A New Record for Scolopia Sp. Nov. (Salicaceaesensulato) from the Early Miocene of Ethiopia: Identification and Classification of Fossil Leaves into their Living Relatives

Abstract

Early Miocene sedimentary layersfrom the Mush Valley in the Ethiopian Highlands located northeast of Addis Ababa preserve compressed fossil leaves and seeds with abundant organic matter in carbonaceous lacustrine shales. Fossil leaf morphotypes, preserved in those sediments, allow compilation of data and provide information on species richness and species relative abundance without the need for lengthy investigation to determine their taxonomic identity. Moreover, identification of fossil plants with respect to their nearest living relatives provides additional data critical for understanding their phylogenetic history, paleoclimate, and concentration of atmospheric CO₂. Here a previously unnamed taxon from the Mush Valley site was described; the taxon has a distinctly acrodromous primary venation, a pulvinus base, brachyparacyticstomatal complexes, and a suite of other characters of higher order venation and epidermal cells. The comparison of venation patterns and cuticular features among the fossil leaves and herbarium leaves using categorical variables shows that the fossil shares several characters with trinerved species in the genus Scolopia, but does not share all characters with any living species. Therefore, an extinct species found in the genus Scolopia (Scolopieae, Salicaceae) is described; the genus Scolopia is found today in Madagascar, the Solomon Islands, the Comoros, Southeast Asia, and eastern Australia. Two species of Scolopia occur today in Ethiopia, but these appear distantly related to the fossil based upon their pinnate, rather than acrodromous, primary venation. The fossil leaves are 21.73 ± 0.03 million years old, based upon ²⁰⁶Pb/²³⁸U geochronology, which can provide a precise date pointfor phylogenetic analysis. The taxonomic identification of this fossil leaf has important implication for reconstructing concentration of atmospheric CO₂ and paleo temperature of the time and hence, to understand the response of plants to the early Miocene global warming.