A densely feathered ornithomimid (Dinosauria: Theropoda) from the Upper Cretaceous Dinosaur Park Formation,Alberta, Canada     

《Cretaceous Research》2016

A recently discovered articulated partial skeleton of Ornithomimus from the Upper Cretaceous Dinosaur Park Formation of Alberta, Canada is remarkable in the extent and quality of preservation of integumentary structures including feathers. It is the first ornithomimid to preserve a tail bearing extensive plumaceous feathers that are slightly more elongate in comparison to those present on the remainder of the body. However, the underside of the tail and the hind limb distal to the middle of the femur appear devoid of plumage. Overall, the plumage pattern in Ornithomimus is similar to that of Struthio camelus (ostrich) and other large palaeognaths, indicating a probable function in thermoregulation. The specimen also preserves the body outline around the legs, including a skin contour anterior to the femur, analogous to skin webs in extant birds. Whereas the knee web of birds bridges the knee to the abdomen, in Ornithomimus it spans from the mid-femoral shaft to the abdomen, and is herein referred to as an anterior femoral web. This is the first report of such soft tissue structures in non-avian theropods. It may indicate that the resting position of the femur was positioned more anteroventrally in ornithomimids than in most theropods, and in that sense may have been transitional to the situation in modern birds.  相似文献   

On the determination of the mesh size for numerical simulations of shock wave propagation in near field underwater explosion     

《Applied Ocean Research》2016

It is well known that the accuracy of mesh-based numerical simulations of underwater explosion strongly relies on the mesh size adopted in the analyses. Although a numerical analysis of underwater explosion can be performed with enough accuracy by using considerably fine meshes, such fine meshes may lead to substantially increase in the CPU time and the usage of computer memory. Thus, how to determine a suitable mesh size in numerical simulations is always a problem confronted when attempting to study the shock wave propagation resulting from underwater explosion and the subsequent response of structures. Considering that there is currently no universally accepted method for resolving this problem, this paper aims to propose a simple method to determine the mesh size for numerical simulations of near field underwater explosion. To this end, the mesh size effects on the shock wave propagation of underwater explosion are carefully investigated for different charge weights, through which the correlation between mesh sizes and charge weights is identified. Based on the numerical study, a dimensionless variable (λ), defined as the ratio of the radius of charge to the side length of element, is introduced to be the criterion for determining the mesh size in simulations. It is interesting to note that the presented method is suitable for various charge weights. By using the proposed meshing rule, adequate balance between solution accuracy and computational efficiency can be achieved for different blast scenarios in numerical simulations of underwater explosion.  相似文献   

Man and Nature: George Perkins Marsh and Alexander Von Humboldt          下载免费PDF全文

Andrea Wulf 《Geographical review》2017,107(4):593-607

George Perkins Marsh's book Man and Nature was the first work of natural history to fundamentally influence American politics. This paper establishes the powerful impact that Alexander von Humboldt's writings had on Marsh. Marsh took ideas that Humboldt introduced across his books and synthesized them into a single powerful argument regarding the dangers of deforestation. These warnings eventually led to policies that sought to more carefully manage forestland, plant trees, and spawn the 20^th century conservation movement.  相似文献   

文蛤消化系统的形态学研究   总被引：8，自引：0，他引：8  

应雪萍  张永普  宋晓东  蔡希武  陈鸟 《海洋湖沼通报》2001,(2):60-67

文蛤消化系统包括唇瓣、口、食道、胃、晶杆、肠、肛门以及肝脏等部分，无唾液腺和食道腺，也无齿舌，其消化管壁的结构与一般软体动物的消化管壁基本相同，主要由粘膜层、粘膜下层、肌层及外膜组成，但肌层很薄。本文对文蛤消化系统各部分的解剖和组织结构进行了较详细的描述，并利用组织化学方法测定晶杆与肝脏中的几种成分。  相似文献   

An Introduction to Observations Relevant to Astrophysical Jets and Nebulae     

P. Hartigan 《Astrophysics and Space Science》2007,307(1-3):3-6

This article reviews the basic physics and jargon associated with astronomical observations of nebulae, with an emphasis on processes relevant to shock waves in astrophysical jets.  相似文献   

New evidence for strong nonthermal effects in Tycho’s supernova remnant     

H. J. Völk  E. G. Berezhko  L. T. Ksenofontov 《Astrophysics and Space Science》2007,309(1-4):389-394

For the case of Tycho’s supernova remnant (SNR) we present the relation between the blast wave and contact discontinuity radii calculated within the nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. It is demonstrated that these radii are confirmed by recently published Chandra measurements which show that the observed contact discontinuity radius is so close to the shock radius that it can only be explained by efficient CR acceleration which in turn makes the medium more compressible. Together with the recently determined new value E _sn=1.2×10⁵¹ erg of the SN explosion energy this also confirms our previous conclusion that a TeV γ-ray flux of (2–5)×10⁻¹³ erg/(cm² s) is to be expected from Tycho’s SNR. Chandra measurements and the HEGRA upper limit of the TeV γ-ray flux together limit the source distance d to 3.3≤d≤4 kpc.  相似文献   

