共查询到20条相似文献，搜索用时 640 毫秒
1.
The separation of the middle ear bones from the mandible is considered a defining feature of all modern mammals. But did this event happen once in a primitive mammalian ancestor or independently in the monotreme lineage and therian (marsupial and placental) lineage? As Martin and Luo discuss in their Perspective, a new fossil-the dentary bone of an ancient toothed monotreme-suggests that the middle ear bones formed independently in these two mammalian lineages, providing a remarkable example of homoplastic evolution. 相似文献
2.
An ossified Meckel's cartilage has been recovered from two early Cretaceous mammals from China. This element is similar to Meckel's cartilage in prenatal and some postnatal extant mammals and indicates the relationship of Meckel's cartilage with the middle ear in early mammals. The evidence shows that brain expansion may not be the initial factor that caused the separation of postdentary bones from the dentary as middle ear ossicles during mammalian evolution. The failure of the dentary to seize reduced postdentary elements during ontogeny of early mammals is postulated as an alternative mechanism for the separation. Modifications of both feeding and hearing apparatuses in early mammals may have led to the development of the definitive mammalian middle ear. 相似文献
3.
A fossil from the Early Jurassic (Sinemurian, approximately 195 million years ago) represents a new lineage of mammaliaforms, the extinct groups more closely related to the living mammals than to nonmammaliaform cynodonts. It has an enlarged cranial cavity, but no postdentary trough on the mandible, indicating separation of the middle ear bones from the mandible. This extends the earliest record of these crucial mammalian features by some 45 million years and suggests that separation of the middle ear bones from the mandible and the expanded brain vault could be correlated. It shows that several key mammalian evolutionary innovations in the ear region, the temporomandibular joint, and the brain vault evolved incrementally through mammaliaform evolution and long before the differentiation of the living mammal groups. With an estimated body weight of only 2 grams, its coexistence with other larger mammaliaforms with similar "triconodont-like" teeth for insectivory within the same fauna suggests a great trophic diversity within the mammaliaform insectivore feeding guild, as inferred from the range of body sizes. 相似文献
4.
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
5.
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
6.
New specimens of middle Eocene Basilosaurus isis from Egypt include the first functional pelvic limb and foot bones known in Cetacea. These are important in corroborating the intermediate evolutionary position of archaeocetes between generalized Paleocene land mammals that used hind limbs in locomotion and Oligocene-to- Recent whales that lack functional pelvic limbs. The foot is paraxonic, consistent with derivation from mesonychid Condylarthra. Hind limbs of Basilosaurus are interpreted as copulatory guides. 相似文献
7.
Many mammalian species rely on pheromones-semiochemicals produced by other members of the same species-to communicate social status and reproductive readiness. To assess how the central nervous system integrates the complex repertoire of pheromones, we recorded from single neurons in the accessory olfactory bulb, a nucleus that processes pheromonal signals, of male mice engaged in natural behaviors. Neuronal firing was robustly modulated by physical contact with male and female conspecifics, with individual neurons activated selectively by specific combinations of the sex and strain of conspecifics. We infer that mammals encode social and reproductive information by integrating vomeronasal sensory activity specific to sex and genetic makeup. 相似文献
8.
Scaling body support in mammals: limb posture and muscle mechanics 总被引：12，自引：0，他引：12
A A Biewener 《Science (New York, N.Y.)》1989,245(4913):45-48
The scaling of bone and muscle geometry in mammals suggests that peak stresses (ratio of force to cross-sectional area) acting in these two support elements increase with increasing body size. Observations of stresses acting in the limb bones of different sized mammals during strenuous activity, however, indicate that peak bone stress is independent of size (maintaining a safety factor of between 2 and 4). It appears that similar peak bone stresses and muscle stresses in large and small mammals are achieved primarily by a size-dependent change in locomotor limb posture: small animals run with crouched postures, whereas larger species run more upright. By adopting an upright posture, large animals align their limbs more closely with the ground reaction force, substantially reducing the forces that their muscles must exert (proportional to body mass) and hence, the forces that their bones must resist, to counteract joint moments. This change in limb posture to maintain locomotor stresses within safe limits, however, likely limits the maneuverability and accelerative capability of large animals. 相似文献
9.
