Paleoenvironmental changes in the Hiwegi Formation (lower Miocene) of Rusinga Island, Lake Victoria, Kenya

The Early Miocene of Rusinga Island (Lake Victoria, Kenya) is best known for its vertebrate fossil assemblage—particularly of early hominoids and catarrhines—but the multiple stratigraphic intervals with well-preserved fossil leaves have received much less attention. The Hiwegi Formation has three fossil leaf-rich intervals: Kiahera Hill, R5, and R3. Here, we made new fossil collections from Kiahera Hill and R3 and compared these floras to previous work from R5 as well as modern African floras. The Kiahera Hill flora was most similar to a modern tropical rainforest or tropical seasonal forest and was a warm and wet, closed forest. This was followed by a relatively dry and open environment at R5, and R3, which was most similar to a modern tropical seasonal forest, was a warm and wet spatially heterogenous forest. Floral composition of these floras differed dramatically but Kiahera Hill and R3 were more similar to each other than either flora was to R5. The Kiahera Hill flora had few monocots or herbaceous taxa and was dominated by large leaves and had a higher species richness and greater evenness than the R3 flora. Our work, coupled with previous studies, suggests that R3 had a landscape of both closed forest and more open areas with seasonal ponding. The absence of morphotypes from the R5 flora that were present in the Kiahera Hill and R3 floras provides evidence for local expatriation during the R5 time interval. These results demonstrate that there was a considerable change in both climate and vegetation over an ~500 kyr interval of the Kiahera Hill, R5, and R3 floras. Thus, this work suggests that the Hiwegi Formation on Rusinga Island samples multiple environments in the Early Miocene and provides important context for the evolution and habitat preference of early apes.

The only complete articulated early Miocene chameleon skull (Rusinga Island, Kenya) suggests an African origin for Madagascar’s endemic chameleons

We here present the first detailed study of the specimen KNM-RU 18340 from Rusinga Island (Kenya), the only known complete early Miocene chameleon skull, using micro-CT. This specimen represents one of the oldest chameleon fossils ever recovered. For the first time, the skull bone internal surfaces, their sutures, and elements contained inside the rocky matrix are observed. Our morphological comparisons and phylogenetic analyses place this specimen confidently in the genus Calumma and a new species, Calumma benovskyi sp. nov., is erected for it. Since all species of this genus are endemic to Madagascar, this fossil uniquely demonstrates the existence of Calumma on continental Africa in the past. Our results challenge the long-held view that chameleons originated on Madagascar and dispersed over water to Africa, and provide a strong evidence of an African origin for some Malagasy lineages. The Oligocene–early Miocene dispersal to Madagascar, using oceanic currents that favoured eastward dispersal at that time, is a highly supported scenario matching the suggested dispersal of lemurs to this island. This is consistent with a previously suggested hypothesis based on molecular data.

Sedimentological and paleoenvironmental study from Waregi Hill in the Hiwegi Formation (early Miocene) on Rusinga Island, Lake Victoria, Kenya

Paleontological deposits on Rusinga Island, Lake Victoria, Kenya, provide a rich record of floral and faunal evolution in the early Neogene of East Africa. Yet, despite a wealth of available fossil material, previous paleoenvironmental reconstructions from Rusinga have resulted in widely divergent results, ranging from closed forest to open woodland environments. Here, we present a detailed study of the sedimentology and fauna of the early Miocene Hiwegi Formation at Waregi Hill on Rusinga Island, Kenya. Our new sedimentological analyses demonstrate that the Hiwegi Formation records an environmental transition from the bottom to the top of the unit. Lower in the Hiwegi Formation, satin-spar calcite after gypsum in siltone deposits are interpreted as evidence for open hypersaline lakes. Moving up-section, carbonate deposits – interpreted previously as evidence of aridity – are actually diagenetic calcite cements, which preserve root systems of trees; further up-section, the upper-most paleosol layer contains abundant root traces and tree-stump casts, previously interpreted as evidence of a closed-canopy forest. These environmental differences are reflected by differences in faunal composition and abundance data from Hiwegi Formation fossils sites R1 and R3. Taken together, this work suggests that divergent paleoenvironmental reconstructions in previous studies likely suffered from time-averaging across multiple environments. Further, our results demonstrate that during the early Miocene habitats in Rusinga’s Hiwegi Formation varied both spatially and temporally. From a regional perspective, it has been argued that during the early Neogene a broad forested environment stretched across the African continent, transitioning later to predominately open landscapes that characterizes the region today. Our results challenge this simple model, suggesting instead that local or regional habitat heterogeneity already existed in the early Miocene. This has important implications for interpretations of the selective pressures faced by early Miocene fauna, including Rusinga Island’s well-preserved ape and catarrhine primates.