Sweet or bitter Bennettitales? Chemistry, biomechanics, and physiology of Ptilophyllum micropapillosum (Cretaceous, Argentina)

D`ANGELO J A; LAFUENTE DIAZ, M; DEL FUEYO, G

Simposio; XVIII Simposio Argentino de Paleobotánica y Palinología; 2022

The aim of this contribution is to study the relationships between chemical composition and biomechanical/physiological characteristics of foliar cuticles of Ptilophyllum micropapillosum (Springhill Formation, Lower Cretaceous of Santa Cruz, Argentina). Analyzed properties of apical, middle, and basal frond leaflets include: density, tensile strength (resistance to fracture), tensile modulus of elasticity (stiffness), leaf mass per area (metabolic cost of tissue construction), and photosynthetic capacity. They were calculated using a 3D-multivariate model based on data obtained by Fourier transform infrared spectroscopy, and trait relationships that link density and the properties mentioned above, which have been previously established for leaf tissues of living plants. Cuticles of P. micropapillosum leaflets have a predominantly aromatic chemical composition with lower contents of aliphatics. The chemical groups detected are related to diagenetically resistant molecules, e.g., lignin-, tannin-, and resin-like polymeric structures. Calculated mean values (including apical, middle, and basal leaflets) of biomechanical/ physiological properties are: density = 0.94 g/cm3, resistance to fracture = 4.54 MPa, stiffness = 810 MPa, metabolic construction costs = 0.76 g/cm2, and photosynthetic capacity = 53 nmol g-1s-1. Comparisons indicate that rigid, tough, and brittle apical and basal leaflets may have had considerably higher density, resistance to fracture, stiffness, and metabolic construction costs than leaflets from the middle frond parts. On the other hand, the highest photosynthetic capacity was determined for the relatively more flexible and cheaply constructed leaflets from the middle frond part. Results suggest that, theoretically, the once-living plants bearing P. micropapillosum leaves may have invested considerable amounts of resources for the construction of aromatic, high-density, biomechanically resistant, and metabolically expensive foliar cuticles. P. micropapillosum pinnae may have employed a combination of biological and biochemical characteristics that optimized their biomechanical stability and physiological activities. Cutinization and lignification implied variable concentrations of chemical structures related to phenols (e.g., free phenolic compounds, phenylpropanoids, lignins, and tannins). They are often astringent and bitter to taste, and so possibly reduced the palatability of P. micropapillosum leaflets, particularly those located in apical and distal frond parts, or even imbued them with life-threatening toxicity to herbivores (i.e., a chemical defense strategy). This case study illustrates the utilization of chemically-based, 3D-multivariate models to perform detailed and realistic studies on the biomechanics, physiology, autecology, and synecology of extinct plants.