Biomechanics and physiology of Odontopteris cantabrica and Odontopteris schlotheimii (Medullosales, Pennsylvanian, Canada)

As part of the ongoing research project “Chemistry, Biomechanics, and Physiology of Carboniferous plants”, this study investigates two of the largest Canadian specimens of Odontopteris cantabrica Wagner, 1969 (22-cm long) and Odontopteris schlotheimii Brongniart, 1828-1831 (7-cm long) from the Pennsylvanian Sydney Coalfield. Specimens of both species co-occur in the roof shale of the Lloyd Cove coal seam (Asturian-Cantabrian boundary; vitrinite reflectance Ro% = 0.65) at Brogan's pit locality, Cape Breton Island, Nova Scotia, Canada. Well-preserved compressions of pinnules (rachises were not preserved) are analyzed by Fourier transform infrared (FTIR) spectroscopy. These data are examined with the purpose of (i) calculating pinnular density (δ) and associated biomechanical and physiological properties, i.e., tensile strength (TS = resistance to fracture), tensile modulus of elasticity (TME = stiffness), and leaf mass per area (LMA = biomass investment and construction cost); (ii) comparing the data for δ, TS, TME, and LMA of O. cantabrica and O. schlotheimii at two levels: interspecific differences and intraspecific variations between pinnules representing different (proximal and distal) parts of each frond; (iii) comparing data for δ, TS, TME, and LMA of O. cantabrica and O. schlotheimii with those of previously studied medullosalean frond taxa, namely Alethopteris ambigua and Neuropteris ovata var. simonii; and (iv) obtaining new insights into the possible growth habit of both fronds. Results indicate that pinnular density, resistance to fracture, stiffness, and leaf mass per area vary continuously throughout the fronds and indicate biomechanical resistance and physiological efficiency. ANOVA tests show that the pinnules of O. cantabrica and O. schlotheimii are not significantly different in terms of density and biomechanical/physiological characteristics overall. However, within each species, proximal pinnules exhibit statistically higher density, stiffness, resistance to fracture, and metabolic construction cost compared to distal pinnules. The δ, TS, TME, and LMA data for O. cantabrica and O. schlotheimii are notably lower than those of N. ovata var. simonii and A. ambigua. The order of values for δ, TS, TME, and LMA follows: N. ovata var. simonii > A. ambigua > O. cantabrica = O. schlotheimii. Considering the values for δ, TS, TME, LMA, and general morphology, it is possible that O. cantabrica and O. schlotheimii had a semi-self-supporting to self-supporting (arborescent) growth form.