In a large set of data, Eocene outcrops around the Sundaland craton are characterized by the presence of the related genera Assilina d'Orbigny 1939, Pellatispira Boussac 1906 and Biplanispira Umbgrove 1937 [the "APB fauna"]. Another large set of data shows that Eocene limestones on the Australian margin, in south and west Papua New Guinea, west Papua and surrounding islands are characterized by the genus Lacazinella Crespin 1962. The record of Crespin (1938) proves that the APB and Lacazinella faunas are not totally exclusive of each other. However, the occurrences of the APB fauna on the Australian plate margin are very rare, as Crespin's record is the only one with illustrations. In contrast the Lacazinella fauna has not been observed thus far in any plate or micro-plate considered to have been attached to the Sundaland craton or at low latitudes in Eocene times. Therefore, the APB fauna has a distinct preference for low latitude (western Tethys, Sundaland, and Pacific island) locations in the Eocene, but occurs rarely on parts of the Australian margin. More work in the Papua New Guinea (Omati to Chimbu area) and eastern Sulawesi areas should help quantify how often, or how rare, the APB fauna are found to occur in Australian derived locations.

This model was tested by a visit to Christmas Island which lies between the two realms and was not fully documented within such a paleobiogeographic context. In the process, a new Eocene biogeographic cue was identified, as represented by the higher latitude European form Grzybowskia. This taxon was at an estimated palaeolatitude comparable to the northern hemisphere records. This specific distribution supports the notion of latitudinal variation in larger foraminiferal assemblages in warm Eocene seas.