The present study is a further contribution to the systematic knowledge of the ostracods from the southern Brazilian continental shelf. Twelve species of Trachyleberididae are recorded, including the following six new taxa: Cativella ornelasae sp. nov., C. sudbrasilienis sp. nov. sp., Henryhowella inflata sp. nov., Henryhowella verrucosa sp. nov., Ambocythere venusta sp. nov. and Basslerites costata sp. nov. The other six species have been recorded in previous studies, either in shallow or bathyal depths in both Brazilian waters and adjacent areas. Most species recorded herein are distributed in the neritic zone, except three, i.e., Trachyleberis aorata, Ambocythere venusta sp. nov. and C. sudbrasilienis sp. nov. The first two species are eurybathic, while C. sudbrasilienis sp. nov. is restricted to the inner shelf.

REDESCRIPTION AND PALEOENVIRONMENTAL SIGNIFICANCE OF HELEOBIA AMEGHINI (DOERING, 1884) (GASTROPODA: RISSOOIDEA) FROM THE LATE PLEISTOCENE OF BUENOS AIRES PROVINCE, ARGENTINA. New remains of Heleobia ameghini (Doering, 1884) are analysed. This is an extinct late Pleistocene gastropod species recovered from alluvial sections along the middle course of the Luján River. Detailed observations allowed establishing the main diagnostic characteristics of the species; these are compared with the data provided in the original description of this taxon. Its paleoenvironmental significance is also discussed in context with the stratigraphic setting and the accompanying fauna. Heleobia ameghini is characterized by its small size, which varies between 0.75 and 6.75 mm in length. Shells are globose, with the last whorl highly developed, in a similar way to the modern Heleobia piscium (d'Orbigny, 1835), from which it differs by their relative higher size and the lower convexity of the whorls. The poor preservation of the specimens and the bell-shaped size-frequency distribution suggest an important taphonomic alteration, which may be consequence of the dissolution processes that occurred during the biostratinomic or diagenetic stage. It is concluded that Heleobia ameghini tolerated significant salinity fluctuations, inhabiting fluvial environments subjected to recurrent episodes of storm-flooding alternated with moments of waterflow decrease. The latter were responsible for the development of ponds and eventually dry ponds during drought periods and thus higher salinity values.

PALYNOLOGICAL STUDY OF SPORES FROM SALTO DEL MACHO FORMATION, PALEOGENE OF THE ÑIRIHUAU BASIN, ARGENTINA. The first palynological data from the type locality of the Salto del Macho Formation, at the Río Foyel section, Ñirihuau Basin, Argentina, is here presented. Nineteen taxa of trilete and monolete spores of Bryophyta and Tracheophyta (Lycopsida and Polypodiopsida) belonging to the following genera were recorded: Anthocerisporis, Azolla, Baculatisporites, Biretisporites, Cingutriletes, Cyatheacidites, Cyathidites, Ischyosporites, Laevigatosporites, Matonisporites, Muricingulisporis, Peromonolites, Polypodiisporites, Reboulisporites, Retitriletes, Trilites and Tuberculatosporites. The palynoflora is dominated by representatives of the Class Polypodiopsida, with predominant Cyatheaceae and Dicksoniaceae, in asociation whith Osmundaceae and Blechnaceae. The comparison of the record of spores with that of other Paleogene/early Neogene basins from Argentinian Patagonia, Chile and Antartica, made through the PAST program, shows remarkable palynofloristic similarities between Salto del Macho Formation with the spore associations recorded in the Río Foyel, Chenque and San Julián formations. The accompanying palynoflora (Nothofagaceae, Podocarpaceae, Araucariaceae, Myrtaceae, Botryococcus braunii and low diversity of dinocysts) suggests a marginal-marine enviroment, in conditions of sub-normal salinity and temperate-warm water temperature.

