and carbon were investigated to determine the isotopic difference between juvenile and adult chambers, and to determine when in the lineage isotopic bimodality appears between the coiling types of the species. Test micro-amputation was done to remove adult final chambers and isolate juvenile from adult chamber calcite. Coiling direction changes were tracked and compared with morphologic and isotopic changes to investigate the origin of coiling direction reversals and dominance.; Results indicate that the Truncorotalia have had morphologically distinct coiling types since the inception of the lineage from the Hirsutella ancestor. Isotopic analysis reveals that the group has always been deep dwelling, with left coiling groups recording colder isotopic temperatures. Juveniles typically show warmer isotopic temperatures than adult specimens do. At a species FAD, coiling is either 100% sinistral or dextral, rarely mixed suggesting that the coiling types have been present since the beginning of the lineage. From the analysis it appears that left coiling ancestors give rise to left coiling descendents and vice versa if right coiling dominates at a sight. Coiling direction and morphologic similarity are maintained through speciation events. The coiling types of Gr. truncatulinoides have been shown to be genetically different (de Vargas et al., 2001) and may even be distinct species. Whether or not the rest of the species coiling types in the lineage are distinct remains unclear, although they appear to be 'cryptic species'. The mode of speciation is depth parapatric and the tempo is speciation is gradual in the Truncorotalia.