HighP–TNano-Mechanics of Polycrystalline Nickel

We have conducted highP–Tsynchrotron X-ray and time-of-flight neutron diffraction experiments as well as indentation measurements to study equation of state, constitutive properties, and hardness of nanocrystalline and bulk nickel. Our lattice volume–pressure data present a clear evidence of elastic softening in nanocrystalline Ni as compared with the bulk nickel. We show that the enhanced overall compressibility of nanocrystalline Ni is a consequence of the higher compressibility of the surface shell of Ni nanocrystals, which supports the results of molecular dynamics simulation and a generalized model of a nanocrystal with expanded surface layer. The analytical methods we developed based on the peak-profile of diffraction data allow us to identify “micro/local” yield due to high stress concentration at the grain-to-grain contacts and “macro/bulk” yield due to deviatoric stress over the entire sample. The graphic approach of our strain/stress analyses can also reveal the corresponding yield strength, grain crushing/growth, work hardening/softening, and thermal relaxation under highP–Tconditions, as well as the intrinsic residual/surface strains in the polycrystalline bulks. From micro-indentation measurements, we found that a low-temperature annealing (T < 0.4 Tm) hardens nanocrystalline Ni, leading to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of impurity segregation to the grain boundaries of the nanocrystalline Ni

Middle Triassic Pentasauropus-dominated ichnofauna from western Gondwana: Ichnotaxonomy, palaeoenvironment, biostratigraphy and palaeobiogeography

© 2019 Elsevier B.V. Quadruped trackways of large pentadactyl footprints are reported from the Middle Triassic (Anisian-Ladinian) Cerro de las Cabras Formation of the Cuyo Basin, Mendoza Province, central-western Argentina. The track-bearing strata are interpreted as deposited by sheetfloods in a mixed flat where water was ponded in a playa-lake setting. The vertebrate trackways are assigned to the ichnogenus Pentasauropus, originally described from the Upper Triassic Elliot Formation of South Africa, based on the presence of five equally spaced digit imprints that form an anteriorly convex broad arcuate pattern. A new ichnospecies, Pentasauropus argentinae nov. isp., is erected for the Argentinian material because of the distinct heteropody, inward rotation of the pes and outward rotation of the manus imprints, and the presence of palm/sole traces, and the diagnosis of the ichnogenus is emended. Pes/manus set arrangement and trackway patterns indicate a sprawling limbed trackmaker with an abducted posture for the fore-limbs and at least a semi-abducted posture for the hindlimbs, which suggests that the trackmaker was a kannemeyeriiform dicynodont. Associated archosaur ichnotaxa (Chirotherium barthii, Chirotherium cf. C. rex, Isochirotherium cf. I. coureli) point to a Middle Triassic age for the trackway-bearing strata in agreement with bracketing geochronological data. The rare occurrence of the therapsid footprint Dicynodontipus isp. is also compatible with the inferred age. The moderate abundance and oldest occurrence of Pentasauropus from three areas in Argentina suggest an origin for this ichnogenus in southwestern Gondwana, and presumably this area was a faunal exchange gate between southeastern and southwestern Gondwana and south Gondwana and North America

The earliest burst of necrophagous dung beetles in South America revealed by the Cenozoic record of Coprinisphaera

New ichnological, sedimentological and palaeobotanical information from a Miocene palaeosol succession from Patagonia bearing abundant fossil brood balls of dung beetles (Coprinisphaera) allow inferring novel aspects of the evolutionary history, biology and feeding habits of Scarabaeinae, along with the palaeoenvironmental and palaeoclimatic conditions in which they nested. Coprinisphaera tonnii and Coprinisphaera akatanka, both attributed to necrophagous dung beetles, represent 40.2% of the specimens. Considering their scarce or null record in older Cenozoic units from South America, these high values reveal that the first burst of necrophagous Scarabaeinae took place during the Miocene. Some Coprinisphaera preserve characters that indicate the developmental stage of the ball occupant and the adult emergence success. Both ichnospecies show the egg chamber isolated from the provision chamber, which was associated with a higher preference of cleptoparasites for necrophagous balls. The comparison among the abundance of traces of nest intruders in Coprinisphaera attributed both to necrophagous and to coprophagous beetles from different units of South America indicates that the presence of such intruders would be independent of the nature of the organic matter contained within the balls. Phytolith analyses performed in Coprinisphaera and extant necrophagous and coprophagous balls indicate that the comparison between the relative abundance of phytoliths in the wall of the brood ball, their infilling and the bearing palaeosol is a useful tool for inferring the feeding behaviour of the trace makers and support the attribution of Coprinisphaera tonnii and Coprinisphaera akatanka to necrophagous Scarabaeinae. Sedimentary and palaeosol analyses indicate that the beetles would have nested in well-drained soils showing mollic-like features, in grass-dominated habitats, under a seasonal sub-humid, temperate–warm climate.Fil: Cantil, Liliana Fernanda. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Bellosi, Eduardo Sergio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Sánchez, María Victoria. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Gonzalez, Mirta Gladys. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Genise, Jorge Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentin