Missing data in craniometrics: a simulation study

Craniometric measurements represent a useful tool for studying the differentiation of mammal populations. However, the fragility of skulls often leads to incomplete data matrices. Damaged specimens or incomplete sets of measurements are usually discarded prior to statistical analysis. We assessed the performance of two strategies that avoid elimination of observations: (1) pairwise deletion of missing cells, and (2) estimation of missing data using available measurements. The effect of these distinct approaches on the computation of inter-individual distances and population differentiation analyses were evaluated using craniometric measurements obtained from insular populations of deer micePeromyscus maniculatus (Wagner, 1845). In our simulations, Euclidean distances were greatly altered by pairwise deletion, whereas Gower’s distance coefficient corrected for missing data provided accurate results. Among the different estimation methods compared in this paper, the regression-based approximations weighted by coefficients of determination (r ²) outperformed the competing approaches. We further show that incomplete sets of craniometric measurements can be used to compute distance matrices, provided that an appropriate coefficient is selected. However, the application of estimation procedures provides a flexible approach that allows researchers to analyse incomplete data sets.     

Craniometric Data Supports Demic Diffusion Model for the Spread of Agriculture into Europe

Background

The spread of agriculture into Europe and the ancestry of the first European farmers have been subjects of debate and controversy among geneticists, archaeologists, linguists and anthropologists. Debates have centred on the extent to which the transition was associated with the active migration of people as opposed to the diffusion of cultural practices. Recent studies have shown that patterns of human cranial shape variation can be employed as a reliable proxy for the neutral genetic relationships of human populations.

Methodology/Principal Findings

Here, we employ measurements of Mesolithic (hunter-gatherers) and Neolithic (farmers) crania from Southwest Asia and Europe to test several alternative population dispersal and hunter-farmer gene-flow models. We base our alternative hypothetical models on a null evolutionary model of isolation-by-geographic and temporal distance. Partial Mantel tests were used to assess the congruence between craniometric distance and each of the geographic model matrices, while controlling for temporal distance. Our results demonstrate that the craniometric data fit a model of continuous dispersal of people (and their genes) from Southwest Asia to Europe significantly better than a null model of cultural diffusion.

Conclusions/Significance

Therefore, this study does not support the assertion that farming in Europe solely involved the adoption of technologies and ideas from Southwest Asia by indigenous Mesolithic hunter-gatherers. Moreover, the results highlight the utility of craniometric data for assessing patterns of past population dispersal and gene flow.     

Intra- and interobserver error in craniometry: A cautionary tale

This study investigates intra- and interobserver measurement error in craniometry. Data consists of 72 craniometric measurements taken on a series of 28 Sadlermuit Eskimo crania. Utermohle measured the series twice; Zegura measured it once. Statistical procedures used to demonstrate measurement imprecision include the mean difference, the method error statistic, two-way anova without replication, the t-test for paired comparisons, Fisher's distribution-free sign test, and the t-test for independent samples. The results indicate less intraobserver repeatability than expected as well as an alarming lack of interobserver reproducibility for many of these craniometric measurements. We hope these results will serve as a caution against the widespread belief that craniometric measurements are always produced with a high degree of precision by experienced craniometrists. In addition, these results suggest that investigators employing craniometric measurements to study population affinities, functional morphology, forensics, fossil primates, and human microevolution might profit from conducting a measurement error analysis as an important baseline for the interpretation of the biological significance of their results.     

Craniometric Relationships of Plains Populations: Historical and Evolutionary Implications

Abstract

Large skeletal samples from cemeteries in the Northern Plains near Mobridge, South Dakota have made possible the accurate determination of craniometric relationships between populations ancestoral to the historic Arikara. There is considerable change throughout the approximately 200 years represented, due primarily to gene flow from adjacent Siouan speaking groups. Arikara crania from the Northern Plains are seen to be morphologically similar to earlier crania further south, in particular St. Helena materials in northeastern Nebraska. This offers Support for the archaeological hypothesis that Coalescent Tradition cultures grew out of the Central Plains Tradition. Early crania from the Northern Plains are markedly different from those in the Central Plains and more similar to historic Mandan. The craniometric evidence argues for biological continuity rather than replacement in the Plains area.     

