Phyllotaxis, ontogeny and CT imaging: old and new approaches to understanding optimal seed packing in Middle Jurassic...

Fibonacci spiral phyllotaxis is overwhelmingly the most common leaf arrangement among plants, both today and in the fossil record. Spiral phyllotaxis appeared as early as the Late Devonian, some 380 Mya. This mathematical property has been studied rigorously in living conifer species; however, the phyllotaxis of fossil conifer seed cones remains under-studied, including that of the Middle Jurassic seed cones of Araucaria mirabilis.

Twenty-one A. mirabilis seed cones from the Bosques Petrificados de Jaramillo National Park, Argentina, were analysed for their morphometrics, phyllotaxis and seediness. For each cone, the number of seeds in the cone, in addition to the number of clockwise and anticlockwise parastichies, was counted on the well-preserved cone surface. Micro-CT was used to observe the internal arrangement of the seeds in the cone in a non-destructive manner. Then, Avizo was used to visualize one clockwise and one anticlockwise parastichy from each cone to determine the number of seeds in the spiral and the angle of rotation of the spiral.

Here, we show that clockwise ontogenetic chirality and greater axis length and width are positively correlated with greater seediness in A. mirabilis seed cones. Cones with clockwise ontogenetic chirality have moderately tighter spirals (P < 0.05, r = 0.2) with more seeds in them (P < 0.05, r = 0.3). Likewise, a taller axis is associated with a greater circumference (P < 0.01, r = 0.6), which, in turn, is correlated with more bract/scale complexes (P < 0.05, r = 0.05). The most common phyllotaxis is 13,21 (64 %), with 360° spirals (48 %) and clockwise ontogenetic chirality (66 %).

Visualizing internal morphometrics and seed arrangements show which characteristics contribute to optimal seed packing. Thus, micro-CT imaging, in addition to traditional methods, enables a deeper study of conifer cone morphology and construction.