Cranial suture closure and its implications for age estimation

htternafional Journal of Osteoarcbaeology, vof. 4: 193-207 (1 994)

Cranial Suture Closure and Its Implications for Age Estimation CATHERINE A. KEY', LESLIE C. AIELLOl AND THEYA MOLLESON2 Department of Anthropology, University College London, Gower Street, London WC1 E

6BT, UK; and 2Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW75BD. UK

ABSTRACT Three age estimation techniques using ectocranial andlor endocranial suture closure are tested on a sample of known age from Spitalfields, London in order to determine the value of cranial suture closure as an indicator of age at death. The three techniques are those proposed by Acsadi and Nemeskeri, Meindl and Lovejoy and Perizonius. Results indicate that the A d d i and Nemeskeri technique, which is based on endocranial sutures, can be used to distinguish young and middle- aged individuals in the Spitalfields sample but gives no information for crania over the age of 50 years. Age estimation using the Meindl and Lovejoy and Perizonius (Old system) techniques, which use ectocranial sutures, was found to be subject to a number of complicating factors, of which sexual dimorphism in the rate and pattern of closure is the most significant. A method of estimating age at death based on both endocranial and ectocranial suture closure is developed on the basis of the Spitalfields sample. The technique attempts to overcome some of the problems associated with both intra- and interpopulation variation in cranial suture closure. For a truly accurate age- estimation technique based on cranial suture closure we would need to know more about the causes and functions of suture closure in human populations.

Keywords: Cranial suture closure, age estimation, sexual dimorphism, variation.

Introduction

Cranial suture closure has been used for estimation since the sixteenth century,' yet its reliability is still debated. Although various methods using endocranial (internal) and/or ectocranial (external) fusion have been used to mark the progress of age, several studies have found age estimation based on cranial suture closure to be unreliable.2-8 Despite possible sexua12t9 and i n t e r p o p u l a t i ~ n ~ , ~ , ~ variation, three separate modern studies have resurrected cranial suture closure as an accurate indicator of age. Acsadi and Nemeskeri'O base their technique on endocranial union but warn that determination of age from suture closure is possible only between wide age limits. None the less they maintain that suture closure is an important age indicator, especially when combined with other age estimation methods. Although endocranial union is usually held to have the tighter relationship with age,' Meindl and Lovejoy' advocate the use of ectocranial sutures and suggest that their technique is accurate,

particularly in the older age ranges. Perizonius" suggests that older ( > 50 years) and younger ( < 50 years) individuals warrant different methods and has proposed a system that uses different endocranial and ectocranial sutural sites according to approximate age group.

T h e purpose of this paper is to test these three methods of age estimation on a large sample of human crania of known age and sex. This exercise involves analysis and discussion of the relationship between suture closure and age in the sample, taking into consideration pattern of closure, sexual dimorphism of closure and variation in the rate and pattern of closure both within and between populations. Furthermore, broader issues in the prediction of age from skeletal parameters will be addressed. Based on the results of this analysis a new ageing technique using suture closure is proposed.

Sample The sample of I83 skulls used in this study was drawn from a total sample of 387 named individuals

CCC 1047-428X/94/030193 - 15 0 1994 by John Wiley Sons, Ltd.

Received 27 lune 1993 Accepted 10 March 1 9 9 4

194 C. A. Key, L. C. Aiello and T. Molleson

Table 1. Age and sex distribution of 183 crania from the Soitalfields collection used in this study.

~ ~ ~~ ~~

Age group Males Females Total 10-14 1 2 3 15-19 2 5 7 20-24 2 1 3 25-29 2 8 10 30-34 3 2 5 35-39 6 5 11 40-44 1 2 3 45-49 2 8 10 50-54 10 9 19 55-59 8 12 20 60-64 10 8 18 65-69 13 11 24 70-74 11 7 18 75-79 6 11 17 80-84 2 5 7 85-95 2 6 8

Total 81 102 183

that were excavated between 1984 and 1986 from the crypt of Christ Church, Spitalfields, London. The sample of 183 includes all those adults that had complete enough crania to collect a full set of suture closure data. It is unlikely that this introduced a significant bias into the sample. There is no evidence to suggest that the state of preservation in adult crania is related to cranial strength and that this is related to degree of suture closure. l 2

The Spitalfields sample dates from the eighteenth and early nineteenth centuries and consists of a mixture of Londoners and descendants of French Huguenot immigrants to England. Names (and therefore sexes) and ages were derived directly from coffin plates, and ages were confirmed where possible by cross-checking dates in baptismal and burial registers. It has also been possible to reconstruct many aspects of the personal history of these people such as parity, social class, occupation and the cause of death.I2 The Spitalfields collection is currently housed at the Natural History Museum, London. The age and sex distribution of the sample is given in Table 1, which presents the sample divided into arbitrary five-year age categories.

