New species of Primozygodactylus from Messel and the ecomorphology and evolutionary significance of early Eocene zygodactylid birds (Aves, Zygodactylidae)

Abstract

Zygodactylids (Zygodactylidae) are the extinct sister taxon of passerines (Passeriformes) and among the more abundant small arboreal birds in the early Eocene German fossil site Messel. Four species of the taxon Primozygodactylus have previously been identified and here two new species are described. In addition, new fossils of the poorly known P. eunjooae are reported. The fossils corroborate the presence of two very long central tail feathers for Primozygodactylus, and the feathering of the taxon corresponds with that of extant birds foraging in scrub and undergrowth. Species diversity of zygodactylids falls short of that of passerines, the most species-rich extant avian clade, and in bill shapes they also do not reach the diversity seen in extant Passeriformes. Because zygodactylids closely resemble passerines in skeletal morphology, the evolutionary success of passerines is unlikely to be due to particular skeletal features. Passerines and zygodactylids coexisted in Europe from the early Oligocene to the middle Miocene, and both groups probably differed in ecological characteristics. The zygodactyl foot of zygodactylids may have represented an adaptation for clinging to tree trunks, and if nesting in tree cavities, zygodactylids may have succumbed to competition for safe nesting places with avian or mammalian competitors.

http://zoobank.org/urn:lsid:zoobank.org:pub:7F142141-7E1F-4568-89BF-E2363D128C36

Keywords:

• Fossil birds
• avian diversity
• ecological competition
• Zygodactylidae
• Zygodactylus

Acknowledgements

I thank E. Brahm, A. Vogel, and S. Schaal (all SMF), N. Micklich (HLMD), and E. Frey (SMNK) for access to the fossils, and S. Tränkner for taking the photographs. Comments of an anonymous reviewer improved the manuscript.