Population‐specific deviations of global human craniometric variation from a neutral model

Past studies have revealed that much of human craniometric variation follows a neutral model of population relationships. At the same time, there is evidence for the influence of natural selection in having shaped some global diversity in craniometrics. In order to partition these effects, and to explore other potential population‐specific influences, this article analyzes residuals of craniometric distances from a geographically based neutral model of population structure. W.W. Howells' global craniometric data set was used for these analyses, consisting of 57 measurements for 22 populations around the world, excluding Polynesia and Micronesia because of the relatively recent settlement of these regions. Phenotypic and geographic distances were derived between all pairs of populations. Three‐dimensional multidimensional scaling configurations were obtained for both distance matrices, and compared using a Procrustes rotation method to show which populations do not fit the geographic model. This analysis revealed three major deviations: the Buriat, Greenland Inuit, and Peru. The deviations of the Buriat and Greenland Inuit appear to be related to long‐term adaptation to cold environments. The Peruvian sample is more similar to other New World populations than expected based on geographic distance alone. This deviation likely reflects the evolutionarily recent movement of human populations into South America, such that these populations are further from genetic equilibrium. This same pattern is seen in South American populations in a comparative analysis of classical genetic markers, but not in a comparative analysis of STR loci, perhaps reflecting the higher mutation rate for the latter. Am J Phys Anthropol, 2010. © 2009 Wiley‐Liss, Inc.     

Estimating the probability that the sample mean is within a desired fraction of the standard deviation of the true mean

The use of small sample sizes in human and primate evolutionary research is commonplace. Estimating how well small samples represent the underlying population, however, is not commonplace. Because the accuracy of determinations of taxonomy, phylogeny, and evolutionary process are dependant upon how well the study sample represents the population of interest, characterizing the uncertainty, or potential error, associated with analyses of small sample sizes is essential. We present a method for estimating the probability that the sample mean is within a desired fraction of the standard deviation of the true mean using small (n < 10) or very small (n ≤ 5) sample sizes. This method can be used by researchers to determine post hoc the probability that their sample is a meaningful approximation of the population parameter. We tested the method using a large craniometric data set commonly used by researchers in the field. Given our results, we suggest that sample estimates of the population mean can be reasonable and meaningful even when based on small, and perhaps even very small, sample sizes.     

Studies of ancient crania from northern Africa

Historical sources and archaeological data predict significant population variability in mid-Holocene northern Africa. Multivariate analyses of crania demonstrate wide variation but also suggest an indigenous craniometric pattern common to both late dynastic northern Egypt and the coastal Maghreb region. Both tropical African and European metric phenotypes, as well intermediate patterns, are found in mid-Holocene Maghreb sites. Early southern predynastic Egyptian crania show tropical African affinities, displaying craniometric trends that differ notably from the coastal northern African pattern. The various craniofacial patterns discernible in northern Africa are attributable to the agents of microevolution and migration.     

Patterns of sexual dimorphism in the Persian Leopard (Panthera pardus saxicolor) and implications for sex differentiation

Extant felids show a high degree of inter-sexual dimorphism, meaning significant size differences between males and females. Such a differentiation may have various ecological, behavioural and evolutionary implications, at both species and subspecies levels. We have investigated the sexual size differences in one of the most dimorphic felids, i.e. the Leopard (Panthera pardus), based on 63 craniometric and 55 morphometric samples from Iran which belong to the subspecies Persian Leopard (P. p. saxicolor). In order to explore patterns of sexual dimorphism, multivariate statistical analysis on 24 skull variables as well as univariate approaches for two body measurements were applied. We found significant inter-sexual differences in skull size whereas it was not meaningful after removing the effect of size to address skull shape. Moreover, inter-sexual differentiation was also remarkable when comparing morphometric body measurements in adults, showing that the males possess a larger head mass and longer body, but sub-adults did not show any remarkable differentiation between sexes. A combination of craniometric and morphological features is proposed for sex differentiation in Leopards.     

