Basics in paleodemography: a comparison of age indicators applied to the early medieval skeletal sample of Lauchheim

Recent advances in the methods of skeletal age estimation have rekindled interest in their applicability to paleodemography. The current study contributes to the discussion by applying several long established as well as recently developed or refined aging methods to a subsample of 121 adult skeletons from the early medieval cemetery of Lauchheim. The skeletal remains were analyzed by 13 independent observers using a variety of aging techniques (complex method and other multimethod approaches, Transition Analysis, cranial suture closure, auricular surface method, osteon density method, tooth root translucency measurement, and tooth cementum annulation counting). The age ranges and mean age estimations were compared and results indicate that all methods showed smaller age ranges for the younger individuals, but broader age ranges for the older age groups.     

Model life table fitting by maximum likelihood estimation: a procedure to reconstruct paleodemographic characteristics from skeletal age distributions

A procedure is presented that uses the regression coefficients for the Coale and Demeny west model life tables to model selected demographic characteristics from skeletal age-at-death distributions. Model death distributions were constructed and compared to a given skeletal distribution, using methods of maximum likelihood estimation to determine the best fit. Two chi-square tests are employed to evaluate the degree of fit. The resulting model includes estimates of demographic characteristics including gross reproductive rate, crude birth rate and life expectancy. The procedure is applied to three archaeological skeletal samples as test cases: two from eastern North America and one from Mexico. These display a range of correspondence (between the best fitting model and the data) from good to poor. The proposed procedure is a potentially powerful tool for both reconstructing paleodemographic rates and illuminating differences between typical human patterns and those found in archaeological populations.     

Maximum likelihood estimation of human craniometric heritabilities

This study presents univariate narrow-sense heritability estimates for 33 common craniometric dimensions, calculated using the maximum likelihood variance components method on a skeletal sample of 298 pedigreed individuals from Hallstatt, Austria. Quantitative genetic studies that use skeletal cranial measurements as a basis for inferring microevolutionary processes in human populations usually employ heritability estimates to represent the genetic variance of the population. The heritabilities used are often problematic: most come from studies of living humans, and/or they were calculated using statistical techniques or assumptions violated by human groups. Most bilateral breadth measures in the current study show low heritability estimates, while cranial length and height measures have heritability values ranging between 0.102-0.729. There appear to be differences between the heritabilities calculated from crania and those from anthropometric studies of living humans, suggesting that the use of the latter in quantitative genetic models of skeletal data may be inappropriate. The univariate skeletal heritability estimates seem to group into distinct regions of the cranium, based on their relative values. The most salient group of measurements is for the midfacial/orbital region, with a number of measures showing heritabilities less than 0.30. Several possible reasons behind this pattern are examined. Given the fact that heritabilities calculated on one population should not be applied to others, suggestions are made for the use of the data presented.     

Brief Communication: Timing of spheno‐occipital closure in modern Western Australians

The spheno‐occipital synchondrosis is a craniofacial growth centre between the occipital and sphenoid bones—its ossification persists into adolescence, which for the skeletal biologist, means it has potential application for estimating subadult age. Based on previous research the timing of spheno‐occipital fusion is widely variable between and within populations, with reports of complete fusion in individuals as young as 11 years of age and nonfusion in adults. The aim of this study is, therefore, to examine this structure in a mixed sex sample of Western Australian individuals that developmentally span late childhood to adulthood. The objective is to develop statistically quantified age estimation standards based on scoring the degree of spheno‐occipital fusion. The sample comprises multidetector computed tomography (MDCT) scans of 312 individuals (169 male; 143 female) between 5 and 25 years of age. Each MDCT scan is visualized in a standardized sagittal plane using three‐dimensional oblique multiplanar reformatting. Fusion status is scored according to a four‐stage system. Transition analysis is used to calculate age ranges for each defined stage and determine the mean age for transition between an unfused, fusing and fused status. The maximum likelihood estimates for the transition from open to fusing in the endocranial half is 14.44 years (male) and 11.42 years (female); transition from fusion in the ectocranial half to complete fusion is 16.16 years (male) and 13.62 years (female). This study affirms the potential value of assessing the degree of fusion in the spheno‐occipital synchondrosis as an indicator of skeletal age. Am J Phys Anthropol 153:132–138, 2014. © 2013 Wiley Periodicals, Inc.     

