Miocene Volcaniclastic Environments Developed in the Distal Sector of the Bermejo Basin, Argentina

During the Miocene, in the distal sectors of the Bermejo Basin, a complex relationship developed between a floodplain and contemporary volcanic activity. Seven stages of sedimentation are established to interpret this paleoenvironmental relationship. Stage I corresponds to the development of the floodplain previous to pyroclastic activity; in Stage II, pyroclastic activity is manifested by fall deposits and dry pyroclastic surges. A probable calm in the volcanic activity, associated with exceptional rains, generates laharic deposits (Stage III). Stage IV is dry pyroclastic surges that collapse the floodplain. Subsequently, the river system is reestablished (Stage V) under a regime of low to null volcanic activity. During Stages VI and VII, thick deposits of dry and wet pyroclastic surges, which have records of contemporary seismic activity. The presence of deformational structures within the pyroclastic deposits and lahars indicate that the volcanic centers were in distant areas. The volcanism that generated these deposits is probably associated with the migration to the east of the Miocene volcanic arc of the Cordillera de Los Andes or could be associate with the volcanism of the Sierra de Famatina.

Geographic variation of Moenkhausia bonita (Characiformes: Characidae) in the rio de la Plata basin, with distributional comments on M. intermedia

ABSTRACT Moenkhausia bonita occurs in numerous additional localities from the Bermejo, Paraná, Paraguay, and Uruguay river basins. Given that this finding greatly expands the distributional range of M. bonita, we carried out an intraspecific comparison, using multivariate methods for 18 morphometric and eight meristic characters taken from a comprehensive sample of 536 specimens. All localities were distributed in four major geographic groups as follows: Bermejo, Paraná, Paraguay, and Uruguay. Results of the morphometric comparisons showed significant differences among the studied groups except between the Paraguay and Uruguay groups. Statistical differences in meristic values were found for most between-group comparisons, especially in those resulting from discriminant canonical analyses (DCA). Specimens from the Bermejo basin were the most distinct group in most morphological comparisons. However, the overall subtle differences found in body morphology likely reflect intraspecific variation within M. bonita and seem to be mainly influenced by spatial and environmental features of drainages. As M. bonita was previously identified as M. intermedia in the río de La Plata basin, distributional comments on the latter species in that basin are provided.

Freshwater actinopterygians of the Los Rastros Formation (Triassic), Bermejo Basin, Argentina

The present paper includes detailed descriptions of all fossil fishes known from the Los Rastros Formation (Ladinian; Bermejo Basin, Argentina). Four taxa are identified: Gualolepis carinaesquamosa n. gen. n. sp., Rastrolepis n. gen. with two species R . riojaensis n. sp. and R. latipinnata n. sp., and Challaiaelongata n. comb. Gualolepis n. gen. and Rastrolepis n. gen. are incertae sedis actinopterygians. The opercular bones of Gualolepis resemble those in peipiaosteid acipenseriforms and the fish might be related with the Chondrostei. The most distinctive features of Rastrolepis are the narrow opercular region and the presence of a very large plate-like branchiostegal bone resembling the condition in the Redfieldiiformes. Challaiaelongata, originally described in the Australian genus Myriolepis, is here refered to the genus Challaia, previously known from the nearby Cuyo Basin and representing the first fish taxon common to both continental sequences. Furthermore, Challaia is shown to be a member of the Acrolepidae and represents the youngest record of the family in Gondwana. Alle fossilen Fische, die bisher von der Los Rastros Formation (Ladin; Bermejo-Becken, Argentinien) bekannt sind, werden im Detail beschrieben. Vier Taxa können identifiziert werden: Gualolepis carinaesquamosa n. gen. n. sp., Rastrolepis n. gen., mit zwei Arten, R . riojaensis n. sp. und R . latipinnata n. sp., und Challaiaelongata comb. nov. Gualolepis n. gen. und Rastrolepis n. gen. Actynopterygier unsicherer systematischer Stellung. Die Operkular-Knochen von Gualolepis ähneln jenen der peipiaosteiden Acipenseriformen und das Taxon mag mit den Chondrostei verwandt sein. Die auffälligsten Merkmale von Rastrolepis sind die sehr schmale Operkularregion und das Vorhandensein eines sehr großen, plattigen Branchiostegale, das dem Zustand in Redfieldiiformen ähnelt. Challaiaelongata, ursprünglich zu der australischen Gattung Myriolepis gestellt, wird hier der Gattung Challaia zugeordnet, die bisher nur aus dem nahegelegenen Cuyo-Becken bekannt war und repräsentiert somit das erste Fisch-Taxon, das in beiden Sequenzen vorkommt. Zudem kann gezeigt werden, dass Challaia zu den Acrolepidae gehört und somit den jüngsten Nachweis dieser Gruppe in Gondwana darstellt. doi:10.1002/mmng.200600011