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71.
Abstract The well preserved cranium of Protodobenus japonicus, a new genus and species of odobenine walrus, is from the lower part of the Tamugigawa Formation at Ooshima-mura, Higashi Kubiki-gun, Niigata Prefecture, central Japan. The lower part of the formation that yielded P. japonicus is Early Pliocene, dating approximately from 5.0 to 4.9 Ma. The skull of P. japonicus is generally shaped like that of the modern walrus, Odobenus rosmarus (Linnaeus, 1758), but is much more primitive, especially in the dentition. Protodobenus japonicus is derived from the Imagotariinae. It is similar to such primitive fossil walruses as Prorosmarus alleni from the Western Atlantic and Aivukus cedrosensis from Baja California, and has some similarities as well as significant differences from them. Protodobenus japonicus could have evolved directly into Odobenus in the North Pacific Ocean, and could have dispersed directly to the Arctic Ocean. This is contrary to the scenario proposed by Repenning and Tedford, in which primitive odobenids, like Aivukus cedrosensis, migrated to the Atlantic Ocean through the Central American Seaway and returned to the North Pacific Ocean as O rosmarus via the Arctic about 0.6 Ma. Regarding the first lower premolar of the living walrus, Fay concluded after studying fetal O. rosmarus that the first premolar (P1) of the mandible is absent, and that the lower tooth row consists of: C1 P2, P3, P4. Protodobenus japonicus has in its mandible I 1-2 C1 P2, P3, P4, and this is the primitive pinniped formula. Pinnipeds typically lose the molars, not the anterior premolars, and the present author believes that the correct tooth formula for O rosmarus is C1 P1, P2, P3. Its skull, palate, and mandible suggest that P. japonicus had morphology pre-adapted to an early stage of benthic suction feeding, but it probably was unable to rely on that method of feeding. The discovery of P. japonicus demonstrates that the odobenine lineage (true walruses) goes back in time at least 5 Ma. Other more aberrant odobenine walruses, such as Aivukus and Prorosmarus, and pseudo-walruses of the subfamily Dusignathinae, are more distantly removed from the lineage leading to living walruses than has previously been suspected. 相似文献
72.
The influence of the attenuation model used in seismic hazard assessment in terms of intensity and acceleration is studied. For two sites in central Italy, the catalogue of the actual observed intensities during the last three centuries has been recovered. In the study region, the data collected during a recent seismic sequence give the basis for relating intensity and acceleration. The results show the importance of establishing statistical relationships among the used quantities, based on a representative set of data. 相似文献
73.
冀中中奥陶统高频率旋回层序的基本特征 总被引:2,自引:0,他引:2
王立峰 《沉积与特提斯地质》1994,(6)
通过野外实测剖面及路线观察,河北中部中奥陶统马家沟群的高频率旋回可划分为三个级别,并与层序地层学术语可以对比,由低到高为;六级旋回──韵律层;五级旋回──准层序;四级旋回──准层序组。这些由米兰柯维奇驱动力所控制的旋回层序在不同的沉积环境中具有不同岩石组合特征,反映了高频率旋回的沉积模式。 相似文献
74.
南岭成矿带是太平洋及印度洋板块对中国大陆板块联合作用的结果。囊括大量稀有-有色金属矿床的中心弧形构造-岩浆岩系正是这个“钳式”应力场作用的集中体现,“深部叠加-重熔”——以板块持续运动和相对高速率运动产生并贮集能量为前提,以俯冲板块不断带入新的挥发物质为条件,随着动力热和挥发组分的不断回流—汇集—贮集而使仰冲板块深部预热的原地或途中固化-半固化的先期熔融的岩浆岩发生了二次甚至多次的熔融作用。 相似文献
75.
将中国沿海省区17个中心城市服务业水平分为全面发展型、发达型、次发达型和欠发达型。通过服务业以及服务业与城市之间的关系分析得出:发达型、次发达型服务业的人均服务产品占有量与增加值比重以及就业比重均呈正相关关系,且相关性随着服务业绝对水平的提高而增强;欠发达型服务业的人均服务产品占有量与增加值比重以及就业比重均呈负相关的关系;任何水平类型的服务业的增加值比重与就业比重之间都为正相关。服务业绝对水平与城市人均GDP、城市综合竞争力、城市规模之间都呈正相关;比重规模与城市经济水平之间无明显相关性。 相似文献
76.
We infer the groundwater-level anomalies associated with a hypothetical preslip prior to the anticipated M 8 Tokai earthquake,
and evaluate the detectability of the anomalies using data from seven groundwater wells. We evaluate the detectability of
the anomalies under the following assumptions: (1) an Mw 5.5–6.5 aseismic preslip event occurs at the plate boundary in and
around the hypothetical focal zone of the Tokai earthquake; (2) the total amount of the strain step at each observation associated
with the preslip can be calculated by tensile and shear faulting based on the dislocation model; (3) a normalized strain history
associated with the preslip is defined from the results of numerical simulations based on rate- and state-dependent friction
laws; and (4) the groundwater-level anomaly prior to the earthquake is proportional to the estimated history of the strain
change associated with the preslip. We investigate the detection time of the anomaly at seven wells given an Mw 5.5, 6.0,
or 6.5 aseismic preslip at one of the 272 grid points in and around the area of the hypothetical focal zone of the Tokai earthquake.
As a result, over the time interval between 1 and 48 hours prior to the hypothetical Tokai earthquake, we are able to detect
at each of the seven wells a hypothetical Mw 6.5 preslip at 10–86 of the 272 grid points, an Mw 6 preslip at 0–19 grid points,
and an Mw 5.5 preslip at 0–5 grid points. 相似文献
77.
羌塘中部雀莫正长斑岩体特征及构造意义 总被引:2,自引:0,他引:2
雀莫正长斑央全碱含量较高且K2O>Na2O,富集大离子亲石元素,轻重稀土元素比值较高,稀土元素配分曲线呈斜率较大的右倾式,副矿物组合为磷灰石-榍石-磁铁矿,时代属喜马拉雅期,构造上属造山晚期侵入体。 相似文献
78.
该文阐述了透视坐标系的概念 ,列举了多种透视坐标网中量化信息的图解实例 ,力求进一步丰富和提高专题地图图形可视化的表现能力。 相似文献
79.
80.