Phytochemistry of Xylopteris (Corystospermales, Corystospermaceae) from the Triassic of Mendoza, Argentina

D`ANGELO, JOSÉ A

Neuquén

Congreso; XVIII CONGRESO GEOLOGICO ARGENTINO; 2011

Despite the wide variety of instrumental chemical analyses currently available, Fourier Transform infrared spectroscopy (FTIR) is one of the most powerful and versatile techniques. Commonly used for the characterization of coal and related sedimentary materials (e.g., Ahmed et al., 2003; Geng et al., 2009), FTIR offers a unique possibility to obtain new information on the structure of the organic matter preserved in fossil remains (e.g., Zodrow et al., 2009, 2010). In this study, several specimens of Xylopteris argentina (Kurtz, 1921) Frenguelli, 1943 from the Triassic of Cacheuta, Mendoza, Argentina were studied by solid-state FTIR-Attenuated Total Reflection (FTIR-ATR) technique. Fossil remains are preserved as compressions (Fig. 1) in dark pelites of the upper Potrerillos - lower Cacheuta Formation (Cerro Cacheuta, 33°04’74’’ S, 69°07’18’’ W, 1413 m above sea level; D’Angelo, 2006). Cuticles obtained from compressions by Schulze’s maceration process exhibited an excellently preserved morphological structure as evidenced by scanning electron microscopy. Though the phytochemistry of some members of the Corystospermaceae has been recorded in previous contributions (D’Angelo, 2004, 2006; D’Angelo and Volkheimer, 2007; D’Angelo et al., 2009), nothing was known about the chemical composition of Xylopteris, an important floristic element of the terrestrial habitats of Gondwana during the Triassic. In this first chemical study of Xylopteris remains (including both compressions and cuticles); different treatments of digitized spectra were applied to improve the information provided by FTIR-ATR technique. Thus, Fourier self-deconvolution (Kauppinen et al., 1981a, b) and curve-fitting methods (Maddams, 1980; Ibarra et al., 1996) were used to study Xylopteris spectra. Figure 2 shows FTIR-ATR spectra (region 1900-900 cm-1) of compressions and cuticles, revealing several functional groups in the preserved organic structure of Xylopteris remains. The most important functionalities are the following: C-H in alkyl and aryl ethers and esters; C-O in phenols and aryl and alkyl ethers; C-O and O-H in phenoxy structures and ethers; carbonyl (C=O) groups in carboxylic acids and conjugated and highly conjugated structures such as ketones and C=C in aromatic carbon structures. Table 1 summarizes the main functional groups found X. argentina specimens with indication of the center of the peak (cm-1) and the likely class of organic compound containing the functional groups. From the results it is concluded that compressions and cuticles, either as one entity or separately,  preserve complementary information, which includes different IR spectra (Fig. 2). Compressions are mainly characterized by aromatic compounds, showing a high contribution of aromatic carbon bands (C=C) including benzene rings in both aromatic compounds and phenoxy structures. On the contrary, cuticles exhibit a more aliphatic overall structure with distinct carbonyl groups and some other C-O groups in aliphatic and aromatic ethers. These novel determinations of chemical structures by FTIR-ATR in compressions along with data from the biochemistry of the cuticles may assist in unraveling not only taphonomic aspects, but also preservation bias, i.e., fidelity of the biochemistry of fossil plants. Finally, FTIR-ATR information if characteristic could be useful for chemotaxomic studies in problematic fossil plant groups such as the Corystospermaceae.