Research Article

On the interrelationships of early Eocene “parrot-like” and “near-passerine” zygodactyl birds (Aves: Psittacomimidae fam. nov.)

Gerald Mayr^‡

‡ Senckenberg Research Institute and Natural History Museum Frankfurt, Frankfurt am Main, Germany

Corresponding author: Gerald Mayr ( gerald.mayr@senckenberg.de )

Academic editor: Martin Päckert

This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Citation: Mayr G (2026) On the interrelationships of early Eocene “parrot-like” and “near-passerine” zygodactyl birds (Aves: Psittacomimidae fam. nov.). Vertebrate Zoology 76: 121-134. https://doi.org/10.3897/vz.76.e182505

ZooBank: urn:lsid:zoobank.org:pub:10B109A0-6AE0-4D56-A9BD-668C5618F5D0

Abstract

A diverse array of early Eocene zygodactyl birds has been assigned to the Psittacopasseres, the clade including parrots and passerines, but the exact affinities of the fossils are controversially resolved. Here, new analyses are performed based on a revised character matrix. Concerning critical taxa, the results of the primary analysis and that of the analyses constrained to a molecular backbone phylogeny show disparate tree topologies, and probably none correctly reflects the true interrelationships of the fossil taxa. The new taxon Psittacomimidae fam. nov. is introduced for a clade formed by the taxa Psittacomimus and Parapsittacopes, which were before assigned to the Psittacopedidae. The Psittacomimidae fam. nov. are likely to be the sister taxon of the Parapasseres, that is, the clade formed by the Zygodactylidae and Passeriformes, with the Morsoravidae branching next. A clade formed by all or most Halcyornithidae and the Messelasturidae is termed Messelasturiformes. A derived morphology of the proximal tarsometatarsus is reported, which may support a clade including the Vastanavidae and Messelasturiformes. However, although psittacopasserine affinities of the Vastanavidae conform to the overall osteology of these birds, the higher-level affinities of the Messelasturiformes are more elusive.

Keywords

Aves, Eocene, evolution, fossil birds, phylogeny, systematics

Introduction

One of the most unforeseen results of sequence-based analyses of avian interrelationships, and one that has not been proposed by earlier anatomists, is a sister group relationship between the Passeriformes (passerines) and the Psittaciformes (parrots). A clade including both taxa was first found in analyses of Ericson et al. (2006) and has since been recovered in all phylogenies based on nuclear gene sequences (Hackett et al. 2008; Jarvis et al. 2014; Prum et al. 2015; Kuhl et al. 2021; Stiller et al. 2024; Wu et al. 2024). A clade including the Psittaciformes and Passeriformes is also supported by retroposon data and was termed Psittacopasserae (Suh et al. 2011), which was emended to the grammatically correct spelling Psittacopasseres by Sangster et al. (2022). Sequence-based studies, furthermore, identify the Falconiformes (falcons) as the sister taxon of the Psittacopasseres. Together with the Cariamiformes (seriemas), the Falconiformes and Psittacopasseres form the clade Australaves (Ericson et al. 2006; Hackett et al. 2008; Jarvis et al. 2014; Prum et al. 2015; Kuhl et al. 2021; Stiller et al. 2024; Wu et al. 2024).

Because stem group representatives of the Passeriformes, the aptly named Zygodactylidae (Fig. 1C), have zygodactyl feet (i.e., the fourth toe was permanently reversed as it is in parrots), it was hypothesized that a zygodactyl foot may be a plesiomorphic trait of passeriform birds (Mayr 2009: 184). A developmental mechanism for a reversal of this foot morphology into the plesiomorphic anisodactyl configuration was subsequently identified by Botelho et al. (2014; see, however, Mayr 2017: 213f. for a potential caveat regarding this hypothesis). As detailed by Mayr (2015), the new view on the evolutionary history of passeriforms has implications for the affinities of various early Cenozoic birds with “parrot-like” feet and led to a reinterpretation of various parrot-like taxa.

Figure 1.

