Much of the world's ocean bottoms have been undisturbed for tens of millions of years, so complete sets of intermediate fossils have been seen at many locations around the world. This means that the speciation event occurred worldwide, rather than in a specific geographic area. There are a number of ideas as to how the two species may have become isolated in non-geographical ways.
Click to see the geological evidence
Click to see the fossils
The two species may have become separated in a number of ways. The changes to the shell might have had some as-yet-unknown advantage. Orbulina might have switched diet, or switched symbiotic allegiance, although the isotope evidence argues against that. It may have moved its reproductive cycle to a different time of day, or to a different part of the lunar month: and the two modern species are indeed different in that way. The two modern species also live at different average depths, so that may have been the difference that came first.
This web page is based on:
Stable isotopic evidence for the sympatric divergence of Globigerinoides trilobus and Orbulina universa (planktonic foraminifera), P.N. Pearson, N.J. Shackleton and M.A. Hall, Journal of the Geological Society, London, Vol. 154, 1997 pp. 295-302but I have simplified quite a lot. The paper is just the most recent of the many trilobus-to-orbulina papers that have been published in the last forty-odd years. The article concludes:
Planktonic foraminifer evolution has often been gradual and anagenetic, rather than saltational and cladogenetic. Thus, profound morphological evolution within unbroken lineages is commonly observed.In short, there are many sequences of foram fossils which show slow change across time.