A Fossil Anthonotha (Leguminosae: Detarioideae: Amherstieae) Species from the Early Miocene (21.73 Ma) of Ethiopia

Abstract

Premise of research. Several morphotypes of well-preserved legume leaflet compressions are known from the early Miocene Mush plant assemblages from the northwestern plateau of Ethiopia. One of these morphotypes represents a member of the Amherstieae tribe (subfamily Detarioideae) and is prevalent in the assemblage. Morphological and epidermal micromorphological characteristics are sufficient to identify it to genus. The evolutionary and biogeographic significance of the early Miocene occurrence of this genus in the uplands of Ethiopia is explored. Methodology. Fossil laminae were compared with herbarium specimens and leaf cuticle preparations to determine their likely identity. Pivotal results. Morphological and micromorphological characteristics support recognition of a new fossil species of Anthonotha (Leguminosae: Detarioideae: Amherstieae: Berlinia clade), herein named Anthonotha shimaglae Pan, Jacobs, Currano, Estrella, Herendeen et Burgt sp. nov. Characteristics include symmetrical, untoothed leaflets with pulvinate petiolules, short acuminate leaflet tips, eucamptodromous secondary venation, an abaxial epidermal surface possessing sinuous anticlinal cell walls, paracytic stomatal complexes, abundant trichome bases, papillate periclinal cell walls, and an adaxial epidermal surface possessing highly sinuous anticlinal cell walls. A. shimaglae represents the earliest-known macrofossil of the genus. The occurrence of this new species with other Detarioideae legumes, including Englerodendron mulugetanum, and a morphotype representing Berlinia or Isoberlinia in the early Miocene Mush plant assemblages is more analogous to high-rainfall evergreen forests of Central Africa today than East African forest types. Conclusions. Early Miocene Mush fossils include the common occurrence of an extinct species of legume, herein named Anthonotha shimaglae. Prehistoric forests of the Ethiopian plateau may represent ancestral sources of currently disjunct and perhaps relictual wet forest taxa in tropical montane and coastal forests of eastern Africa today, as well as potential evolutionary sources of the Guineo-Congolian forested region.