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Borings in the Eocene Orthophragminids, Stratigraphic and/or paleocologic features?

The upper most part of the Eocene shallow-water carbonate succession, deposited on the Adriatic Carbonate Platform, is represented by Orthophraminae-bearing limestones (ranging in age from the Lutetian to Bartonian, SBZ 14-SBZ 17). They are mainly composed of orthophragminid and nummultid tests, the former ones with abundant traces of bioerosion. A statistical analysis of affected specimens, altogether 60 foraminiferal tests (drilling frequency of 10%, this estimation may be seriously affected by selective sampling biases), indicates that these datasets are quantitatively consistent and provides interpretable numerical data on drilling pattern. The foraminiferal tests bear either isolated elliptical marks (major axis from 0.4 to 1 mm) located in the central part of the test or series of elliptical marks (spaced at regular intervals of about 1 mm, with major axis from 0.4 to 1.25 mm ) lined along the equatorial layer (in Discocyclina individuals with Diameter/Thickness ratio greater than 2.2) or round, circular holes (range in size from 0.1 to 0.2 mm) in lateral layers of inflate Orbitoclypeus specimens (Diameter/Thickness ratio less than 2.2). Despite various limitation (such as only axial sections were available for study), the non –randomly distributed traces record: 1) predators (singular circular holes) or parasites (elliptical, singular to multiple holes) in orthophragminids ; 2) drilling attacks being size selective (traces show a relatively narrow size range) ; 3) the position of traces within the test suggests that they were made to gain access to inside of the host. The distributional pattern of ellipsoidal traces implies that microorganisms broke into the test along the equatorial layer benefiting from the cytoplasm concentration and complex stolon system as “possible routes”. Some “intruders” continued to grow as the “host” grew, living the set of marks evenly distributed along the equatorial layer, whole some “invaders” slicked with the embryonic part of test, including successive two chambers ; 4) temporal distribution of affected orthophragminids is potential tool as a paleoecological marker (Middle Eocene) ; 5) the occurrences of the traces in suggest that their distribution was controlled by environmental parameters (low sedimenatition rate, mesotrophication, lower photic zones, low energy).

Eocene; orthophragminids; stratigraphy; paleoecology

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