Chemotaxonomy, biomechanics, and paleophysiology of Alethopteris ambigua and Neuropteris ovata var. simonii (Late Pennsylvanian, Canada): A chemometric approach

Abstract

Medullosales was a highly diverse and ecologically differentiated group of plants that flourished in the Late Carboniferous and Early Permian tropical wetlands of Euramerica and China. Although the Medullosales have been extensively studied, some aspects of their life habits are still not fully understood. This study focuses on the relationships between chemical composition and biomechanical /physiological characteristics of the two largest-known frond segments of Alethopteris ambigua (45 cm long) and Neuropteris ovata var. simonii (65 cm long) from the Late Pennsylvanian of the Sydney Coalfield, Canada. Additionally, this study provides new data obtained from Fourier transform infrared spectroscopy (FTIR) analysis that, in conjunction with morphological features, could be used to efficiently distinguish species and genera of fossil plants. The density of the different frond parts is calculated using a 3D multivariate model derived from FTIR data. Tensile strength, tensile modulus of elasticity, flexural stiffness, and leaf mass per area for the different frond parts are estimated using trait relationships linking density and structural properties established for modern plant-leaf tissues. Key results include the cognition of continuous variation of geomacromolecular compounds throughout the different-order rachises and pinnules from proximal to distal frond sections. In the proposed, theoretical fronds, these diagenetically-modified biomacromolecules would represent tannin-, lignin-, and resin-related compounds. Overall, pinnules and rachises of A. ambigua are characterized by high contents of aliphatic-rich structures, whereas those of N. ovata are mostly aromatic in composition. The chemical composition of the two fronds is hierarchically partitioned, indicating density dependence on the frond parts. This is reflected in the metabolic construction costs that, in the two species, differ significantly among pinnules and rachises. These results indicate “flexibility” of resource allocation, resulting into two different, architectural frond designs. The costs of building harder, stiffer, and more expensive pinnules in the N. ovata frond are compensated for by the metabolically inexpensive rachises. The opposite holds for the A. ambigua frond, i.e., relatively cheaper pinnules and more expensive rachises. This reveals the complexity of the biomechanical and physiological strategies employed by these two taxa to construct mechanically resistant and physiologically efficient, longer-lived fronds. Results suggest that these plants were likely stress-tolerant and opportunistic colonizers that dominated eutrophic habitats, which were typical of some tropical ecosystems of the Late Pennsylvanian.