What the frogs say: New insight into climate and seasonality of the early Pliocene fossil site of Langebaanweg (West coast, South Africa) 

Thalassa Matthews

Centre of Excellence for Paleontology, Iziko South African Museum, 25 Queen Victoria Street, Cape Town 8000, South Africa, Email:[email protected]

John Measey 

Centre for Invasion Biology, Department of Botany & Zoology, University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africa

The frog fauna from the 5.1 million year old fossil site of Langebaanweg (LBW) has provided novel information on climate and rainfall on the southwestern coast of South Africa during the early Pliocene. Frogs are particularly sensitive to climatic change, especially to fluctuations in warming and moisture levels,  due to the fact that the timing of breeding is driven by these same environmental cues.  The mega fauna and the small mammals (rats, mice and shrews) at LBW indicated more humid conditions than at present but the nature of the environment in the area remained uncertain and there was no suitable direct proxies for the amount of rainfall, or seasonality, at the site.

Research on the LBW fossil frog assemblages revealed a rich and diverse frog community, with some 19-21 different taxa (eight genera), and indicated high rainfall. The majority of frog taxa at LBW are currently found in the Summer Rainfall Zone (SRZ) and the Winter Rainfall Zone (WRZ) in southern Africa and consequently are not diagnostic in terms of the rainfall regime. However, the identification of two SRZ genera, namely Ptychadena (2 species of Grass  frogs) and Kassina (1 species of Running frog), provides new evidence of a summer rainfall regime, or of at least some significant summer rainfall, at ~5.1 Mya on the south-western coast of southern Africa.

The presence of three SRZ breeding taxa challenges the commonly held assumption in the literature that the WRZ has been established on the west coast since the later Miocene/Early Pliocene. The initiation of the Benguella Upwelling System (BUS) is frequently linked in the literature to summer aridity and the entrenchment of the current winter rainfall pattern on the west coast, however these are assumptions for which there is no hard evidence.^.  Based on palaeoclimatic proxies situated much further to the north of LBW in Namibia and the Orange River^, the onset of aridity on the west coast is suggested to pre-date the LBW fossils by 3-5 Mya. The fossil frog fauna from LBW strongly contradicts assertions that aridification of the west coast in the region was well established by 5.1 Mya and clearly indicates that at the area was still receiving a relatively high rainfall, which fell partly, if not entirely, in summer.

Please find a link to an article  on the Langebaanweg frogs:  Implications of summer breeding frogs from Langebaanweg, South Africa: Regional climate evolution at 5.1 mya published online at www.sajs.co.za.

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