共查询到20条相似文献，搜索用时 812 毫秒
1.
Radinsky L 《Science (New York, N.Y.)》1983,221(4616):1189-1191
Allometric analysis of skull proportions in 25 species of fossil equids indicates that both scaling effects (allometry) and reorganization were factors in the evolutionary transformation of horse skulls. A relatively longer preorbital portion of the skull resulted from the ventral and forward displacement of the tooth row relative to the jaw joint and the orbit when high-crowned teeth evolved. Correlated with the increased distance between jaw joint and tooth row is an increase in the relative size of the attachment areas of masseter and internal pterygoid muscles. 相似文献
2.
A S Romer 《Science (New York, N.Y.)》1969,166(907):881-882
A diagnostic mammalian character is jaw articulation between squamosal and dentary bones, replacing the quadrate-articular joint of reptiles. A newly discovered Argentinian Middle Triassic form shows, for the first time in an ancestral reptile, definite evidence of a squamosal-dentary articulation supplementary to the persistent primitive connection. 相似文献
3.
Ward PD Botha J Buick R De Kock MO Erwin DH Garrison GH Kirschvink JL Smith R 《Science (New York, N.Y.)》2005,307(5710):709-714
The Karoo basin of South Africa exposes a succession of Upper Permian to Lower Triassic terrestrial strata containing abundant terrestrial vertebrate fossils. Paleomagnetic/magnetostratigraphic and carbon-isotope data allow sections to be correlated across the basin. With this stratigraphy, the vertebrate fossil data show a gradual extinction in the Upper Permian punctuated by an enhanced extinction pulse at the Permian-Triassic boundary interval, particularly among the dicynodont therapsids, coinciding with negative carbon-isotope anomalies. 相似文献
4.
Thomson KS 《Science (New York, N.Y.)》1966,153(3739):999-1000
Analysis of the jaw mech anism of the coelacanth Latimeria shows' that the action of the intracranial artic ulation and the associated subcephalic muscles (a system unique to the Cros sopterygii) is important in increasing the angle of ihe gape and the power of the bite. Maximum rotation at the in tracranial joint is approximately 15 de grees. 相似文献
5.
Cisneros JC Abdala F Rubidge BS Dentzien-Dias PC Bueno Ade O 《Science (New York, N.Y.)》2011,331(6024):1603-1605
Anomodonts, a group of herbivorous therapsid "mammal-like reptiles," were the most abundant tetrapods of the Permian. We present a basal anomodont from South America, a new taxon that has transversally expanded palatal teeth and long saber canines. The function of the saber teeth is unknown, but probable uses include deterring attack from predators and intraspecific display or combat. The complex palatal teeth were used to process high-fiber food and represent early evidence of dental occlusion in a therapsid. This discovery provides new insight into the evolution of heterogeneous dentition in therapsids and broadens our understanding of ecological interactions at the end of the Paleozoic. 相似文献
6.
A new fossil of the primate Amphipithecus mogaungensis Colbert from the late Eocene of Burma shows that this species has a mandibular and molar morphology very similar to Oligocene and post-Oligocene higher primates. It has an exceptionally deep jaw. Its brachybunodont first and second molars have smooth enamel but lack hypoconulids. The shape of its second molar is nearly square-an advanced higher primate feature. Amphipithecus mogaungensis and related taxon Pondaungia cotteri Pilgrim are the earliest known higher primates. They suggest that Southeast Asia was an early theater of higher primate diversification. 相似文献
7.
