Using Radiocarbon Dating
and Paleontological Extraction Techniques in the Analysis of a Human
Skull in an Unusual Context
Department of Anthropology
National Museum of Natural History
Max M. Houck
Forensic Science Initiative
West Virginia University
Morgantown, West Virginia
cranium and mandible partially encased in an extremely hard plastic
material were discovered in a riverbed in Pennsylvania. Several
traditional fossil-preparation methods were used to extract the
fragile skull from the plastic. Anthropological analysis of the
skull indicated it was a male of African ancestry with an age at
death greater than 50 years. To clarify time since death, radiocarbon
analysis was conducted. The results were compared using the modern
bomb curve. The skull revealed pre-1950 levels of radiocarbon, and
thus it was not of recent origin.
On August 1,
1999, a poorly preserved metal bucket was recovered from a river
in Lancaster, Pennsylvania. Inside the metal bucket was a smaller,
white plastic bucket containing a human skull partially embedded
in a gray plastic material. The recovered objects were taken to
the local medical examiner to have the skull removed from the plastic
matrix and to be analyzed. The medical examiner found the plastic
to be very difficult to remove, to the extent that the motor on
a bone saw burned out in an attempt to extract the skull. The skull
and plastic were then sent to the FBI Laboratory to determine the
composition of the matrix and to attempt to extricate and analyze
the human remains.
revealed a skull with the cranium and mandible disarticulated within
the plastic matrix (Figure 1). The disarticulation suggested that
the cranium and mandible were most likely skeletonized at the time
of their immersion into the liquid plastic. The plastic itself was
potentially an important clue. Its chemical composition indicated
that it was similar to the plastic used in kitchen countertops,
not commonly sold to the public in the quantity that was used to
encase the skull.
removal of the matrix proved to be quite a challenge. The resources
of the Smithsonian Institution's Vertebrate Paleontology Preparation
Laboratory were drawn upon to remove the plastic atrix with a pneumatic
scribe. Several sessions of intensive, meticulous work with the
so-called air scribe, used to separate fossils from their geological
matrices, were required to extract the delicate skull from the surrounding
plastic (Figures 2 and 3).
from their plastic context, the cranium and mandible were examined
for geological materials which, if present, might indicate if the
remains had been buried prior to submersion in the plastic. No soil,
however, was found on either item. The cranium and the mandible
were then analyzed for anthropological information.
The bone was
well preserved with no soft tissue or hair present. The posterior
portion of the left ramus of the mandible and the bones of the right
cheek area were missing. These areas did not show obvious signs
of recent fracture, but it could not be determined if the damage
was peri- or postmortem. However, the coloration of the broken margin
of the right zygomatic suggested relatively recent fracture. Numerous
teeth were missing both ante- and postmortem.
robusticity of the remains, especially the large supraorbital ridges,
suggested male sex. Discriminant function analysis of cranial measurements
(Ousley and Jantz 1996) also suggested male sex.
The extent of
cranial suture closure, antemortem tooth loss, and age-related changes
in tooth structure suggested a relatively old age at death. In particular,
application of the Lamendin technique (Lamendin et al. 1992; Prince
and Ubelaker 2002) for age estimation from characteristics of single
rooted teeth to the right maxillary canine indicated an age at death
greater than 50 years. This technique produced an age estimate of
60.2 years, plus or minus about eight years. All of the collectively
available information for estimating age suggested the remains originated
from a mature adult, probably greater than 50 years of age.
African ancestry consisted of relatively wide interorbital distance,
wide nasal aperture, prognathism, and other features. A discriminant
function analysis using the FORDISC 2.0 system (Ousley and Jantz
1996) also strongly supported African ancestry. No measurements
were used that had been compromised by disease, age, or taphonomic
Time since death
was more problematic. The lack of soft tissue and hair and the desiccated
nature of the remains were consistent with considerable antiquity,
but the possibility that the remains were sufficiently recent to
be of forensic interest could not be ruled out. To address this
question, radiocarbon analysis was conducted.
dating is based on the work of Nobel Laureate Willard Frank Libby
(1908-1980) (Libby 1946), which was inspired by the research of
a cosmic-ray physicist, Serge A. Korff (Korff and Danforth 1939).
Although Libby initially kept his findings secret because he thought
the idea was too ludicrous to gain financial support, 14C dating
has ultimately revolutionized archeology, geology, geophysics, and
other branches of science (Taylor 2000).
dating method recognizes that living plants and animals maintain
amounts of the carbon isotope 14C, which are close to atmospheric
levels. After death, the amount of 14C gradually declines, with
a half-life of about 5,730 years. By measuring the amount of radiocarbon
remaining in the material to be dated, the analyst can calculate
the antiquity. Traditionally, radiocarbon dating has been applied
to materials older than 300 years because in younger materials,
minimal radiocarbon decline and other factors usually allow only
a modern label to be applied.
One of the variables
involved in the interpretation of the so-called modern radiocarbon
readings is the bomb 14C, or artificial radiocarbon, released into
the atmosphere during the testing of thermonuclear devices between
1950 and 1963. Such testing, which increased to a peak in the mid-1960s,
then gradually declined, left terrestrial plants and animals that
were alive at the time with artificially elevated levels of 14C.
The presence of greater than 1950 levels of 14C can be plotted on
the earlier and later slope to suggest alternative post-1950 dates.
For these reasons, bomb curve radiocarbon information is not useful
in archaeological or geological investigations but is very helpful
in forensic analysis (Ubelaker 2001; Wild et al. 2000).
To clarify the
antiquity of the materials from Pennsylvania, an approximately square
bone sample (measuring about two inches) taken from the cranial
vault was sent to a commercial radiocarbon dating laboratory for
of the radiocarbon testing indicated that 14C values were below
the 1950 levels, thus the individual's death (and production of
the bone collagen) preceded 1950. Although this analysis establishes
the important fact that the death was not recent, the exact antiquity
of the remains is still not clear. The analysis produced a conventional
radiocarbon age of 230 years, plus or minus 40 years, with concentrations
approximately 98 percent that of modern (1950) levels. Calibration
of these results using 95 percent probability levels reveals four
periods when the individual could have died (Figure
4), which begin at AD 1530 and end at AD 1950. Using the less
precise 68 percent probability levels, the range is shortened to
between AD 1650 and AD 1800. The C13/C12 ratio of -13.6 suggests
that the diet contained substantial amounts of marine foods and/or
C4 plants such as corn.
is not possible from these data to determine the precise antiquity
within the broad interpretative framework outlined above, it is
clear that the individual died prior to 1950. This information provides
useful perspective to the investigation of the case.
This case represents
an example of how techniques employed in paleontology can be useful
in extracting human remains from a challenging modern context and
how radiocarbon dating, especially comparison with the modern bomb
curve, can clarify time since death issues in forensic situations.
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