79-7 Modeled Ecophysiology of Non-Angiosperm Mesozoic Floras

Abstract:

Mesozoic plant communities gave rise to modern floral ecosystems. One prominent extinct clade, the bennettitaleans, were key components of paleoecosystems during this period and remained dominant until the rapid diversification of angiosperms in the Cretaceous. While the anatomy and morphology of the reproductive structures of these enigmatic taxa have been extensively studied, very little is understood about their vegetative structure functions, or their ecophysiology, throughout the Mesozoic. 

Permineralized wood is common in the fossil record, preserving large quantities of plant vascular tissue. Analysis of fossilized secondary xylem characteristics provides insight into how plants interacted with their environments and indicates survival through climatic perturbation. Xylem hydraulic characteristics such as pit density, tracheid diameter, and pit area are used to quantify wall-specific conductivity (K_SP) and mean cavitation pressure (Ψ_P50). These parameters, when coupled, create a hydraulic ecospace in which plants occupy specific zones relative to their water use efficiency and stress resistance. This study focuses on bennettitalean wood permineralizations in the Wieland Type Collection housed at the Yale Peabody Museum and the Smithsonian Type Collection. We considered four bennettitalean species (Cycadeoidea dartoni, Cycadeoidea wielandii, Cycadeoidea dacotensis, and Cycadeoidea paynei), all from the Valanginian-Barremian aged Lakota Formation in southwestern South Dakota. We analyzed thin sections of permineralized trunk material which still preserve measurable vascular tissue that were originally prepared in the early 20^th century. Preliminary results from these specimens reveal that all four Bennettitales species cluster together in hydraulic ecospace (i.e., K_SP vs. Ψ_P50), in which they show low conductance and relatively moderate water stress vulnerability. This region of ecospace occupied by these bennettitalean taxa is distinct from extant plants with similar morphological features (e.g., cycads) and other Mesozoic taxa that lived with similar environments (e.g., conifers). These results suggest that bennettitalean hydraulic capabilities may have been the possible cause of their extinction. Further, this investigation into bennettitalean ecophysiology is a step toward understanding their unique role throughout deep time. Quantification of the hydraulic capabilities of this once-dominant clade provides important constraints ecosystem dynamics, including water and carbon cycling, in Mesozoic terrestrial environments.

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