Earlier I glazed over the word ‘extinct.’
Zygodactylidae is a clade of extinct birds, not including any extant birds with zygodactyl feet. Hence the confusion. Here’s the author’s diagnosis verbatim: “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.” They also report, “The results of that analysis provided further justification for a sister-taxon relationship between Passeriformes and Zygodactylidae.” That is not supported by the LRT. Rather tested members of the Zygodactylidae arise near roadrunners (genus: Geoccocyx, the cuckoo clade), not near sparrows, barbets, and woodpeckers. This was a poorly named clade. Moreover it is likely a junior synonym and a paraphyletic clade.
A new paper by Hieronymus, Waugh and Clarke 2019,
supports the hypothesis that extinct zygodactylid birds (Zygodactylidae, Brodkorb 1971) are monophyletic. Extant zygodactyl-style birds rotate pedal digit 4 posteriorly. Such birds include parrots, roadrunners, woodpeckers, barbets and several fossil taxa. Among these, only parrots are related to sparrows (genus: Passer).
Using one or a dozen traits to determine a clade
is “Pulling a Larry Martin“. You don’t want to do that. You get false positives, like dorsal fins on whales, fish and ichthyosaurs.
Only a comprehensive (wide gamut) phylogenetic analysis
can determine the relationships of any and all taxa. The large reptile tree (LRT, 1373 taxa) nests each of these zygodactyl-footed birds in a separate clade. So that’s four convergent occurrences of this trait (Fig. 1).
Brodkorb P 1971.Catalogue of fossil birds: part 4 (Columbiformes through Piciformes) Bulletin of the Florida State Museum, Biological Sciences. 1971;15:163–266.
Hieronymous TL, Waugh DA and Clarke JA 2019. A new zygodactylid species indicates the persistence of stem passerines into the early Oligocene in North America. BMC Evolutionary Biology (2019) 19:3 https://doi.org/10.1186/s12862-018-1319-6
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. 2018; 6: e4950.