VERTEBRATE SPECIES RICHNESS CHANGE FROM THE LATE MIOCENE TO EARLY PLIOCENE OF LOTHAGAM, TURKANA BASIN, KENYA

Turkana Basin in Kenya, Africa, is home to many discoveries of hominin and non-hominin fossils, and a myriad of isotopic paleoenvironmental interpretations has been conducted. Stable carbon isotope studies on fossil tooth enamel and eggshell indicate that an expansion of C₄ biomass occurred across the Miocene-Pliocene boundary, and stable carbon and oxygen isotope studies on fossil soil carbonates indicate increased temperatures in the Pliocene. The purpose of this research is to ascertain if paleoenvironmental shifts from the late Miocene to early Pliocene are associated with changes in vertebrate diversity at Lothagam in the Turkana Basin. The upper Miocene is represented by the Lower and Upper Members of the Nawata Formation, and the lower Pliocene is represented by the Apak and Kaiyumung Members of the Nachukui Formation. The fossils utilized were collected from fluvial deposits, although details about collecting methods and depositional environments were unavailable. I compiled specimen counts for terrestrial, semi-aquatic, and aquatic fossil species for each member, excluding fish and birds, sub-dividing into total, mammalian, and reptilian groups. The total rarefaction curve displays a significant (p < 0.05) decrease between the Apak and Kaiyumung, as do species richness, suggesting a noticeable drop in species richness in the early Pliocene. Diversity indices calculated for total and mammalian data display a decrease from the Apak to Kaiyumung consistent with the decrease in species richness, while dominance indices for total, mammalian, and reptilian data display a significant inverse relationship with diversity. Isotopic interpretations from large-bodied herbivores show a switch from C₃ to C₄ vegetation and a transition is observed from browsing to grazing in ungulates. With the rise of C₄ grasses, several herbivore families decline or disappear. A consistent inverse relationship between mammalian and reptilian dominance data is observed, indicating that reptiles responded to environmental changes differently. This study provides a deeper understanding of diversity and dominance changes in mammals and reptiles across the Miocene-Pliocene boundary in the Turkana Basin, and suggests that these groups responded differently to expanding C₄ grasslands and associated climate change.