A comparative study of collections from the S.W. Pacific (Saipan to Tonga), with the descriptions of Gambiella caudata (Brady, 1890) and a new species of Pterobairdia (Ostracoda)

Abstract. Gambiella caudata (Brady, 1890) and Pterobairdia briggsae sp. nov. are described from collections made in the S. W. Pacific (Saipan, Onotoa, Ontong-Java/Kula Gulf, Noumea, Cook Islands, Fiji, Samoa, Tonga); and the lectotypes of several species described in a major early paper by Brady (1890) are illustrated. The carbonate compensation depth in this region lies at around 4500 m. Comparison of the Ontong-Java in Kula Gulf samples reinforces consideration of depth as a factor of ecological importance. A similarity matrix for the several faunas shows factors in common at species level ranging from 22% (Onotoa/Noumea) to nearly 60% (Samoa/Onotoa); while endemism ranges from 8.5% (Samoa) to nearly 33% (Tonga). Most endemic species belong in a limited number of podocopid families, in particular Bairdiidae, Trachyleberididae, Paradoxostomatidae and Leptocytheridae. These results appear consistent with an hypothesis that continued tectonics-driven changes in the regional marine topography and sedimentation, i.e. niche development, could have triggered speciation along the regional plate margins.