Shinji Nagaoka

Bio: Shinji Nagaoka is an academic researcher from Nagasaki University. The author has contributed to research in topics: Tephra & Holocene. The author has an hindex of 15, co-authored 41 publications receiving 639 citations.

The first skull of Australopithecus boisei

Abstract: Australopithecus boisei was first described from a cranium recovered in 1959 from Olduvai Gorge, Tanzania1,2. This and subsequent finds, mostly from Kenya's Turkana basin3,4,5, resulted in its characterization as a specialized Australopithecus species with a hyper-robust masticatory apparatus2,4,6. A distinct A. boisei facial morphology has been emphasized to differentiate robust Australopithecus lineages from East and South Africa6. A preference for closed and/or wet habitats has been hypothesized7. Here we report some new A. boisei specimens, including the taxon's first cranium and associated mandible, from Konso, Ethiopia. These fossils extend the known geographical range of A. boisei. They provide clear evidence for the coexistence of A. boisei and Homo erectus within a predominantly dry grassland environment. The A. boisei specimens from Konso demonstrate considerable morphological variation within the species. The unexpected combination of cranial and facial features of this skull cautions against the excessive taxonomic splitting of early hominids based on morphological detail documented in small and/or geographically restricted samples.

Melting of the uppermost metasomatized asthenosphere triggered by fluid fluxing from ancient subducted sediment: Constraints from the quaternary basalt lavas at chugaryeong Volcano, Korea

Abstract: Major and trace element and Sr^Nd^Pb isotope data for wholerocks and major element data for minerals within basalt samples from the Chugaryeong volcano, an intra-plate back-arc volcanic centre in the central part of the Korean Peninsula, are used to address the process of magma genesis in the deep back-arc region of eastern Asia.There are two lava flow units at Chugaryeong volcano: the Chongok (0·50Ma) and the Chatan (0·15Ma) basalts. These basalts have similar MgO (9·1^10·4 wt %) but exhibit differences in their major and trace element and isotope compositions. The Chongok basalt has higher TiO2, Al2O3, Na2O, K2O, P2O5, Cr2O3, large ion lithophile elements (LILE), high field strength elements (HFSE), and rare earth elements (REE), and lower FeO*, SiO2, and CaO than the Chatan basalt. In addition, the Chongok basalt has more radiogenic Nd/Nd and Pb/Pb, and less radiogenic Sr/Sr and Pb/Pb than the Chatan basalt. Chi-square tests for the major elements indicate that crystal fractionation can explain the chemical variations within each basalt suite; intra-crustal processes, including crystal fractionation and assimilation of continental crust, cannot result in the formation of one basalt suite from the other. The Sr^ Nd^Pb isotopic compositions of the Chongok and Chatan basalts plot on mixing hyperbolae between peridotite mantle xenoliths from the area and a fluid flux derived from a mixture of ancient and recent sediments. The trace element compositions of the estimated primary melts for the two basalt suites suggest different degrees of partial melting of a common enriched mantle source that was metasomatized by a Ba-, K-, Pb-, and Sr-rich fluid. The estimated degree of melting increased with time from 7·5% for the Chongok basalt to 10% for the Chatan basalt. The source mantle for the Chatan basalt is more enriched in Ba and Pb, indicating a greater fluid flux than for the Chongok basalt. This suggests that melting of the source mantle increased with time, sustained by an increased sediment-derived fluid flux from the deeper upper mantle.

Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa

Abstract: Homo naledi is a previously-unknown species of extinct hominin discovered within the Dinaledi Chamber of the Rising Star cave system, Cradle of Humankind, South Africa. This species is characterized by body mass and stature similar to small-bodied human populations but a small endocranial volume similar to australopiths. Cranial morphology of H. naledi is unique, but most similar to early Homo species including Homo erectus, Homo habilis or Homo rudolfensis. While primitive, the dentition is generally small and simple in occlusal morphology. H. naledi has humanlike manipulatory adaptations of the hand and wrist. It also exhibits a humanlike foot and lower limb. These humanlike aspects are contrasted in the postcrania with a more primitive or australopith-like trunk, shoulder, pelvis and proximal femur. Representing at least 15 individuals with most skeletal elements repeated multiple times, this is the largest assemblage of a single species of hominins yet discovered in Africa.

Subsidence of ash-flow calderas: Relation to caldera size and magma-chamber geometry

Abstract: Diverse subsidence geometries and collapse processes for ash-flow calderas are inferred to reflect varying sizes, roof geometries, and depths of the source magma chambers, in combination with prior volcanic and regional tectonic influences. Based largely on a review of features at eroded pre-Quaternary calderas, a continuum of geometries and subsidence styles is inferred to exist, in both island-arc and continental settings, between small funnel calderas and larger plate (piston) subsidences bounded by arcuate faults. Within most ring-fault calderas, the subsided block is variably disrupted, due to differential movement during ash-flow eruptions and postcollapse magmatism, but highly chaotic piecemeal subsidence appears to be uncommon for large-diameter calderas. Small-scale downsag structures and accompanying extensional fractures develop along margins of most calderas during early stages of subsidence, but downsag is dominant only at calderas that have not subsided deeply. Calderas that are loci for multicyclic ash-flow eruption and subsidence cycles have the most complex internal structures. Large calderas have flared inner topographic walls due to landsliding of unstable slopes, and the resulting slide debris can constitute large proportions of caldera fill. Because the slide debris is concentrated near caldera walls, models from geophysical data can suggest a funnel geometry, even for large plate-subsidence calderas bounded by ring faults. Simple geometric models indicate that many large calderas have subsided 3–5 km, greater than the depth of most naturally exposed sections of intracaldera deposits. Many ring-fault plate-subsidence calderas and intrusive ring complexes have been recognized in the western U.S., Japan, and elsewhere, but no well-documented examples of exposed eroded calderas have large-scale funnel geometry or chaotically disrupted caldera floors. Reported ignimbrite "shields" in the central Andes, where large-volume ash-flows are inferred to have erupted without caldera collapse, seem alternatively interpretable as more conventional calderas that were filled to overflow by younger lavas and tuffs. Some exposed subcaldera intrusions provide insights concerning subsidence processes, but such intrusions may continue to evolve in volume, roof geometry, depth, and composition after formation of associated calderas.

Australopithecus garhi: A New Species of Early Hominid from Ethiopia

Abstract: The lack of an adequate hominid fossil record in eastern Africa between 2 and 3 million years ago (Ma) has hampered investigations of early hominid phylogeny. Discovery of 2.5 Ma hominid cranial and dental remains from the Hata beds of Ethiopia's Middle Awash allows recognition of a new species of Australopithecus. This species is descended from Australopithecus afarensis and is a candidate ancestor for early Homo. Contemporary postcranial remains feature a derived humanlike humeral/femoral ratio and an apelike upper arm-to-lower arm ratio.