薏苡(Coix)作为我国最早栽培的作物品种,这一事实可从许多证据得到证考;甲骨文作为最早记载人类活动的文字之一,不仅推动了人类文明的进程,也最早记载了作物产量株的构成。文章通过大量文献资料对这一事实进行了判定及证考,推定出了甲骨文记载的作物产量公式的要素,即密度(穗数)、穗粒数、籽粒重(千粒重),是最早记载作物产量构成公式的三要素,鉴于篇幅所限,本文只讨论密度(株、穗数)之内容。 相似文献
10.
Dentary-squamosal joint and the origin of mammals 总被引：1，自引：0，他引：1
The dentary-squamosal jaw joint evolved more than once in advanced cynodont therapsids or their descendants, probably as a buttress against the reaction force created at the articulation by the adductor jaw musculature. The multiple origin of this joint suggests that additional criteria are required to separate early mammals from advanced therapsids. 相似文献
11.
Spontaneous middle ear muscle activity in man: a rapid eye movement sleep phenomenon 总被引：1，自引：0，他引：1
Changes in compliance of the tympanic membrane have been detected in normal human sleep, presumably due to spontaneous contraction of the stapedius and tensor tympani muscles of the middle ear. In the waking state, these muscles generally respond to loud sound (middle ear reflex). Middle ear muscle activity typically erupts before or at the onset of rapid eye movement (REM) sleep and persists throughout the REM period in a discontinuous pattern resembling that exhibited by rapid eye movements. Approximately 80 percent of all nocturnal middle ear muscle activity is contained in REM sleep. Half of the remaining 20 percent occurs in the 10-minute intervals just prior to the onset of REM sleep. Middle ear muscle activity is often associated with other phasic events such as momentary enhancement of electromyogram inhibition, apnea, and K complexes. Rapid eye movements and middle ear muscle activity, though significantly correlated in REM sleep, are not always simultaneous. 相似文献
12.
K G Chandy C B Williams R H Spencer B A Aguilar S Ghanshani B L Tempel G A Gutman 《Science (New York, N.Y.)》1990,247(4945):973-975
To understand the molecular mechanisms responsible for generating physiologically diverse potassium channels in mammalian cells, mouse genomic clones have been isolated with a potassium channel complementary DNA, MBK1, that is homologous to the Drosophila potassium channel gene, Shaker. A family of three closely related potassium channel genes (MK1, MK2, and MK3) that are encoded at distinct genomic loci has been isolated. Sequence analysis reveals that the coding region of each of these three genes exists as a single uninterrupted exon in the mouse genome. This organization precludes the generation of multiple forms of the protein by alternative RNA splicing, a mechanism known to characterize the Drosophila potassium channel genes Shaker and Shab. Thus, mammals may use a different strategy for generating diverse K+ channels by encoding related genes at multiple distinct genomic loci, each of which produces only a single protein. 相似文献
13.
14.
不同播期播量对不同类型超级小麦产量构成因子的影响 总被引：24，自引：1，他引：24
以大穗型(01-35、8355)、中间型(62008、4133-2)和多穗型(988044、02-48)超级小麦品种(系)为试材,研究其适宜播期和播量。结果表明,大穗型品种最佳播期10月15日,中间型、多穗型品种最佳播期10月5日,播期对单位面积穗数的影响差别最高可达15%以上,对穗粒数、千粒重和产量的影响差别均在13%以下;播量对其各项指标的影响差别均在5%以下,3种类型的品种均以中、高密度产量较高。播期对超级小麦产量各构成因素的影响大于播量。 相似文献
15.
