Determining an accurate method for estimating the post-mortem interval of decomposed remains found in a temperate Australian environment

Abstract

Estimating the post-mortem interval (PMI) is one of the most important determinations to make in a forensic investigation. However, at present, an accurate and reliable forensic anthropological method for estimating the PMI, based on the gross morphological changes occurring during decomposition, is currently unavailable. This is due to a multitude of variables influencing the rate and processes of decomposition in any given environment. Forensic anthropologists have traditionally relied on their knowledge and experience of the decomposition stages to make an assessment of the time since death. However, recently new, quantitative methods that are not solely based on the anthropologists observations, have been developed in a number of regions that have been proposed to accurately determine the PMI based on the observed decomposition changes alongside important taphonomic variables. The aim of the current study was to examine and document the decomposition process of pig carcasses, as an analogue for human remains, in the summer and winter climate of the Greater Western Sydney region. Secondly, the study aimed to evaluate the accuracy and replicability of the Megyesi et al. [1] ADD method, the Marhoff et al. [2] formula and the Vass [3] universal PMI formula, for their applicability as PMI methods in this region. Thirdly, should the methods mentioned above fail to accurately determine the PMI of remains within this region, a new method for PMI determinations will be created based on the observed decomposition changes and the most influential taphonomic variables affecting decay rates within the Greater Western Sydney region. Over an 18 month period, from June 2014 to March 2016, four experimental trials were undertaken: two summer trials and two winter trials. Eight adult pig carcasses per trial were left to decompose naturally on a soil surface at Western Sydney University's Hawkesbury campus. During each trial, four carcasses were left to decompose in the shade under the canopy of trees and the other four carcasses were deposited in the open, with direct exposure to the sun. This was to examine the differences in decay rates between a sun and shaded microclimate. The published methods [1-3] and their associated scoring protocols were applied to determine the PMI of the remains. Through linear mixed modelling, the variation between the true PMI and the estimated PMI. The results showed that of the three methods validated in the present study, none could accurately determine the PMI in the Greater Western Sydney region. The Vass [3] formula overestimated the PMI during the winter trials but underestimated the PMI of the summer remains. The Megyesi et al. [1] and Marhoff et al. [2] methods were both found to underestimate the PMI when they were applied during the winter but overestimated the PMI when they were applied during the summer. As it was found that the currently published protocols for PMI estimates could not accurately determine the PMI of remains found within this region, a new method (the Marhoff-Beard method) for PMI determinations specific to the Western Sydney region was created. Using the degree of soft tissue decomposition observed at the time of discovery alongside the climatic variables humidity, wind speed, and rainfall, new regression equations were created. To determine if the new Marhoff-Beard formulae were accurately estimating the PMI for the Western Sydney region, the method was validated retrospectively from photographs of pig and human remains, and was applied longitudinally from the start to the end of the decomposition process on a donated human body. The validation showed this new method can accurately determine the PMI in a Western Sydney winter and summer climate and results were comparable when it was applied to both human remains and pig carcasses. The method performed consistently well during the fresh and early decomposition stages with a maximum error of eight days. As the remains dried and progressed through the advanced and skeletonisation stages, the accuracy of the method became compromised. It is likely the Marhoff-Beard method failed after this time point, as the decomposition process during the later stages is affected by further variables which were not accounted for by this method. Continued testing of the Marhoff-Beard method for PMI determinations should be undertaken both within this region and other temperate Australian locations. It should also be determined what variables are affecting decay rates during the more advanced stages of decomposition as this will help refine the PMI formula for its use during these stages.

Date of Award	2017
Original language	English