EVOLUTIONARY CHANGES IN SUPPLEMENTARY APERTURAL CHARACTERISTICS OF THE LATE NEOGENE SPHAEROIDINELLA-DEHISCENS LINEAGE (PLANKTONIC-FORAMINIFERA)

With its thick, glassy cortex and wide open, coalescing primary and su pplementary apertures Sphaeroidinella dehiscens is a conspicuous compo nent of low-latitude planktonic foraminifer faunas. Sphaeroidinella ev olved fi om Sphaeroidinellopsis through the development of a supplemen tary aperture on the spiral side of the test. The Sphaeroidinellopsis- Sphaeroidinella lineage is important in tropical late Neogene stratigr aphy. Changes in. the proportion of specimens equipped with a suppleme ntary aperture and the size of this aperture have been analyzed in ter minal Miocene through basal Pleistocene sections from DSDP sites 502A fi om the Caribbean Sea, 503 from the eastern equatorial Pacific Ocean and 214 from the equatorial Indian Ocean. Contrary to previous sugges tions, the development of a supplementary aperture (the Sphaeroidinell a morphotype), marking the base of the basal Pliocene Zone N19 15.1 Ma ) in the standard tropical planktonic foraminifer zonation, took place already in the terminal Miocene (before 5.5 Ma). Following the evolut ionary appearance of a supplementary aperture, the proportion in the p opulations of such an aperture remained low (10-30%) in. the earlier p arts of the Pliocene. The proportion increased abruptly within a short interval (<0.1 m.y.) in the middle parts of the Pliocene (about 3.4-3 .2 Ma), and after 3.0 Ma more than 85% of the specimens had a suppleme ntary aperture Specimens without a supplementary aperture (the Sphaero idinellopsis morphotype) persisted all through the Pliocene. However, after 3.0 Ma they represented the smallest specimens in particular pop ulations. During the terminal Miocene and earlier Pliocene the supplem entary aperture was constantly very small (<2.62 . 10(3) mu m(2)). In the middle Pliocene (3.4-3.3 Ma) it rapidly increased in size within a short interval of time (<0.09 m.y.) to post-transitional values of 4. 25-22.20 . 10(3) mu m(2) (with one exception). The portion of the spir al test area covered by a supplementary aperture increased from <1% in the earlier Pliocene to 2.80-7.46% in. the later Pliocene, marking a 3-8-fold increase in the relative size of the supplementary aperture f rom the earlier to later Pliocene. Hence, the differences in size of t he supplementary aperture were not related to changes in the mean test size. Consequently the transitional event in the middle Pliocene (3.3 -3.4 Ma) reflects the onset of a true evolutionary innovation within t he Sphaeroidinella Lineage, most probably representing an example of p unctuated anagenesis.