The genus Tribrachiatus (all species of which are widely used stratigraphic markers) and its related genus Rhomboaster are studied in type samples and in five DSDP/ODP sites that have magnetostratigraphies and range from low through high latitudes. Rhomboaster calcitrapa, R. spineus, and R. bitrifida are shown to be junior synonyms of R. cuspis. Rhomboaster transforms into T. bramlettei by flattening of the nannolith. This makes the location of the first occurrence (FO) of T. bramlettei difficult and imprecise. The FO of Rhomboaster, however, is less ambiguous and more reliable for global stratigraphic correlation. Tribrachiatus contortus has a very short (< 1 m.y.) and consistent stratigraphic range. The FO of T. orthostylus is located very near the top of Chron C24r and slightly below the last occurrence of T. contortus. This study also presents detailed morphometric data on the changes from T. bramlettei to T. contortus to T. orthostylus, and calibrates their changes through time with magnetostratigraphy. This shows a detailed picture of the progressive evolution in this group as well as offers an age curve for high-resolution stratigraphic correlation.

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