Resurrecting Pompeii (40 page)

Metopism is a hypostotic trait. Hypostotic traits involve the retention of forms that are usually visible in the embryonic or early infant state, such as metopic sutures or inca bones. They involve arrested development or incomplete ossification. At birth, the frontal bone is made up of two halves separated by a suture. This suture functions as an area where growth can occur and also enables the two frontal halves to move relative to each other during birth. If this suture persists after the first two to three years of life, it is scored as present. Studies of mice and macaques as well as x-ray examination of families suggest that there is a genetic component to metopism. It has been suggested that this trait occurs with a higher frequency in females.
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Observations for metopic suture were made on 121 Pompeian skulls. Twelve cases or 9.9 per cent of the sample retained complete metopic sutures, 13.2 per cent of the sample exhibited some degree of metopism. One case displayed partial persistence at the parietal end of the suture and there were three cases of partial persistence at both ends of the suture. It is notable that Nicolucci reported a frequency of 11 per cent in the Pompeian sample he studied; a figure he considered to be high. Capasso recorded seven cases in the Herculaneum sample which reflected a frequency of 4.4 per cent in the sample he examined. Six of the cases were complete and one was an incomplete superior metopic suture. The frequencies observed in Pompeii and Herculaneum can be compared with those found in other populations (Table 9.4). Much of the comparative material was scored using the method of Berry and Berry but their definition is not clear about whether partial persistence would be scored as present.
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If one assumes that only total persistence was scored then the frequency of metopism in the Pompeian sample would be only 9.9 per cent.

The results of this study are relatively consistent with those of Nicolucci. The Pompeian incidence of this trait sits within the range reported for other Italian populations and is higher than the reported African samples. Possibly the most interesting observation is the fact that its frequency is more than double that of the Herculaneum sample, regardless of whether partial presence is included.

In the foetus, the inferior and superior squama of the occipital bone are separated by a suture that runs from asterion to asterion. This suture generally closes prior to birth, but if it persists into adult life it is classified as an inca bone. If the foetal sutures on the superior squama fail to unite, bipartite, tripartite or inca bones divided into four parts can result.
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It is important to note that inca bones were treated as a separate entity from ossicle at lambda in this study as the latter tend to be fontanelle bones and result from a different cause (see below). It is possible to distinguish inca bone variants from sutural bones by morphological features but there is considerable ambiguity in recording in the literature.
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Six cases of inca bone variants were observed in a sample of 116 Pompeian skulls, which means that this trait occurred with a frequency of 5.2 per cent. Only one case, or 0.9 per cent of the sample, exhibited full expression of this trait. Nicolucci observed one case of an inca bone on a skull he interpreted as female in his sample of 100 Pompeian skulls.
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In most populations this trait has been observed more frequently in males than females,
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though no relationship between this trait and gender could be ascertained for the

Pompeian sample. This is in no small part due to the dif ficulty in establishing sex from the skulls in this sample. As a result of this problem, there was usually no attempt to establish sex association for cranial non-metric traits in the disarticulated Pompeian sample.

Capasso recorded five cases of inca bone variants in the Herculaneum crania. He is not clear about the size of the sample upon which this study was based. Based on the frequencies he documented for other cranial nonmetric traits, he apparently examined 159 individuals, which means that the incidence of inca bones was 3.3 per cent. He observed that one case was a rare bipartite inca bone variant.
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It was extremely dif ficult to obtain appropriate comparative material for this trait (Table 9.5), which is testimony to its rarity. Non-metric studies of other Italian populations tended to only record ossicle at lambda, which may include inca bone variants, though the scoring system that was employed suggests that only fontanelle bones were recorded.
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Comparison of this trait with other samples is complicated by inconsistencies of scoring between scholars. The comparative data presented by Hauser and De Stefano gives a range of 3.7 per cent to 18 per cent for various European populations dating from the 1st–2nd millennium
_BC
to the medieval period. Interpretation of these data, however, requires some consideration of the definitions for inca bone for each sample. For example, a medieval French sample was recorded as having an incidence of 11.6 per cent based on the definition of Berry and Berry, which is very vague and does not distinguish between the ossicle at lambda, the preinterparietal bone, an inca bone variant that should be scored separately, and the inca bone. Similarly, the definitions for the medieval Bohemians and Alamannes also pooled these three variants. If, for comparative purposes, the preinterparietal bones were pooled with the inca bones in the Pompeian sample, the incidence would increase to 6.9 per cent. Inclusion of the ossicle at lambda as well would raise the Pompeian incidence to 25 per cent. Only inca bone variants were scored for prehistoric Ukrainians and first to second millennium
_BC
Lithuanians. The European and North African male and female samples and the Italian sample were all recorded in a consistent manner with the Pompeian sample.
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The Pompeian and Herculaneum incidence is higher than the Italian and European frequencies recorded for these samples.