Biomechanics of mammalian terrestrial locomotion 总被引：9，自引：0，他引：9
A A Biewener 《Science (New York, N.Y.)》1990,250(4984):1097-1103
Mammalian skeletons experience peak locomotor stresses (force per area) that are 25 to 50% of their failure strength, indicating a safety factor of between two and four. The mechanism by which animals achieve a constant safety factor varies depending on the size of the animal. Over much of their size (0.1 to 300 kilograms), larger mammals maintain uniform skeletal stress primarily by having a more upright posture, which decreases mass-specific muscle force by increasing muscle mechanical advantage. At greater sizes, increased skeletal allometry and decreased locomotor performance likely maintain stresses constant. At smaller sizes, skeletal stiffness may be more critical than strength. The decrease in mass-specific muscle force in mammals weighing 0.1 to 300 kilogram indicates that peak muscle stresses are also constant and correlates with a decrease in mass-specific energy cost of locomotion. The consistent pattern of locomotor stresses developed in long bones at different speeds and gaits within a species may have important implications for how bones adaptively remodel to changes in stress. 相似文献
16.
测定了大麦不同穗数株及5个品种（系）不同粒位（顶部、中部和底部）籽粒的粒重、蛋白质含量和发芽率。结果表明：不同穗数株及品种（系）各粒位间籽粒千粒重的差异表现相同的趋势，即中部最大，顶部和底部较小。不同穗数株主茎穗各粒位籽粒蛋白质含量的最大值，1穗株为顶部，2穗株为中部，3穗株则为底部；各品种（系）主茎穗和分蘖穗不同粒位蛋白含量加权平均值均以秀麦3号为最大，97－33最小，且主茎穗各粒位蛋白含量以顶部为最高，不同穗数株各粒位籽粒发芽速度均以顶部最快，中部和底部相对较慢；品系99－18的发芽整齐度最好，吕系92－11发芽最慢且发芽率最低。 相似文献
17.
黑龙江省玉米品种农艺性状与单株粒重的灰色关联分析 总被引：1，自引：0，他引：1
选取黑龙江省中、晚熟玉米对照品种及杂交组合共160份,采用灰色关联分析方法,对影响玉米产量(单株粒重)的主要农艺性状进行研究。结果表明,与玉米单株粒重相关的农艺性状中,穗粗、百粒重和穗长最为重要,整体关联度排序为:穗粗>百粒重>穗长>生育期>行粒数>吐丝期>散粉期>穗位>株高>穗行数>出苗期>出籽率,这说明目前玉米新品种选育应以果穗粗、长,百粒重大且穗行数多为主,以其他农艺性状适中为目标,协调各相关因素,提高产量。本试验利用大量品种(组合)明确玉米主要农艺性状对产量的贡献,为选育高产玉米新品种提供了理论参考依据。 相似文献
18.
1984至1986年对垦区参加全省联合区域试验的早熟、中早熟、中熟三个熟期组39个组合,进行“湿果穗称重取样法”和“实收法”计算产量数据进行差异显著性测定、变量分析、回归系数显著性测定、相关系数显著性测定。两种计算产量方法精确度基本相同,在玉米产量试验中可用“湿果穗称重取样法”代替“实收法”计算产量。 相似文献
19.
20.
Asher RJ Meng J Wible JR McKenna MC Rougier GW Dashzeveg D Novacek MJ 《Science (New York, N.Y.)》2005,307(5712):1091-1094
We describe several fossils referable to Gomphos elkema from deposits close to the Paleocene-Eocene boundary at Tsagan Khushu, Mongolia. Gomphos shares a suite of cranioskeletal characters with extant rabbits, hares, and pikas but retains a primitive dentition and jaw compared to its modern relatives. Phylogenetic analysis supports the position of Gomphos as a stem lagomorph and excludes Cretaceous taxa from the crown radiation of placental mammals. Our results support the hypothesis that rodents and lagomorphs radiated during the Cenozoic and diverged from other placental mammals close to the Cretaceous-Tertiary boundary. 相似文献