Darío G. Lazo

Barremian Bivalves from the Huitrín Formation, West-Central Argentina: Taxonomy and Paleoecology of a Restricted Marine Association

Abstract The Cretaceous Huitrín Formation in west-central Argentina records the final connection of the Neuquén Basin to the Pacific Ocean. This formation is comprised of a variety of continental to marginal-marine sediments deposited behind an Andean volcanic arc under warm, arid paleoclimatic conditions. Here we focus on a bivalve fauna from carbonate ramp deposits within the Barremian La Tosca Member of the Huitrín Formation. This fauna is very abundant and widely distributed within the basin but, surprisingly, it has not yet been studied in detail. In addition, paleoenvironmental affinities remain unresolved, with the fauna variously interpreted as having freshwater, brackish, and marine affinities. We studied the faunas taxonomy and paleoecology based on more than 500 specimens collected at ten fossil localities in combination with new field observations. The bivalve assemblage was recorded from middle to outer carbonate ramp deposits and is composed of five taxa of marine affinity: Phelopteria huitriniana n. sp., Isognomon cf. I. nanus (Behrendsen), Placunopsis? pichi n. sp., Anthonya jarai n. sp., and Argenticyprina mulensis n. gen. n. sp.; the first three may be regarded as eurytopic and/or opportunistic. Reduced diversity, low evenness, overall small size (length <4 cm), thin shells, eurytopic or opportunistic life strategies, and high endemism point to a restricted marine setting for the La Tosca Member. The most important limiting factors likely were low primary productivity and fluctuating salinity and temperature, as conditions inferred for the unit include high evaporation rates combined with low continental runoff and reduced rainfall. Thick evaporite deposits below and above La Tosca Member and thin intercalated gypsum beds support a restricted, hypersaline setting.

Data from: Morphological evolution of the bivalve Ptychomya through the Lower Cretaceous of Argentina

Abstract. The complex morphological evolution of the bivalve Ptychomya throughout the well-studied Agrio Formation in the Neuquén Basin (west-central Argentina, lower/upper Valanginian-lowest Barremian) constitutes an ideal opportunity to study evolutionary patterns and processes occurring at geological timescales. Ptychomya is represented in this unit by four species, the morphological variation of which needs to be temporally assessed to obtain a thorough picture of the evolution of the group. Here we use geometric morphometrics to measure variation in shell outline, ribbing pattern, and shell size in these species. We bracket the ages of our samples using a combination of ammonoid biostratigraphy and absolute ages and study the anagenetic pattern of evolution of each trait by means of paleontological time-series analysis and change tracking. We find that evolution in Ptychomya is mostly speciational, as the majority of traits show stasis, with the exceptions of shell size in P. coihuicoensis and shell outline in P. windhauseni, which seem to evolve directionally toward larger and higher shells, respectively. Ptychomya displays changes in its average morphology and disparity, which are the result of amixture of taxonomic turnover and mosaic evolution of traits. Pulses of speciation would have been triggered by ecological opportunity, as they occur during the recovery of shallow-burrowing bivalve faunas after dysoxic events affecting the basin. On the other hand, the presence of directional patterns of evolution in P. coihuicoensis and P. windhauseni seems to be the result of a general shallowing-upward trend observed in the basin during the upper Hauterivian-lowest Barremian, as opposed to the cyclical paleoenvironmental stability inferred for the early/late Valanginian-early Hauterivian, which would have prompted stasis in P. koeneni and P. esbelta.

TAPHONOMY AND PALEOECOLOGY OF LOWER CRETACEOUS OYSTER MASS OCCURRENCES FROM WEST-CENTRAL ARGENTINA AND EVOLUTIONARY PALEOECOLOGY OF GREGARIOUSNESS IN OYSTERS

Abstract Lower Cretaceous (lower Hauterivian) oyster mass occurrences (OMOs) dominated by the gryphaeid small oyster Ceratostreon from the Neuquén Basin (west-central Argentina) are analyzed in terms of taphonomy and paleoecology in order to characterize their origin, reconstruct the oyster-dominated paleocommunity, and assess their paleoenvironmental implications. A laterally extensive oyster-bearing sedimentary interval with high oyster abundance was analyzed at three localities situated along a 75 km N-S transect. Three different types of OMOs were differentiated: biogenic bioherms with dominance of encrusting life habits, biogenic autobiostromes with dominance of soft bottom recliners, and mixed sedimentologic-biogenic parabiostromes with signs of reworking. The development of these different types of OMOs indicates a high nutrient input that favored high oyster proliferation, whereas the different life habits adopted by the oysters indicate a difference in sedimentation rate throughout the studied level: reclining oysters indicate a higher sedimentation rate than cementing ones. This Lower Cretaceous study case is framed in a general context of global shift of OMOs from a Tethyan distribution during the Jurassic to a mainly eastern Pacific distribution in the Cretaceous, occupying both coastal and deep continental shelf. Also, there is a taxonomic shift from primarily gryphaeid OMOs during the Jurassic and Lower Cretaceous to primarily ostreid OMOs during the Upper Cretaceous. Cenozoic OMOs are found in marginal marine environments and are exclusively built by ostreids. An increase in predation pressure from the mid-Mesozoic onwards could have pushed OMOs to marginal marine environments where predators cannot thrive and force a taxonomic shift towards brachyhaline taxa.