Rupelramphastoides knopfi Mayr  

Well, the head appears to have also been found, yet in another specimen, so here’s the reconstruction after measuring the skull.:

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It does not really differ that much from the previous version.: 

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References:

[1] Gerald Mayr: A tiny barbet-like bird from the Lower Oligocene of Germany: the smallest species and earliest substantial fossil record of the Pici (woodpeckers and allies). The Auk 122(4): 1-9. 2005 
[2] Gerald Mayr: Avian Evolution: The Fossil Record of Birds and its Paleobiological Significance. Wiley-Blackwell 2016

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edited: 18.07.2017