Many studies on avian microanatomy have established a relationship between high bone compactness (i.e., considerable degree of osteosclerosis) and diving behavior. Greatest degrees of compactness have been observed in the femora and humeri of fossil and extant penguins, femora of Hesperornis Marsh, and Polarornis gregorii Chatterjee, and to a somewhat lesser degree, in the humeri of flightless Pan-Alcidae. Within Anatidae, humeral and femoral compactness among diving species is greater than among non-diving ones, whereas flightless diving species have a greater degree of compactness than their volant close relatives. In Cayaoa bruneti Tonni, an extinct flightless foot-propelled, diving anatid with extreme forelimb reduction, femoral osteosclerosis is as great as that of penguins. Osteosclerosis in the femur of both foot- and wing-propelled divers could be part of the consequences of flightlessness and a result of selection to counter buoyancy.

In this contribution we present the first record of a member of the family Argyrolagidae (Metatheria) recovered from the India Muerta Formation (late Miocene) exposed near San Miguel de Tucumán, Tucumán Province, Argentina. The specimen is represented by a fragment of right maxilla with M1–4 and is referred here as Microtragulus Ameghino. It clearly differs from other members of the genus that preserve the upper dentition. Nevertheless, the impossibility of comparing this material with species only known by lower dentition, such as Microtragulus catamarcensis (Kraglievich) prevent us from establishing whether this is a new species or not. The comparisons performed allow us to identify characters from the upper dentition differing between Microtragulus and Argyrolagus Ameghino. These characters, in turn, led to the reassignment of a species previously assigned to Microtragulus. The new record reported here, the third known of Microtragulus from northwestern Argentina, broadens the geographic range of the genus, known from the Atlantic coast of Argentina and several localities of the Andean highlands and the foothills of the cordilleran range.

The new combination Phugatherium dichroplax nov. comb. (Ahearn and Lance) is proposed for the North American species “Neochoerus” dichroplax Ahearn and Lance, and “N.” cordobai Carranza-Castañeda and Miller. Its age range is here expanded (late Early Blancan—latest Blancan; Piacenzian—Gelasian), being the youngest species of Phugatherium and thus extending the biochron of the genus into the early Pleistocene. This is the first mention of a Pliocene South American lineage of capybaras crossing the Panamanian bridge, implying the recognition of a third lineage of capybaras involved in the Great American Biotic Interchange. The proposed North American origin of the genus Neochoerus is discussed.

Middle Devonian bivalves from Argentina are reported for the first time, based on records of the Precordilleran Punta Negra Formation (Emsian—Frasnian?), San Juan Province. This finding is the youngest Devonian record of bivalves from southernmost South America. The impoverished bivalve association in the Punta Negra Formation includes five shallow infaunal deposit feeders, i.e. Nuculites argentinum Sánchez, Anthracoleda (Pseudoleda) minuta Sánchez, Praenucula sp., Deceptrix sp. and Praectenodonta sp. The distribution of the Silurian—Devonian bivalves from Precordillera shows a decreasing diversity trend during this interval, due to potential biases in sampling and paleoecology and thus suggesting that this trend might be apparent. Early and Middle Devonian bivalves from Precordillera show a strong cosmopolitan paleobiogeographic signature, in contrast with other geographically related basins showing endemic (Malvinokaffric Realm) signals during the Devonian. Contrarily, an endemic composition is recorded in the Precordillera Basin when based on coeval Malvinokaffric groups such as ostracods and trilobites. Although more information is needed, the cosmopolitan character of Devonian bivalve assemblages from the Argentine Precordillera Basin might be interpreted under the light of a unique history driving biogeographic processes affecting different taxonomic groups in different ways.

A new megaflora composed of fossil woods and leaves is described. The bearing sediments overlie the Santa Cruz Formation (early Miocene), making it one of the youngest fossil megafloras described from southern Patagonia. The fossil woods is carbonized and found as clasts within a conglomerate. It includes a few specimens representing Araucariaceae (Agathoxylon sp.), Podocarpaceae (Phyllocladoxylon sp.), Cupressaceae (Cupressinoxylon sp.) and two indeterminable angiosperms with anatomical features consistent with Nothofagaceae. Most leaves are assigned to Nothofagaceae while a few specimens are related to Lauraceae, Typhaceae, Leguminosae, and a conifer. The recovered assemblage suggests a temperate climate similar to that of northern Patagonia today, inhabited by extant relatives of the fossils described herein.