Method

The sutures of each skull were examined ectocranially and endocranially under a strong

light using a hand-held magnifying glass. Endocranial assessments required the additional use of a small torch and a dentist’s mirror. Thirty- six sites were chosen for analysis ectocranially and I8 endocranially (even with the use of a dentist’s mirror only the sutures of the vault could be seen clearly) as shown in Table 2 and Figure 1 . All assessments of suture closure were taken by the same individual (CK).

Twenty-five of the crania were re-scored for degree of suture closure 18 months after the original assessment in order to assess intra-observer error. This was determined using Martin’s’3 system of scoring suture closure, i.e. the system used by Acsidi and NemeskCri and by Perizonius. Each suture site was scored on a scale of 0-4, where 0 represents a suture that shows no trace of closure, t represents incipient closure, 2 represents closure in progress, 3 represents advanced closure and 4 represents a suture that is completely fused. I t should be pointed out that these categories are subjective non-measurable quantities which simply use numerals as class identifiers. Diagrams of these closure stages can be found in Martin13 and in the recommendations for age and sex diagnosis of the skeleton prepared by the ‘Workshop of European Anthropologists’.’4 The assessment error for each suture closure site was determined for each cranium by subtracting the closure score obtained in the second assessment exercise from that achieved in the first. The mean assessment score was determined by averaging the absolute deviation for each assessment site across the sample:

where E, is the mean assessment error, S1 the original assessment score, S2 t he second assessment score and N the sample size.

The mean assessment error at each site is shown in Table 3. The average error over all the endocranial sites is 0.24, with a range of 0.04-0.52. Ectocranially, the mean assessment error is 0.47, with a range of 0.08- 1.12. T h e greater accuracy of the endocranial assessments most probably reflects the ’all or nothing’ nature of closure at these sites. Endocranial sutures tend to be either

Cranial Suture Closure 195

Table 2. Suture sites used in this analysis.' Checks indicate the specific suture sites used by the various techniques. Where suture sites occur on both sides of the skull (e.9. C2) the mean score is used, unless indicated by an asterisk.

AN ML: V ML: LA P: Y P: 0

Suture site END ECT END ECT END ECT END ECT END ECT

Coronal: bregmatica c1 J *

complicata c 2 I / *

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Bregma B Sagittal:

bregmatica s1 vertis s2 obelica s3 lambdica s4

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lambdica media asterica

L1 L2 L3

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Sagirral suture F s2

l-21 L11

Squamous suture Coronal suture I

Lambdoid suture Sphenoremporal suture

Figure 1. The cranial sutures, indicating the sites chosen for analysis. Endocranial sutures were assessed at endocranial sites corresponding to the ectocranial sites shown.

196 C. A. Key, L. C. Aiello and T. Mofleson

Table 3. Mean assessment error for the suture closure sites used in this analysis. Assessment error is based on the Martin'3 scoring system. Two figures are given where the error was assessed on both sides of the skull, in these cases the figure prior to the slash corresponds to the right-hand side.

Suture Endocranial Ectocranial site error error

c1 0.2410.08 0.0810.12 c2 0.2810.08 0.3610.32 c3 0.04lO.04 0.4810.32 B 0.12 0.48 s1 0.24 0.24 s2 0.20 0.44 s3 0.28 0.32 s4 0.20 0.36 L 0.40 0.92 11 0.5210.44 0.8810.76 L2 0.2810.1 6 1.1 210.68 L3 0.3610.28 1.0010.64 oc1 0.1 210.12 oc2 0.6410.68 OC3 0.6410.72 SQ1 0.0810.12 SQ2 0.2410.08 P 0.5210.64 SF 0,4010.36 ST1 0.2810.72 ST2 0,3610.56

completely open or completely closed, whereas ectocranially sutures close more gradually. Error i s most common when distinguishing the stages of closure in between open and fully closed. The assessment error also includes a proportion of error involved in the relocation of the specific part of the suture for assessment. The location of the closure sites was determined visually and as a result the assessment in the second exercise may not have been at the precise location of the first assessment.

The estimated age at death based on the degree of suture closure was determined for each skull using the techniques described below.

Analysis 1: the Acsadi and Nemeskeri system

The Acsidi and Nemeskeri (AN) system is based on endocranial suture closure and utilizes those endocranial sites indicated in Table 2. Each endocranial suture site is scored on the basis of Martin's scale'3 as described above. The AN ages for each skull are determined on the basis of the mean closure score averaged over the 16 endocranial sites used in this technique. Table 4 gives the AN key for equating the mean closure scores to real ages. For example, in the AN reference sample those skulls with mean closure scores between 0.4 and 1.5 (mean closure stage 1) have a mean real age of 28.6k 13.1 years (range 15-40 years). Using the AN technique the closure scores were determined for each of the 183 Spitalfields crania. For each AN mean closure stage, the mean age, standard deviation and age range for the Spitalfields crania are also given in Table 4. It is apparent from this table that the greatest difference between the AN sample and the Spitalfields sample occurs in age-stages 2 and 3, where the Spitalfields ages are considerably, although not significantly, higher than the AN mean ages. In fact there is no significant difference in mean ages or age variance between the Spitalfields and AN samples for any of the age stages.

