In this section, we’re going to focus on estimating the age-at-death of non-adult skeletal remains, so infant and juvenile skeletal remains. During this growth period, there’s a huge number of changes happening in the skeleton. So we can usually use these to establish a fairly reliable chronological age estimate from our non-adult remains. There are three broad categories of technique that we use and that I’ll be going through today. These include dental development and eruption, long bone growth, and skeletal maturity indicators, such as the appearance of ossification centers and the fusion of these. So here we’ve got the skeletal remains of the perinatal skeleton, and I just want to show you that the bones do survive quite well in the ground.
There is a tendency to think that the bones of infants just dissolve somehow, but that’s not the case. However, they are quite small. You can see from the colour of them as well, they take on the staining of the soil and because of this and their size, they can be quite easily missed at excavation. I’m just going to highlight a few of the bones that are on this tray. So here we’ve got the long bones. So this is actually the femur, and that’s about seven to eight centimeters in size at birth. Here’s the mandible and you can see that the mandible is actually in two halves, and this will fuse about six to nine months of age.
This bone here is called the pars basilaris and these bones either side of the pars lateralis and together these will fuse to form the foramen magnum. The pars basilaris is actually quite a useful bone for age estimation, because it’s quite robust and it survives well in the ground. And if you measure both the length and the width, it can give you an approximate age estimation. Here we have the vertebrae and you can see that they’re actually in three separate pieces rather than one. From about one to three years of age, the arches of the vertebrae will fuse together. These will then fuse to the vertebral body between about five to seven years of age.
Here we have the left side of the pelvis. So we’ve got the ilium, the ischium and the pubis. And as you can see they’re in three separate pieces at birth. This is the lower jaw or mandible at around the time of birth. And as you can see, these are the small tooth crowns that are forming within the jaw. These form in utero, although they don’t erupt until about six months of age. At birth, the crowns of the teeth are about 60% formed and you can see the cusps of the molars have formed. You can also see that they’re quite dark in color and this is because they’re not fully mineralized yet, compared to an older child.
You can also see the small size of the crowns in comparison to the large size of the tooth sockets. It means that they can quite easily fall out into the surrounding soil and frequently they’re lost at excavation. There are four lower jaws here, each of them are showing a different stage of dental development, going from 18 months of age here, this lower jaw here is approximately four to five years and this individual here is about eight or nine years of age. Starting on this side, at 18 months, the deciduous teeth start to erupt at about six months of age, and then they’re fully errupted by two years of age. There are 20 deciduous teeth in total.
These are gradually replaced by the permanent teeth. There are 32 permanent teeth in total, the first permanent molar will erupt at about six years of age. That’s this one here. On this individual here you can see it forming in the jaw. This is followed by the permanent incisors which erupt between about six to eight years of age and see these here, followed by the canine, premolars and second molar. On this individual here at the end, you can see that the second permanent molar has erupted. So we know that this individual is about 12 years of age. The third permanent molar or wisdom teeth don’t erupt until about 18 to 22 years of age, but this is really variable.
Some people don’t have third molars at all whilst for others, the teeth are impacted, or just fail to erupt. Teeth formation is the most accurate and reliable method of age estimation because dental development is strongly correlated with chronological age. If the child is ill or has a poor diet, the teeth will continue to form and erupt as usual. The method most commonly used for recording the stage of tooth development is the AlQahtani 2010 method. It’s really important to gain a good understanding of tooth morphology in order to correctly identify each of the deciduous and permanent teeth at the different stages of development.
Remember, it’s common for teeth to fall out of the sockets in the burial environment, particularly in those fetal and infant remains. Therefore relying on position within the dental arcade for identification is not always an option. Long bone growth, measured as diaphyseal length, is useful for estimating age-at-death, particularly of younger individuals. Going from left to right, these individuals are a newborn, an 18 month-old, a three-to-four-year-old, an eight- to -nine-year-old, a 16- to- 17- year-old and an adult. When we’re measuring the long bones we want to just focus on the diaphyseal length, so not including the ends of the bones or epiphyses. During growth, you have this unfused epiphyses surface at the ends of the long bone.
So you can see it’s very wavy and then this is the epiphysis and this is separate. And then when growth is stopped, this will fuse on to the end of the bone. So if we look at this one here that is fused, you can see that the fusion line is still very clearly visible. So fusion has been quite recent here. Once the bones start to fuse, as in this instance, you can see that the proximal end of the femur has fused, it no longer yields a useful measurement. The long bones begin to ossify in utero between the eighth and 12 gestational weeks.
Diaphyseal growth continues to be rapid during the first year, but the rate of growth declines after about three years of age. There are often temporary increases in growth rate between the ages of about six- to-eight years and then again around puberty. It’s important to remember that growth isn’t linear, and instead occurs in a series of growth spurts followed by periods of stasis. In infant and juvenile skeletal remains, we do not know if the individual died during a period of stasis or just after a growth spurt, and this is a source of inaccuracy in the method.
Upon reaching puberty, there’s an increasing divergence between the sexes, and unless we can ascertain the sex of the skeleton, this will cause further imprecision in the age estimates. And of course, as we discussed earlier, we cannot reliably sex non-adult skeletons. Unlike the dentition, if the child is ill, or has a poor diet, growth will be affected and this will affect the age estimation. Long bone growth is most useful for estimating the ages of infants and very young children. Because of the increasing divergence between individuals in long bone length with age and because of the differences between boys and girls, it is less accurate in older children.
I wouldn’t recommend using long bone length as an age indicator beyond the age of about 10 years. So we’ve gone through all of the different techniques now. And I think the key thing to remember is that dental development and eruption is the most accurate age indicator. So even if you have a complete skeleton and you’ve got long bone length and other maturity indicators, always prioritize dental development over the others, in terms of establishing your age-at-death. In the next section, we’re going to focus on estimating the age-at-death of adult skeletons. There’s a very different suite of techniques that we use for adults because once skeletal maturity has been achieved, most of the changes are to do with degeneration rather than development.