Fossil scales of the enigmatic teleost Acregoliath rancii Richter, collected in 1945 and 1946, are described for the first time from fine-grained sediments of the La Victoria Formation of the Honda Group in central Colombia. The specimens include one isolated but nearly complete scale from the anterior region of the body, and one isolated scale fragment. Although fragmentary, these specimens provide relevant anatomic and biogeographical information. Both specimens agree in most details with the type material of A. rancii; however, the almost complete scale differs in focus position and outline, thus suggesting topological variation. This aspect of lepidosis in A. rancii was previously unknown and could be relevant for future reassessment of the interrelationships of this taxon. The presence of A. rancii in the middle to late Miocene fossil assemblages from La Venta, Fitzcarrald, and Acre suggests a relationship between these areas during the middle Miocene. The relevance for paleodrainage evolution in northern South America is discussed in the context of these findings.

The marginal-marine, low-energy, carbonate ramp deposits of the La Tosca Member (Huitrín Formation, Barremian) in southern Mendoza Province, Argentina, bear an abundant though low-diversity macrobenthic invertebrate fauna that is scarce or absent in the outcrops of the unit in the Neuquén Province. Here we describe new taxa from the Quebrada del Gastrópodo and Ranquil 1 sections and provide taxonomic revisions for other key faunal elements. The species discussed herein are the gastropods Paleoanculosa macrochilinoides (Doello-Jurado), Provalvata minor sp. nov., and Provalvata maior sp. nov., the bivalve Rutitrigonia cintarojensis sp. nov., and the serpulid Parsimonia aff. laevis (Goldfuβ). Additionally, in order to analyze the observed changes in faunal content along the studied section, a paleoecological analysis was performed combining estimation of diversity indices and the paleoautoecology of the fauna recorded in Quebrada del Gastrópodo. Three macrobenthic assemblages were qualitatively recognized in the field; from base to top: 1) Paleoanculosa macrochilinoides, 2) serpulid-gastropod, and 3) bivalve-serpulid assemblages. Species richness and equitability increase upwards. This trend suggests a reduction in environmental stress from bottom to top. The first two assemblages indicate settings with fluctuating environmental parameters, whilst the third points to higher environmental stability. Euryhaline taxa dominate, whilst stenohaline species are scarce or absent in the lower two thirds of the succession. The nature of the stress is interpreted herein to be mainly related to high-frequency salinity variations, especially in the first and second assemblages in which temporal deviations from euhaline to meso-brachyhaline and brachyhaline conditions are hypothesized.

Diatom dissolution is controlled by environmental conditions prevailing during fossilization. In Pampean shallow lakes, diatom dissolution showed important correlations with gradients of salinity, pH, carbonate, and bicarbonate, being these the most probable causes of the observed shifts in Holocene assemblages preservation. In the present contribution, a series of experiments were conducted in order to demonstrate the effect that physical-chemical lake characteristics exert on Pampean diatom assemblages preservation. Three experimental assemblages were subjected to the effect of three concentrations of two salts, NaCl (0.6, 1.2 and 3M) and NaHCO₃ (0.6, 0.9, and 1.2M), and two pH values (7 and 10). Aliquots of the experimental solutions were removed once each five days for 20 days, and analyzed for changes in dissolved silica concentration (SiDi), relative and absolute abundances of diatoms, and dissolution indices (DDI) based on the target taxon Cyclotella meneghiniana. All the experimental solutions increased the SiDi significantly, particularly since day 10. These increased SiDi values were accompanied by significant changes in the DDI, which reached maximum values at pH 10, and by evidence of dissolution observed in SEM images, whereas no significant changes in relative or absolute abundances of diatoms were registered. These experimental results demonstrated the impact that water chemistry can exert on diatom dissolution in Pampean shallow lakes, even during short-term exposures. Given the naturally high pH, NaCl and NaHCO₃ concentrations characteristic of many of these lakes, these experimental findings can be confidently extrapolated to the interpretation of the dissolution trends found in modern and fossil sedimentary assemblages.