New early Miocene 40Ar/39Ar ages for Nakwai, northwest Kenya, Africa, and paleontological analysis of Meroehyrax kyongoi dentition.

摘要

Nakwai is a fossiliferous region in the Turkana Basin of northwestern Kenya. Until recently the only ages for the Nakwai formation were based on biostratigraphic correlation with Losodok; a late Oligocene site located to the north of Nakwai. The new 40Ar/39Ar ages in this study range from 23 Ma to 15 Ma and constrain the upper portions of the Nakwai section to the early Miocene. These ages make it the only securely dated fossil locality in the region that represents the earliest Miocene; all of the other early Miocene fossil localities date back to ~20 Ma or younger. The time period following the Oligocene -- Miocene boundary is crucial for understanding the dynamics and timing of the great faunal transition that happened between the endemic fauna of island Afro-Arabia as it made contact with Eurasia and brought many of the animals into Africa that we associate with the continent today. These new dates help constrain the timing of this transitional event, which was previously thought to have been at, or near the Oligocene -- Miocene boundary.;Hyracoidea is a group of mammals native to Africa, which were very diverse throughout the early Cenozoic. During the late Oligocene and early Miocene some species of hyracoid appear to be developing hypsodont and selenodont features in their cheek teeth. Dental features such as hypsodonty were long thought to have evolved in response to an increase in silica-rich C4 grasses. However, in Africa these grasses did not arrive until the Late Miocene. The species Meroehyrax kyongoi, which has been recovered from Nakwai, is a member of the Pliohyracidae family; which has been described as having these dental features. Dental measurements from Meroehyrax kyongoi along with two younger species of Pliohyracidae were compared to try to assess the changes in these features through time. The degree of hypsodonty was also established within these taxa by determining a Hypsodonty Index (HI) for each specimen. These dental data suggest that, although tooth size does increase through time, the degree of hypsodonty remains relatively constant. When compared to other hypsodont herbivores, the HI of these hyracoids does not indicate true hypsodonty.