Palaeobotanical studies from tropical Africa: relevance to the evolution of forest, woodland and savannah biomes

Abstract

Fossil plants provide data on climate, community composition and structure, all of which are relevant to the definition and recognition of biomes. Macrofossils reflect local vegetation, whereas pollen assemblages sample a larger area. The earliest solid evidence for angiosperm tropical rainforest in Africa is based primarily on Late Eocene to Late Oligocene (ca. 39-26 Myr ago) pollen assemblages from Cameroon, which are rich in forest families. Plant macrofossil assemblages from elsewhere in interior Africa for this time interval are rare, but new work at Chilga in the northwestern Ethiopian Highlands documents forest communities at 28 Myr ago. Initial results indicate botanical affinities with lowland West African forest. The earliest known woodland community in tropical Africa is dated at 46 Myr ago in northern Tanzania, as documented by leaves and fruits from lake deposits. The community around the lake was dominated by caesalpinioid legumes, but included Acacia, for which this, to my knowledge, is the earliest record. This community is structurally similar to modern miombo, although it is different at the generic level. The grass-dominated savannah biome began to expand in the Middle Miocene (16 Myr ago), and became widespread in the Late Miocene (ca. 8 Myr ago), as documented by pollen and carbon isotopes from both West and East Africa.

Forest refugia in Western and Central Africa as ‘museums’ of Mesozoic biodiversity

“…This period corresponds to a time of diversification among angiosperms and associated reduction in gymnosperm diversity, documented in the Cenomanian stage of the Late Cretaceous [29]. Even if the Mesozoic rainforests were structurally and compositionally different from those in present-day tropical Africa, stratified forests may have been present since the Late Cretaceous, based on the presence of large seeds and fruits in the fossil record [30].…”

Large-scale phylogeny of chameleons suggests African origins and Eocene diversification

“…Transition out of habitats with large perches into habitats with fine structure (i.e. grassland and heathland) has only occurred for a few clades [32,34], and primarily since the Pliocene (figure 2) when these habitats became present in the African landscape [69][70][71][72].…”

Remnants of an ancient forest provide ecological context for Early Miocene fossil apes

“…Leaf mass per area (M A ) of the flora suggests the forest comprises predominantly evergreen taxa, but deciduous taxa with short-lived leaves would have also been an important component (site mean M A ¼ 125.0, þ 114.0, À 57.9 gm À 2 , errors represent 95% prediction interval, range ¼ 70-196 gm À 2 , n ¼ 16; Supplementary Table 2). The average leaf area (site mean ¼ 7.83 log e mm 2 , range ¼ 6.0-9.1 log e mm 2 , n ¼ 24; Supplementary Table 2) and the proportional abundance of large leaves (100% Z microphyll sized; Supplementary Table 2) is characteristic of closed forested environments 20 .…”