The use of nonmetric variation in estimating human population admixture: a test case with Brazilian blacks, whites, and mulattos

Measurements in populations which serve as valid indicators of biological relationship should be proportional to genetic distance. In order to test the utility of discrete cranial traits for estimating genetic distances among populations, estimates of admixture are obtained for gene frequency data and nonmetric cranial data in São Paulo mulattos (M). The gene frequency data serve as a control that the three populations are related as stated: estimates of admixture are obtained by using São Paulo whites (W) and blacks (B) as parental populations and by estimating the parameter of admixture, m, in the model p_M = (1 ? m) p_W + mp_B (Elston, 1971) where the p's are either gene frequencies or nonmetric trait frequencies. A test of goodness of fit of the model provides a means of ascertaining whether or not the data fit this linear model. While the gene frequency data indicate distances among the three populations which are highly compatible with the linear model of admixture, the nonmetric data show significant deviations from the model. This implies that the frequencies of the nonmetric traits in the populations used in this analysis are not a linear function of genetic distance. This discourages the use of nonmetric traits in making quantitative conclusions about genetic relationships. It also suggests the need for investigation of the use of other skeletal characters for estimating genetic distance, as well as approaches for such investigations through the study of hybrid individuals.     

Preoperative anthropometric dysmorphology in metopic synostosis

Anthropometric identification of dysmorphology in craniofacial anomalies, including the craniosynostoses, provides invaluable assistance in clinical diagnosis as well as offering a technique for interpreting possible deformities in skeletal remains. Premature closure of the metopic suture is a rare form of craniosynostosis, representing about 4% of clinically diagnosed synostoses. Accompanying this closure are defects of the head and face, particularly the upper face and orbits. To identify quantitatively the craniofacial dysmorphology associated with metopic synostosis, 50 patients with a diagnosis of primary (nonsyndromal) metopic synostosis were examined using a battery of 24 anthropometric measurements from which 11 proportion indices were calculated. The data were compared to sex- and age-matched normal standards and converted to standard (Z) scores before being analyzed using Student's t-test. The data indicate a complex pattern of dysmorphology arising from the synostosis which affects the upper face and orbits as well as the cranial vault. The entire fronto-orbito-zygomatic complex is narrowed, and vertex is reduced. There is compensatory sagittal and transverse growth of the posterior neurocranium and compensatory vertical and sagittal growth of the upper face. There are statistically significant differences in the pattern of dysmorphology between patients presenting prior to 6 months of age and those older but no significant differences between sexes. Am J Phys Anthropol 103:341–351, 1997. © 1997 Wiley-Liss, Inc.     

Relative dental maturity and associated skeletal maturity in prehistoric native Americans of the Ohio valley area

Estimation of age-at-death of subadults in prehistoric skeletal samples based on modern reference standards rests on a number of assumptions of which many are untestable. If these assumptions are not met error of unknown magnitude and direction will be introduced to the subadult age estimates. This situation suggests that an independent estimate or estimates of age-related features, free of most of the assumptions made when using modern reference standards may be useful supplements in evaluating the age of subadults in prehistoric samples. The present study provides an internally consistent, population-specific measure of maturity for prehistoric Ohio valley Native Americans based on the seriation of dental development that may be used as a supplement to age-estimation. The developing dentition of 581 subadults from eight Ohio valley prehistoric-protohistoric groups was seriated within and among individuals resulting in a sequence of tooth development and a sequence of individuals from least to most mature. Dental maturity stages or sorting categories were then defined based on exclusive, easily observable, and highly repeatable tooth-formation stages. Tooth eruption (into occlusion), bone lengths, and fusion of skeletal elements are summarized by dental maturity stage. This procedure provides maturity estimates for skeletal features ordered by dental maturity stages derived from the same sample thus making explicit the relationship between dental and skeletal maturity.     