本研究旨在揭示斑马鱼下颌再生中神经和肌肉的关系。通过HE染色技术、免疫荧光组织化学等实验技术分别观察斑马鱼下颌神经和肌肉的再生过程,考察神经和肌肉的关系。研究结果显示,截面附近的神经纤维在剪切后5 h降解,剪切后2 d时完全消失。剪切后3 d,整个芽基所在位置已出现新生的神经纤维。剪切5 d时,芽基上布满大量的神经纤维。肌纤维在剪切后12 h已发生断裂,剪切后5 d形成芽基细胞并分化成各种前体细胞如肌肉前体细胞,剪切后9 d出现新生的肌细胞,剪切后14 d新生的肌细胞与新生的神经纤维建立神经肌肉接头联系,30 d后成为成熟的肌肉组织。结果表明:如果斑马鱼1/3的下颌被人为切除,下颌肌肉可以实现功能性再生,同时神经极可能在肌肉再生中发挥作用。但是如果切除部分超过1/3,则下颌无法再生复原,其原因有待进一步研究。 相似文献
8.
王景欣 《东北林业大学学报》1988,(1)
本文采用了三角形单元对抓爪进行了离散,在格楞型抓取工况下计算了在外力函数作用下抓爪的结点应力。其计算结果与实测结果一致。最后分析了抓爪的应力分布。 相似文献
9.
Comparative analyses of behavior have an underappreciated potential for revealing the role of ethoecological factors in the origins of higher taxa. Twenty-seven species (13 genera) in the advanced family Colubridae exhibited 19 patterns of coil application; one or two patterns were usually consistent within a genus. Forty-eight species (26 genera) in the primitive families Acrochordidae, Aniliidae, Boidae, and Xenopeltidae usually used a single pattern, despite differences in age, size, shape, habitat, and diet. This implies the shared retention of an action pattern used by their common ancestor no later than the early Paleocene. Constriction must have been used as a prey-killing tactic very early in the history of snakes and might have been a behavioral "key innovation" in the evolution of their unusual jaw mechanism. 相似文献
10.
夹爪结构的设计与分析 总被引：2，自引：2，他引：0
为了加强夹爪的结构,提高其承载能力,使用Solid Works软件建立夹爪的三维模型,并分析夹爪满载时液压缸和钳口的受力情况,阐述模型静力学分析的边界条件.通过Hyper Works软件对模型进行有限元分析,对钳口的强度进行校验.通过分析钳口的应力云图,对模型进行改进和重新设计,并对新模型进行验证.结果表明,改进模型2(增大侧板外弧半径)的结构设计比改进模型1(增加侧板厚度)的更加合理. 相似文献
11.
Knowledge of early mammalian evolution has been based on Old World Late Triassic-Early Jurassic faunas. The discovery of mammalian fossils of approximately equivalent age in the Kayenta Formation of northeastern Arizona gives evidence of greater diversity than known previously. A new taxon documents the development of an angular region of the jaw as a neomorphic process, and represents an intermediate stage in the origin of mammalian jaw musculature. 相似文献
12.
陈松波 《仲恺农业技术学院学报》1990,(1)
在食品加工中,各单元操作已使用各种形式的操作单机。如和面机、搅拌机、打蛋机、绞肉机等。目前,像饮食行业和职工食堂等部门,由于加工批量不大,采用单机操作,则机械利用率低,且投资大,管理和使用不方便。如果用一台具有多种功能的加工机代替各种单机,是有一定的经济和实用价值的。因此,我国已开始研制多种功能食品加工机。为了交流信息,加速多功能食品加工机的发展,本文介绍一种具有和面、拌肉馅、打蛋花、打奶酪、绞肉末、挤面条等多种功能的食品机。 并扼要说明该机的整体结构和传动系统,及进行不同作业时所选用的各种附件的结构和工作原理。 相似文献
13.
为有效减小复摆式破碎机动颚衬板磨损,可通过降低动颚运动特性值实现.为提高颚式破碎机工作性能,以PE250×400型颚式破碎机为优化对象,以动颚运动特性值最小作为优化设计目标,以各构件基本尺寸作为优化设计变量,建立包含构件尺寸、工作参数及腔形参数的约束条件,构建优化设计数学模型,采用遗传算法对数学模型进行优化,优化后的动颚运动特性值降低13.84%,同时生产率提高12.73%,满足颚式破碎机的优化设计要求,为颚式破碎机优化设计从理论上提供了方法指导. 相似文献
14.
