Paleocene-earliest Eocene Larger Benthic Foraminifera and Ranikothalia-bearing carbonate paleo-environments of Costa Rica (South Central America)
Authors: Baumgartner-Mora Claudia , Baumgartner Peter O. 
Published: April 2022

Here we present a biostratigraphic and systematic revision of American Ranikothalia, based on outcrop collections form Costa Rica end westernmost Panama and on type collections of the Naturhistorisches Museum Basel. Paleocene-lowermost Eocene Ranikothalia-bearing shallow-water limestones occur in three distinct palaeogeographic settings along the Southern Central American convergent margin: 1. The Barra Honda Limestone (Tempisque Basin, Costa Rica) is, according to 87Sr/86Sr-ratios and biostratigraphy of mid Selandian to late Thanetian (? earliest Ypresian) age. Tectonic uplift and temporary demise of a Middle American Arc segment during the Selandian allowed the formation of the pure carbonates of the Barra Honda Limestone. The upper well-bedded subunit exposes a higher energy facies, in which Neodiscocyclina spp. and Ranikothalia catenula group (redefined here) are locally abundant. Open marine facies with Morozovella velascoensis interfinger in a small scaled facies pattern in the topmost part. Tectonic subsidence and resumed arc activity resulted in rapid eutrophication/drowning. 2. Herradura â€“ Quepos carbonate shoals on oceanic seamounts. The Herradura and Quepos terranes may represent the earliest seamounts formed by the Galapagos hotspot plume tail activity, >1000km to the SW of their present-day position. Pillowed lava flows and subaerial volcanic breccias occur with Upper Palaeocene-lowermost Eocene shallow-water limestones rich in Ranikothalia gr. catenula, Neodiscocyclina spp. and coralline algae. 3. Burica Peninsula â€“ carbonate shoal on an accreted oceanic plateau. An Upper Paleocene carbonate shoal established on part of the Inner Osa Igneous Complex after its accretion to the western edge of the CLIP (Caribbean Large Igneous Province) and its possible emersion. In situ rhodoid limestones of the RÃ­o Palo Blanco (W-Panama) contrast with redeposited material in turbidites and debris flow breccias, interbedded with siliceous pelagic limestones and arc-derived mudstones. A morphometric study on 150 axial sections of Ranikothalia spp. from all studied settings reveals a morphologic continuum between megalospheric (A) forms and microspheric (B) forms previously described as several species, here grouped as the R. gr. catenula. We conceive that these co-occurring morphotypes belong to one biological species. The separation of this group into 2 or 3 genera seems highly artificial, since many samples contain transitional forms between the â€œsub-evoluteâ€ microsphreric forms â€œCaudrinaâ€ soldadensis (R. catenula soldadensis herein) and the involute â€œChordoperculinoidesâ€ bermudezi (R. c. bermudezi herein). Neither from previous studies nor from our material we can conclude on a biostratigraphic relevance of the various morphotypes. In particular, we cannot confirm a stratigraphic succession from flatter (R. c. soldadensis) to more robust (R. c. bermudezi) forms. Shape and shell thickness distribution of morphotypes of the Ranikothalia gr. catenula can be interpreted in terms of hydrodynamic energy and light availability in their habitat, in comparison with observations made on living LBF. This interpretation matches the hydrodynamic energy deduced from the microfacies and the abundance of photosynthetic red algae in our thin sections. Flat lenticular delicate A-forms (R. c. catenula) occur together with delicate B-forms (R. c. soldadensis) both in shallow protected and deeper off-shore calm water areas. The most robust B-form R.c. bermudezi may be a good indicator of shallow, high energy palaeo-environments. Possible Paleoceme dispersal routes for Amerucan Ranikothalia are discussed.