Paleodemographic age‐at‐death distributions of two Mexican skeletal collections: A comparison of transition analysis and traditional aging methods

Traditional methods of aging adult skeletons suffer from the problem of age mimicry of the reference collection, as described by Bocquet‐Appel and Masset (1982). Transition analysis (Boldsen et al., 2002) is a method of aging adult skeletons that addresses the problem of age mimicry of the reference collection by allowing users to select an appropriate prior probability. In order to evaluate whether transition analysis results in significantly different age estimates for adults, the method was applied to skeletal collections from Postclassic Cholula and Contact‐Period Xochimilco. The resulting age‐at‐death distributions were then compared with age‐at‐death distributions for the two populations constructed using traditional aging methods. Although the traditional aging methods result in age‐at‐death distributions with high young adult mortality and few individuals living past the age of 50, the age‐at‐death distributions constructed using transition analysis indicate that most individuals who lived into adulthood lived past the age of 50. Am J Phys Anthropol 152:67–78, 2013. © 2013 Wiley Periodicals, Inc.     

On estimating mortalities from osteological age data

A statistical method (based on the maximum likelihood principle) for estimating mortalities of skeletal populations is presented. The method can be applied both when osteological age groups are not overlapping as well as when they are. The results are presented as point estimates and as confidence intervals around these. The method is applied to two series from the Middle Ages: Westerhus and Helgeandsholmen.     

Modeling senescence changes of the pubic symphysis in historic Italian populations: A comparison of the Rostock and forensic approaches to aging using transition analysis

Age‐related anatomical changes to the surface of the pubic symphysis are well‐documented in the literature. However, aligning these morphological changes with chronological age has proven problematic, often resulting in biased age estimates. Statistical modeling provides an avenue for forensic anthropologists and bioarchaeologists to increase the accuracy of traditional aging methods. Locating appropriate samples to use as a basis for modeling age estimations can be challenging due to differing sample age distributions and potentially varying patterns of senescence. We compared two approaches, Rostock and Forensic, coupled with a Bayesian methodology, to address these issues. Transition analysis was run specific to each method (which differ by sample selection). A Gompertz model was derived from an informative prior that yielded the mortality and senescence parameters for constructing highest posterior density ranges, i.e., coverages, which are analogous to age ranges. These age ranges were generated from both approaches and are presented as reference tables useful for historic male and female Italian samples. The age ranges produced from each approach were tested on an historic Italian sample, using cumulative binomial tests. These two approaches performed similarly, with the Forensic approach showing a slight advantage. However, the Forensic approach is unable to identify varying senescence patterns between populations, thus preference for one approach over the other will depend on research design. Finally, we demonstrate that while populations exhibit similar morphological changes with advancing age, there are no significant sex differences in these samples, and the timing of these changes varies from population to population. Am J Phys Anthropol 156:466–473, 2015. © 2014 Wiley Periodicals, Inc.     

Semiparametric method for estimating paleodemographic profiles from age indicator data.

