Abstract:
Cainozoic deep-sea ostracod assemblages from the summits of Mid-Pacific guyots point to high levels of endemism possibly as a result of their bathymetric separation from the surrounding sea floor. However, the interpretation of these fossil assemblages is hampered by the paucity of comparative material from surrounding non-guyot sites. Fifteen ostracod assemblages from DSDP site 463 (Late Cretaceous-Pleistocene) were studied to compare with those from nearby guyots. Three distinct faunal assemblages are recognised at site 463: Assemblage A (Maastrictian-Eocene), Assemblage B (Oligocene-Upper Miocene) and Assemblage C (Upper Miocene-Pleistocene) although the palaeoenvironmental significance of these units is unclear. Sixty-two ostracod species are identified, the thirteen most abundant are discussed in the taxonomic section, five of which are described as new. Between 30 and 100% of the species encountered in each sample are considered as endemic to site 463, while some of the remaining species were previously thought to be endemic to individual guyots. Similarly high levels of endemism on nearby guyots reflect an incomplete knowledge of deep-sea ostracod faunas rather than the establishment of geographically or bathymetrically restricted populations. The presence of globally pandemic and geographically widespread taxa on sites such as the mid-Pacific mountains, surrounded by abyssal depths which lie below the CCD, indicates that some faunal exchange or migration of ostracods does take place. This must be achieved within the intermediate waters and probably occurs passively.