The systematic position of Hemicircus and the stepwise evolution of adaptations for drilling, tapping and climbing up in true woodpeckers (Picinae, Picidae)

摘要

The cladistic analysis of 69 morphological and behavioural characters supports the recent DNA sequencebased hypothesis that Hemicircus forms the sister group of all the remaining true woodpeckers (Picinae), but also indicates a sister-group relationship between Dendropicini and Malarpicini, which challenges the results of previous analyses. The present phylogeny further allows a more detailed reconstruction of the stepwise evolution of adaptations for drilling, tapping and climbing up head first on vertical surfaces. The last common ancestor of woodpeckers (Picidae) was neither capable of excavating nest cavities by drilling with its beak nor of climbing up tree trunks. First adaptations for drilling such as reinforced rhamphotheca, frontal overhang and proc. dorsalis pterygoidei evolved in the ancestral lineage of piculets (Picumninae) and true woodpeckers (Picinae s.l.). Further adaptations for drilling and tapping are an enlarged condylus lateralis of the quadrate and fused cotylae mediales and laterales of the lower jaw, but these characters evolved in the ancestral lineage of Picinae s. str. and are primarily lacking in Hemicircus. The inner rectrix pairs became stiffened, and the lamina pygostyli was enlarged in the ancestral lineage of true woodpeckers (Hemicircus + Picinae s. str.). These features can be regarded as first adaptations for climbing up head first and were retained by Hemicircus. In the ancestral lineage of Picinae s. str., however, the tail feathers became further transformed into a specialized support tail, the discus pygostyli became greatly enlarged, and the ectropodactyl toe arrangement evolved. The last mentioned characters might have been the prerequisites for the enormous increase in body size in different lineages of Picinae s. str., namely Megapicini such as Campephilus and Malarpicini such as Dryocopus and Mulleripicus.