This paper addresses the problem of estimating an age-at-death distribution or paleodemographic profile from osteological data. It is demonstrated that the classical two-stage procedure whereby one first constructs estimates of age-at-death of individual skeletons and then uses these age estimates to obtain a paleodemographic profile is not a correct approach. This is a consequence of Bayes' theorem. Instead, we demonstrate a valid approach that proceeds from the opposite starting point: given skeletal age-at-death, one first estimates the probability of assigning the skeleton into a specific osteological age-indicator stage. We show that this leads to a statistically valid method for obtaining a paleodemographic profile, and moreover, that valid individual age estimation itself requires a demographic profile and therefore is done subsequent to its construction. Individual age estimation thus becomes the last rather than the first step in the estimation procedure. A central concept of our statistical approach is that of a weight function. A weight function is associated with each osteological age-indicator stage or category, and provides the probability that a specific age indicator stage is observed, given age-at-death of the individual. We recommend that weight functions be estimated nonparametrically from a reference data set. In their entirety, the weight functions characterize the relevant stochastic properties of a chosen age indicator. For actual estimation of the paleodemographic profile, a parametric age distribution in the target sample is assumed. The maximum likelihood method is used to identify the unknown parameters of this distribution. As some components are estimated nonparametrically, one then has a semiparametric model. We show how to obtain valid estimates of individual age-at-death, confidence regions, and goodness-of-fit tests. The methods are illustrated with both real and simulated data.     

Paleodemography of the Carlston Annis (Bt-5) Late Archaic skeletal population

The study presents a demographic assessment of the Carlston Annis (Bt-5) Late Archaic hunting and gathering population recovered from the banks of the Green River in west-central Kentucky. The shell midden habitation and cemetery site originally yielded the remains of 390 individuals. Radiocarbon dates place site occupation between 3,000 and 4,500 y.b.p. The skeletal sample consisted of 354 individuals ranging in age from 7 months in utero to 70 + years. Subadults were aged by seriation of dental and skeletal developmental criteria. Adult ages were determined by the multifactorial summary age method that employed 1) five indicators of adult skeletal age at death, 2) the procedure of age indicator seriation, and 3) differential weighting of age assessments as determined by principal components analysis. Adult sex diagnoses were based on qualitative assessment of pelvic and cranial morphological criteria. The Bt-5 life table analysis yields an E0 of 22.4 years, crude birth of 45, mean family size of 3.3, gross reproductive rate of 2.7, generation length of 26.6 years, and B of .076, indicating a healthy population with a substantial capability to replace succeeding generations. Survivorship profiles and demographic parameters that compare the Carlston Annis (Bt-5) and Indian Knoll (Oh-2) skeletal series are presented. Both populations display type II survivorship curves, with high infant mortality and early onset of elevated mortality rates in adults. Major differences between Bt-5 and Oh-2 demographic parameters concern adult sex ratio and adult age distribution over 30 years. These differences are interpreted to reflect census errors in the Oh-2 demographic reconstruction that were possibly introduced by selective methodological biases and/or taphonomic factors.     

Consequences of Secondary Calibrations on Divergence Time Estimates

Secondary calibrations (calibrations based on the results of previous molecular dating studies) are commonly applied in divergence time analyses in groups that lack fossil data; however, the consequences of applying secondary calibrations in a relaxed-clock approach are not fully understood. I tested whether applying the posterior estimate from a primary study as a prior distribution in a secondary study results in consistent age and uncertainty estimates. I compared age estimates from simulations with 100 randomly replicated secondary trees. On average, the 95% credible intervals of node ages for secondary estimates were significantly younger and narrower than primary estimates. The primary and secondary age estimates were significantly different in 97% of the replicates after Bonferroni corrections. Greater error in magnitude was associated with deeper than shallower nodes, but the opposite was found when standardized by median node age, and a significant positive relationship was determined between the number of tips/age of secondary trees and the total amount of error. When two secondary calibrated nodes were analyzed, estimates remained significantly different, and although the minimum and median estimates were associated with less error, maximum age estimates and credible interval widths had greater error. The shape of the prior also influenced error, in which applying a normal, rather than uniform, prior distribution resulted in greater error. Secondary calibrations, in summary, lead to a false impression of precision and the distribution of age estimates shift away from those that would be inferred by the primary analysis. These results suggest that secondary calibrations should not be applied as the only source of calibration in divergence time analyses that test time-dependent hypotheses until the additional error associated with secondary calibrations is more properly modeled to take into account increased uncertainty in age estimates.     

