A new species of Simiolus from the middle Miocene of the Tugen Hills, Kenya

A new species of the "small-bodied ape" Simiolus is described here that extends the temporal range of the genus to the end of the Middle Miocene. As such, it is one of the few species of fossil primates known from East Africa during a time of significant change in which Old World monkeys and crown hominoids replaced the primitive ape-like primates that had dominated the early Miocene. The dynamics of this important event in our evolutionary history are obscured by the small number of fossil primates known from Africa between 14 and 6 million years ago, as well as persistent ambiguity regarding the phylogenetic status of the ape-like Miocene primates. The new species described here helps to fill this temporal gap, and our analysis of its phylogenetic position suggests that Simiolus and many other Miocene primates were not only ape-like, they were, indeed, stem hominoids. Judging from the available material, the new species may be the smallest known ape.

Early cercopithecid monkeys from the Tugen Hills, Kenya

The modern Old World Monkeys (Superfamily Cercopithecoidea, Family Cercopithecidae) can be traced back into the late Miocene, but their origin and subsequent diversification is obscured by the scarcity of terrestrial fossil sites in Africa between 15 and 6 Ma. Here, we document the presence of cercopithecids at 12.5 Ma in the Tugen Hills of Kenya. These fossils add 3 My to the known antiquity of crown Cercopithecidae. The two specimens represent one or possibly two species of early colobine, and their morphology suggests that they were less folivorous than their modern relatives.

40Ar/(39)Ar dating of the Kapthurin Formation, Baringo, Kenya

The(40)Ar/(39)Ar radiometric dating technique has been applied to tuffs and lavas of the Kapthurin Formation in the Tugen Hills, Kenya Rift Valley. Two variants of the(40)Ar/(39)Ar technique, single-crystal total fusion (SCTF) and laser incremental heating (LIH) have been employed to date five marker horizons within the formation: near the base, the Kasurein Basalt at 0.61+/-0.04 Ma; the Pumice Tuff at 0.543+/-0.004 Ma; the Upper Kasurein Basalt at 0.552+/-0.015 Ma; the Grey Tuff at 0.509+/-0.009 Ma; and within the upper part of the formation, the Bedded Tuff at 0.284+/-0.012 Ma. The new, precise radiometric age determination for the Pumice Tuff also provides an age for the widespread Lake Baringo Trachyte, since the Pumice Tuff is the early pyroclastic phase of this voluminous trachyte eruption. These results establish the age of fossil hominids KNM-BK 63-67 and KNM-BK 8518 at approximately 0.510-0.512 Ma, a significant finding given that few Middle Pleistocene hominids are radiometrically dated. The Kapthurin hominids are thus the near contemporaries of those from Bodo, Ethiopia and Tanzania. A flake and core industry from lacustrine sediments in the lower part of the formation is constrained by new dates of 0.55-0.52 Ma, a period during which the Acheulian industry, characterized by handaxes, is known throughout East Africa. Points, typical of the Middle Stone Age (MSA), are found in Kapthurin Formation sediments now shown to date to between 0.509+/-0.009 Ma and 0.284+/-0.012 Ma. This date exceeds previous estimates for the age of the MSA elsewhere in East Africa by 49 ka, and establishes the age of Acheulian to MSA transition for the region. Evidence of the use of the Levallois technique for the manufacture of both small flakes and biface preforms, the systematic production of blades, and the use and processing of red ochre also occurs in this interval. The presence of blades and red ochre at this depth is important as blades signify a high degree of technical competence and red ochre suggests symbolic behavior.

New cercopithecoids and a hominoid from 12.5 Ma in the Tugen Hills succession, Kenya

The early evolutionary history of the cercopithecoids is poorly understood, primarily due to a lack of fossil material from between 15 and about 9 Ma. Cercopithecoid primate specimens from a fossil site in the Ngorora Formation of the Tugen Hills, Kenya, belong to the genus Victoriapithecus, possibly a new species. These fossils are associated with a hominoid specimen that resembles Proconsul, and another tooth of a catarrhine, also probably hominoid. The locality is BPRP#38, in the Kabasero type section of the Ngorora Formation, and well dated at 12.5 Ma. If the hominoid specimen is confirmed as Proconsul, it would be one of the most recent recorded. The relatively diverse mammal fauna from the site in some ways resembles that of Fort Ternan. The site is underlain, and not far removed in time, from one of the best fossil macro-floras in Africa, which indicates lowland rainforest conditions in this portion of the Rift Valley at 12.6 Ma.

The Neogene: Part 2 Neogene geochronology and chronostratigraphy

We present a revised Neogene geochronology based upon a best fit to selected high temperature radiometric dates on a number of identified magnetic polarity chrons (within the late Cretaceous, Paleogene, and Neogene) which minimizes apparent accelerations in sea-floor spreading. An assessment of first order correlations of calcareous plankton biostratigraphic datum events to magnetic polarity stratigraphy yields the following estimated magnetobiochronology of major chronostratigraphic boundaries: Oligocene/Miocene (Chron C6CN): 23.7 Ma; Miocene/Pliocene (slightly younger than Gilbert/Chron 5 boundary): 5.3 Ma; Pliocene/Pleistocene (slightly younger than Olduvai Subchron): 1.6 Ma.

Changes to the marine time-scale are relatively minor in terms of recent and current usage except in the interval of the middle Miocene where new DSDP data reveal that previous correlations of magnetic anomalies 5 and 5A to magnetic polarity Chrons 9 and 11, respectively, are incorrect. Our revized magnetobiostratigraphic correlations result in a 1.5-2 m.y. shift towards younger magnetobiochronologic age estimate in the middle Miocene. Radiometric dates correlated to bio- and magnetostratigraphy in continental section generally support the revized marine magnetobiochronology presented here. Major changes, however, are made in marine-non-marine correlations in the Miocene in Eurasia which indicate African-Eurasian migrations through the Persian Gulf as early as 20 Ma. The 12.5 Ma estimate of the Hipparion datum is supported by recent taxonomic revisions of the hipparions and magnetobiostratigraphic correlations which show that primitive hipparions first arrived in Eurasia and North Africa at c. 12.5 Ma and a second wave in the tropics (i.e. Indian and central Africa) at c. 10 Ma.