## 4 Discussion  
The present study adds to the works previously performed by other authors on the origins of metallurgy through the study of bone surface modifications with lithic or metal industry [38; 39; 77; 78; 79; 80; 81; 82], though, in this case, we analyse a site from the Iron Age instead of from the Bronze Age or the Chalcolithic period. The results obtained in this study are quite surprising since our primary hypothesis was that considering the site's chronological context (c.II-I BC) and the presence of iron knives and other iron or bronze tools at the site, there should be a predominance of bones with cut marks produced with metal tools. However, the results summarized in Figures 4-6 and Table 3 show that most of the Ulaca cut marks might have been produced with lithic tools, whereas only a small percentage might have been produced using metal tools. On the basis of these results, several questions have been raised. Figure 6: Boxplots comparing the centroid sizes of the Ulaca cut mark sample and the two experimental samples of cut marks generated with metal and flint tools. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Model** & **Accuracy** & **Kappa** & **Sensitivity** & **Specificity** & **Bal. Accuracy** \\ \hline  & & & **shape** & & \\ \hline RF & 0.9221 & 0.838 & 0.9032 & 0.9348 & 0.9190 \\ GBM & 0.9351 & 0.8698 & 0.9444 & 0.9268 & 0.9356 \\ SVM & 0.9221 & 0.838 & 0.9032 & 0.9348 & 0.9190 \\ NN & 0.974 & 0.946 & 0.9677 & 0.9783 & 0.9730 \\ \hline  & & & **form** & & \\ \hline RF & 0.9221 & 0.8397 & 0.9355 & 0.9130 & 0.9243 \\ GBM & 0.9481 & 0.892 & 0.9355 & 0.9565 & 0.9460 \\ SVM & 0.9091 & 0.81 & 0.8710 & 0.9348 & 0.9029 \\ NN & 0.974 & 0.946 & 0.9677 & 0.9783 & 0.9730 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Predictive models’ performance. The sample handled in this study includes 30 bones with cut marks along the diaphysis; that means that only 30 cut marks out of the 99 marks documented in [41] have been analysed. Thus, it is possible that some cut marks made with metal tools were not included in the present study. However, for the evaluation of the remaining cut marks, new experimental analyses on axial elements and long bone epiphyses would be necessary. So far, all the experimental studies developed by the authors of this research to analyse cut marks made with metal or lithtic artefacts are focused on diaphyseal sections [8; 9; 11; 15; 16; 17; 18; 19; 20; 23; 25; 27; 28]. Although some studies show that cut marks do not change significantly when the size of the animal varies [20], changes in the density, texture, and shape between axial, long bone epiphyses, and diaphysis are significant [83]. Thus, we still do not know if this condition affects the morphological characteristics of the cut marks. For this reason, the study of the cut marks of the Ulaca _oppidum_ found on axial elements or long bone epiphyses, requires new referential frameworks. On the other hand, some chop marks observed in Ulaca have not been analysed in this work. Chop marks are a somewhat V-shaped impact marks with impact fractures and/or splinters that are produced by beating to facilitate the disarticulation or fracture of the bone. Their morphology contrasts with the fine, thin, and deep V-section traces observed among cut marks, which can have different orientations, variable lengths, and can appear in groups or isolated after activities of skinning, defleashing, disarticulation, or visceration. Probably, some chop marks observed in Ulaca could have been produced by metal tools, as shown by [41] (Figure 4a). In some recent prehistoric contexts of the Bronze Age, mixed practices have been observed in which chop marks are produced by metal and also lithtic tools [82]. The analysis of chop marks also requires extra experimental works to create accurate referential frameworks. In the particular case of the Ulaca _oppidum_, the skeletal distribution of the cut marks and the functionality associated with these marks requires future analyses as stated in the Supplementary file of [41].