Abstract
We document the presence of the bald ibis genus Geronticus Wagler, 1832 (Aves: Threskiornithidae) from the mid-Pliocene (ca. 3–3.5 Ma) of South Africa based on an incomplete skull from the Bolt’s Farm Cave System (Cradle of Humankind, Gauteng, South Africa). The fossil cranium is distinct on morphometric and structural grounds from Geronticus apelex, the only other Pliocene Geronticus described from Southern Africa, but is very close in dimensions and general morphology to the extant G. calvus of South Africa, and the Bolt’s Farm fossil ibis is therefore attributed to G. cf. calvus. Modern Geronticus ibises are localised to temperate, open grasslands and semi-arid steppe, and nest exclusively on cliffs and similar rocky eminences. Given its attribution as G. cf. calvus, the Bolt’s Farm ibis was likely similar in ecology to the extant G. calvus, suggesting that the habitat surrounding the Bolt’s Farm fossil site during the mid-Pliocene featured open grassland and presence of cliffs. This record constrains the divergence between G. calvus and its putative ancestor G. apelex to the mid-Pliocene and implies that G. calvus has possibly been subject to “evolutionary/morphological stasis” for more than 3 million years. This postulated stasis would be consistent with the notion that extant genera with few species (i.e. high genus-to-species ratios) show low rates of phenotypic diversification and change through the Neogene.
Kurzfassung
Wir dokumentieren die Anwesenheit der Ibis-Gattung Geronticus Wagler, 1832 (Aves: Threskiornithidae) aus dem Mittel-Pliozän (ca. 3–3.5 Ma) von Südafrika, basierend auf einem unvollständigen Schädel aus dem Bolt’s Farm-Höhlensystem (Cradle of Humankind, Gauteng, Südafrika). Der fossile Schädel unterscheidet sich bezüglich morphometrischer und struktureller Anlagen von Geronticus apelex, dem einzigen anderen aus Südafrika beschriebenen pliozänen Geronticus, steht jedoch aufgrund der Dimensionen und der generellen Morphologie dem heutigen südafrikanischen G. calvus sehr nahe; somit ist der fossile Bolt’s Farm-Ibis G. cf. calvus zuzuordnen. Moderne Geronticus-Ibisse sind örtlich auf gemäßigte, offene Graslandschaften und semi-aride Steppen begrenzt und nisten ausschließlich auf Felsen und ähnlichen steinernen Erhebungen. Wird die Zuordnung zu G. cf. calvus vorgenommen, dann war die Lebensweise des Bolt’s Farm-Ibisses vermutlich ähnlich der des rezenten G. calvus, was nahelegt, dass das umgebende Habitat der Bolt’s Farm-Fossilfundstelle während des Mittel-Pliozäns offene Graslandschaft sowie die Präsenz von Felsen bot. Dieser Nachweis belegt die Divergenz zwischen G. calvus und seinem mutmaßlichen Vorfahren G. apelex im Mittel-Pliozän und beinhaltet, dass G. calvus möglicherweise Gegenstand einer über 3 Millionen Jahre andauernden „evolutionären/morphologischen Stasis“ war. Diese postulierte Stasis wäre vereinbar mit der Vorstellung, dass rezente Gattungen mit wenigen Arten (d. h. hohe Gattung: Art-Verhältnisse) geringe Raten phänotypischer Diversifikationen und Veränderungen während des Neogens zeigen.
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Acknowledgements
We thank S. Potze for access to the Plio-Pleistocene collection at the Ditsong National Museum of Natural History. We thank the following individuals and institutions for assistance during this project: C. Klinkert for permission to conduct fieldwork on her property; F. Sénégas for the personal communication; the South African Heritage Resource Agency (SAHRA) for the continued support in providing the excavation permits. We would also like to thank F. Thackeray and M. Unsöld for their comments, which improved the manuscript, and the Editor M. Reich for his support. The French Embassy in South Africa, the Ditsong National Museum of Natural History, the CNRS (LIA No. 1041 HOMEN), the French Ministry of Foreign Affairs and International Development (Sous-Direction de la recherche et des échanges scientifiques—Pôle sciences humaines et sociales) funded fieldwork or/and preparation and laboratory studies. M.P. was funded by the Kromdraai Research Project led by J. Braga and supported by the French Ministry of Foreign Affairs, by the Italian Ministry for Education and Research (PRIN2012-2012MY8AB2), and by the Torino University Research Grants 2015 and 2016.
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Pavia, M., Davies, G.B.P., Gommery, D. et al. Mid-Pliocene bald ibis (Geronticus cf. calvus; Aves: Threskiornithidae) from the Cradle of Humankind, Gauteng, South Africa and its environmental and evolutionary implications. PalZ 91, 237–243 (2017). https://doi.org/10.1007/s12542-017-0346-8
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DOI: https://doi.org/10.1007/s12542-017-0346-8