Density and biomechanical properties of fossil fronds. A case study of Neuropteris ovata (seed fern, Late Pennsylvanian, Canada)

Abstract

A theoretical biomechanical model is proposed for the largest known 650 mm-long frond segment Neuropteris ovata var. simonii (Medullosales, Pennsylvanian, Sydney Coalfield, Canada). The study procedure includes, amongst others, a novel methodology for calculating density of fossil pinnules and rachides. This, in conjunction with the trait relationships and mathematical models that linked density and material/structural properties established for modern plant-leaf tissues, permits the estimation of tensile strength, tensile modulus of elasticity, flexural stiffness, and leaf mass per area. Results suggest that, theoretically, (a) the living frond invested a considerable amount of resources for the construction of metabolically expensive, hard, tough, and resistant pinnule and rachial tissues, which were “made to last”, and (b) were used as part of a combination of strategies that increased the plant's biomechanical stability. This translated in resistance to damage during the application of external loads such as those exerted by strong winds or tropical storms. High leaf mass per area (dry-mass investment) of pinnules and rachial tissues points to a slow photosynthetic return that was sustained for long periods of time (long leaf lifespan). The latter possibly indicates a plant adaptation to wet, levee-top habitats, which were characterized by nutrient-deficient soils exposed to intensive solar irradiation. The 3D, chemistry-based model of the N. ovata specimen provided new insights into the hitherto poorly-known biomechanical and related traits of this iconic medullosalean plant that inhabited Pennsylvanian wetlands. Other implications of using our chemistry-based methodology include the estimation of (i) frond size and shape (plant reconstructions), (ii) plant ecophysiological adaptations to different wetland environments, and (iii) plant phylogenetic and ontogenetic adaptations.