Mc Dean

Growth layers and incremental markings in hard tissues; a review of the literature and some preliminary observations about enamel structure in Paranthropus boisei

Abstract The general factors underlying the formation of growth layers and incremental markings in hard tissues are reviewed with particular reference to fossil hominid tooth enamel. The experimental and circumstantial evidence that point to a slowing of enamel matrix secretion in a daily (circadian) and near weekly (circaseptan) mode during tooth formation is also reviewed. Data from previous studies in which the number of daily increments between adjacent striae of Retzius have been recorded in primates are reviewed and new data are presented for this repeat interval in fossil hominids. The factors likely to influence the number of striae of Retzius beneath the cuspal regions of anterior teeth are outlined and the limitations of employing surface incremental features to obtain estimates for age at death of an individual are also discussed. It is concluded that there is good evidence to support the hypothesis that perikymata are near weekly incremental phenomena with a likely periodicity of 7,8 or 9 days in fossil hominids. It can also be concluded that at present, better estimates for the age at death of an individual during early phases of the growth period can be obtained from studies of perikymata than by any other non-destructive technique.

Developing Pongid Dentition and Its Use for Ageing Individual Crania in Comparative Cross-Sectional Growth Studies

This study of the developing pongid dentition is based on cross-sectional radiographic data of juvenile Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus skulls. Comparisons with developmental features of the human dentition are made, and possible explanations for the formation of larger teeth within the reduced pongid growth period are discussed. The data presented in this study provide an alternative method for ageing individual pongid crania in comparative cross-sectional growth studies. The advantages of this method are demonstrated by ageing individual Gorilla crania form radiographs and plotting relative dental age against length of the jaw.

Brief communication: the timing of linear hypoplasias on human anterior teeth.

One hundred and fifteen unworn anterior teeth were sectioned longitudinally with a diamond saw and prepared for histological examination by polarized light microscopy. Incremental markings in the enamel of each tooth were used to estimate the average total crown formation times of each tooth type. The total time taken to form the crowns of each tooth type was apportioned by 1) cuspal enamel formation and 2) each tenth percentile of total tooth height. Based on these data, and on histological estimates for the time of initiation of mineralization in each anterior tooth, the following conclusions can be drawn. Little if any visible surface enamel is likely to form before the end of the first year after birth in any anterior tooth type. No relation exists between tooth crown height and the total time taken to form enamel. Anterior crown formation is nonlinear and slows towards the cervix in all teeth. The estimated mean chronological age at crown completion ranged in this study from between around 4 years for lower central incisors to around 6 years for lower canines. We suggest that these combined findings will be useful for devising more reliable ways to estimate the timing of linear enamel hypoplasias than some methods currently in use.

Morphological variation in great ape and modern human mandibles

Adult mandibles of 317 modern humans and 91 great apes were selected that showed no pathology. Adult mandibles of Pan troglodytes troglodytes, Pongo pygmaeus pygmaeus and Gorilla gorilla gorilla and from 2 modern human populations (Zulu and Europeans from Spitalfields) were reliably sexed. Thirteen measurements were defined and included mandibular height, length and breadth in representative positions. Univariate statistical techniques and multivariate (principal component analysis and discriminant analysis) statistical techniques were used to investigate interspecific variability and sexual dimorphism in human and great ape mandibles, and intraspecific variability among the modern human mandibles. Analysis of interspecific differences revealed some pairs of variables with a tight linear relationship and others where Homo and the great apes pulled apart from one another due to shape differences. Homo and Pan are least sexually dimorphic in the mandible, Pan less so than Homo sapiens, but both the magnitude of sexual dimorphism and the distribution of sexually dimorphic measurements varied both among and between modern humans and great apes. Intraspecific variation among the 10 populations of modern humans was less than that generally reported in studies of crania (74.3% of mandibles were correctly classified into 1 of 10 populations using discriminant functions based on 13 variables as compared with 93% of crania from 17 populations based on 70 variables in one extensive study of crania). A subrecent European population (Poundbury) emerged as more different from a recent European population (Spitalfields) than other more diverse modern populations were from each other, suggesting considerable morphological plasticity in the mandible through time. This study forms a sound basis on which to explore mandibular variation in Neanderthals, early Homo sapiens and other more ancient fossil hominids.

Unlocking evidence of early diet from tooth enamel

Recent developments in microspatial analysis of enamel chemistry provide the resolution needed to reconstruct detailed chronological records of an individuals early life history. Evidence of nutritional history, residential mobility, and exposure to heavy metals can potentially be retrieved from archaeological and even fossil teeth. Understanding the pattern and timing of incorporation of each trace element or stable isotope into enamel is crucial to the interpretation of the primary data. Here, we use laser ablation inductively coupled plasma mass spectrometry and ArcGIS software to map variation in calcium-normalized strontium intensities across thin sections of enamel from exfoliated deciduous teeth. Differences in calcium-normalized strontium intensities across each tooth reflect variation in tooth mineralization, implying that sampling location must be taken into account in interpreting results. Chronologically consistent shifts in calcium-normalized strontium intensities in teeth from children with known nursing histories reflect the onset and duration of breastfeeding and the introduction of nonmaternal sources of food. This tool is likely to be valuable for studying weaning and nursing behavior in the past. The distribution of normalized strontium intensities presented here is consistent with a model for the differential incorporation of strontium and calcium into enamel during the secretory and maturational phases of formation.

