A number of non-metric traits are commonly recorded in order to examine population differences and associations. The traits generally have no known biological significance.
The frontal bone of the cranium is divided vertically in the mid-line at birth by the metopic suture [Photo 0042]. The suture usually fuses in early childhood but occasionally the two halves remain separate in a condition known as metopism. Metopism occurs in approximately 10% of modern Caucasoids and Mongoloids and in 2% of Negroids (Krogman and Iscan 1986, 119) although casual observation by a Dumfries consultant radiologist suggests a much lower frequency in modern Southwest Scotland (Dr D Jones, pers. comm.). Retention of the suture is usually held to be genetically determined (e.g. Torgerson 1951).
Complete metopism was found in 20% (17/86) of frontal bones of those over six years of age and is compared with those of other groups (Table 36). This table indicates a high rate of metopism in Aberdeen compared with English sites, and a general increase in frequency with northern latitude. The outstanding exception is Glasgow Cathedral where only 3% displayed metopism. The reason for this is unclear although it does seem that the age distribution of the Glasgow sample was relatively old, with approximately one quarter of the total sample living to be 'mature adults', the oldest age category employed in that study (King 2002). Cranial sutures become obliterated with age and it seems possible that some metopic sutures may have been 'lost' in this way.
Data from the Isle of Ensay and Linlithgow may be seen to support the genetic argument. One quarter of crania from the earliest 3 levels at Ensay (approximately 1500 AD to 1800 AD) were metopic, reducing to about 5% thereafter. Miles postulates that the high frequency reflects a period of geographic and genetic isolation which ended in the 19th century when the 'metopic gene' of the indigenous population was 'diluted' by genes of incomers (Miles 1989). A high frequency of metopism was observed in the Linlithgow series where 18 of the 21 cases were recovered from the chancel, cloister or nave of the church (Cross and Bruce 1989) perhaps indicating that families carrying the 'metopic gene' were buried in these areas.
The spatial distribution of metopism was examined. Whilst the trait appears to be evenly distributed amongst the internal burials, the three examples in the graveyard outwith the church were in a row of four burials, perhaps indicating a family plot.
The spatial distribution of the conditions noted in this section were plotted as were those of certain humeral non-metric traits. They all appeared to be randomly distributed.
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