高校校办产业模式辨析 总被引：1，自引：0，他引：1
本文对新兴的高校校办产业模式进行了较系统的分析，归纳出自我发展、联合开发、集团化、转让开发及建立高科技园区等几种模式。阐明了校办产业在依托本校科技优势，正确处理社会需求与社会限制、主动开拓与被动适应矛盾等方面的共性特点，从中得出推动校办产业发展的有益启迪。 相似文献
15.
16.
Shigetani Y Sugahara F Kawakami Y Murakami Y Hirano S Kuratani S 《Science (New York, N.Y.)》2002,296(5571):1316-1319
Genes involved in late specification of the mandibular arch, the source of the vertebrate jaw, are expressed with similar patterns in the oral regions of chick and lamprey embryos. However, morphological comparisons indicate that apparently orthologous homeobox genes were expressed in different subdivisions of the ectomesenchyme in the two species. Therefore, the homology and gene expression of the oral region are uncoupled during the transition from agnathan to gnathostome; we conclude that a heterotopic shift of tissue interaction was involved in the evolution of the jaw. 相似文献
17.
Ants of the ponerine genus Odontomachus use a trap jaw mechanism when hunting fast prey. When particular trigger hairs, located on the inner edge of the mandibles, are touched by prey, the jaws close extremely rapidly and trap the target. This trap jaw response lasts only 0.33 to 1 millisecond. Electrophysiological recordings demonstrated that the trigger hairs function as mechanoreceptors. Associated with each trigger hair are large sensory cells, the sensory axons of which measure 15 to 20 micrometers in diameter. These are among the largest sensory neurons, and their size implies that these axons conduct information very rapidly. 相似文献
18.
Rich TH Hopson JA Musser AM Flannery TF Vickers-Rich P 《Science (New York, N.Y.)》2005,307(5711):910-914
A dentary of the oldest known monotreme, the Early Cretaceous Teinolophos trusleri, has an internal mandibular trough, which in outgroups to mammals houses accessory jaw bones, and probable contact facets for angular, coronoid, and splenial bones. Certain of these accessory bones were detached from the mandible to become middle ear bones in mammals. Evidence that the angular (homologous with the mammalian ectotympanic) and the articular and prearticular (homologous with the mammalian malleus) bones retained attachment to the lower jaw in a basal monotreme indicates that the definitive mammalian middle ear evolved independently in living monotremes and therians (marsupials and placentals). 相似文献
19.
对一只死亡的野生雌性大熊猫进行了系统的病理学检查。主要病变：右侧下颌有一洞创深达皮下结缔组织；咽部食道与颈椎间有一形态不规则的大脓肿，颈下结缔组织中散布多个小脓肿，脓液灰白糊状；颌下、肺门等处淋巴结肿大化脓；右后肢腓骨和右侧第９，１０肋骨呈未愈合陈旧性骨折；肝门区有两个已钙化脓肿，肝细胞及间质局灶性变性坏死，少量炎细胞浸润；化脓性支气管肺炎，颈下皮下蜂窝织炎；多种组织器官内可见球菌样物质和微血栓。病理学诊断为脓毒败血性休克。 相似文献
20.
杉木不同水平试验林的遗传参数估算和高世代育种的亲本评选 总被引：2，自引：0，他引：2
利用杉木种源试验、杉木半同胞子代自由授粉试验和杉木全同胞子代试验3类共6片试验林中材积、树高、胸径、枝下高、枝高比和木芯密度共6个性状的资料,采用转化分析原理,进行方差分析以获得各种参数,在此基础上采用主成分指数和基本指数公式,按较大的入选率进行逆向选择,以获得若干优良种源和优良家系(半同胞或全同胞),然后采用优良家系... 相似文献