ONTOGENETIC VARIATION IN THE BONE HISTOLOGY OF CAYPULLISAURUS BONAPARTEI FERNÁNDEZ, 1997 (ICHTHYOSAURIA: OPHTHALMOSAURIDAE). In order to explore the osteohistological ontogenetic variation in ichthyosaurs, we studied the bone microstructure of the ophtalmosaurid Caypullisaurus bonapartei Fernandez. The analyzed thin sections were obtained from three ribs belonging to different individuals (MLP85-I-15-1, 83-XI-15-1 and 83-XI-16-1), from the Vaca Muerta Formation (Tithonian, Late Jurassic), in the Neuquén Basin (Argentina). The ontogenetic stages of these specimens have previously been determined based on the morphology of the humerus and the sclerotic rings. The first specimen is composed entirely of finely spongy tissue distributed around the medullary cavity. Microscopically, the spongy bone is secondary, with abundant inner spaces delimited by bone trabeculae. There are numerous overlapping generations of lamellar bone. The second specimen (a juvenile) shows primary spongy bone surrounding the medullary cavity and Sharpey's fibers. The third specimen lacks a medullary cavity; instead, the medullary region is occupied by secondary spongy bone, uniformly distributed through the entire section. These results suggest that the macroscopic changes classically attributable to ontogeny have a correlation in bone microstructure. The bones of immature individuals show some primary bone, while the bones of the mature specimens are characterized by the exclusive presence of secondary bone. These features are easily identifiable, and provide an auxiliary criterion for the determination of ontogenetic stages in incomplete and fragmentary specimens.

The study of palaeodemecological features requires some particular taphonomic conditions. These conditions were met in the Mulichinco Formation (Valanginian), where burrowing bivalve trace fossils are widespread and often appear in cross section on bedding surfaces. Two groups of such beds were analyzed, measuring population density, spatial distribution, size distribution and horizontal orientation of the burrows. The palaeoenvironment was established by means of a detailed sedimentological analysis, and the bivalve fauna present was checked, in order to attempt identifying their potential producers. High population densities were found in the two groups, indicating favourable physical conditions and good food supply, while differences in both spatial and size distributions were noticed between them; on most surfaces there was no preferred orientation. The first group (group A) showed a uniform pattern of spatial distribution and larger traces, with a remarkable absence of small sizes. In the second group (group B), the spatial distribution pattern is indistinguishable from a random distribution (except one case in which the pattern appears to be aggregated). Group A is interpreted as a set of escape traces made by deep burrowers in response to storm deposition, while group B is considered as resting/escape traces made by shallow burrowers in tide-dominated environments. Palaeodemecological studies of this kind are potentially useful tools for sedimentary and basin analyses.

REVISION OF THE GENUS TRANSPITHECUS AMEGHINO, 1901 (NOTOUNGULATA, INTERATHERIIDAE) FROM THE MIDDLE EOCENE OF PATAGONIA, ARGENTINA. The genus Transpithecus Ameghino is up to now known from middle Eocene levels of Santa Cruz and Chubut Provinces, Argentina. Only a single species is known, T. obtentus Ameghino, which is represented by very scarce remains. Traditionally, this species has been placed in the Subfamily “Notopithecinae” (Notoungulata, Interatheriidae) together with Notopithecus Ameghino, Antepithecus Ameghino, and Guilielmoscottia Ameghino. According to recent phylogenetic analyses, this group constitutes a paraphyletic assemblage conformed by basal interatheriids. Since Ameghino's description and except for the contribution made by Simpson, no recent studies on Transpithecus have been performed. In this contribution, the associated upper and lower dentition of a same individual is assigned for the first time to T. obtentus. This specimen comes from Valle Hermoso (Chubut Province, Argentina), and has led to the recognition of other unpublished materials as T. obtentus. The present study allows: (1) extending the diagnosis of T. obtentus; (2) defining an ontogenetic sequence of its dental morphology; (3) formally proposing Patriarchippus annectens Ameghino as synonymous to T. obtentus; (4) establishing similarities between Transpithecus obtentus and the specimen MNHN CAS 69, previously assigned to Paginula parca Ameghino; and (5) discarding the presence of Transpithecus in the Kibenikhoria faunal zone (Itaboraian South American Land Mammal Age), restricting its biochron to the Casamayoran South American Land Mammal Age (Barrancan subage; middle Eocene).

