Molecular structure of the cuticles of Dicroidium and Johnstonia (Corystospermaceae, Triassic, Argentina). Ecophysiological adaptations of two chemically indistinguishable, morphology-based taxa

Abstract

Cuticles of compression-preserved Dicroidium odontopteroides and Johnstonia coriacea (Corystospermaceae, Upper Triassic, Mendoza, Argentina) are spectrochemically analyzed. The objectives included (i) studying the chemical resistance of cuticles to different oxidative conditions to gain new insights into their fine molecular structure and its likely (ii) chemotaxonomical and (iii) paleoecophysiological implications. Two experimental procedures are employed to obtain the cuticles from the compressions: (a) room-temperature (25o C and up to 75 min) and (b) high-temperature (500o C and up to 50 min) oxidative reactions using Schulze's reagent. Details of the molecular structure (i.e., functional groups) of cuticles are studied using Fourier transform infrared (FTIR) spectroscopy followed by data evaluation using principal component analysis and one-way ANOVA test. Morphological changes as a function of different oxidative conditions are monitored by scanning electron microscopy. Results indicate that the geomacropolymers composing the cuticles of both taxa are chemically characterized by high contents of aliphatic compounds with relatively smaller amounts of aromatic hydrocarbons. The presence of considerable contents of carbonyl groups in the cuticles indicates likely ester chemical “bridges” that cross-link aliphatic and aromatic hydrocarbons. These chemical bonds “strengthen” the molecular structure, thus increasing the overall mechanical and chemical resistance of the cuticle. Conclusions include: (i) the cuticles of both taxa are extraordinarily resistant to extremely harsh chemical conditions, which modified neither morphology nor chemical structure; (ii) independently of the oxidative procedure employed, the two taxa cannot be statistically differentiated using the cuticular FTIR information; (iii) the high chemical resistance of the studied cuticles represents likely ecophysiological adaptations of the once living plants to survive stressful environmental conditions. They could have included high temperatures, elevated CO2 concentrations, seasonal drought, and nutrient-deficient, acidic soils exposed to intensive solar irradiation, and eventual acid (H2SO4) precipitations. Chemical results are in agreement with taphonomic, sedimentological, paleopedological and (micro- and macro-) morphological data, which indicate that the studied plants were likely opportunistic and stress-tolerant colonizers that dominated flood-disturbed, waterlogged lowlands. This combination of chemical and statistical tools enhances our understanding of the fine details of corystosperm cuticles regarding their structure, taxonomy, and paleoecophysiology.