Two extant species of spinose planktonic foraminifera, Globigerinoides trilobus and Orbulina universa, shared a common ancestor in the early Miocene, as studied here in well-preserved fossil material from Limalok Guyot in the tropical Pacific (Ocean Drilling Program Site 871). The first appearance of O. universa (15.1 Ma) was preceded by an increase in the morphological variance of the ancestral lineage, including the origin of several new and short-ranging Praeorbulina morphospecies. Biological speciation (cladogenesis) probably occurred before 15.1 Ma. New oxygen (δ18O) and carbon (δ 13C) stable isotopic results are compared with analyses of two reference species, Globigerinoides ruber (shallow water) and Globoquadrina venezuelana (deep water). Oxygen isotopic ratios of G. trilobus, Praeorbulina spp., and O. universa indicate that the entire evolutionary transition took place within mixed layer habitats similar to those occupied by modern G. trilobus. A slight separation in the δ 18O of O. universa and G. trilobus later in the mid-Miocene may indicate subsequent habitat partitioning. Carbon isotopes suggest that no significant change in the carbon metabolism or degree of photosymbiosis occurred. The origin of Orbulina therefore appears to have been a case of sympatric speciation at shallow depths in the open ocean. The causes of the speciation and morphological transition are unknown.