Three stratigraphically successive species of Bradleya Hornibrook, 1952 from abyssal sediments of the central equatorial Pacific (Leg 85, Deep Sea Drilling Project) form an unbranching evolutionary sequence designated as the B. johnsoni Benson lineage. These taxa are B. sp. 1 (Lower Oligocene), B. johnsoni (Upper Oligocene – Middle Miocene) and B. thomasi n. sp. (Upper Miocene – Recent). A shared synapomorphic character, the posterior bridge complex, establishes the kinship of these species. Their stratigraphic ranges and a traceable series of changes in the organisation and mass of the reticulum are consistent with the hypothesis of direct ancestor/descendant relationships between them. Bradleya johnsoni is distinguished from B. sp. 1 by the appearance of new primary muri in the posteroventral region and by a general increase in reticular mass. In B. thomasi, the overall reticular pattern is maintained but it becomes more delicate and less rectilinear. Each species of this lineage shows similar variation in the posteroventral reticulum.
- Revision received March 1, 1988.
- © 1988 The Micropalaeontological Society