A Re-Appraisal of the Early Andean Human Remains from Lauricocha in Peru

The discovery of human remains from the Lauricocha cave in the Central Andean highlands in the 1960’s provided the first direct evidence for human presence in the high altitude Andes. The skeletons found at this site were ascribed to the Early to Middle Holocene and represented the oldest known population of Western South America, and thus were used in several studies addressing the early population history of the continent. However, later excavations at Lauricocha led to doubts regarding the antiquity of the site. Here, we provide new dating, craniometric, and genetic evidence for this iconic site. We obtained new radiocarbon dates, generated complete mitochondrial genomes and nuclear SNP data from five individuals, and re-analyzed the human remains of Lauricocha to revise the initial morphological and craniometric analysis conducted in the 1960’s. We show that Lauricocha was indeed occupied in the Early to Middle Holocene but the temporal spread of dates we obtained from the human remains show that they do not qualify as a single contemporaneous population. However, the genetic results from five of the individuals fall within the spectrum of genetic diversity observed in pre-Columbian and modern Native Central American populations.     

Estimation of body weights from craniometric variables in baboons

Body weights of adult baboons (genera Papio, Mandrillus, and Theropithecus) were gathered from notes of collectors and museum records. However, these data were insufficient to establish mean body weights for all baboon groups. Thus, log cube roots of mean body weights were regressed as functions of the logs of several cranial and dental variables. The resulting least squares regression coefficients were used to estimate weights for 503 adult baboons from cranial measurements. The ability of the various regression functions to assess baboon body weight was determined by comparing reported and estimated mean and individual body weights. The best estimator of baboon body weights was the function derived from the factor scores of a principal components analysis of seven craniometric variables regressed on body weight. However, each of these craniometric variables singly was nearly as precise an estimator of body weight as the multivariate combination of all seven. Other measurements such as dental dimensions and foramen magnum area estimated weight less accurately. Body weight estimates derived from the regression analyses coupled with museum and literature records allowed an assessment of size relationships among all baboon groups.     

The Stasis and Possible Patterns of Selection in Evolution of a Group of Related Species from the Bat Genus <Emphasis Type="Italic">Myotis</Emphasis> (Chiroptera,Vespertilionidae)

Phenotypic evolutionary rates were measured for craniometric characters in five extant closely related OTUs from the bat genus Myotis (Chiroptera, Vespertilionidae): M. myotis, M. blythii oxygnathus, M. b. omari, M. b. blythii, and M. nattereri using Lynch’s and Gingerich’s approaches. Cranial shape appeared to be more conservative than cranial size. Estimates for evolutionary rates were found to be lower than expected if the divergence had been produced solely by mutation and random drift. So, it can be concluded that stabilizing selection was the principal factor that maintained craniometric characters during the evolution of the studied species and prevented their greater diversification. The observed differences between the OTUs could be established by random drift or directional selection of rather moderate intensity. The rates of divergence between the ancestors of M. nattereri and the common ancestors of M. blythii and M. myotis apparently were higher than the rates of following divergence between M. myotis, M. b. oxygnathus, M. b. omari and M. b. blythii.     