Brief communication: A probabilistic approach to age estimation from infracranial sequences of maturation

Infracranial sequences of maturation are commonly used to estimate the age at death of nonadult specimens found in archaeological, paleoanthropological, or forensic contexts. Typically, an age assessment is made by comparing the degree of long‐bone epiphyseal fusion in the target specimen to the age ranges for different stages of fusion in a reference skeletal collection. While useful as a first approximation, this approach has a number of shortcomings, including the potential for “age mimicry,” being highly dependent on the sample size of the reference sample and outliers, not using the entire fusion distribution, and lacking a straightforward quantitative way of combining age estimates from multiple sites of fusion. Here we present an alternative probabilistic approach based on data collected on 137 individuals, ranging in age from 7‐ to 29‐years old, from a documented skeletal collection from Coimbra, Portugal. We then use cross validation to evaluate the accuracy of age estimation from epiphyseal fusion. While point estimates of age can, at least in some circumstances, be both accurate and precise based on the entire skeleton, or many sites of fusion, there will often be substantial error in these estimates when they derive from one or only a few sites. Because a probabilistic approach to age estimation from epiphyseal fusion is computationally intensive, we make available a series of spreadsheets or computer programs that implement the approach presented here. Am J Phys Anthropol 142:655–664, 2010. © 2010 Wiley‐Liss, Inc.     

Radiographic changes in the clavicle and proximal femur and their use in the determination of skeletal age at death

Visually seriated radiographs of the proximal femur, proximal humerus, clavicle, and calcaneus from 130 individuals from the Hamann-Todd collection were examined as indicators of skeletal age at death. The clavicle demonstrated the most consistent relationship to age in both sexes. The same radiographs were also seriated by size-normalized optical density as a means of establishing relative radiolucency. In this context, visual seriation proved superior. The four sites studied showed strong divergence in response to age. Since each was sampling bone response from the same individual, it is concluded that bone loss is highly site specific. This demonstrates the individual character of specific skeletal sites. Visual inspection of clavicular radiographs, seriated on a populational basis, provides age estimates that are comparable to anatomical age indicators and provides independent estimates of skeletal age when included in the summary age method (1985: Am. J. Phys. Anthropol. 68:1–14).     

Age of epiphyseal closure in tamarins and marmosets

Estimates of the chronological age for animals of unknown age provide useful information for medical, demographic, and evolutionary studies. Skeletal development, as indicated by epiphyseal closure, can be used to estimate an animal's chronological age or specify its stage of development. Many studies of Primate skeletal development have used animals of unknown age, with the order of epiphyseal closure providing a relative age for each animal. This study examines the age of epiphyseal closure at 22 epiphyseal sites using animals of known age at death in three calitrichid species (Saguinus fuscicollis, Saguinus oedipus, and Callithrix jacchus). The observed average age of epiphyseal closure is similar in these tamarins and marmosets. There is a significant difference in rate of development between the species. Regression equations can predict the age of unknown animals to within 4.8 months for S. fuscicollis, 8.6 months for S. oedipus, and 7.6 months for C. jacchus (twice the standard error of the estimate). These age estimates allow us to determine if an animal is relatively mature or immature, but are largely unacceptable for studies in which precise age estimates are necessary. The order of epiphyseal closure is similar across 11 monkey species (using additional data from published literature) and supports the suggestion of a general pattern in Primate skeletal development. Am. J. Primatol. 41:129–139, 1997. © 1997 Wiley-Liss, Inc.     