Pleurotomaria gerthi Weaver, 1931 (Gastropoda, Vetigastropoda) from the Pilmatué Member of the Agrio Formation is redescribed and reillustrated on the basis of 38 newly collected specimens from nine localities in Neuquén Province, Argentina, including its type locality. The holotype housed in the C.E. Weaver collection is also refigured. Pleurotomaria gerthi-bearing levels, dated using a detailed ammonoid zonation, are late Valanginian—early Hauterivian. Taphonomic and sedimentological data gathered at three of the nine studied sections indicate that P. gerthi dwelled in shallow marine settings above the storm-weather wave-base. Associated palaeoenvironments are interpreted as inner shelf and inner carbonate ramp settings deposited under the influence storm currents and waves. The species lived on well-oxygenated soft sandy and patchy bioclastic substrates in normal marine warm-temperate waters within the photic zone, associated with a high-diversity macrofossil assemblage. Our study shows that its preferred habitat was the middle carbonate ramp, below the fair-weather wave-base, under low to moderate energy from episodic storm waves and currents. Pleurotomaria gerthi is endemic to the marine Andean basins of the western margin of southern South America and its oldest record is from the Tithonian of the Aconcagua-Central Chile Basin. It bears the closest morphological resemblance to Early Cretaceous species from the Tethyan Realm. During the Berriasian—Barremian interval, Pleurotomaria was a pandemic genus, with records in peri-Tethyan basins of Europe and India, as well as in north and south temperate marine basins in England and the western margin of South America.

The genus Steinmanella Crickmay is recorded in several Lower Cretaceous marine units of the Neuquén Basin. It is particularly abundant at the transition between the Vaca Muerta and Mulichinco formations. This paper presents a taxonomic revision of this fauna, discusses the stratigraphic range of each species, and describes and interprets the associated lithofacies based on detailed field sections, newly collected specimens and revision of previous fossil collections. Four sections were measured in northern Neuquén comprising the upper beds of the Vaca Muerta Formation and the lower ones of the Mulichinco Formation. The Steinmanella fauna was dated accurately on the basis of a precise ammonoid zonation. The species were found in the Neocomites wichmanni and Lissonia riveroi zones of early Valanginian age. Specimens were collected in situ from dark grey shales, alternating siltstones, wackestones and mudstones, and rudstones. Three species were identified: Steinmanella curacoensis (Weaver), S. quintucoensis (Weaver), and S. subquadrata sp. nov. Steinmanella curacoensis has an oval outline, poor demarcation of carinae, flank and corselet ribs partially intercalated at the median part of the valve, and oval escutcheon. Steinmanella quintucoensis has a furrow below the marginal carina, subrectangular outline, robustly tuberculated flank ribs, and lanceolate escutcheon. Steinmanella subquadrata sp. nov. has a subquadrate outline, well-marked carinae, and oval escutcheon ornamented with thin ribs and small elongated tubercles.

Asteromys punctus Ameghino was only known from the holotype, found in the Deseadan locality Cabeza Blanca (Chubut Province, Argentina). This species was included in the Family “Eocardiidae” because of its high-crowned cheek teeth and reduction of the mesoflexid-mesofossettid. In turn, “Eocardiidae”, together with Caviidae and Hydrochoeridae, has been regarded as part of Cavioidea sensu stricto, a large group of rodents characterized by double heart-shaped occlusal surface, moderate hystricognathy, and short extension of the lower incisors. The aim of this work is to report a new specimen of Asteromys punctus found in the Deseadan locality Laguna de Los Machos, Chubut Province, and to discuss the affinities of this taxon within Cavioidea s.s. in the context of a recently published phylogenetic analysis. In that analysis, four unambiguous synapomorphies support the inclusion of A. punctus within this clade: presence of mandibular foramen below the m3, root of the lower incisors extending up to the level of the posterior lobe of m2, lateral crest dorsally convex and deflecting anteroventrally from the base of the coronoid process, and horizontal crest as a low and broad ridge. Furthermore, the information provided by the holotype and the new material within a phylogenetic framework allows a discussion of the evidence that supports this taxon as the basalmost member of Cavioidea s.s., placing Asteromys in a critical position for understanding the early evolutionary history of Cavioidea s.s..