哈密天山北路墓地出土颅骨的测量性状

本文是对出土于新疆哈密地区天山北路青铜时代墓地颅骨测量性状的研究。文中公布了24例天山北路墓地古代人群77项颅骨测量性状基本数据, 在此基础上, 对男、女两性的测量性状特点进行了统计分析, 并结合考古学文化研究和分子生物学研究的相关结论对实验结果进行了讨论和推论。主要研究结论如下: 一是该人群虽已经形成了具有自身特点的、共性的体质特征, 但人群内部也存在些许差异, 尤其是在男性个体数据之中。二是该人群是由分别具有东、西方体质特征的祖先人群混杂融合而成, 是处于当时大人种分布过渡地带的过渡人群。     

Craniometric Variation In The Northern And Central Plains

Abstract

Cranial measurements of 13 male and 12 female samples from the Central and Northern Plains region were subjected to canonical analysis. The samples include historic or protohistoric crania that can be ascribed to the Arikara, Mandan, Pawnee, Ponca and Omaha tribes. In addition, two samples belong to the archaeologically defined St. Helena Focus. Both sexes yielded five significant canonical variates, although only four were readily interpretable. The first canonical variate is clearly a Siouan-Caddoan discriminator and reflects variation in cranial vault height. St. Helena sites associate with the Arikara on this axis, supporting previous craniometric analyses which suggest a relationship between these two groups. Subsequent canonical variates deal with more particular aspects of craniometric variation among groups, but are still interpretable in historic or evolutionary terms. The classificatory analysis shows that the Arikara sites are closely related. A major exception to this is the Sully site, which frequently misclassifies with non-Arikara groups. This suggests that the Sully crania have little collective reality; and that there may be non-Arikara components represented at the Sully Site.     

Differentiation under isolation and genetic structure of Sardinian hares as revealed by craniometric analysis,mitochondrial DNA and microsatellites

Hares (Lepus capensis Linnaeus 1758) were probably introduced into Sardinia in historical times. Previous studies indicated North Africa as the most likely source area but did not exclude the occurrence of hybridization events with continental brown hares (L. europaeus Pallas 1778) perhaps introduced for hunting purposes. We implemented both morphometric and genetic approaches to verify the genetic isolation of the Sardinian population. Specifically, we conducted a multivariate analysis of craniometric data and analysed 461 bp of the mitochondrial control region and 12 autosomal microsatellites in Sardinian hares, using North African cape hares and European brown hares as reference populations. Sardinian hares displayed a peculiar skull shape. In agreement, both nuclear and mitochondrial markers remarked the distinctiveness of this population. Observed and expected heterozygosity were 0.52 and 0.61, while haplotype and nucleotide diversity were 0.822 and 0.0129. Self‐assignment based on Bayesian cluster analysis was high (average membership 0.98), and no evident signs of introgression from continental brown hares were found. Our results support the hypothesis that the Sardinian hares have been introduced from North Africa, remained genetically isolated since the founding event and evolved independently from the source population. This long‐lasting isolation and the consequent genetic drift resulted in a differentiation, perhaps accompanied by an adaptation to local environmental conditions.     

Variation in soft-tissue thicknesses on the human face and their relation to craniometric dimensions

The average thickness of soft tissues on parts of the face is known, but its variation has not been related to cranial morphology. To investigate this relationship, measurements of facial soft-tissue depths and craniometric dimensions were taken on adult, white Australian cadavers (17 male and 23 female). Significant correlations between many soft-tissue depths and craniometric dimensions were found, suggesting a relationship between the amount of soft tissue present on the face and the size of the underlying bony skeleton. Soft-tissue depths were highly positively correlated with each other; craniometric dimensions were correlated but to a lesser extent. Males had thicker soft tissues and larger craniometric dimensions than females; considerable overlap of ranges was also noted. Multiple regression analysis was used to produce equations predicting the soft-tissue depth at specified areas of the face from craniometric dimensions. A subsample of nine cadavers was examined for the effects of tissue embalming. Embalming caused significant initial increases in facial soft-tissue depths. Cadavers embalmed for less than 6 months had soft-tissue depths significantly greater than for fully embalmed cadavers. The evidence that facial soft-tissue thicknesses vary with craniofacial dimensions has implications for forensic identification, facial aesthetic surgery, and approximation of the facial features of extinct individuals.     

