| Australopithecus bahrelghazali | |
|---|---|
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| Life restoration | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Suborder: | Haplorhini |
| Infraorder: | Simiiformes |
| Family: | Hominidae |
| Subfamily: | Homininae |
| Tribe: | Hominini |
| Genus: | †Australopithecus |
| Species: | †A. bahrelghazali |
| Binomial name | |
| †Australopithecus bahrelghazali Brunet et al., 1995 | |
| Synonyms | |
Praeanthropus bahrelghazali | |
Australopithecus bahrelghazali is anextinctspecies ofaustralopithecine discovered in 1995 atKoro Toro,Bahr el Gazel, Chad, existing around 3.5 million years ago in thePliocene. It is the first and only australopithecine known fromCentral Africa, and demonstrates that this group was widely distributed across Africa as opposed to being restricted to East and southern Africa as previously thought. The validity ofA. bahrelghazali has not been widely accepted, in favour of classifying the specimens asA. afarensis, a better known Pliocene australopithecine from East Africa, because of the anatomical similarity and the fact thatA. bahrelghazali is known only from 3 partial jawbones and an isolatedpremolar. The specimens inhabited a lakeside grassland environment with sparse tree cover, possibly similar to the modernOkavango Delta, and similarly predominantly ateC4 savanna foods—such as grasses, sedges,storage organs, orrhizomes—and to a lesser degree alsoC3 forest foods—such as fruits, flowers, pods, or insects. However, the teeth seem ill-equipped to process C4 plants, so its true diet is unclear.
In 1995, two specimens were recovered fromKoro Toro,Bahr el Gazel, Chad: KT12/H1 or "Abel" (a jawbone preserving the premolars,canines, and the right secondincisor) and KT12/H2 (an isolated first upperpremolar). They were discovered by the Franco-Chadian Paleoanthropological Mission, and reported by French palaeontologistMichel Brunet, French geographer Alain Beauvilain, French anthropologistYves Coppens, French palaeontologist Emile Heintz, Chadian geochemist engineer Aladji Hamit Elimi Ali Moutaye, and British palaeoanthropologistDavid Pilbeam. Based on the wildlife assemblage, the remains were roughly dated to the middle to latePliocene 3.5–3 million years ago; consequently, the describers decided to preliminarily assign the remains toAustralopithecus afarensis, which inhabited Ethiopia during that time period, barring more detailed anatomical comparisons.[1] In 1996, they allocated it to a new species,A. bahrelghazali, naming it after the region; Bahr el Gazel means "River of the Gazelles" inclassical Arabic. They denoted KT12/H1 as theholotype and KT12/H2 aparatype.[2] Another jawbone was discovered at the K13 site in 1997,[3] and a third from the KT40 site.[4]
In 2008, apelite (a type ofsedimentary rock) recovered from the same sediments as Abel wasradiometrically dated (using the10Be/9Be ratio) to have been deposited 3.58 million years ago.[5] However, Beauvilain responded that Abel was not foundin situ but at the edge of a shallow gulley, and it is impossible to figure out from whatstratigraphic section the specimen (or any other fossil from Koro Toro) was first deposited in, in order to accurately radiometrically date it.[6] Nonetheless, Abel was redated in 2010 using the same methods to about 3.65 million years ago,[7] and Brunet agreed with an age of roughly 3.5 million years ago.[8]
A. bahrelghazali was the firstaustralopithecine recovered from Central Africa, and disproved the earlier notion that they were restricted to east of the eastern branch of theEast African Rift which formed in theLate Miocene. Koro Toro is situated about 2,500 km (1,600 mi) from the Rift Valley, and the remains suggest australopithecines were widely distributed in grassland and woodland zones across the continent. The lack of other Central and West African australopithecines may be due tosampling bias, as similarly aged fossil-bearing sediments are more or less unknown beyond East Africa.[1] The ancestors ofA. bahrelghazali may have left East Africa via theCentral African Shear Zone.[9] In 2014, the first australopithecine in the western branch of the East African Rift was reported inIshango, Democratic Republic of the Congo.[10]
