(Nigrini, 1971, who termed it a 'Concurrent Range Zone')
The top of the zone is defined by the morphotypic first appearance of Buccinosphaera invaginata and is coincident with the lower limit of the Buccinosphaera invaginata Zone.
Events included in the zone are:
- Tm Stylatractus universus (= Axoprunum angelinum)
The base of the zone is defined by the morphotypic first appearance of Collosphaera tuberosa and is coincident with the upper limit of the Amphirhopalum ypsilon Zone.
* In the tropical Indian Ocean, Johnson et al. (1989) used the upper limit of Stylatractus universus to divide this zone into two zones, Collosphaera tuberosa (NR2) and Stylatractus universus (NR3).
Reference slides R 2.1, R 2.2 and R 2.3 from Sample 85-573-2-3, 130-131 cm.