The presence of living specimens of Spongaster pentas and related spongadiscid forms, Buccinosphaera invaginata, and certain other radiolarians in plankton samples from the Gulf of Mexico is evidence of a unique radiolarian population that is composed in part of relict and/or expatriate forms. These populations may have survived in the Gulf because: (1) the closure of the Tethys seaway by the uplift of the Panamanian block isolated the equatorial and temperate Atlantic waters and blocked radiolarian faunas from entering the Pacific

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and Indian Oceans; (2) temperate and perhaps equatorial radiolarian faunas of the Pacific and Indian Oceans have contributed to the temperate and ?equatorial Atlantic radiolarian faunas since the closure of Panama; (3) the ability of relict and related forms to carry on symbiotic relations with algal associates may have enabled these forms to adapt and survive. This information adds insight into our understanding of the evolution of Cenozoic radiolarian faunas, and perhaps also Mesozoic and Paleozoic faunas.

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