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The Archaeomonadaceae are a family of fossil marine chrysomonadins, presumably planktonic in life habitat, characterized by siliceous cysts of spherical or ovoid form with a minute round opening or pore on their top side (Stradner, 1971). Archaeomonads are known as fossils from the Late Cretaceous to Miocene, only recently having been reported from the Oligocene by Perch-Nielsen (1975). Tynan (1971), in a report on the known geologic occurrences of archaeomonads, noted that "there are no reported occurrences of the archaeomonads in the Oligocene." He attributed this "absence" of Oligocene archaeomonads to a lack of study of sediments of that age. This appears to have been the case as PerchNielsen (1975) has recently reported the first occurrences of Oligocene archaeomonads from two DSDP Leg 29 sites: Site 278 (56°33.42'S, 160°04.29'E) in the Emerald Basin and Site 280 (48°57.44'S, 147°14.08'E) south of the South Tasman Rise. Three new archaeomonad species of the genus Archaeosphaeridium were described by Perch-Nielsen (1975) from the late Eocene and Oligocene at these sites. The three species, Archaeosphaeridium australensis, A. tasmaniae, and A. dumitricae, consist, basically, of smooth spheres to which spines of varying lengths and orientations are attached. One of the species, A. tasmaniae, is the first reported archeomonad with very long spines. During the preparation of the diatom report for this volume, the occurrence of the three above-mentioned archaeomonad species was noted in late Eocene and Oligocene samples from Site 328 (49°48.67'S, 36°39.53'W) in the Malvinas Outer Basin in the South Atlantic Ocean (Figure 1). These, quite distinctive species, appear to have potential as guide fossils to Eocene and Oligocene age sediments; therefore, their distribution at Site 328 is presented here.

Archaeomonads as Eocene and Oligocene Guide Fossils in Marine Sediments