At present, the classification ofAustralopithecus andParanthropus species is in disarray.Australopithecus is considered agrade taxon whose members are united by their similar physiology rather than how close they are to each other in the hominin family tree. In an attempt to resolve this, in 2003, Spanish writerCamilo José Cela Conde and evolutionary biologistFrancisco J. Ayala proposed splitting off the genus "Praeanthropus" and includingA. bahrelghazali alongsideSahelanthropus (the only other fossil ape known from Chad),A. anamensis,A. afarensis, andA. garhi.[11]
The validity ofA. bahrelghazali has not been widely accepted given how few remains there are and how similar they are toA. afarensis.[12]
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The teeth of KT12/H1 are quite similar to the jawbone ofA. afarensis, with large and incisor-like canines and bicuspid premolars (as opposed tomolar-like premolars). UnlikeA. afarensis, thealveolar part of the jawbone where thetooth sockets are is almost vertical as opposed to oblique, possesses poorly developed superior transverse torus and moderate inferior torus (two ridges on the midline of the jaw on the tongue side), and thinenamel on the chewing surface of the premolars.[1] Brunet and colleagues had listed the presence of 3 distinct tooth roots as a distinguishing characteristic, but the third premolar of theA. afarensis LH-24 specimen fromMiddle Awash, Ethiopia, was described in 2000 as having the same feature, which shows that premolar anatomy was highly variable forA. afarensis.[13] Themandibular symphysis (at the midline of the jaw) of KT40, especially, as well as KT12/H1 have the same dimensions as the symphysis ofA. afarensis, though theirs is relatively thick compared to the height.[4]
Carbon isotope analysis indicates a diet of predominantlyC4 savanna foods, such as grasses, sedges, undergroundstorage organs (USOs), orrhizomes. There is a smallerC3 portion which may have comprised more typical ape food items such as fruits, flowers, pods, or insects. This indicates that, like contemporary and future australopiths,A. bahrelghazali was capable of exploiting whatever food was abundant in its environment, whereas most primates (including savanna chimps) avoid C4 foods.[14] However, despite 55–80% of δ13C deriving from C4 sources similar toParanthropus boisei and the moderngelada (and considerably more than any testedA. afarensis population),A. bahrelghazali lacks the specialisations for such a diet. Because the teeth are nothypsodont, it could not have chewed large quantities of grass, and because the enamel is so thin, the teeth would not have been able to withstand the abrasive dirt particles of USOs. In regard to C4 sources, chimps andbonobos (which have even thinner enamel) consume plantmedullas as a fallback food and sedges as an important energy and protein source; however a sedge-based diet likely could not have sustainedA. bahrelghazali.[9]
During the Pliocene around the expandedLake Chad (or "Lake Mega-Chad"), insecttrace fossils indicate this was a well-vegetated region, and the abundance ofrhizomes may suggest a seasonal climate with wet and dry seasons.[15] Koro Toro has yielded several large mammals, including severalantelopes, of which some wereendemic, theelephantLoxodonta exoptata, thewhite rhinocerosCeratotherium praecox, thepigKolpochoerus afarensis, aHipparionhorse, aSivatherium, and agiraffe. Some of these are also known from Pliocene East African sites, implying that animals could freely migrate between east and west of the Great African Rift.[1] The K13 site features, in regard tobovids, an abundance ofReduncinae,Alcelaphinae, andAntilopinae, whereasTragelaphini is much rarer, which indicates an open environment which was drier than Pliocene East African sites.[16] In total, the area seems to have been predominantly grasslands with some tree cover.[14] In addition, the area featured aquatic creatures, predominantlycatfish, and also 10 other kinds of fish, thehippoHexaprotodon protamphibius, anotter, aGeochelonetortoise, aTrionyxsoftshell turtle, afalse gharial, and ananatidwaterbird. These aquatic animals indicate Koro Toro had open-water lakes or streams with swampy grassy margins, connected to the Nilo-Sudan waterways (including theNile,Chari,Niger,Senegal,Volta, andGambia Rivers).[17] Koro Toro, during Mega-Chad events (which have been cyclical for the last 7 million years), may have been similar to the modernOkavango Delta.[8]
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