Marcela Cichowolski

Lithofacies, palaeoecology and palaeoenvironments of the Agrio Formation, Lower Cretaceous of the Neuquén Basin, Argentina

Abstract The lithofacies and macrofossil guilds of the Agrio Formation (Upper Valanginian—Lower Barremian) have been analysed using evidence from sedimentological, taphonomic and palaeoecological studies. The study area is Agua de la Mula and adjacent regions in central Neuquén. Seven lithofacies have been recognized in the field, which indicate that the Agrio Formation was deposited in an open-marine, ramp depositional system under storm influence. Lithofacies indicate conditions that range from low-energy basin to high-energy inner ramp. Outer and mid-ramp deposits are the most abundant. Macrofossils have been grouped into 16 guilds based on tiering, life habit and feeding category. The guilds indicate normal benthic oxygen level, normal salinity, and soft—firm muddy and sandy bottoms. Suspension-feeders are more common than deposit-feeders suggesting the predominance of suspended food particles over deposited food resources. A low input of siliciclastics and, possibly, other palaeoceanographic conditions allowed the development of oolitic facies in the inner ramp and coral patch reefs in the upper mid-ramp for a limited period of time.

The nautiloid genus Cymatoceras from the Cretaceous of the Neuquén and Austral basins, Argentina

Abstract Until now, Cretaceous nautiloids from Argentina have been studied only briefly. Here Cymatoceras is described in detail, based on specimens found in the Neuquen and Austral basins, located in west-central and southern Argentina, respectively. Those from the Neuquen Basin are of Valanginian–Barremian age and belong to a single species, Cymatoceras perstriatum (Steuer). Those from the Austral Basin range in age from Hauterivian to Campanian and represent the first record of Cymatoceras from this region. It is represented by a new species, Cymatoceras patagonicum from the Turonian, and by isolated specimens, which are assigned to this genus in open nomenclature, owing to their poor preservation state; their ages range from Early Hauterivian to Early Campanian.

Nautilids from the Upper Cretaceous of the James Ross Basin, Antarctic Peninsula

To date, Cretaceous nautilids from the Antarctic Peninsula have received little attention and only a single species had been reported, Eutrephoceras simile Spath, from Seymour, Snow Hill, and James Ross islands. Currently, it is considered a synonym of Eutrephoceras subplicatum (Steinmann), which has also been described from the Upper Cretaceous of central Chile, southern Argentina and Angola. Here, we report and describe E. subplicatum in detail, based on specimens from the Lower Campanian–Maastrichtian of Vega, Seymour and James Ross islands, presenting, for the first time, embryonic conch features related to the palaeoecology of these organisms. The nauta of this species had a diameter of approximately 30 mm with 5–6 septa. In addition, we describe a new species, Eutrephoceras antarcticum, and one specimen assigned to the same genus in open nomenclature, both recovered from the Lower Campanian beds of James Ross Island.

El género Cymatoceras (Nautilida) en el Cretácico Inferior de la Cuenca de Atacama, norte de Chile

Se describen restos fragmentarios de nautilidos del genero Cymatoceras Hyatt, provenientes de estratos barremianos de la Cuenca de Atacama, norte de Chile. Los especimenes corresponden a dos taxones: Cymatoceras sp. del Barremiano tardio alto, cuya preservacion es deficiente, aunque su tipo de ornamentacion, con costillas gruesas y un seno ventral muy marcado, es solo conocida, hasta el momento, en este taxon y Cymatoceras cf. perstriatum (Steuer), del Barremiano tardio. El material chileno es comparado, principalmente, con taxones de Cymatoceras presentes en las cuencas Neuquina y Austral, en Argentina.

First report of Devonian bactritids (Cephalopoda) from South America: paleobiogeographic and biostratigraphic implications

Abstract. Devonian bactritids are described for the first time from South America. They come from siliciclastic rocks of the Talacasto Formation in the Precordillera Basin, west-central Argentina. The host strata span the Lochkovian—Emsian and contain other non-ammonoid cephalopods as well, thus refuting the alleged virtual absence of cephalopods in circumpolar Devonian basins from southwestern Gondwana (the Malvinokaffric Realm). We report Bactrites gracilis and Devonobactrites? sp., whose wide distribution contrasts with the endemic paleobiogeographic signature of some other taxonomic groups in these basins. Furthermore, new Lochkovian and Pragian records of Bactrites sp. provide new insights into the earliest bactritid records worldwide.