There is a progressive increase in mean known age for the Spitalfields crania from age-stage 1 to age-stage 5. However, only three of these stages are significantly different from each other. Kruskal-Wallis tests of significance demonstrate that age-stage 1 is significantly different from all of the other age stages ( p < 0.05) and age-stage 2 is significantly different from the oldest age-stage, age-stage 5 (p<0.05). There is no significant

Table 4. The Acsadi and Nemeskbri system.

Acsadi and Nemeskeri Spitalfields

Stage Mean closure Mean age Range Mean age Range number score (years) SD N (years) (years) SD N (years) 1 0.4-1.5 28.6 13.1 16 15-40 30.9 11.6 8 18-55 2 1.6-2.5 43.7 14.5 29 30-60 52.9 16.3 14 27-79 3 2.6-2.9 49.1 16.4 17 35-65 59.7 18.0 9 39-86 4 3.0-3.9 60.0 13.2 162 45-75 60.9 13.1 33 31 -88 5 4.0 65.4 14.1 61 50-80 65.6 12.5 61 35-92

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difference between age-stages 2, 3 and 4, or between age-stages 3, 4 and 5. At best then, the AN technique provides only an ageing system that roughly sorts the Spitalfields crania into younger, middle-aged and older grades. If a skull falls into age-stage 3 or 4 the only conclusion that can be drawn is that it is probably older than skulls scored at age-stage 1. The relatively poor correspondence between the AN age stages and known age reflects the overall poor relationship between the AN mean closure scores and known age (Figure 2).

This variation in the relationship between mean closure score and known age does not have an obvious explanation, such as sexual dimorphism, in pattern of closure or the lack of correlation between closure at individual suture sites and age. Sexual dimorphism in closure was tested for each suture site using Wilcoxon signed rank tests. The results indicate that there is no significant difference in rate of closure between the sexes at any of the suture sites. The relationship between suture closure score and age was tested for each suture site using Spearman rank correlations and dividing the sample into 16 arbitrary age categories of 5 years each. All correlations between age rank and mean A N closure score for that rank were highly significant.

Another source of error that could be related

to the variation in the overall relationship between AN mean closure score and known age (Figure 2) is assessment error. The mean and range of error in the assessment of endocranial suture closure is relatively small (Table 3) and, in itself, is not sufficient to explain the relatively poor relationship between the overall mean closure score and known age. The only conclusion that can be drawn at the present time is that the variation seen in this relationship represents individual variation for which there is no obvious explanation.

An interesting feature of endocranial suture closure emerges when the pattern of suture closure is examined (Figure 3). The Spitalfields data suggest that endocranial suture union begins at a young age, and is completed early relative to overall lifespan. Endocranial closure of the coronal and sagittal sutures begins at around 20 years and is complete in most individuals (80 per cent) by 30-35 years of age!. Closure of the lambdoid suture appears to start about a decade later, and is completed in most members of this population by 50 years of age. From these observations it would be expected that endocranial suture closure would only reflect age in younger individuals. The pattern of suture closure shown in Figure 3 illustrates another important feature. Even though endocranial suture closure is complete in most of the population by 50 years of age, there are

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individuals with open sutures who are over 50 years of age. Such individuals are subject to gross underestimation of age. There are also some individuals that show accelerated closure and whose ages are likely to be overestimated.

In summary, the Spitalfields sample confirms that the AN technique of determining age from the degree of closure of the endocranial sutures can be applied with some confidence in determining age at death. However, this study also points out its limitations and emphasizes the fact that the AN technique can be used to provide only very broad age grades, distinguishing young from middle-aged from older crania. It is not a technique with which to assign specific age in years to individual crania.

Analysis 2: the Meindl and Love joy system

In contrast to the Acsadi and Nemeskeri technique, which uses the endocranial sutures for age estimation, the Meindl and Lovejoy (ML) technique uses the ectocranial sutures. Meindl and Lovejoy argue that the endocranial sutures are not

only easier to examine but also are better for ageing older individuals. In devising their technique, Meindl and Lovejoy defined two sets of sutures; the lateral-anterior (LA) system, which is based on coronal and sphenoid suture sites (C2, P, SF, ST1 and ST2), and the vault (V) system, which is based on lambdoid, sagittal and coronal suture sites (L2, L, S3, S1, B, C2 and P) (Table 2 and Figure 1). Meindl and Lovejoy favour the LA system, suggesting that the suture sites used in this system demonstrate a protracted sequence of closure, correlate moderately with age and that each suture site gives information about different age periods.

