CUTICLE CHARACTERS ARE ESSENTIAL FOR THE CLASSIFICATION OF TROPICAL FOSSIL LEAVES: A CASE STUDY FROM THE EARLY MIOCENE OF ETHIOPIA

Many tropical plant species have elongate, elliptic leaves with smooth margins, drip tips, and grossly similar primary and secondary venation patterns. Thus, it is often difficult to divide tropical leaf macrofossils into unique morphotypes relying solely on shape and venation characters. Fossilized leaf cuticle retains a record of important epidermal traits, which are often diagnostic for distinguishing among taxa. Cuticle is more resistant to decay than other parts of the leaf, which means it can be preserved in the fossil record, especially in reducing conditions. It also provides proxies for light availability and CO₂ concentrations. Cuticle thickness correlates to the amount of light the plant receives, and stomatal density is inversely related to atmospheric CO₂ concentration. Here, we test the utility of cuticle to aid in taxonomic identification of tropical plants from the early Miocene Mush Valley site in Ethiopia.

Small chips of individual leaf fossils were subjected to a series of chemical baths, including 0.1M tetrasodium pyrophosphate decahydrate and 48% hydrofluoric acid, to isolate the cuticle from the matrix. Cuticle pieces were analyzed using a Nikon LV100 compound microscope with Intenslight C-HGFI Illumination unit (Endow GFP longpass green filter: exciter HQ470/40 ex, dichroic 495, emitter 500LP). Photographs were taken with a Nikon Digital Sight DS-Ri1 camera and NIS-Elements v. 3.22.11 with extended depth of focus capabilities. Specimens were described using the traits of Dilcher (1974).

Cuticle allowed us to distinguish between morphotypes with similar shapes and venation. The images revealed thickened trichome bases on one species of Euphorbeaceae but not another. Undulation Index (UI) of epidermal cells was an effective metric to distinguish between two similar legume morphotypes (2.2 vs. 1.6 UI respectively). Lastly, the cuticle allowed for identification of fossils that were poorly preserved. 19% of the images allowed for identification of unknowns, 32% of the macrofossils’ identities were corrected with cuticle, and 21% of the macrofossils’ initial identities were confirmed.