Cheilostome bryozoan diversity from the southwest Atlantic region: Is Antarctica really isolated?

During the Cenozoic, the break-up of Gondwana was accompanied by a gradual separation of its components and the subsequent establishment of the Antarctic Circumpolar Current, leading to a relative thermal and biogeographic isolation of the Antarctic fauna. However, the zoogeographical affinities of several taxa from South America and Antarctica have been subject to debate, bringing into question the extent of Antarctic isolation. Here we present new data on bryozoan species and their spatial distribution in the Argentine Patagonian (AP) region, as well as an analysis of the bryozoological similarities between deep ranges from Argentina and neighboring regions. A total of 108 species of cheilostome bryozoans (378 samples), belonging to 59 genera was found. Five new genera and 36 new species were found in the AP region, while 71 species were reported for the first time from Argentina. The bathymetric ranges of 94 species (87%) were expanded and a high proportion of the identified species (44.4%) also had an Antarctic distribution. The bryozoological affinities found in the current study between the nearest geographical neighbors are in agreement with the hypothesis of the sequential separation of Gondwana during the Cenozoic. Moreover, a high number of shared species, mainly from the slope, were found in this study between the AP region and Antarctica, thus supporting the idea that the Southern Ocean may have been less isolated over geological time than once thought.