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Responses of African bovids to Pliocene climatic change
Published online by Cambridge University Press: 05 October 2018
Abstract
The record of fossil mammals from the Shungura Formation, lower Omo Valley of southern Ethiopia, represents one of the largest and most carefully controlled samples for deciphering the responses of land faunas to global-scale climatic change. We use the abundant and continuous fossil record of the family Bovidae to analyze the effects of a late Pliocene climatic shift toward increased aridity in Africa beginning at 2.8 Ma and intensifying at about 2.4 Ma. A database consisting of 4233 specimen-based records collected under well-defined procedures is used to define patterns through time in bovid abundances while also controlling for taphonomic and other potential biases. Univariate and multidimensional (correspondence analysis) methods are used to study changes in bovid abundances through time. Our results indicate that bovids experienced an increase in species richness and a rapid episode of change in taxonomic abundances at 2.8 ± 0.1 Ma (between Members B and C), and that this shift was followed by gradual and prolonged changes in abundance between 2.8 and 2.0 Ma (between Member C and upper Member G). An analysis of skeletal-element abundances through the Shungura sequence shows that only moderate changes in taphonomic conditions occurred between 3.0 and 2.1 Ma, when the lower Omo Valley was dominated by a large, meandering river, but that significant changes in the mode of preservation accompanied the onset of lacustrine depositional environments at 2.1 Ma (between lower and upper Member G). A juxtaposition of taxonomic with taphonomic patterns shows that the shift in taxonomic abundances at 2.8 Ma occurred in the absence of significant changes in taphonomic conditions. The main changes in bovid relative abundances and diversity appear to have been driven by broad environmental and climatic changes in Africa. As environmental indicators, bovids show a transition in the Omo at about 2.8 Ma from closed and wet environments in Member B to closed but dry environments in Member C. This drying trend intensified in Members D, E, and F, between about 2.5 and 2.3 Ma. In lower Member G, between 2.3 and 2.1 Ma, there was an increase in bovid abundance and diversity, which may be a result of greater environmental heterogeneity. The pattern of environmental change depicted by Shungura bovids is consistent with independently derived evidence of Omo paleoenvironments (from paleosols, paleoflora, and micromammals), and with regional and global evidence of climatic changes, especially acute between 2.8 and 2.3 Ma, that caused the initiation of glacial cycles in the north and drier climate in the tropics of Africa. Even though the Omo bovids showed distinct responses to large-scale climatic and environmental change, the Omo bovid community also had important attributes of long-term stability: two species, Aepyceros shungurae and Tragelaphus nakuae, dominated the community for nearly one million years. This study highlights the importance of carefully controlled collection procedures of fossil vertebrates and provides an important demonstration of the potential complexity in mode and rate of responses of land faunas to climatic change.
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