Leaf surface development and the plant fossil record: stomatal patterning in Bennettitales

Area of Science:

• Plant developmental genetics
• Paleobotany
• Evolutionary developmental biology (evo-devo)

Background:

• Stomata are crucial for plant-environment interactions, with guard-cell pairs conserved across land plants.
• Epidermal cell development and patterning follow distinct pathways in different plant taxa.
• Recent advances in molecular genetics illuminate plant epidermis development in model organisms.

Purpose of the Study:

• To apply a palaeo-evo-devo perspective to extinct Bennettitales using comparative morphology.
• To identify homologous epidermal traits in Bennettitales and living taxa for phylogenetic insights.
• To infer developmental mechanisms and evolutionary relationships of Bennettitales.

Main Methods:

• Comparative morphological analysis of fossil Bennettitales and extant plant taxa.
• Inference of developmental pathways ('fossil fingerprints') from epidermal traits.
• Analysis of cuticular features, epidermal cell shapes, stomatal orientation, and subsidiary cell development.

Main Results:

• Bennettitales possess a distinctive combination of epidermal traits: cuticular guard-cell thickenings, lobed abaxial epidermal cells, transverse stomatal orientation, and paracytic stomata.
• Lobed epidermal cells suggest adaxial-abaxial leaf polarity and optimized photosynthesis.
• Paracytic stomata in Bennettitales show strong similarity to those in Gnetum, indicating mesogene origin of subsidiary cells.

Conclusions:

• Epidermal traits provide reliable identification of fossil Bennettitales, even from vegetative material.
• The transverse stomatal orientation in Bennettitales likely resulted from a unique leaf expansion mechanism.
• The mesogene origin of subsidiary cells in Bennettitales, similar to Gnetum, suggests shared developmental pathways and necessitates revision of their use in phylogeny reconstruction.