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The Napak Primate Fossils of Uganda

Napak in Northern Uganda is regarded amongst the richest fossil locations for well-preserved hominoid ("ape") primates dating back over 20 million years during the Miocene Epoch.

African gorilla - Image by Mike Arney
African gorilla - Image by Mike Arney

In 2022 Napak was named among the “First 100 Geological Heritage Sites” designated by the International Union of Geological Sciences (IUGS).

Since the early 1950s the volcanic ash sedimentary tuff successions including the Iriri (20.5 Ma), Napak (19-20.5 Ma) and younger Akisim (16 Ma) Members and palaeosols (fossil soils) exposed on the slopes of the Akisim Mountain (02° 08' 30'' N034° 16' 44'' E) has revealed teeth, jaws and skull fossils from 15 taxa of early ape species ranging in size from a small bush baby to a much larger gorilla.

The Miocene Epoch dated between 23.03 - 5.3 million years ago forms part of the current Cenozoic Era and known for the dominance of mammals. The Miocene was first of the two epochs of the Neogene Period preceded by the Oligocene Epoch and succeeded by the Pliocene Epoch.

The terms Miocene, Palaeozoic and Cenozoic, were first mentioned by Sir Charles Lyell 1st Baronet FRS (1797 –1875) a British lawyer and the foremost geologist when he was examining rocks of the Paris Basin.

During the Miocene the climate was generally warmer and continental ice sheets were only present on Antarctica and not in the Artic region of the northern hemisphere.

Notably the Miocene saw the first appearances of two major ecosystems including the kelp forests and the expansion of grasslands as the continental interiors dried.

During the Miocene Africa experienced tectonic movement with rifting in East Africa and the collision of the African-Arabian Eurasian tectonic plates.

The closing of the continents of Africa and Eurasia also contracted the Tethys Sea, depleting the main source of atmospheric moisture and rainfall creating a dry region. The joining of the continents also enabled the radiation and diversity of flora and fauna between Africa and Eurasia.

The fossils of Napak indicate that during the Miocene the fauna and flora was living in a tropical woodland to forest environment.

Mount Napak (2,103m ASL) is the third highest mountain and extinct volcano in the Napak District of the Karamoja region in Uganda. Located on the northern edge of the Bokora Game Reserve it is an isolated region with a history of tribal conflicts and elevated security concerns.

Mount Napak (2,103m ASL) is the third highest mountain and extinct volcano in the Napak District, Uganda
Mount Napak (2,103m ASL) is the third highest mountain and extinct volcano in the Napak District, Uganda

Mount Napak is alkaline volcano that periodically erupted and vented carbonatite-nephelinite material during the Early Miocene and sits on the margin of the Great Rift Valley.

The succession of volcanic ash deposits that built up and the soils that developed are responsible for preserving a range of both animal and plant fossils.

Afropithecus turkanensis - Image by Ghedoghedo
Afropithecus turkanensis - Image by Ghedoghedo

The Miocene was a period of expanding areas of vegetation and the diversification of mammals, birds and grasses.

Ugandapithecus maxilla bone and teeth
Ugandapithecus maxilla bone and teeth

Napak has yielded three species of large apes throughout the local succession of volcano-sedimentary rock.

The basal Iriri Member was deposited in a freshwater marshy or palustral environment where eroded rock or terrigenous sediments and vented volcanic ash or tuff settled and red coloured palaeosol developed.

Within this layer the primate species of Ugandapithecus legetetensis and Ugandapithecus major have been discovered alongside gastropods, bivalves, crocodiles, fish and turtles.

The Napak Member is comprised of a dark volcanic ash and black palaeosol laying on the volcano slopes where small streams flowed downwards.

This succession is where the bulk of the fossil hominoid primates have been discovered including Ugandapithecus legetetensis, Ugandapithecus major and Afropithecus turkanensis. These apes were discovered alongside a few crocodiles and diverse assemblage of plants and mammals.

The steep outcrops of volcanic ash agglomerate of the Akisim Member that caps the succession sequence has yielded few primates.

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