Each of the 183 Spitalfields crania was scored at all of the suture closure sites specified in both the LA and the V systems using the four-point system devised by Meindl and Lovejoy (0 = completely open, 1 = up to 50 per cent fusion, 2 = 51 -99 per cent fusion, 3 =complete fusion). For paired suture closure sites (such as L2 or S3), the closure score was determined individually for the sites on both sides of the cranium and then averaged to produce a single score. The Spitalfields crania were analysed for differences between the right and left sides using the Wilcoxon signed rank

Table 5. The Meindl and Lovejoy systems.

(a) The lateral anterior system.

Meindl and Lovejoy: LA Spitalfields

Stage Composite Mean age Range Mean age Range numbers closure score (years) SD N (years) (years) SD N (years)

1 32.0 8.3 2 36.2 6.2

3-5 41.1 10.0 6 43.4 10.7

7-8 45.5 8.9 9-10 51.9 12.5

11-14 56.2 8.5

(b) The vault system

18 19-48 54.8 16.1 17 21 -85 18 25-49 68.7 16.0 12 37-92 56 23-68 58.5 14.3 29 34-86 17 23-63 66.5 10.9 12 48-80 31 32-65 63.5 10.8 24 47-77 29 33-76 65.9 14.6 26 34-85 24 34-68 71.2 9.4 5 60-81

Meindl and Lovejoy: V Spitalfields

Stage Composite Mean age Range Mean age Range number closure score (years) SD N (years) (years) SD N &ears)

1 1-2 30.5 9.6 12 18-45 51.4 21.3 16 18-85 2 3-6 34.7 7.8 30 22-48 55.3 14.7 33 27-92 3 7-1 1 39.4 9.1 50 24-60 59.0 16.2 43 19-88 4 12-15 45.2 12.6 50 24-75 62.0 11.7 25 35-79 5 16-18 48.8 10.5 31 30-71 64.1 13.0 19 34-86 6 19-20 51.5 12.6 26 23-76 71.9 9.1 14 53-85

SD =standard deviation; N = sample size.


test. Although skulls of both sexes show considerable asymmetries, there was found to be no significant bias towards advanced closure on the right or left sides. For each system, the Meindl and Lovejoy composite score was calculated as the sum of the individual closure scores for each site. inferred age was determined for each Spitalfields cranium by comparing the composite score to the key provided by Meindl and Lovejoy (Table 5a and b).

For each Meindl and Lovejoy composite score stage, the mean known age, standard deviation and range for the Spitalfields sample is given in Table 5a for the LA system and in Table 5b for the V system. It is immediately clear from both of these tables that the Spitalfields mean ages are considerably different from the Meindl and Lovejoy mean ages for each composite score stage. For both the LA and the V systems t-tests establish that the Spitalfields mean ages are significantly higher than the Meindl and Lovejoy mean ages (p< 0.01). This indicates considerable interpopulation difference between the Meindl and Lovejoy sample and the Spitalfields crania. The Spitalfields sample shows delayed suture closure in relation to Meindl and Lovejoy's sample and would therefore be subject to underestimation of age using this technique. In fact the maximum mean age for the Meindl and Lovejoy techniques is 56.2 years, whereas over half the Spitalfields population is older than this (Table 1). Further- more, for the LA system the Spitalfields mean ages show no trend of increasing age from stage 1 to stage 7. The V system does show a trend of increasing mean age. However, Kruskal-Wallis tests indicate that the only significant differences between the composite score stages are between stage 1 and stage 6, the oldest and the youngest stages. Stages 2-5 are not significantly different from each other or from stages 1 and 6, therefore they carry no relevant age information. The weak relationships between composite suture closure score and known age in the Spitalfields sample in both the LA and V systems are illustrated in Figures 4a and 4b. As with the Acsidi and Nemeskeri system, these results could also be related to assessment error, to sexual difference in closure rate, to poor relationships between rate of closure and age in certain sutures and to the fact that some closure sites may be better or worse indicators of age than other sites.

The mean assessment error of the ectocranial sutures is 0.47 of an assessment category. Considering only those suture sites used in these techniques, the average assessment errors are 0.56 for the V system and 0.45 for the LA system. The error test was carried out on the Martin'3 system of assessment. Although the ML systems use a four-point rather than a five-point scale of closure, the error would be expected to be comparable. The lambdoid suture is particularly subject to error (Table 3), which would account for the higher figure for the V system. Such a level of error should be considered an important factor. However, it is unlikely to be the sole cause of the poor relationship between composite score and age found in this sample.

