Fossil land snails of East Africa and their palaeoecological significance

Abstract

This study deals with the Neogene and extant land snails of tropical East Africa and their implications for interpreting the paleoenvironments of the numerous localities at which they have been found. Of major significance to the study is the intimate association between the terrestrial molluscs and the rich mammalian faunas, hominoids included, of East Africa. Thus, palaeoecological reconstructions based on land snails are directly applicable to the mammalian faunas. Palaeoecological reconstructions are proposed for most of the Lower and Middle Miocene hominoids, including Proconsul, Rangwapithecus, Limnopithecus, Micropithecus, Nyanzapithecus, Kenyapithecus and others, and for the mid-Pliocene Australopithecus from Laetoli, Tanzania. The departure point for the palaeoecological reconstructions is a comprehensive study of extant terrestrial molluscs of East Africa, the habitat preferences of which are well documented. All the fossil gastropods studied comprise extant genera and even species, so the usual problems regarding the application of actualism to fossil assemblages is avoided. Furthermore, the fossil gastropod assemblages resemble extant ones, confirming their utility for such reconstructions. Among the parameters examined are rainfall, altitude, vegetation cover and type and zoogeography. A further point of interest is that the samples are more than adequate for the purposes of the study, many of the fossil localities having yielded several thousand specimens. Finally, more than 40% of the extant genera of East Africa have now been recognized in the fossil state. The molluscs are thus, by far, the best represented biological group known in the fossil record of Africa and as such hold great potential for understanding the past. This study ends with reconstructions of the palaeoecology of numerous fossiliferous localities in East Africa which have yielded molluscs and mammals. Changes through the geological column are documented and the habitat preferences of hominoid taxa are proposed. The study suggests, for example, that Proconsul major lived in humid tropical forest, Proconsul nyanzae inhabited dry tropical forest or closed woodland, Kenyapithecus africanus occurredin semi-arid steppe to woodland, and Australopithecus afarensis at Laetoli lived in savannah/steppe in which there were watercourses fringed by riparian forest. This paper is illustrated with distribution maps and drawings of the more than 40 snail taxa represented in the collections. The distribution maps illustrate the extant distribution of taxa, as well as the fossil localities at which they have been recovered. It represents a major advance in the study of tropical African Neogene palaeoecology, and will undoubtedly have many applications in future studies of African palaeoeforas and palaeofaunas.