Giant Warthog – Metridiochoerus sp.
Fig 1. Metridiochoerus jacksoni, a species from the Early Pleistocene
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Taxonomy
Metridiochoerus is commonly dubbed the ’giant warthog’ and was a genus of African suid which ranged through much of Africa in the Pliocene and Pleistocene (5). It is the likely ancestor of the common warthog (Phacochoerus africanus) and desert warthog (Phacochoerus aethiopicus) (3), but also had species that were contemporaneous with the extant warthogs, making it a paraphyletic genus. Late Pleistocene remains are known but restricted to a few dental remains, the species level identity is therefore difficult to discern. Several species of Metridiochoeurus have been named, Harris & White 1979 considered the following to be valid: M. andrewsi, M. hopwoodi, M. modestus, and M. compactus (5), Cooke 2005 further recognised M. jacksoni and M. shawi (1). Of these M. compactus appears the youngest with definite records stretching into the middle Pleistocene (5) and therefore seems the likeliest taxon to assign Late Pleistocene specimens to, though a distinct species can’t be precluded.
Distribution and Age
Metridiochoerus is known exclusively from a single confirmed Late Pleistocene occurrence, which is Redcliff, Zimbabwe, a site dated to 25,650 BP (6). A further three sites containing Metridiochoerus remains have been reported from South Africa, but more recent dating suggests these sites may be middle-Pleistocene and one site contained misidentified desert warthog fossils (2). Due to the scarce material from this time, it is difficult to ascertain when the species went extinct, especially since there is no clear Late Pleistocene extinction events in Africa. Likewise, the geographic distribution can’t be determined, but based on the climatic conditions at Redcliff it may be reasonable to suggest a distribution in Eastern and Southern Africa, (9). This was similar to the range of the Metridiochoerus genus during the Plio-Pleistocene (5).
The scarcity of Metridiochoerus at Redcliff (6) compared to other taxa suggests the genus was quite rare at this time, perhaps slowly giving way to the extant warthogs. This is a strong contrast to their earlier abundance observed in places like Lake Turkana, where they constitute about 17% of mammal specimens during the Middle Pleistocene (8).
Fig 2. Estimated Natural Range of Late Pleistocene Metridiochoerus based on modeling by PHYLACINE (9) as well as the confirmed and putative fossil sites from the time period and samples of habitat within the natural range.
Ecology and Morphology
The size of Late Pleistocene Metridiochoerus was somewhat larger than Phacochoerus, averaging about 142kg (9). Metridiochoerus andrewsi from the Late Pliocene to Early Pleistocene shows clear signs of sexual dimorphism in the form of two ‘warts’ present only in males, but it is unclear if this also occurs in later taxa (5). Size differences between males and females were not noticeable in the fossil record (5).
Much like the extant warthogs, Metridiochoerus possessed enlarged canines in the form of tusks, these were comparatively longer and larger. (5). Amongst warthogs Incisors are only widely present in the extant common warthog. Metridiochoerus is somewhat of an intermediate between the two extant species in this regard, there appear to have three pairs of incisors present in the lower jaw, however, the upper jaw lacks them entirely (5). Metridiochoerus has highly hypsodont molars, meaning the crown of the teeth is very tall and can therefore afford to be worn down, this is a dental trait usually indicative of grazing since grass is tough vegetation to consume (5). This is corroborated by isotopic data and morphometric analysis of the teeth of Plio-Pleistocene specimens which both indicate grass as the main dietary component (4, 7). In that sense, Metridiochoerus is similar to modern warthogs in diet, though it is unclear if it was also an opportunistic carnivore as has been observed in the extant relatives. Rannikko et al 2020 suggest that Metridiochoerus due to its size may have been a bulk feeder which consumed a wide range of grasses, unlike extant warthogs which are selective feeders (7).
The faunal assemblage at Redcliff is fairly well studied and a number of other megafaunal species co-existed with Metridiochoerus (6). In the same layers, the most common megafauna are the Alcephalines, including the Hartebeest (Alcelaphus buselaphus), common tsessebe (Damaliscus lunatus), and bontebok (Damaliscus pygargus), a large number of other antelope species were also common at the site (6). In addition, the site contained Plains Zebra (Equus quagga), cape buffalo (Syncerus caffer), the African elephant (Loxodonta africana), and significantly the African warthog (Phacochoerus africanus). The large abundance of grazers suggests that Redcliff during the time of the giant warthog was an open landscape, perhaps Savannah, though both cooler and wetter than the area today (6). Large predators known from the site are the lion (Panthera leo), leopard (Panthera pardus), spotted hyena (Crocuta crocuta), and African wild dog (Lycaon pictus) (6), all of which may have been potential predators of Metridiochoerus.
Citations
1. Cooke, H.B.S.. (2005). Makapansgat suids and Metridiochoerus. Palaeontologica Africana 41,131-140.
2. Faith, J.T.. (2014). Late Pleistocene and Holocene mammal extinctions on continental Africa", Earth-science reviews 128, 105-121.
3. Gongora, J., Cuddahee, R.E., Nascimento, F.F.d., Palgrave, C.J., Lowden, S., Ho, S.Y.W., Simond, D., Damayanti, C.S., White, D.J., Tay, W.T., Randi, E., Klingel, H., Rodrigues-Zarate, C.J., Allen, K., Moran, C., Larson, G.. (2011). Rethinking the evolution of extant sub-Saharan African suids (Suidae, Artiodactyla). Zoologica scripta 40(4), 327-335.
4. Harris, J.M., Cerling, T.E.. (2002). Dietary adaptations of extant and Neogene African suids. Journal of zoology 256(1), 45-54.
5. Harris, J.M. & White, T.D.. (1979). Evolution of the Plio-Pleistocene African Suidae. Transactions of the American Philosophical Society 69 (2), 46-63.
6. Cruz-Uribe, K.. (1983) The Mammalian Fauna from Redcliff Cave, Zimbabwe. South African archaeological bulletin 38(137), 7-16.
7. Rannikko, J., Adhikari, H., Karme, A., Žliobaitė, I., Fortelius, M.. (2020). The case of the grass‐eating suids in the Plio‐Pleistocene Turkana Basin: 3D dental topography in relation to diet in extant and fossil pigs. Journal of morphology 281 (3), 348-364.
8. Rannikko, J., Žliobaitė, I., Fortelius, M.. (2017) Relative abundances and palaeoecology of four suid genera in the Turkana Basin, Kenya, during the late Miocene to Pleistocene. Palaeogeography, palaeoclimatology, palaeoecology 487, 187-193.
9. Faurby, S., Pedersen, R. Ø., Davis, M., Schowanek, S. D., Jarvie, S., Antonelli, A., & Svenning, J.C. (2020). PHYLACINE 1.2.1: An update to the Phylogenetic Atlas of Mammal Macroecology. doi:10.5281/zenodo.3690867