Allia Bay is located on the east side of Lake Turkana, 40 km south of Koobi Fora, in northern Kenya. The surrounding area is very open and dry. The vegetation consists of arid and subarid scrub with large stretches of C₄ grassland. Many of the mammalian species found near this area are drought-adapted. Baboons and vervet monkeys are often present near the limited number of waterholes.

The sediments at Allia Bay, which are part of the Koobi Fora Formation, are mainly fluvial. At the time of their deposition, the ancestral Omo River dominated the Turkana Basin. After the filling of Lonyumun Lake at around 4 Myr, the Omo, along with the Turkwel and Kerio Rivers, deposited sediments across their expansive floodplains. The most important fossil bearing site at Allia Bay, formally known as 261-1, consists of a series of exposures deposited by these fluvial channels. The deposits underlay the Moiti Tuff, which has been dated to ca. 3.9 Myr through single-crystal dating.

Around 5 m below the Moiti Tuff lays a bone bed. These fossils are thought to have been accumulated relatively quickly. They are badly rolled and weathered, which indicates that they were transported from a distance by seasonal floodwaters. Fossil taxa consist mainly of fish and reptiles, such as Protopterus (lungfish), Crocodylus (crocodile) and Chelonia (turtle). Additional mammalian taxa include Parapapio (genus of extinct baboons), Dinofelis (saber-toothed cat), Hipparion (extinct genus of horse), Elephas and Giraffa.

Fossils from the earliest species of Australopithecus have also been discovered at 261-1. These fossils, once attributed to Australopithecus cf. A. afarensis, have been assigned to Australopithecus anamensis. These hominins are only represented at one other site in Kanapoi. Australopithecus anamensis possesses a combination of primitive and derived traits shared with later australopithecines. It differs from A. afarensis in several features, such as: narrower and more parallel jaws, slightly more ape-like canine and premolar morphology, slightly larger lower lateral incisors and a different structure of the lateral nasal aperture. Features that Australopithecus anamensis share with A. afarensis include: relatively thick enamel, the degree of postcanine megadontia, the level of canine size dimorphism and the unique scaphoid and lunate surface morphology of the distal radius. The postcranial morphology of Australopithecus anamensis indicates that this species was a habitual biped. Their thick enamel has been interpreted as an adaptation to feeding on hard objects. However, studies of molar microwear suggest that these hominins had a diet similar to gorillas and chimpanzees.

Australopithecus anamensis from Kanapoi (4.20-4.17 Myr) and Allia Bay (3.9 Myr), plus the A. afarensis samples from Laetoli (3.7-3.5 Myr) and Hadar (3.4-3.0 Myr) represent four temporally disjunct sites. Their distributions do not overlap. Although distinct from other australopithecines, Australopithecus anamensis from Allia Bay bears closer morphological affinities to A. afarensis from Latetoli, than it does to A. afarensis from Hadar. In addition, the Australopithecus anamensis sample from Kanapoi does not share as close an affinity to A. afarensis as the Allia Bay sample does. This has led researchers to conclude that Australopithecus anamensis and A. afarensis represent a single evolving lineage.

Paleoenvironmental studies of Allia Bay during the Pliocene have reconstructed this area as riverine woodland with gallery forests. There was probably more extensive canopy cover than found today as well as increased moisture levels. Dry C₄ grasslands were also present, but these environments were less widespread than present day conditions. The faunal composition at Allia Bay also suggests a mosaic environment. The presence of rivers is supported by the dominance of fish bones. The presence of the African knifefish, Gymnarchus, suggests that these waters were quite muddy. This species hunts using electrosensory prey location similar to the electric eel. Floodplain swamps are supported by the abundance of the lungfish, Protopterus. Gallery forests are supported by the presence of Thallomys (acacia rat) and arboreal colobine monkeys. In addition, dry savanna and woodland are supported by the presence of the gerbil, Tatera. All these lines of evidence suggest an environment very different from the present. However, some work on stress lines in mammal teeth indicates that conditions were actually quite similar to those seen today.

References

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