Of even more significance are the sexual differences in the timing of closure of specific ectocranial suture sites. Wilcoxon signed rank tests reveal significant sex differences at several of the ectocranial sites: C2r, S 3 , S4 ( p < O . O l ) ; C l r , S2, L1 r, L31, PI, SFr ( p < 0.05); L, L3r, OC2r, 001, Pr, SF1 ( p < O . l ) . At many other sites the probabilities were low enough to suggest sex differences but not low enough to be significant. When these probabilities are combined's a clear overall sex difference was found whereby males exhibit more advanced closure than females

The sex differences found in the Spitalfields sample go beyond a mere divergence in the rate of closure. The relationship between known age and suture closure was determined for each suture site, for each sex, by dividing the sample into 16 age categories and determining the Spearman rank correlation coefficient, rs, between age rank and composite suture closure stage (Table 6). In females suture closure progresses with age at most ectocranial suture sites, where 17 out of the 21 (81 per cent) sites showed significant correlations ( p < 0.05). However, males have significant correlations ( p < 0.05) at only 1 1 out of the 2 1 sites (52 per cent). Closure sites on the coronal and sagittal sutures show the greatest difference between males and females. For those specific sites used in the Meindl and Lovejoy V system, females have significant correlations at six out of seven (86 per cent) sites, whereas males have significant , correlations at only two out of the seven sites (29 per cent). For the LA system, males have

( p < 0.01).

Cranial Suture Closure

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significant correlations in four out of the five sites (80 per cent), whereas females show significant correlations a t three out of the five sites (60 per cent). In the Spitalfields sample, therefore, a considerable sexual dimorphism in ectocranial suture closure affects the applicability of the ML techniques.

In both sexes, but especially in males, there are many individuals whose ectocranial sutures do not close until fairly old age and incomplete closure is common. It is important to realize that there is considerable variation in ectocranial suture

closure in almost all age ranks. In particular, there are some people whose sutures close very early (i.e. in their late teens) and there are others who reach old age with completely open sutures.

In summary, these results indicate that sexual dimorphism in the pattern and rate of ectocranial suture closure is a characteristic of the Spitalfields sample. This is particularly true at closure sites on the coronal and sagittal sutures, where only females tend to show a significant relationship of closure stage and known age. These sex differences, alongside measurement error,

202

Table 6. Correlation between age and closure score for each ectocranial suture site. Spearman rank correlation coefficients for the relationship between age and suture closure score at the ectocranial sites used in this analysis. V = those sites used by the Meindl and Lovejoy (ML) vault system and LA = those sites used by the M L lateral-anterior system. The sample was divided into 16 age classes of 5 years each. The age breaks and sample sizes for each age class are given in Table 1. Correlations are between age class and composite suture score for each class.

Spearman rank correlation coefficient Suture ML site system All crania Females Males

c1 c2 V,LA c3 B V s1 V s2 s3 V s4 L V L1 L2 V L 3 oc1 o c 2 OC3 SQ 1 SQ2 P V,LA SF LA ST1 LA ST2 LA

0.85' 0.91"' 0.91"' 0.94"' 0.74." 0.78"' 0.84' 0.87"' 0.83' 0.89' * 0.94' 0.82"' 0.56' 0.84*" 0.90* 0.59' 0.32 0.79' * * 0.75' 0.74' 0.76"'

0.91"' 0.91"' 0.91"' 0.94"' 0.86"' 0.71"' 0.84"'

0.81 * * * 0.93"' 0.96'"' 0.91"' 0.57' 0.77"' 0.90"' 0.00 0.00 0.20 0.78"' 0.58' 0.37

0.829 * *

0.30 0.42 0.63' 0.31 0.05 0.33 0.18 0.36 0.33 0.50" 0.70" 0.52' 0.44 0.49 0.74"' 0.39 0.35 0.54' 0.57' 0.72' 0.80"'

***p<0.001; **p<o.oi; *p<o.o5.

contribute to the low correlation between composite score and age in the entire sample and seriously affect the utility of either the V system or the LA system (Fig. 4).

Analysis 3: the Perizonius system

The previous two analyses suggest that techniques based on endocranial suture closure might be more universally applicable than techniques based on ectocranial suture closure. That is to say that ectocranial suture closure is more sensitive to interpopulational variation than is endocranial closure. This suggestion is also supported by the Perizonius system.

The Perizonius system uses the Martin five- point scoring system and utilizes both endocranial and ectocranial sites. It differs from the previous two ageing systems in being two separate systems,

C. A. Key, L. C. Aiello and T. Molleson

one for younger individuals (< 50 years) and the other for older individuals (>SO years). The system for the younger individuals (Y system) is based on the closure of selected endocranial suture sites, whereas the system for older individuals (0 system) is based on a combination of endocranial and ectocranial suture sites (Table 2, Figure 1). Age is determined on the basis of the composite suture closure score. For the Y system this score is the mean score for the relevant endocranial closure sites. For the 0 system it is the mean score for L3r and L31 subtracted from the mean score for the remaining relevant closure sites. Table 7 gives the Perizonius key for translating mean closure score into age at death. As in the previous analyses, this table also gives the mean age, standard deviation and range for the Spitalfields crania for each closure stage.

