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Abstract: Micropaleontological analyses based on calcareous nannofossils were conducted on samples taken from the Agencia Nacional de Hidrocarburos (ANH)-Piedras Blancas-1 Well, which was drilled in the Sinú–San Jacinto Basin, Caribbean Region of Colombia. A summary of published literature on fossil coccolithophores from the marine Cenozoic record of South America reveals that our study is the first to report Lower Eocene coccolith-bearing sedimentary rocks in the substratum of the Caribbean inland margin. The assemblages are characterized by common occurrences of Reticulofenestra dictyoda and Sphenolithus radians and sporadic occurrences of Girgisia gammation, Heliodiscoaster lodoensis , and Sphenolithus orphanknolli. The co-occurrence of H. lodoensis, R. dictyoda , and S. orphanknolli indicates that the core between 1190 ft (362 m) and 279 ft (85 m) encompasses Zone NP12 to Subzone NP14a and spans the interval between ~53 to ~48.3 Ma of the latest Early Eocene (Late Ypresian Age). Small placoliths ( R. dictyoda and Reticulofenestra <3 μm) and sphenoliths ( S. radians and Sphenolithus spp.) dominate the tropical assemblages. Other species include Campylosphaera dela, Coccolithus formosus, Coccolithus pelagicus, Helicosphaeroides lophota, Helicosphaeroides seminulum , and Pontosphaera spp. Investigations on calcareous nannofossil distribution in Cenozoic marine sediments from South America are few, and most published information concerns the Brazilian continental margin and inland Argentina. The Man Aike Formation from the Austral Basin of Argentina is the only Lower Eocene formation available for comparison.

Patterns of diversification and longevity in Paleogene coccolithophorids are analyzed by combining the temporal history of selected genera, families, and orders with the number of discrete morphospecies in them. The coccolithophorids underwent an abrupt mass extinction at the Cretaceous/Paleogene boundary, and a rapid (~1 m.y.-long) global turnover at the Paleocene/Eocene boundary. In contrast, they underwent a diachronous turnover at the Eocene/Oligocene boundary that spread over 6–7 m.y. at mid- and low-latitudes. The turnover included sequential extinctions and speciations of short-lived taxa, beginning slightly before 37 Ma, and losses of taxa that dominated mid- and low latitudes at 34.2 Ma and high latitudes at 32.3 Ma. It is also marked by a few evolutionary appearances, in particular, that of the Family Syracosphaeraceae, which is the most diversified of the living coccolithophorids. Most importantly, the turnover resulted in a shift in the balance between families across several orders, such that families that dominated during the Eocene dwindled during the turnover, and, conversely, families that were little diversified during the Eocene became dominant. Thus, members of Family Coccolithaceae and the genus Heliodiscoaster typify Eocene communities; members of the Family Calcidiscaceae and the genus Eudiscoaster characterize Neogene communities. This shift was accompanied by a decrease in the robustness of coccoliths, suggesting that the Eocene/Oligocene event had a marked effect on the physiology of Eocene coccolithophorids. Bolide impacts and the emplacement of large basaltic provinces provide mechanisms to explain large biotic events. Such mechanisms, however, can be ruled out in the case of the Eocene/Oligocene turnover, which was undoubtedly related to climatic cooling and glaciation. The filtering effect of environmental stress on late Eocene diversity remains to be explained.

Abstract: A revision of the coccolithophores in the extinct Order Discoasterales is made possible by the realization that the coccoliths in this order were delicate skeletal entities similar in every way to the coccoliths secreted in today’s oceans, and not the massively crystallized objects that are found as fossils in deep-sea calcareous oozes. Phylogenetic reconstruction based on morphostructural analysis of the original coccoliths properly oriented with regard to the live cell leads to the establishment of the earliest Paleocene genus Biantholithus at the root of the Cenozoic proliferation of the order into 16 mostly highly diverse genera. Identification of the stem genus, with its spherical coccosphere, low coccolith count, and no flagella opening, permits a reconstruction of the coccospheres in different lineages leading to the crown taxa of the late Neogene. Due to the unusual texture or shape of their coccoliths, the Discoasterales coccospheres were remarkably cavernous, with minimal calcified matrix, particularly in these crown taxa. Several lines of evidence indicate that this was part of an adaptive strategy towards oligotrophy. The >64-million-year longevity of the Cenozoic Discoasterales, to which about a quarter of all Cenozoic fossil species belongs, reflects the adaptive potential of the basic wedge-shaped element of its coccoliths in an ever-changing ocean.