Meeting date: September 29, 2008
Discussion 1: Aging techniques of the auricular surface
Method:
Lovejoy CO, Meindl RS, Pryzbeck TR and Mensforth RP. 1985. Chronological metamorphosis of the auricular surface of the ilium: a new method for the determination of adult skeletal age at death. American Journal of Physical Anthropology 68:15-28.
Buckberry JL and Chamberlain AT. 2002. Age estimation from the auricular surface of the ilium: a revised method. American Journal of Physical Anthropology 119:231-239.
Test:
Falys CG, Schutkowski H, and Weston DA. 2006. Auricular surface aging: worse than expected? A test of the revised method on a documented historic skeletal assemblage. American Journal of Physical Anthropology 130:508-513.
Schmitt A. 2004. Age at death assessment using the os pubis and the auricular surface of the ilium: a test on an identified Asian sample. International Journal of Osteoarchaeology 14: 1-6.
Application:
Buikstra JE, Milner GR, and Boldsen JL. 2006. Janaab‘ Pakal: the age at death controversy revisited. In: Janaab’ Pakal of Palenque: reconstructing the life and death of a Maya ruler. Tiesler V and Cucina A, eds., Tucson: University of Arizona Press.
Nagaoaka T and Hirata K. 2008. Demographic structure of skeletal populations in historic Japan: a new estimation of adult age at death distributions based on the auricular surface of the ilium. Journal of Archaeological Science 35: 1370-1377.
Background Summary:
The ability to reliably and accurately assess age-at-death for skeletal remains is essential to the study of past populations. Many techniques exist for determining age-at-death, but it should be noted that nearly all of these methods estimate biological age, rather than chronological (calendar) age. This is similar to the difference between radiocarbon years and calendar years: they are similar, but during some periods the correlations are more precise than others, and after a certain age it just doesn’t work anymore.
In general, children can be aged with much higher accuracy and precision than adults. This is because aging markers in children are based on predictable developmental changes, rather than variable degenerative changes in adults. In general, precision in aging decreases with age. For example, infants can often be aged accurately to ±3 months. This error range increases to a decade or more in adults.
Developmental changes used to age sub-adults and young adults can be divided into two categories: dental development patterns and epiphyseal union patterns. Dental development is genetically hard-wired and progresses in a fixed order on a fairly rigid time frame that is relatively uninfluenced by environmental factors such as health, diet, and activity patterns. Dentition is an effective indicator of developmental age from near birth to approximately 21 years of age. Aging by dental development is considered to be the most accurate of all aging methods. Most people use the dental maturation chart by Ubelaker (1989) to score dental development.
Aging by epiphyseal union patterns (growth plate fusion) is also an accurate, but somewhat less precise, method of aging. One advantage is that the last epiphysis does not fuse until approximately age 30, thus providing a developmental marker of age into young adulthood. One disadvantage of this method is that the timing of epiphyseal fusion is more variable among individuals and differs according to sex. Sex cannot be determined in children and most young adolescents (without aDNA testing). This is because male and female skeletons are virtually indistinguishable before hormonal changes that take place during puberty that cause changes in pelvic shape in females and cranial robusticity in males. The inability to sex sub-adults can decreases the precision of the technique because the ranges for both males and females must be included in the estimation. The most recent and complete epiphyseal fusion chart is by Scheuer and Black (2000). Most researchers measure both dental development and epiphyseal union patterns to produce a best estimate of biological age in sub-adults and young adults.
Age estimates in adults are based primarily on degenerative changes since nearly all development is now complete. The timing of degenerative changes in adults varies on the basis of activity level, health status, and other environmental factors, but there is a general trend toward joint degeneration, arthritis, dental wear and loss, and osteoporosis with advancing age. Degenerative changes become so variable among older adults that claims to accurate age assessments over the age of 45 are often highly controversial.
Degenerative changes used to age adults can be divided into three categories: tooth wear/loss, cranial suture closure, and joint degeneration. Tooth wear/loss is heavily dependent on diet (i.e. sugar intake that causes cavities, grit that wears down enamel surfaces, etc.) and use (often in tool-making or the manufacture of goods). Because of these factors, tooth wear patterns can vary dramatically between populations and within populations across categories such as status and occupation. Most work on tooth wear has been conducted on past and present British populations. Researchers who score tooth wear usually employ either Brothwell 1981, Hilson 1986, or Miles 1962.
Cranial suture closure patterns are also highly variable, especially (and unfortunately) within populations. At birth, the bones of the cranial vault are unfused. This allows the head to change shape and fit through the birth canal (which is why newborns often have lopsided heads) and it also allows for the tremendous brain growth to occur during the first few years of a child’s life. At birth, a baby’s brain is only 23% of full size, but it will effectively reach adult size by age 4. Premature suture closure can result in severe mental retardation. Once growth is complete, the bones of the cranium and hard palate begin to fuse. This is a long process, and complete fusion may never occur in some individuals, while others close all their sutures a young age. Cranial suture closure is considered to be one of the least precise aging methods. To score cranial suture closure, most researchers follow Meindl and Lovejoy 1985.
The final category, degenerative joint change, relies on four immobile joints: the pubic symphysis, the auricular surface, the first rib, and the fourth rib. These immobile joints are used in order to mitigate the bias of activity levels on the degeneration of the joints. Degenerative joint changes are the gold standard in adult aging. The first degenerative joint aging technique was developed by Todd (1921a,b) for the pubic symphysis, and it was later refined by Brooks and Suchey (1990) into the Suchey-Brooks Pubic Symphysis Scoring System. This system divides adult pubic symphyses in to six age classes, segregated by sex. Casts of the type symphyses can be ordered for more accurate comparison. This is the most commonly cited aging technique.
The medial aspects of the 1st and 4th ribs can also be examined for age indicators. Rib end aging was developed by Iscan et al. (1984) for the fourth rib. However, ribs can often be very difficult to distinguish from one another, especially in archaeological collections, and relatively few people used this technique. Recently, DiGangi et al (2008) have developed a method for aging from the first rib (which is much more diagnostic). Although this new method is claimed to be able to capture age-related change into the ninth decade, its precision is at best ±14 years, at worst ±30 years.
Lovejoy et al. (1985) developed an aging technique for the auricular surface, which is the iliac surface of the immobile sacroiliac joint. Lovejoy et al originally claimed that it was equally accurate to the pubic symphyseal method, but that the auricular surface had a higher preservation rate in archaeological contexts and that it continued to show changes beyond the 5th decade. The authors admit, however, that the method is difficult to apply. The original method divides auricular surfaces into 8 age classes. It has been modified several times by different researchers, including Buckberry and Chamberlain (2002).
Most recently, transition analysis and Bayesian methods of aging have come into vogue. These methods combine aging observations from one or more of the skeletal features described above and use statistics and probability to refine age estimations and determine confidence intervals. There are two main disadvantages. First, most of these mathematical models assume that the age distribution around a “phase” is normal, when empirical tests generally show that they are not; and second, many of the statistical methods require very good preservation across multiple age indicators. These tests may not be able to handle null values for missing/damaged elements that cannot be measured due to bone taphonomy. Comparisons of these multifactorial methods to single factorial methods have been shown to slightly improve mean accuracy of age estimations, but residual biases still skew age distribution patterns (e.g., Bedford et al 1993).
How do you safely excavate friable bone? What are the current standard consolidation techniques? Do consolidants affect isotope and/or ancient DNA analysis? What on earth are they doing at Teotihuacan?
