## 2 Materials and Methods
The standard uncertainty is \(\pm\)0.11% for \(\delta^{13}\)C and \(\pm\)0.15% for \(\delta^{15}\)N.  
All modern plant samples were ground into a fine, homogeneous powder with a particle size of less than 250 \(\upmu\)m. The plant material was dried in an oven at 60-70 \({}^{\circ}\)C for 48-72 h or until completely desiccated. The dried plant was ground to a fine, homogenous powder using a ball mill, mortar, and pestle. Isotopic analysis was performed using 1-3 mg of the plant powder using a microbalance. Approximately 1.6 mg of each sample was weighed and placed in tin capsules for simultaneous measurement of sulfur, carbon, and nitrogen isotopes using EA-IRMS (Elemental Analyzer--Thermo DeltaPlus V Plus Isotope Ratio Mass Spectrometer) connected via a Conflo III interface at the Stable Isotope Unit in the National Centre for Scientific Research (NCSR) in Greece. To study the isotope shift between bone and flesh, muscle tissue was collected from each specimen and immediately frozen for all modern fish samples. This ensures the preservation of the original chemical composition of the tissue for later analyses. The analysis referred to in Table 3 pertains to bone collagen. Since collagen is the main organic component of bone, it was used as a significant marker for the isotopic studies in the frame of this work, as it preserves information about the diet and environment of the organism. Moreover, collagen was utilized for isotopic studies, as it can be compared with archaeological bone remains. The isotopic ratios for carbon are expressed as \(\delta^{13}\)C versus PDB (Pee Dee Belemnite, a marine carbonate), while for nitrogen, they are expressed as \(\delta^{15}\)N versus atmospheric N\({}_{2}\) (AIR) and for \(\delta^{34}\)S as V-CDT: X = [(Rsample \(-\) Rstandard)/Rstandard] \(\ast\) 1000 (1), where X is the \(\delta^{13}\)C or \(\delta^{15}\)N or \(\delta^{34}\)S value and R = \({}^{13}\)C/\({}^{12}\)C, \({}^{15}\)N/\({}^{14}\)N and \({}^{34}\)S/\({}^{32}\)S, respectively. The analytical precision was 0.1% for both the \(\delta^{13}\)C and \(\delta^{15}\)N values and 0.3% for \(\delta^{34}\)S.  
In particular, to ensure the accuracy of measurements, instruments were calibrated using certified standards for each isotope (e.g., \({}^{12}\)C/\({}^{13}\)C, \({}^{14}\)N/\({}^{15}\)N, \({}^{32}\)S/\({}^{34}\)S), while during each analysis, instrument accuracy is reviewed through reference samples. Additionally, instruments underwent scheduled checks and maintenance to ensure they were operating with high sensitivity, and daily inspections were conducted to verify that results remained within acceptable error limits. Clear guidelines are established for proper sample collection, storage, and preparation to prevent contamination or isotope loss, and each sample was inspected to ensure proper preparation, including the removal of contaminants or foreign elements before analysis. Blank samples were used to monitor potential contamination, ensuring that measurements were not influenced by external factors, while blank analysis confirmed the absence of contamination or alteration during the analytical process. Certified reference materials (CRM) were employed to guarantee the accuracy and reliability of measurements, and results were compared with those of certified materials to detect any deviations from standards. Standards for repeatability and consistency of measurements were set through multiple measurements per sample, and repeated measurements verified result consistency, preventing random errors. Error analysis was conducted using statistical methods to determine uncertainty ranges, and measurement uncertainty was calculated with results presented alongside their respective error margins. Finally, all results and data were recorded and archived for tracking and review, while data were examined by third parties and the accuracy of analyses was verified before the final presentation of results.