There are a total of 53 individuals in the Spitalfields sample who were under 50 years of age at death. Of these, only 25 crania (47.2 per cent) had Perizonius Y system closure scores (Table 7a) that indicated that they were actually under 50 years of age. This system fails to recognize over half of the young crania in the Spitalfields sample. For those crania that it does recognize as under 50 years of age, there is a close correspondence between suture closure stage and known age. The only significant difference is at suture closure stage 5, where the Spitalfields crania are significantly younger than the Perizonius sample.

When the Perizonius 0 system is applied to the 130 individuals in the Spitalfields sample with known ages over 50 years of age, the Spitalfields mean ages show no pattern of increase of age with increasing suture closure score (Table 7b). This lack of correspondence between Perizonius 0 system closure scores and known age for the Spitalfields sample is not unexpected. The previous analyses have demonstrated that in the Spitalfields sample the endocranial closure sites ( 0 , S l ) are largely closed by age 50 and, therefore, would not be expected to give any significant information after this age. The ectocranial sites (C2, L3) are strongly affected by sexual dimorphism in closure. At C2, in particular, there is no significant relationship between closure score and known age in males (Table 6).


Table 7. The Perizonius system.

(a) The young system.

Stage Closure number score

Perizonius: young Spitalfields

Mean age Range Mean age Range (years) SD N (years) (years) SD N hears)

1 0.6-1.1 2 1.2-1.7 3 1.8-2.3 4 2.4-2.9 5 3.0-3.5 6 3.6-4.0

27.5 0.6 4 27-28 34.5 2 34-35 36.0 1 35.3 1.5 3 34-37 39.4 2.0 8 37-43 47.0 1.2 7 45-48

- 26.3 4.4 4 33.0 5.7 2 38.0 6.6 7 42.0 4.7 6 44.3 4.1 7 45.4 3.5 14

- -

(b) The old system.

Perizonius: old Spitalfields

Stage Closure Mean age Range Mean age Range number score (years) SD N (years) (years) SD N (vearsl

1 < 0.0 59.3 6.9 4 56.7 4.9 6 51-63 70.0 10.8 17 51 -80 66.7 9.7 16 52-88

2 0.0-0.5 59.6 6.6 7

72.4 9.6 12 53-87 3 0.6-1.1 65.2 9.4 40

60.7 8.8 13 51-79 4 1.2-1.7 68.4 9.1 70

75.3 4.7 3 70-79 5 1.8-2.3 67.5 9.2 32

65.1 9.9 28 50-92 6 2.4-2.9 70.7 11.7 32

61.2 12.7 6 53-86 7 3.0-3.5 73.8 8.9 29 8 3.6-4.0 70.0 5.7 2

SD = standard deviation; N = sample size; =data unavailable.

.

In summary, the Perizonius system is of very limited use in ageing the Spitalfields sample. The 0 system shows no relationship with age for the older individuals and the Y system recognizes only 47.2 per cent of the known young individuals in the sample. For those individuals that it does recognize as under 50 years of age, there is a good correspondence between the Perizonius inferred age and known age a t death and this correspondence is based on the closure of the endocranial sutures.

Discussion

The main point that the previous analyses have demonstrated is that suture closure ageing techniques developed on one sample do not necessarily result in accurate ages when applied to another sample. This seems to be particularly true for techniques such as the Meindl and Lovejoy vault and lateral-anterior systems and for the Perizonius system for older individuals, which are based on the closure at ectocranial suture sites. Analysis of the Spitalfields sample suggests that

there can be considerable sexual dimorphism in the rate of closure at these sites. The fact that both the Meindl and Lovejoy and the Perizonius systems use these techniques implies that the correlation between suture score and known age was higher and that the dimorphism may not have been as extreme in their reference populations as i n the Spitalfields sample. Likewise, the discrepancy in the literature, where Todd and Lyon6 and Krogman‘6 maintain that there is no difference between males and females whereas Brooksf2 Singerg and Senyurek” recognize dimorphism, suggests that there is considerable interpopulation difference in the dimorphism of ectocranial suture closure. This problem does not appear to affect the endocranial sutures to the degree that it does the ectocranial sutures and, as a consequence, the endocranially based systems appear to give more accurate ageing results when applied to different populations.

It is clear that the endocranial suture sites as well as the ectocranial sites show a considerable range of closure scores in any given age group and, conversely, a considerable range of known ages for any closure score. It is also clear from the


Spitalfields analysis that this variation cannot be explained entirely by assessment error or by the lack of overall correlation between closure and age. This variation has been encountered in most studies of endocranial suture closure and has led to the conclusion that suture closure is one of the least accurate of the skeletal ageing techniques. However, this variation is not necessarily a fatal flaw in determining reasonably accurate age at death from suture closure data. W e believe that a primary problem with existing techniques is their reliance on mean closure scores for age determination, rather than the assessment of individual suture closure sites, which would provide a check o n the consistency of predicted ages. For example, if the individual sites gave conflicting ages, this would indicate that pattern of closure is different from the reference sample and any resulting overall age determinations would be likely to be inaccurate. Such a principal of consistency would help guard against the effects of both inter- and intrapopulation variation in pattern and rate of suture closure.

