Again: Zygodactyl-footed birds are not monophyletic

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).

Figure 1. Several birds with zygodactyl feet (light red) and one member of the clade Zygodactylidae (red).

Figure 1. Several birds with zygodactyl feet (light red) and one member of the clade Zygodactylidae (red).

References
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.

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6 thoughts on “Again: Zygodactyl-footed birds are not monophyletic

  1. You’ve built a bit of a straw man here. Either that, or you’re confusing the terms “Zygodactylidae” (an extinct clade of birds) and “zygodactyl” (the condition found in many birds in which two toes point back).

    Zygodactylids may be passeriforms, but none of the modern birds with a zygodactyl foot you listed is a passeriform. To my knowledge, they have never been considered as members of that group. Roadrunners are cuculiforms, “barbets” (not monophyletic) and woodpeckers are piciforms, and parrots are psittaciforms.

    The Hieronymus et al. paper never makes a claim that birds with the zygodactyl condition are all close relatives. In fact, if you look at their tree, piciforms are not closely related to the zygodactylids.

    Perhaps you should re-read their paper?

    • You are correct! I glazed over the word ‘extinct.’ Their definition 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” is based on traits also found in outgroup taxa, hence the confusion. 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. I will make the notes.

  2. Probably for the same reason they didn’t include Gavialis or Testudo. Have you thought of asking them yourself?

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