Mobridge Site Cemeteries: Controversy Concerning The Location Of The Over And Stirling Burials

Abstract

During the 1920's, W.H. Over and M.W. Stirling excavated skeletal material from cemeteries at the Mobridge site. Their records are not clear as to the exact location of burial recovery. Mobridge has at least three distinct burial areas (Features 1, 2, and 3), and there are craniometric and temporal differences among them. A comparison of crania from Features 1, 2, and 3 with the Over and Stirling samples gives some insight as to the location of their burials. Burials excavated by Over classify primarily with Feature 2, indicating this to be their point of origin. A few burials from Feature 1 also appear to be represented. Stirling's sample groups with areas 1 and 3. His sample possibly originated there, although it seems more likely that they came from a location north of the village which dates to the same time period as Features 1 and 3 and represents the same population.     

Technical note: Comparability of Hrdlička's Catalog of crania data based on measurement landmark definitions

Archival sources of data are critical anthropological resources that inform inferences about human biology and evolutionary history. Craniometric data are one of the most widely available sources of information on human population history because craniometrics were critical in early 20th century debates about race and biological variation. As such, extensive databases of raw craniometric data were published at the same time that the field was working to standardize measurement protocol. Hrdli?ka published between 10 and 16 raw craniometric variables for over 8,000 individuals in a series of seven catalogs throughout his career. With a New World emphasis, Hrdli?ka's data complement those of Howells ( 1973 , 1989 ) and the two databases have been combined in the past. In this note we verify the consistency of Hrdli?ka's measurement protocol throughout the Catalog series and compare these definitions to those used by Howells. We conclude that 12 measurements are comparable throughout the Catalogs, with five of these equivalent to Howells' measurements: maximum cranial breadth (XCB), basion‐bregma height (BBH), maximum bizygomatic breadth (ZYB), nasal breadth (NLB), and breadth of the upper alveolar arch (MAB). Most of Hrdli?ka's measurements are not strictly comparable to those of Howells, thus limiting the utility of combined datasets for multivariate analysis. Four measurements are inconsistently defined by Hrdli?ka and we recommend not using these data: nasal height, orbit breadth, orbit height, and menton‐nasion height. This note promotes Hrdli?ka's tireless efforts at data collection and re‐emphasizes observer error as a legitimate concern in craniometry as the field shifts to morphometric digital data acquisition. Am J Phys Anthropol, 2011. © 2011 Wiley‐Liss, Inc.     

Migration and diversity in Roman Britain: A multidisciplinary approach to the identification of immigrants in Roman York,England

Previous anthropological investigations at Trentholme Drive, in Roman York identified an unusual amount of cranial variation amongst the inhabitants, with some individuals suggested as having originated from the Middle East or North Africa. The current study investigates the validity of this assessment using modern anthropological methods to assess cranial variation in two groups: The Railway and Trentholme Drive. Strontium and oxygen isotope evidence derived from the dentition of 43 of these individuals was combined with the craniometric data to provide information on possible levels of migration and the range of homelands that may be represented. The results of the craniometric analysis indicated that the majority of the York population had European origins, but that 11% of the Trentholme Drive and 12% of The Railway study samples were likely of African decent. Oxygen analysis identified four incomers, three from areas warmer than the UK and one from a cooler or more continental climate. Although based on a relatively small sample of the overall population at York, this multidisciplinary approach made it possible to identify incomers, both men and women, from across the Empire. Evidence for possible second generation migrants was also suggested. The results confirm the presence of a heterogeneous population resident in York and highlight the diversity, rather than the uniformity, of the population in Roman Britain. Am J Phys Anthropol 2009. © 2009 Wiley‐Liss, Inc.     

Microevolution in prehistoric Andean populations: Chronologic nonmetrical cranial variation in northern Chile

Multivariate distance statistics were computed from 14 nonmetrical cranial variables among five prehistoric samples representing steps in the microevolutionary history of a coastal population in northern Chile. Roughly 70% of nonmetrical cranial variation was found to be explained by chronologic distance covering a period of 6,500 years. This estimate is similar to a previous one derived from craniometric data. Implications of these findings are briefly discussed.