Hydrodynamics of Supernova Evolution in the Winds of Massive Stars     

Vikram V. Dwarkadas 《Astrophysics and Space Science》2007,307(1-3):153-158

Core-Collapse supernovae arise from stars greater than 8 M_⊙. These stars lose a considerable amount of mass during their lifetime, which accumulates around the star forming wind-blown bubbles. Upon the death of the star in a spectacular explosion, the resulting SN shock wave will interact with this modified medium. We study the evolution of the shock wave, and investigate the properties of this interaction. We concentrate on the evolution of the SN shock wave in the medium around a 35 solar mass star. We discuss the hydrodynamics of the resulting interaction, the formation and growth of instabilities, and deviations from sphericity.  相似文献   

Obtaining a social licence for MPAs – influences on social acceptability     

《Marine Policy》2015

The biological success of Marine Protected Areas (MPAs) depends to a large extent on their social acceptability, sometimes referred to as a social licence. Local resistance has slowed international progress towards a global network of MPAs. The causes of local resistance and limited social acceptability are poorly known, which constrains the development of new planning paradigms that could address these issues. Two case studies in New South Wales, Australia determined the factors that influenced community attitudes towards MPAs. The Port Stephens-Great Lakes Marine Park (PSGLMP) and Batemans Marine Park (BMP) underwent virtually identical and concurrent planning processes, however resistance to the BMP was more intense and sustained. Differences in the demographics, history, local media coverage and social impacts of each marine park contributed to these different community responses. The BMP demonstrated the ‘perfect storm’ of opposition triggers – a community struggling in the transition away from a primary production economy, a highly politicised media dominated by powerful elites with ideological objections to the park, and social impacts sufficiently profound to motivate local citizens to support an active campaign against the park. These impacts included loss of access, identity and increased competition for resources. This research points to the importance of developing a deeper understanding of the social, cultural and political landscape of the communities in which MPAs are proposed and a rethink of planning processes to better incorporate community objectives and knowledge.  相似文献   

《金史·地理志》城市行政建制疏漏及补正研究     

韩光辉  魏丹  何文林 《地理学报》2012,67(10):1362-1374

唐末五代以来,城市行政管理机构发生了重要变化,这就是金代明确出现的具有独立行政职能,但等级规模不同的城市行政管理机构警巡院、录事司和司候司.作为金代与县平行的城市行政管理机构,其官制在《金史·百官志》中具有明确记载,但与县平行的城市行政建置却被《金史·地理志》疏漏,这一问题值得学界研究并加以补正.  相似文献   

Statistical Analysis of the Relationships among Coronal Holes,Corotating Interaction Regions,and Geomagnetic Storms     

Yunhee Choi  Y.-J. Moon  Seonghwan Choi  Ji-Hye Baek  Sungsoo S. Kim  K.-S. Cho  G. S. Choe 《Solar physics》2009,254(2):311-323

We have examined the relationships among coronal holes (CHs), corotating interaction regions (CIRs), and geomagnetic storms in the period 1996?–?2003. We have identified 123 CIRs with forward and reverse shock or wave features in ACE and Wind data and have linked them to coronal holes shown in National Solar Observatory/Kitt Peak (NSO/KP) daily He i 10?830 Å maps considering the Sun?–?Earth transit time of the solar wind with the observed wind speed. A sample of 107 CH?–?CIR pairs is thus identified. We have examined the magnetic polarity, location, and area of the CHs as well as their association with geomagnetic storms (Dst≤?50 nT). For all pairs, the magnetic polarity of the CHs is found to be consistent with the sunward (or earthward) direction of the interplanetary magnetic fields (IMFs), which confirms the linkage between the CHs and the CIRs in the sample. Our statistical analysis shows that (1) the mean longitude of the center of CHs is about 8°E, (2) 74% of the CHs are located between 30°S and 30°N (i.e., mostly in the equatorial regions), (3) 46% of the CIRs are associated with geomagnetic storms, (4) the area of geoeffective coronal holes is found to be larger than 0.12% of the solar hemisphere area, and (5) the maximum convective electric field E _y in the solar wind is much more highly correlated with the Dst index than any other solar or interplanetary parameter. In addition, we found that there is also a semiannual variation of CIR-associated geomagnetic storms and discovered new tendencies as follows: For negative-polarity coronal holes, the percentage (59%; 16 out of 27 events) of CIRs associated with geomagnetic storms in the first half of the year is much larger than that (25%; 6 out of 24 events) in the second half of the year and the occurrence percentage (63%; 15 out of 24 events) of CIR-associated storms in the southern hemisphere is significantly larger than that (26%; 7 out of 27 events) in the northern hemisphere. Positive-polarity coronal holes exhibit an opposite tendency.  相似文献