In order to test this idea we devised a new suture closure ageing method that is based on the Spitalfields sample and involves those suture sites that show the most significant relationship with age in this sample. This system uses a simpler scoring system than that used in previous analyses. Errors in the assessment of closure primarily result from difficulty in assigning closure scores to sutures in the process of closing. W e have reduced the grades of closure (and thereby the error in distinguishing between them) by using a simple three-point system where 0 = completely open, 1 =closure in progress (equivalent to a Martin score of 1 or 2), and 3 = nearly closed to complete obliteration (equivalent to a Martin score of 3 or 4). This reduction is based on Kruskal-Wallis analyses, which demonstrate that at most sites there is no significant difference in age between Martin scores of 1 and 2 and to a lesser extent between Martin scores of 3 and 4. It is also important to point out that for nearly every suture site, especially ectocranially, a score of 0 (an open suture) occurs with nearly equal probability in all age groups. Because we are just as likely to find open sutures in a 60 year old as a 20 year old, we must conclude that open sites afford little information regarding age.

The technique is based on those suture sites where age was highly correlated with suture closure (Table 6). For each suture, the probability of being age a, given a suture score of i, was calculated for each sex. Because Spitalfields is an aged population, with 71 per cent of the sample older than 50, the probable ages calculated for each indicator stage were biased toward the older age range. This difficulty has been highlighted by Boquet-appel and Masset'* and more recently by Aiello and Molleson'g and Molleson et ~ 1 . ~ 2 , who have found that the age estimation techniques that work best are those where the age distribution of the test sample most closely resembles that of the reference population (that is, the population from which the technique was derived). This problem is avoided if sample sizes from each age range are trimmed so that they are equal, however, this is clearly wasteful of data. Rather than this, the Spitalfields sample was divided into over 50 year olds and under 50 year olds and the probability of being age a given a score of i recalculated for each age-sex subset. By manipulating the data in

Table 8. The ectocranial suture sites and associated scores that can be used to predict age.

(a) Individuals less than 50 years of age.

Females Males

Suture Score Age Suture Score Age

c2 1 >34 c3 1,2 >34 2 >44

S 0 < 35 1 >34 2 >34

L2 1 >34 L2 1,2 >34 2 >44

OC3 1 >34 SF 1 >34

2 >44 P 1 >34 P 1,2 >34

2 >44

(b) Individuals greater than 50 years of age.

Females Males

Suture Score Age Suture Score Age

c 2 2 >69 C3 0 < 65 S 2 >64 2 > 59 L2 2 > 69 P 0 < 75 OC3 1 < 70 1,2 >59

2 >59 OC3 1,2 >59 SQ2 1.2 >59

S = mean closure score of the sagittal suture.


this way the age distribution within each subset becomes more uniform and hence avoids bias towards a particular age range. Endocranial sutures can be used to assign an unknown cranium to the appropriate age group. Those ectocranial sutures that could predict age with a probability of 70 per cent or greater for a given suture score were chosen as useful age indicators (Table 8a and b).

The application of the technique assumes that the sex of the cranium has been determined already. The two steps in age assessment are firstly to determine whether the cranium belongs to the over 50 or under 50 age group, and secondly, to refer to the appropriate table for more specific age assessment. These steps are carried out according to the following method.

(i) If endocranially C2 = 1 , S2 = 0 and/or S3 = 0 then the individual was probably under 50 years of age. Table 8a is the appropriate table for more specific age determination.

(ii) If endocranially C2=2, and/or S2 and S3 show any signs of closure but there is no activity on the lambdoid suture then the individual was probably between 30 and 50 years of age. Table 8a is the appropriate table for more specific age determination.

(iii) If the coronal and sagittal endocranial suture sites are closed and there is any closure at L2 (endocranial), P, SF and/or OC3 (ectocranial) then the individual was probably over 50 years of age. Table 8b is the appropriate table for more specific age determination.

(iv) If the appropriate table gives widely conflicting results, then the skull is displaying an order of closure different to the average pattern found in the Spitalfields sample. In this case age estimation should not be attempted.

This method does not claim to give specific age estimates but is designed to predict age ranges, which could be valuable in conjunction with other ageing techniques. This new suture closure technique has been tested on a population of South African blacks (85 males and 74 females) of known age from the Dart collection at the University of the Witwatersrand, South Africa. Age estimates were not attempted on skulls whose pattern of suture closure was very different to that

found in the Spitalfields sample. These cases were readily apparent because different sites gave quite different age estimates. Eleven per cent of the sample (1 8 out of 159 crania) was not appropriate for age estimation. With these cases removed, age predictions were accurate for 70 per cent of the male sample and 65 per cent of the female sample. These are quite encouraging results, considering the inaccuracies of the established techniques when applied to the Spitalfields sample. The ultimate utility of this new technique will depend on further testing on populations with widely varying geographical, temporal and ethnic associations. None the less, it does have the strong advantage of being able to recognize at least some of the crania in the sample for which the technique is inappropriate.

