Biostratigraphy and taxonomy of eocene-oligocene ebridians from the South Atlantic
Widman, Julie A.
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Ebridians obtained from Eocene-Oligocene sediments recovered from Site 511 (Falkland Plateau), Site 512 (Maurice Ewing Bank) and Hole 513A (Lower Mid-Atlantic Ridge Flank) in the South Atlantic during DSDP Leg 71 were examined for their taxonomy and biostratigraphic distribution. Contrary to results of earlier work, ebridians were found to be quite abundant throughout the Eocene-Oligocene section studied. Highest diversity was observed in the Late Eocene of Site 511, where the assemblage consisted of 16 species in ten genera. However, the Late Oligocene assemblage (Hole 513A) was reduced to four species in four genera. In the Late Oligocene/Early Miocene of Hole 513A, three new species were observed. This change in diversity supported the findings of earlier workers of diverse Eocene assemblages, less diverse Oligocene assemblages, and expanded Miocene assemblages. The initial and final appearances of ebridian taxa which occurred over the Eocene-Oligocene interval allowed for the establishment of the following seven zones in ascending order: the Ebriopsis aplanata Partial Range Zone (from the Middle Eocene of Site 512); the Ammodochium danicum var. curturn Partial Range Zone and A. ampulla Local Range Zone (from the Late Eocene and Early Oligo- cene of Site 511); the Pseudammodochium dictyoides Partial Range Zone, Ebrinula paradoxa Partial Range Zone, Falsebria ambigua forma coralloidea Partial Range Zone, and pyramidale Partial Range Zone (from the Early and Late Oligocene of Hole 513A). Correlation was possible between Site 511 and Hole 513A based on the A. ampulla Local Range Zone. The newly proposed ebridian biostratigraphy was cross-correlated to magnetostratigraphy and to the use of other microfossil groups for these sites, adding further detail to the existing biostratigraphy. The lorica, a unique structure known only to ebridians, was examined. While the exact function of the lorica could not be determined, three different types of loricas were recognized, relative abundance of loricate specimens was presented, and possible functions of the loricate stage were discussed. Two aspects of ebridian evolution were examined. First, an evolutionary lineage between Ebriopsis crenulata and E. antiqua antiqua, initially postulated by Deflandre in 1950, was confirmed; second, based on morphologic variation observed in A. rectangulare, the possible phylogenetic relationship from an ancestral A. rectangulare to the Miocene genus Parathranium was considered. The taxonomy of the ebridians was presented, and geologic occurrences were discussed, including previous records from different localities throughout the world.