Home About us Contact
|
Home About us Contact | ||
|
|
||
Size Shows (size + shows)
Selected AbstractsGEOMETRIC MORPHOMETRICS OF THE SKULL ROOF OF STEREOSPONDYLS (AMPHIBIA: TEMNOSPONDYLI)PALAEONTOLOGY, Issue 2 2006C. TRISTAN STAYTON Abstract:, Geometric morphometric analysis using relative warps is applied to the skull roof of 62 species of stereospondyls and their closest outgroups (i.e. basal archegosauriforms) from among temnospondyl amphibians. Twenty-one landmarks and five taxonomic groups are used for comparisons. Their skull evolution is quantified in a morphospace defined by two relative warps axes. The majority of groups show poor concordance between morphological and phylogenetic distances. The only exception is represented by Yates and Warren's study of stereospondyl relationships, in which concordance is high. Only basal archegosauriforms and rhinesuchids show significant overlap in morphospace, although this might be due to low sample sizes. Regression of estimated mean disparity against taxon sample size shows that species within both the trematosauroid and the rhytidostean groups are more widely dispersed in morphospace than species belonging to any of the remaining stereospondyl groups. Stereospondyl skull evolution was characterized by divergence between major clades and convergence within those clades. Changes in patterns of morphospace occupation through time agree with the hypothesis of an ,explosive' radiation in the early Early Triassic, after the extinction of basal archegosauriforms at the end of the Permian. [source] Human cranial anatomy and the differential preservation of population history and climate signaturesTHE ANATOMICAL RECORD : ADVANCES IN INTEGRATIVE ANATOMY AND EVOLUTIONARY BIOLOGY, Issue 12 2006Katerina Harvati Abstract Cranial morphology is widely used to reconstruct evolutionary relationships, but its reliability in reflecting phylogeny and population history has been questioned. Some cranial regions, particularly the face and neurocranium, are believed to be influenced by the environment and prone to convergence. Others, such as the temporal bone, are thought to reflect more accurately phylogenetic relationships. Direct testing of these hypotheses was not possible until the advent of large genetic data sets. The few relevant studies in human populations have had intriguing but possibly conflicting results, probably partly due to methodological differences and to the small numbers of populations used. Here we use three-dimensional (3D) geometric morphometrics methods to test explicitly the ability of cranial shape, size, and relative position/orientation of cranial regions to track population history and climate. Morphological distances among 13 recent human populations were calculated from four 3D landmark data sets, respectively reflecting facial, neurocranial, and temporal bone shape; shape and relative position; overall cranial shape; and centroid sizes. These distances were compared to neutral genetic and climatic distances among the same, or closely matched, populations. Results indicate that neurocranial and temporal bone shape track neutral genetic distances, while facial shape reflects climate; centroid size shows a weak association with climatic variables; and relative position/orientation of cranial regions does not appear correlated with any of these factors. Because different cranial regions preserve population history and climate signatures differentially, caution is suggested when using cranial anatomy for phylogenetic reconstruction. Anat Rec Part A, 2006. © 2006 Wiley-Liss, Inc. [source] Phytogeographical evidence for post-glacial dispersal limitation of European beech forest speciesECOGRAPHY, Issue 6 2009Wolfgang Willner The post-glacial migration of European beech Fagus sylvatica has been addressed by many studies using either genetic or fossil data or a combination of both. In contrast to this, only little is known about the migration history of beech forest understorey species. In a review of phytosociological literature, we identified 110 plant species which are closely associated with beech forest. We divided the distribution range of European beech forests into 40 geographical regions, and the presence or absence of each species was recorded for each region. We compared overall species numbers per region and numbers of narrow-range species (species present in <10 regions). A multiple regression model was used to test for the explanatory value of three potential diversity controls: range in elevation, soil type diversity, and distance to the nearest potential refuge area. A hierarchical cluster analysis of the narrow-range species was performed. The frequency of range sizes shows a U-shaped distribution, with 42 species occurring in <10 regions. The highest number of beech forest species is found in the southern Alps and adjacent regions, and species numbers decrease with increasing distance from these regions. With only narrow-range species taken into consideration, secondary maxima are found in Spain, the southern Apennines, the Carpathians, and Greece. Distance to the nearest potential refuge area is the strongest predictor of beech forest species richness, while altitudinal range and soil type diversity had little or no predictive value. The clusters of narrow-range species are in good concordance with the glacial refuge areas of beech and other temperate tree species as estimated in recent studies. These findings support the hypothesis that the distribution of many beech forest species is limited by post-glacial dispersal rather than by their environmental requirements. [source] The Mystery of Perpendicular Fivefold Axes and the Fourth Dimension in Intermetallic StructuresCHEMISTRY - A EUROPEAN JOURNAL, Issue 13 2008Robert Abstract The structures of eight related known intermetallic structure types are the impetus to this paper: Li21Si5, Mg44Rh7, Zn13(Fe,Ni)2, Mg6Pd, Na6Tl, Zn91Ir11, Li13Na29Ba19, and Al69Ta39. All belong to the F3m space group, have roughly 400 atoms in their cubic unit cells, are built up at least partially from the ,-brass structure, and exhibit pseudo-tenfold symmetric diffraction patterns. These pseudo-tenfold axes lie in the ,1,1,0, directions, and thus present a paradox. The ,1,1,0, set is comprised of three pairs of perpendicular directions. Yet no 3D point group contains a single pair of perpendicular fivefold axes (by Friedel's Law, a fivefold axis leads to a tenfold diffraction pattern). The current work seeks to resolve this paradox. Its resolution is based on the largest of all 4D Platonic solids, the 600-cell. We first review the 600-cell, building an intuition discussing 4D polyhedroids (4D polytopes). We then show that the positions of common atoms in the F3m structures lie close to the positions of vertices in a 3D projection of the 600-cell. For this purpose, we develop a projection method that we call intermediate projection. The introduction of the 600-cell resolves the above paradox. This 4D Platonic solid contains numerous orthogonal fivefold rotations. The six fivefold directions that are best preserved after projection prove to lie along the ,1,1,0, directions of the F3m structures. Finally, this paper shows that at certain ideal projected cluster sizes related to one another by the golden mean (,=(1+,5)/2), constructive interference leading to tenfold diffraction patterns is optimized. It is these optimal values that predominate in actual F3m structures. Explicit comparison of experimental cluster sizes and theoretically derived cluster sizes shows a clear correspondence, both for isolated and crystalline pairs of projected 600-cells. [source] | ||||||||||