Representatives of early Eocene “near-passerine” (A−C) and “parrot-like” (D−F) zygodactyl birds. A Morsoravis sedilis (Morsoravidae) from the early Eocene of Denmark (holotype, MGUH 28930). B Holotype of Psittacopes lepidus (Psittacopedidae) from the latest early or earliest middle Eocene of Messel in Germany (SMF-ME 1279). C Holotype of Primozygodactylus quintus (Zygodactylidae) from Messel (SMF-ME 11091A). D Holotype of Eurofluvioviridavis robustipes (Vastanavidae) from Messel (SMNK.PAL.3835). E Pseudasturides macrocephalus (Halcyornithidae) from Messel (SMNK.PAL.2373a). F Holotype of Tynskya eocaena (Messelasturidae) from the North American Green River Formation (SNSB-BSPG 1997 I 6). The scale bars equal 10 mm.

The occurrence of true parrots in the early and middle Miocene of Europe is well documented (Milne-Edwards 1867-1871; Cheneval 2000; Mayr and Göhlich 2004; Mayr 2010; Pavia 2014), and fossil Psittaciformes were also reported from the early Miocene of Siberia (Zelenkov 2016). Psittaciform affinities were, furthermore, assumed for a multitude of recently described fossils from the Paleogene of Europe and North America, but the exact phylogenetic positions of these are afflicted with uncertainty.

Harrison (1982) identified fragments of a putative psittaciform from the early Eocene British London Clay and described it as Palaeopsittacus. However, this taxon has since shown to be misclassified (Mayr and Daniels 1998) and may be a stem group representative of the Nyctibiiformes (potoos; Mayr and Kitchener 2025). A decade later, more substantial remains of parrot-like birds were reported by Mourer-Chauviré (1992) from the late Eocene of the Quercy fissure fillings in France; these fossils were assigned to the taxon Quercypsitta.

Mayr and Daniels (1998) described putative stem group Psittaciformes from the latest early/earliest middle Eocene of Messel in Germany and the early Eocene British London Clay. The species from Messel was classified in the taxon Psittacopes, which is the type genus of the Psittacopedidae (Fig. 1B). The fossils from the London Clay, which were revisited by Mayr (2020) and Mayr and Kitchener (2023a), belong to the taxa Parapsittacopes and Psittacomimus; one species from the London Clay was, furthermore, tentatively assigned to Psittacopes. Based on the new hypotheses on the interrelationships of passerines, Psittacopes, Parapsittacopes, and Psittacomimus are now considered to be zygodactyl stem group representatives of the Passeriformes, together with the Morsoravidae and Zygodactylidae (Mayr 2015; Ksepka et al. 2019, 2025; Mayr and Kitchener 2023a, 2023b, 2023c).

The taxon Morsoravis (Fig. 1A) from the Danish Fur Formation – the type genus of the Morsoravidae – was initially described as a charadriiform bird (Bertelli et al. 2010), but Mayr (2009: 115) noted close affinities to the latest early/earliest middle Eocene Pumiliornis from the Messel site, and a phylogenetic analysis performed by Mayr (2015) supported a clade including Morsoravis, Pumiliornis from Messel, Psittacopes, and the Zygodactylidae (see also Mayr 2011a). The Morsoravidae now include the taxa Morsoravis, Sororavis (from the London Clay), Consoravis (from the North American Green River Formation), and Pumiliornis (Mayr and Kitchener 2023b; Ksepka et al. 2025). The Zygodactylidae are well represented in early Eocene sites of Europe and North America and existed in Europe until the middle Miocene (Mayr 2017, 2022). A clade including the Zygodactylidae and the Passeriformes, which was termed Parapasseres by Mayr (2015), is supported by all current analyses, even though monophyly of zygodactylids is not recovered by some of these (Mayr 2015, 2020, 2021; Ksepka et al. 2019, 2025; Mayr and Kitchener 2023a, 2023b, 2023c).

Most current analyses also support a clade including the Morsoravidae, Psittacopes, Psittacomimus, Parapsittacopes and the Parapasseres (Ksepka et al. 2019, 2025; Mayr 2021; Mayr and Kitchener 2023a, 2023b, 2023c), but the interrelationships of these taxa are controversially resolved. Whereas the analysis of Ksepka et al. (2019) found a clade formed by the Morsoravidae and Psittacopedidae, analyses by Mayr (2021) and Mayr and Kitchener (2023b, 2023c) recovered a clade including the Morsoravidae and Parapasseres to the exclusion of Psittacopes, Psittacomimus, and Parapsittacopes. An analysis by Ksepka et al. (2025) resulted in a clade formed by Psittacopes, Psittacomimus, Parapsittacopes, Eofringillirostrum (see below), and the Parapasseres, to the exclusion of the Morsoravidae.

