According to Smith, DeBee and Clarke 2018
“Zygodactylidae are an extinct lineage of perching birds characterized by distinct morphologies of the foot and wing elements.”
According to the LRT
(large reptile tree, 1137 taxa) zygodactyly (digits 1 and 4 retroverted) appeared several times by convergence in four unrelated bird clades, including toucans, parrots, roadrunners and woodpeckers.
“Although the clade has a complex taxonomic history, current hypotheses place Zygodactylidae as the sister taxon to Passeriformes (i.e., songbirds).”
“Given the rather sparse fossil record of early passeriforms, the description of zygodactylid taxa is important for inferring potentially ancestral states in the largest radiation of living birds (i.e., the ~6,000 species of extant passeriforms).”
taxa related to Passer include only Passer at this point in the LRT. Other traditional passeriformes nest elsewhere.
“Despite the exceptional preservation of many specimens and considerable species diversity in Zygodactylidae, the relationships among species have not been previously evaluated in a phylogenetic context.”
the LRT has exposed a problem of taxon exclusion here.
“Herein, we …provide the first hypothesis of the species-level relationships among zygodactylids. The monophyly of Zygodactylidae is supported in these new analyses.”
As defined by the authors,
“Zygodactylidae Brodkorb, 1971 is an extinct, comparatively species-rich clade of enigmatic birds that possess derived morphological features associated with a perching habitus (Mayr, 2008, 2009, 2015). Zygodactylidae is primarily characterized by a zygodactyl conformation of the pedal phalanges—possessing a retroverted fourth toe and associated accessory trochlea on the distal end of the tarsometatarsus (Olson & Feduccia, 1979).”
The authors chose woodpeckers (Piciformes) as the outgroup.
The unrelated basal barbet/toucan, Cyrilavis, nests at the first dichotomy along with the unrelated Nestor, the parrot. If you are starting to sense yet another case of taxon exclusion, then we are thinking along the same lines.
On the plus side, Botelho et al. 2014 reported
the zygodactyl foot evolved independently in different extant bird taxa.
Botelho JF, Smith-Paredes D, Nuñez-Leon D, Soto-Acuña and Vargas AO 2014. The developmental origin of zygodactyl feet and its possible loss in the evolution of Passeriformes. Proceedings Biological Sciences 281(1788):20140765. doi: 10.1098/rspb.2014.0765.
Smith NA, DeBee AM and Clarke JA 2018. Systematics and phylogeny of the Zygodactylidae (Aves, Neognathae) with description of a new species from the early Eocene of Wyoming, USA. PeerJ 6:e4950Â doi: https://doi.org/10.7717/peerj.4950