Specific information levels in relation to fragmentation patterns of shrew mandibles: do fragments tell the same story?
Introduction
Palaeontologists, archaeologists and biologists are regularly confronted with broken or incomplete specimens. Depending on the discipline and its aims, broken or fragmented specimens may be used, but they are frequently discarded from formal analysis. However, if fragments possess specific characteristics, the attribution to a species, genus, or family is sometimes feasible. Such a diagnosis is strongly dependent on the taxon investigated and the associated fragments available. The choice of whether or not to include fragmented material in analyses could create a bias in the results. As a consequence, a potentially large amount of information often remains unexploited.
Studying objects broken due to bad conditions of preservation, linked typically to fossilization or historical deposits, is particularly problematic in the case of morphometric, and especially geometric morphometric approaches where many variables are needed to describe the form of an object. The first choice is typically to optimize the number of variables and the number of specimens based on what is available and intact, but this is often a restrictive solution. The second choice is to estimate missing data. Although many papers have been written on the subject (e.g. Couette and White, 2010), an ideal or optimal solution does not exist. An alternative approach will be developed in this paper using 2D sliding-landmark approaches which allow an enhancement of the description of form using fragmented specimens. The method requires complete specimens derived from a correctly diagnosed inter-specific data-set. This data set is then divided into sub-samples following a pattern of fragmentation that is well characterized in the literature or established with archaeological data. Then, specific classifications are performed for each sub-sample, from the complete object to the smallest fragment. These results are then compared to the known specific classification of species in order to evaluate the level of discrimination of the different fragments.
In this paper we use mandibles of white-toothed shrews belonging to the genus Crocidura as our model. They present all characteristics needed for this approach. They are very abundant in archaeological deposits and have a regular and known pattern of fragmentation (Denys, 1985, Andrews, 1990). In addition, they possess a highly uniform shape which makes the allocation of small fragments nearly impossible. Small mammals are very useful in the understanding of palaeo-environments (Weissbrod et al., 2013, Stoetzel et al., 2013, Escudé et al., 2013) and shrews in particular (Stoetzel et al., 2007, Valenzuela et al., 2009). Indeed, the various species of small mammals and their associated ecological niches allow the characterization of climatic trends due to their microclimatic specificity (Stoetzel et al., 2011, Stoetzel et al., 2013). The exact specific allocation of shrew mandibular fragments recovered from archaeological deposits is therefore of particular interest.
The aim of this study is to calculate and to compare the level of the specific discrimination of these different types of fragments using an extant reference sample. By doing this, we aim to evaluate the utility and the efficiency of using fragmented shrew mandibles. Moreover, a double morphometric approach to describe form is used to compare sliding semilandmarks versus anatomical landmarks. As such this allows us to test and optimize the advantages of different methods based on the absence or scarcity of anatomical landmarks. We predict that sliding semilandmarks will increase the accuracy of the description of form and thus the potential for specific discrimination. Moreover, we predict that this improvement in the description of form will be particularly beneficial for the smallest fragments.
Section snippets
Material
Teeth and hemi-mandibles are among the best-preserved bones found in archaeological sites, especially in shrews (Butler and Greenwood, 1979) and this is also the case in the archaeological site of the El Harhoura II cave (Stoetzel et al., 2011) where the results of this study have been applied (Cornette, 2011). The preservation can be explained by their solidity and the fusion of the different elements into a single unit. Nevertheless, complete mandibles are an exception and in most cases they
Form space for both sliding semilandmarks and anatomical landmarks
With more than 80% for each fragment, the classification level was very high for both methods (Fig. 3; Table 3). In addition, the differences between the two methods were minor, although some differences could be noticed. When comparing the global results of specific classifications (Fig. 3; Table 3) for each fragment based on the two methods (anatomical landmarks and sliding semilandmarks), two results were obtained. First, there were fragments whose specific classification was slightly
Sliding semilandmarks versus anatomical landmarks
Surprisingly, even the smallest fragments retained a high potential for specific identification with anatomical or sliding semilandmarks and especially when using form data as was predicted. However, the best results were obtained when using semilandmarks. In our study the two smallest fragments obtained more than 80% of good classification, which is rather high. Sliding semilandmarks clearly allowed an optimization of classification for some small fragments (Fig. 6) and for some species that
Conclusions
Assuming that this approach offers a global appraisal of the palaeo-environmental specific composition based on phenotypic similarities with extant species, our study showed that sliding semilandmarks clearly increase the correct classification of species, especially with small fragments. Our method demonstrated the usefulness of maintaining such fragments into the analyses because they still possess interesting shape specific information. Moreover, a functional signal is detected by this
Acknowledgements
We thank the Mission Archéologique El Harhoura-Témara, the Ministère des Affaires Etrangères et Européennes, France, the Ministère de la Culture, Morocco (dir. R. Nespoulet and M.A. El Hajraoui), the Institut National des Sciences de l’Archéologie et du Patrimoine (INSAP), Rabat, Morocco and the Institut Scientifique, Rabat, Morocco for research permits and support. This study has been also supported by ANR “MOMHIE” and by ATM-MNHN “Formes possibles, formes réalisées”. We are very grateful to
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