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The hydroxyapatite of fossil bones can provide paleoenvironmental information, because throughout the fossil-diagenetic process, the fossils can incorporate elements from the sedimentary rocks that involved them. In this way, the present study aimed for geochemical patterns in mesosaurid bones and their respective host rocks to project the paleoenvironmental conditions at deposition/diagenesis' time. For this purpose, Raman Spectroscopy and X-Ray Fluorescence analysis were performed. The samples came from Irati ( S 1 – São Matues do Sul, S 2 – Assistência, S 4 – Aceguá e S 5 – São Gabriel) and Mangrullo ( S 3 – Uruguay) Formations. We found similar geochemical composition between S 1 and S 5 ; S 3 and S 4 which also indicates a similar paleoenvironmental context. The samples S 1 and S 5 have relatively high Al, Si and Sr contents which can represent acidic conditions and high depths. The second set (S 3 and S 4) have similar contents of Al, Si and Ca, but it is worth to note that the Si and Ca contents are slightly different. The Si and Ca contents suggest that S 3 was slightly deeper and less acidic when compared to S 4. In general, the first set (S 1 and S 5) were deeper and more acidic than the second (S 3 and S 4). The S 2 sample is out of line with the others, corresponding to shallow or coastal and more basic paleoenvironment. Overall, our results agree with previous studies about the past environment where mesosaurs lived. Also, shows that spectroscopy techniques are reliable and should be used to refine paleoenvironmental interpretations. [ABSTRACT FROM AUTHOR]