15.2: Skeletal Analysis

Forensic anthropology relies on skeletal analysis to reveal information about the deceased. The methodology and approaches outlined below are specific to the United States. Forensic anthropological methods differ depending on the country conducting an investigation. In the United States, there are typically seven steps or questions to the process:

• Is it bone?
• Is it human?
• Is it modern or archaeological?
• How many individuals are present or what is the minimum number of individuals (MNI)?
• Who is it?
• Is there evidence of trauma before or around the time of death?
• What happened to the remains after death?

Is It Bone?

One of the most important steps in any skeletal analysis starts with determining whether or not material suspected to be bone is in fact bone. Though it goes without saying that a forensic anthropologist would only carry out analysis on bone, this step is not always straightforward. Whole bones are relatively easy to identify, but determining whether or not something is bone becomes more challenging once it becomes fragmentary. As an example, in high heat such as that seen on fire scenes, bone can break into pieces. During a house fire with fatalities, firefighters watered down the burning home. After the fire was extinguished, the sheetrock (used to construct the walls of the home) was drenched and crumbled. The crumbled sheetrock was similar in color and form to burned, fragmented bone, therefore mistakable for human remains (Figure 15.1). Forensic anthropologists on scene were able to separate the bones from the construction material, helping to confirm the presence of bone and hence the presence of individual victims of the fire. In this case, forensic anthropologists were able to recognize the anatomical and layered structure of bone and were able to distinguish it from the uniform and unlayered structure of sheetrock.

Figure 15.1: Burned sheetrock used as building material appears similar to human bone but can be differentiated by the fact that it is the same density throughout. Credit: Example of burned sheetrock (Figure 15.1) original to Explorations: An Open Invitation to Biological Anthropology by Alex Perrone is under a CC BY-NC 4.0 License.

As demonstrated by the example above, both the macrostructure (visible with the naked eye) and microstructure (visible with a microscope) of bone are helpful in bone identification. Bones are organs in the body made up of connective tissue. The connective tissue is hardened by a mineral deposition, which is why bone is rigid in comparison to other connective tissues such as cartilage (Tersigni-Tarrant and Langley 2017, 82–83; White and Folkens 2005, 31). In a living body, the mineralized tissue does not make up the only component of bone—there are also blood, bone marrow, cartilage, and other types of tissues. However, in dry bone, two distinct layers of the bone are the most helpful for identification. The outer layer is made up of densely arranged osseous (bone) tissue called compact (cortical) bone. The inner layer is composed of much more loosely organized, porous bone tissue whose appearance resembles that of a sponge, hence the name spongy (trabecular) bone. Knowing that most bone contains both layers helps with the macroscopic identification of bone (Figures 15.2, 15.3). For example, a piece of coconut shell might look a lot like a fragment of a human skull bone. However, closer inspection will demonstrate that coconut shell only has one very dense layer, while bone has both the compact and spongy layers.

Figure 15.2: Cross section of human long bone with compact and cortical bone layers visible. Credit: Cross section of human long bone (Figure 15.2) original to Explorations: An Open Invitation to Biological Anthropology by Mary Nelson is under a CC BY-NC 4.0 License.

Figure 15.3: Cranial anatomy is slightly different as compared to that of a long bone in cross section. The compact (cortical) bone layers sandwich the spongy (trabecular) bone. One layer of compact bone forms the very outer surface of the skull and the other lines the internal surface of the skull. Credit: Anatomy of a Flat Bone (Anatomy & Physiology, Figure 6.3.3) by OpenStax is under a CC BY 4.0 License.

The microscopic identification of bone relies on knowledge of osteons, or bone cells (Figure 15.4). Under magnification, bone cells are visible in the outer, compact layer of bone. The bone cells are arranged in a concentric pattern around blood vessels for blood supply. The specific shape of the cells can help differentiate, for example, a small piece of PVC (white plastic) pipe from a human bone fragment (Figure 15.5).

Figure 15.4: Bone microstructure (osteons). Credit: Bone (248 12) Bone cross section by Doc. RNDr. Josef Reischig, CSc. is under a CC BY-SA 3.0 License.

Figure 15.5: Fragments of plastic PVC pipe, such as those seen in this photo, may be mistaken for human bone. Credit: Example of PVC pipe original to Explorations: An Open Invitation to Biological Anthropology by Alex Perrone is under a CC BY-NC 4.0 License.

Is It Human?

Once it has been determined that an object is bone, the next logical step is to identify whether the bone belongs to a human or an animal. Forensic anthropologists are faced with this question in everyday practice because human versus nonhuman bone identification is one of the most frequent requests they receive from law enforcement agencies.

There are many different ways to distinguish human versus nonhuman bone. The morphology (the shape/form) of human bone is a good place for students to start. Identifying the 206 bones in the adult human skeleton and each bone’s distinguishing features (muscle attachment sites, openings and grooves for nerves and blood vessels, etc.) is fundamental to skeletal analysis.

Nevertheless, there are many animal bones and human bones that look similar. For example, the declawed skeleton of a bear paw looks a lot like a human hand, pig molars appear similar to human molars, and some smaller animal bones might be mistaken for those of an infant. To add to the confusion, fragmentary bone may be even more difficult to identify as human or nonhuman. However, several major differences between human and nonhuman vertebrate bone help distinguish the two.

Forensic anthropologists pay special attention to the density of the outer, compact layer of bone in both the cranium and in the long bones. Human cranial bone has three distinctive layers. The spongy bone is sandwiched between the outer (ectocranial) and inner (endocranial) compact layers. In most other mammals, the distinction between the spongy and compact layers is not always so definite. Secondly, the compact layer in nonhuman mammal long bones can be much thicker than observed in human bone. Due to the increased density of the compact layer, nonhuman bone tends to be heavier than human bone (Figure 15.6).

Figure 15.6: The compact layer of this animal bone is very thick, with almost no spongy bone visible. Compare with Figure 15.2 to visualize the difference in structure between human and nonhuman bone. Credit: Animal bone cross section (Figure 15.6) original to Explorations: An Open Invitation to Biological Anthropology by Alex Perrone is under a CC BY-NC 4.0 License.

The size of a bone can also help determine whether it belongs to a human. Adult human bones are larger than subadult or infant bones. However, another major difference between human adult bones and those of a young individual or infant human can be attributed to development and growth of the epiphyses (ends of the bone). The epiphyses of human subadult bones are not fused to the shaft (Figure 15.7). Therefore, if a bone is small and it is suspected to belong to a human subadult or infant, the epiphyses would not be fused. Many small animal bones appear very similar in form compared to adult human bones, but they are much too small to belong to an adult human. Yet they can be eliminated as subadult or infant bones if the epiphyses are fused to the shaft.

Figure 15.7: An x-ray of a subadult’s ankle with the epiphyses of the tibia and fibula visible. The gap between the shaft of the bone and the end of the bone (epiphysis) is the location of the growth plate. Therefore, the growth plate gap is what separates the shafts from the epiphyses in the image. Credit: Tib fib growth plates by Gilo1969 at English Wikipedia is under a CC BY 3.0 License.

Is It Modern or Archaeological?

Forensic anthropologists work with modern cases that fall within the scope of law enforcement investigations. Accordingly, it is important to determine whether discovered human remains are archaeological or forensic in nature. Human remains that are historic are considered archeaological. The scientific study of human remains from archaeological sites is called bioarchaeology.