Wormian bones are sutural bones or ossicles in the cranial vault. An ossicle is defined as any bone completely surrounded by a suture. They can be found, for example, in the coronal, sagittal and lambdoid sutures. Sutures permit very slight movement of the skull bones during birth to facilitate delivery. They also function as areas where postnatal bone growth can occur and contribute to the final shape and size of the skull. The embryology associated with the development of ossicles and fontanelle bones was discussed by Hauser and De Stefano. There is no satisfactory explanation for the function of ossicles, though there has been considerable discussion as to whether stress or pathology contributes to their presence.
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The practice of cranial deformation is apparently correlated with an increased frequency of lambdoid ossicles, as well as frequency changes in a range of other non-metric traits. It has been argued that this has little impact on the determination of biological distance between populations. El-Najjar and Dawson examined a sample of American Indian skulls with particular reference to the number of ossicles per side. They observed wormian bones on skulls that had not been subjected to cranial deformation but noted that in the cases of asymmetrical deformation, the number of wormian bones was higher on the side of deformation. There also was a positive correlation between an increased number of lambdoid wormian bones and the pressure associated with deformation. No significant side differences were observed in undeformed crania. It was concluded that cranial deformation could influence the development of lambdoid sutural bones but was not the sole factor that determined their presence. Ossicles are often found in undeformed skulls. In addition, skulls that have been intentionally deformed do not always contain wormian bones.
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There is an undoubted association between ossicles and the presence of some pathologies; for example they are almost invariably present and numerous in skulls of hydrocephalous individuals. It has been suggested that there is a link between the size, number and configuration of wormian bones and the presence of specific disorders. Similarly, this can be the case where there is delayed closure of sutures or fontanelle bones. A radiographic study of 81 skulls of individuals with osteogenesis imperfecta yielded a strong correlation between large numbers of wormian bones of a so-called ‘significant’ size, which are defined as more than ten bones of at least 6 mm by 4 mm which were arranged in a mosaic pattern.
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‘ Significant’ numbers of wormian bones have also been found in relation to a host of other disorders including cretinism, familial osteoarthropathy, kinky-hair syndrome and hypogonadism. Infantile-type osteoporosis, Down syndrome and rickets have been found in association with large sutural bones that do not necessarily occur in a particular pattern or number. It has been claimed that the presence of wormian bones is the result of developmental malfunctions, possibly with a genetic component. Cremin
et al.
suggested that the occurrence of more than ten ‘significant’ wormian bones may reflect an underlying environmental or genetic problem that has affected skull growth in the early developmental stages. Pedigree studies in humans suggest that wormian bone expression has a genetic component. Mouse studies appear to confirm that this trait is subjected to normal population variability.
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Fontanelle bones can be found at the bregma, lambda and the asterion. There can also be fontanelle bones in the anterior lateral fontanelles. It has been suggested that the purpose of ossicles at bregma is to protect the brain in late foetal and early natal life. No similar explanation has been advanced to explain the presence of other fontanelle bones.
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Ossicles are age-related in that sutures tend to be obliterated with advancing years.
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When scoring skulls with partially fused sutures in the Pompeian sample, ossicles were only counted as present when it was certain that they could not be artefacts of other phenomena, such as complex suture patterns.

Pardoe found a positive correlation between the six wormian bone traits he used in his study of Australian Aboriginal skeletal populations, from which he concluded that the presence of one sutural bone on a skull would increase the probability of there being further sutural bones on that skull. This correlation has also been reported by other scholars. A correlation has also been observed between the presence of lambdoid ossicles and inca bones. Sutural bone intercorrelation has also been reported by other scholars.
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The choice of which sutural bones would be studied was based on which bones could be identified without ambiguity and which were more likely to have survived, in order to provide the largest possible sample size. Squamo-parietal wormian bones were excluded as they were difficult to differentiate with certainty from post mortem damage. Additionally, the squamous portion of the temporal bone often had ‘sprung’, thus diminishing the possible number of observations.
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