The evolution of the black wildebeest, Connochaetes gnou, and modern largemammal faunas in central Southern Africa

[摘要] This study investigates the evolution of modern mammalian faunas in the central interior ofsouthern Africa by testing the hypothesis that the evolution of the black wildebeest, Connochaetesgnou, was directly associated with the emergence of Highveld-type open grasslands in the centralinterior.Southern Africa can be distinguished from other arid and semi-arid parts of the continent by thepresence of an alliance of endemic grazing ungulates. The black wildebeest is characteristic of thisalliance. Open habitats are essential for the reproductive behaviour of the black wildebeest, becauseterritorial males require an unobstructed view of their territories in order to breed. The specialisedterritorial breeding behaviour of the black wildebeest is the reason why the black wildebeest ishistorically confined to the Highveld and Karoo areas and why it is reproductively isolated fromsympatric blue wildebeest, Connochaetes taurinus. The finds from a number of fossil-richlocalities, dating from the recent past to approximately a million years ago, have been identified.The remains referred to ancestral C. gnou have been subjected to detailed qualitative andquantitative osteological comparisons with cranial and post-cranial elements of modern and fossilreference specimens. This material includes extant southern African alcelaphines and fossilmaterials of C. gnou, the extinct giant wildebeest, Megalotragus priscus, and North African fossilalcelaphines. The results show that cranial changes in fossil C. gnou, particularly the more forwardpositioning of the horns, basal inflation of the horns and the resultant re-organisation of theposterior part of the skull, preceded other skeletal modifications. These cranial changes indicate ashift towards more specialised territorial breeding behaviour in the earliest ancestral blackwildebeest, evident in the specimens of the c. million year old Free State site of Cornelia-Uitzoek.Since the territorial breeding behaviour of the black wildebeest can only function in open habitatand since cranial characters associated with its territorial breeding behaviour preceded othermorphological changes, it is deduced that there was a close association between the speciation of C.gnou from a C. taurinus-like ancestor and the appearance of permanently open Highveld-typegrasslands in the central interior of southern Africa. This deduction is supported by the lack oftrophic distinction between the modern black and blue wildebeest, suggesting that the evolution ofthe black wildebeest was not accompanied by an ecological shift. It is concluded that the evolutionof a distinct southern endemic wildebeest in the Pleistocene was associated with, and possiblydriven by, a shift towards a more specialised kind of territorial breeding behaviour, which can onlyfuntion in open habitat.There are significant post-speciation changes in body size and limb proportions of fossil C. gnouthrough time. The tempo of change has not been constant and populations in the central interiorunderwent marked reduction in body size in the last 5000 years. Vicariance in fossil C. gnou isevident in different rates of change that are recorded in the populations of generally smaller bodysize that became isolated in the Cape Ecozone. These daughter populations, the result of dispersalsfrom the central interior, became extinct at the end of the Pleistocene.

[发布机构] Stellenbosch University