Dating primate divergences through an integrated analysis of palaeontological and molecular data

Estimation of divergence times is usually done using either the fossil record or sequence data from modern species. We provide an integrated analysis of palaeontological and molecular data to give estimates of primate divergence times that utilize both sources of information. The number of preserved primate species discovered in the fossil record, along with their geological age distribution, is combined with the number of extant primate species to provide initial estimates of the primate and anthropoid divergence times. This is done by using a stochastic forwards-modeling approach where speciation and fossil preservation and discovery are simulated forward in time. We use the posterior distribution from the fossil analysis as a prior distribution on node ages in a molecular analysis. Sequence data from two genomic regions (CFTR on human chromosome 7 and the CYP7A1 region on chromosome 8) from 15 primate species are used with the birth-death model implemented in mcmctree in PAML to infer the posterior distribution of the ages of 14 nodes in the primate tree. We find that these age estimates are older than previously reported dates for all but one of these nodes. To perform the inference, a new approximate Bayesian computation (ABC) algorithm is introduced, where the structure of the model can be exploited in an ABC-within-Gibbs algorithm to provide a more efficient analysis.     

Cranial capacity estimates for Olduvai Hominid 7

Estimation of cranial capacity for Olduvai Hominid (OH) 7 is determined from external parietal dimensions using multiple regressions calculated from an australopithecine grade sample. Capacity estimates for OH 7 (580–600 cc) are much lower than usually claimed. While differences in reconstruction may account for the varying estimates, a regression based only on undistorted and unreconstructed values, as well as a direct comparison of dimensions with other Homo habilis specimens, supports the smaller capacity determination.     

A Simple Method for Estimating Informative Node Age Priors for the Fossil Calibration of Molecular Divergence Time Analyses

Molecular divergence time analyses often rely on the age of fossil lineages to calibrate node age estimates. Most divergence time analyses are now performed in a Bayesian framework, where fossil calibrations are incorporated as parametric prior probabilities on node ages. It is widely accepted that an ideal parameterization of such node age prior probabilities should be based on a comprehensive analysis of the fossil record of the clade of interest, but there is currently no generally applicable approach for calculating such informative priors. We provide here a simple and easily implemented method that employs fossil data to estimate the likely amount of missing history prior to the oldest fossil occurrence of a clade, which can be used to fit an informative parametric prior probability distribution on a node age. Specifically, our method uses the extant diversity and the stratigraphic distribution of fossil lineages confidently assigned to a clade to fit a branching model of lineage diversification. Conditioning this on a simple model of fossil preservation, we estimate the likely amount of missing history prior to the oldest fossil occurrence of a clade. The likelihood surface of missing history can then be translated into a parametric prior probability distribution on the age of the clade of interest. We show that the method performs well with simulated fossil distribution data, but that the likelihood surface of missing history can at times be too complex for the distribution-fitting algorithm employed by our software tool. An empirical example of the application of our method is performed to estimate echinoid node ages. A simulation-based sensitivity analysis using the echinoid data set shows that node age prior distributions estimated under poor preservation rates are significantly less informative than those estimated under high preservation rates.     

Cranial base angulation and prognathism related to cranial and general skeletal maturation in human fetuses.

The purpose of the present study was to describe normal midsagittal craniofacial morphology in second trimester human fetuses. Measurements of the cranial base angle and the prognathism of the maxilla and the mandible were performed on radiographs of cranial midsagittal tissue blocks of 52 fetuses with a gestational age from 13 to 27 weeks. Special procedures were developed for the definitions of the nasion and sella reference points on the radiographs in the early stages of fetal development. Mean data were reported for stages of crown rump length (CRL) and maturation of the fetal cranial base (MSS), usable as reference in assessment of pathological fetal crania in reports and autopsy procedures. Regression equations were determined for the regression of the angular values on CRL, MSS, and general skeletal maturation (TNO). The cranial base angle was found to decrease significantly, and the angles of prognathism to increase significantly with increasing CRL, TNO, and MSS values. It was suggested that these simultaneous and similar changes in the three angles could be accounted for by the upwards movement of the sella point produced by a cranial displacement of the pituitary fossa caused by local cartilagenous growth and bony remodelling during the period of study. The study thus reflects the influence of cranial skeletal maturation on the early development in shape of the craniofacial complex.