Comments and conclusions

This study has tested three techniques that use cranial suture closure as an age indicator and also introduced a new method based on closure scores for individual suture sites rather than composite scores. The problems encountered by the use of cranial suture for age estimation, which include variability, dependency on the age distribution of the reference sample and sex differences, plague most methods of age estimation.I7 However, they are not a reason for despair but a challenge for the creation of more sophisticated methodologies. The question of whether endocranial or ectocranial sutures are better indicators of age has obscured the need to use both depending on the approximate age and sex of the individual. This study suggests that suture closure may at least assort skeletal material into age ranges, which may then lend themselves to more specific ageing techniques. A strong reason for using cranial suture closure as an ageing technique is that crania are durable in archaeological populations.

By studying cranial suture closure in its capacity for age estimation, questions arise as to the function of suture closure, reasons for the extreme variability and the extent to which genetic and/or environmental factors affect the rate and order of closure.

It has been assumed in studies such as this that there is a progressive ossification of sutures with


age, resulting in a linear relationship between age and suture closure. However, implicit in this is the assumption that suture closure does not have selective value. Studies such as this do not show how cranial suture closure varies with age (which could be ascertained only via longitudinal studies of live persons) but rather how suture closure varies with age at death. This study found a high frequency of deaths amongst juveniles with advanced suture closure, and a high number of individuals surviving into old age with open sutures. This pattern of precocious (early) and retarded suture closure has been observed in many collections and such cases have often been discarded as deviations.6 Powers5 has described the authenticated skulls of four Dutchmen more than IOO years old at death who died with completely open sutures. Perizonius' found such a marked drop in the degree of closure in older individuals that he was drawn to comment: 'the question forces itself whether selection does occur. Do individuals with open sutures have more chance to grow old?'.

The selection hypothesis also lends insight into the variability of closure. In theory evolution should confer maximum fitness, which in this case would imply that ectocranial sutures should remain open throughout the human lifespan. In reality, evolution has ensured that most humans have open ectocranial sutures up to the age of about 30, by which time most people have produced one or more offspring, and those individuals whose sutures are 'programmed to close later in life have already passed on their 'close suture' genes. Hence 'close suture' genes slip the evolutionary net into the gene pool of the next generation and ensure suture closure variability in the adult population.

If these speculations are correct then what is the function of suture closure? Few studies have addressed this question. Retzlaff et aL20 suggests that the function of cranial suture ligaments in primates is to bind together the adjacent edges of cranial bones whilst permitting bone movement. O n the other hand, environmental determinants may also be important. For instance, Kimbel and Rak2' have looked at the asterionic region in humans, apes and australopithecines and conclude that temporalis strength is important in determining the suture pattern of this region.

Further study is required, especially on other primate species, if we are to understand the function of suture closure, its selective advantages or disadvantages, and its implications for age estimation. This understanding is surely required if we are ever going to be able to use cranial suture closure as an accurate ageing technique.

Summary

This study has demonstrated that cranial suture closure can be used to predict age at death. However, the Spitalfields sample suggests that all sutures, ectocranial or endocranial, are not equally suitable for ageing. The following specific conclusions can be drawn. Firstly, because of the large degree of variability in suture closure with age only broad age ranges can be inferred. Secondly, endocranial sutures can be used as determinants of age only up to about 50 years. After this age there is little activity in endocranial suture closure. Thirdly, ectocranial sutures can show different rates and patterns of closure in males and females. In particular, there is no correlation in the Spitalfields sample between ectocranial suture closure and age at many of the suture sites in males, and fourthly, open ectocranial sutures, in both Spitalfields males and females, occur at all ages with equal frequency and cannot be taken as an indication of a young age.

These analyses suggest that cranial suture closure may be used to age crania if the above points are taken into account, but can give no more than broad age ranges. Based on these conclusions, a new method for inferring age from suture closure has been developed using the Spitalfields data. This method has been tested on a large sample of South African blacks. The results are sufficiently promising to merit further study in this area. It is suggested that research should be directed towards the understanding of the function of suture closure in order to make further progress in age estimation.

Acknowledgements

We would like to thank Maceji Hennenberg for access to the Dart collection housed in the Department of


Anatomy, University of Witwatersrand. W e are grateful to two anonymous referees for helpful comments on earlier versions of this manuscript and would also like to thank Keith Robinson of the University College London central computing services for his crucial computer assistance. Part of this research was funded by Leverhulme grant no. F134BB and a University of London Central Research Fund grant to LA.

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Cranial suture closure and its implications for age estimation