The dental developmental status of six East African juvenile fossil hominids

Abstract Radiographs of five juvenile fossil hominids from Koobi Fora, Kenya are described and presented together with measurements and observations made on the original speciments. Data are also presented for a single specimen from Olduvai Gorge, Tanzania. Four of these specimens are attributed to Paranthropus boisei (KNM ER 812, 1477 1820 and OH 30), and are all of remarkably similar dental developmental status. Conventional age estimates for these specimens of Paranthropus based on the first permanent molar, indicate an age at death of around 2·2 to 3 years. Perikymata counts on permanent lower central incisors of these specimens also indicate an age at death between 2·5 and 3 years. Two specimens attributed to early Homo (KNM ER 820 and 1507), are dentally more mature than specimens of Paranthropus boisei described here being closer to 5 years of age. Differences between the spacing and distribution of perikymata on the surfaces of incisor teeth are now apparent between Homo, Australopithecus. Paranthropus boisei and Paranthropus robustus: these are described in this paper. Details of the dental developmental patterns of these hominids are also discussed in the light of recent publications that have presented data about hominid eruption sequences and fossil hominid growth periods.

The relation between long-period incremental markings in dentine and daily cross-striations in enamel in human teeth

Ground sections of human permanent teeth were chosen where fluorescent labels in the dentine, resulting from repeated doses of tetracycline antibiotic, were unambiguously associated with accentuated markings in the enamel developing at the same time. Counts of daily cross-striations in enamel were continued from one tooth to another in a developmental sequence over a period of some 1200 days such that the time interval between doses of tetracycline could be calibrated. Long-period incremental markings in the dentine, spaced on average between 15 and 30 microns apart (and first described by Andresen in 1898) were easily visible in the coronal dentine when the ground sections were viewed with polarized light. The total number of long-period incremental markings in the dentine between the consecutive fluorescent labels was also counted. A regression plot of daily incremental lines in enamel against long-period lines in dentine demonstrated a regular and consistent relation between the two (r = 0.997) over a 1200-1300-day period. These data support the hypothesis that long-period markings in dentine are in fact regular incremental markings with a constant periodicity in an individual. They also suggest that regular long-period markings in dentine can be used to reconstruct the timing of tooth growth or to retrieve developmental information about dentine formation rates in forensic, archaeological and palaeontological studies with some confidence.

The Developing Dentition and Tooth Structure in Hominoids

This review of hominoid dental development is presented in two parts. The first section reviews (1) the general relationship between dental development and life history in hominoids; (2) the methods used to document dental development, and (3) the nature of incremental growth markings in hominoid teeth. The second section builds on this and reviews the contributions to hominoid dental development that have been made by (1) studies of tooth emergence; (2) studies of tooth calcification stages, and (3) histological studies of incremental growth markings made either from sections of teeth or replicas of early hominid teeth prepared for scanning electron microscopy.

Crown-formation time of a fossil hominid premolar tooth

Studies using surface or internal enamel growth indicators in hominids have suggested that crown-formation times were shorter than those in modern man. The crown-formation time in a robust australopithecine premolar tooth was calculated by counting enamel cross-striations, which correspond to daily increments of formation, on a replica of the fractured internal enamel surface of cuspal enamel using scanning electron microscopy. Cervical enamel completion time was estimated using other growth indicators including striae, and using measured and calculated cross-striation repeat intervals, giving a completion time of approx. 2.4 yr. This is much shorter than reported premolar crown formation times in modern man. These findings support the concept of an abbreviated period of dental development, with implications on the duration of the growth period in early hominids.

The natural history of tooth wear, continuous eruption and periodontal disease in wild shot great apes

Abstract Wild-shot great apes comprising 65 lowland gorillas, 67 orang-utans and 60 chimpanzees were studied to explore the interrelationship between tooth wear, continous eruption and periodontal disease with increasing age. Observations on stages of tooth wear for upper and lower I1, M1, M2 and M3 were used as a broad scale of the increasing time a tooth was judged to have been in functional occlusion during the lifetime of the animal. Three measurements of combined alveolar and basal bone heights were made on mandibles and maxillae of male and female orang-utans and gorillas. These measurements suggest that there was no change in alveolar bone height during the period between young adulthood and old age in either sex or taxon. Measurements of total tooth height above the alveolar crestal bone remained more or less constant in all teeth measured in all taxa through successive stages of wear. Measurements of enamel height and of the amount of root exposed above the level of the alveolar bone demonstrate that with increasing tooth wear, tooth root emerges above the alveolar bone in a compensatory manner to maintain a constant height of tooth tissue. Eventual degeneration of the functioning dentition occurred in older animals when enamel was completely lost from the occlusal surfaces of the molar teeth and from the crowns of the incisors. Combined chronic pulpo/periodontal infections were judged to underlie final vertical alveolar bone and tooth loss in these great apes (probably at about 30–40 years of age).