Dyke and Cooper (2000) described Pulchrapollia, another putative psittaciform bird from the London Clay. This taxon was subsequently assigned to the Halcyornithidae (“Pseudasturidae”; Mayr 2002), which include fossils from Messel (Pseudasturides, Serudaptus; Fig. 1E), the Green River Formation (Cyrilavis), and other early and middle Eocene localities in Europe and North America (Mayr 1998, 2000a, 2026; Ksepka et al. 2011; Mayr 2022; Mayr and Kitchener 2024). At times considered to be true stem group representative of the Psittaciformes (Dyke and Cooper 2000; Mayr 2002), the affinities of halcyornithids are elusive. An earlier analysis with a restricted ingroup sampling supported a sister group relationship to crown group Psittaciformes (Ksepka et al. 2011), whereas more recent analyses did not recover close affinities of halcyornithids to psittaciforms. Some analyses found the Halcyornithidae to be outside crown group Psittacopasseres (Mayr 2015; Ksepka et al. 2019; Mayr and Kitchener 2023c), but others identified them as stem group Passeriformes (Mayr and Kitchener 2023a; Ksepka et al. 2025).

Another taxon for which psittacopasserine affinities were assumed are the Messelasturidae, which include Messelastur from Messel and Tynskya from the London Clay and the Green River Formation (Fig. 1F; Peters 1994; Mayr 2000b, 2005a, 2011b; Mayr and Kitchener 2023c). Messelastur was initially assigned to the Accipitridae (Peters 1994), whereas Tynskya was likened to the Strigiformes (owls; Mayr 2000b). Possible strigiform affinities were also proposed for Messelastur (Mayr 2005a), but messelasturids were subsequently considered to be most closely related to the Halcyornithidae and both taxa were tentatively identified as stem group representatives of the Psittaciformes (Mayr 2011b). However, and like that of halcyornithids, the placement of messelasturids is poorly resolved in current analyses, some of which do not even support psittacopasserine affinities (Mayr 2011b, 2020; Mayr and Kitchener 2023a, 2023b, 2023c).

Vastanavis is a bird with a parrot-like tarsometatarsus from the early Eocene of India (Mayr et al. 2010, 2013), whose phylogenetic affinities are not unambiguously resolved in current analyses. Vastanavis was likened to Quercypsitta by Mayr et al. (2010), and recent studies supported a clade including Vastanavis, Avolatavis from the London Clay and the Green River Formation, and Eurofluvioviridavis from Messel (Fig. 1D), with this clade – the Vastanavidae – being only distantly related to psittacopasserines (Mayr 2015, 2020; Mayr and Kitchener 2023a, 2023b, 2023c; the analysis of Ksepka et al. 2025 did not include representatives of the Messelasturidae and Vastanavidae). Avolatavis resulted as the sister taxon of Quercypsitta in the analysis of Ksepka and Clarke (2012), and the clade formed by both taxa was recovered as the sister taxon of crown group Psittaciformes. The higher-level affinities of Eurofluvioviridavis could not be resolved in the original description of the taxon (Mayr 2005b).

Apart from Vastanavis, the only other taxon from Eocene deposits outside North America and Europe for which psittaciform affinities were assumed is Namapsitta from the middle Eocene of Namibia (Mourer-Chauviré et al. 2015, 2017). This bird is currently considered to be a stem group representative of the Psittaciformes.

A further early Eocene zygodactyl taxon assigned to the Psittacopasseres is Eofringillirostrum from the Green River Formation and Messel (Ksepka et al. 2019), which resulted as the sister taxon of either Pumiliornis (Ksepka et al. 2019), a clade formed by the Zygodactylidae and Passeriformes (Ksepka et al. 2025), or a more inclusive clade of putative stem group Passeriformes (Mayr and Kitchener 2023a).

The above taxa show disparate morphologies and can be grouped into two morphotypes, which are here informally termed “parrot-like” and “near-passerine”. The “parrot-like” morphotype (Vastanavis, Avolatavis, Eurofluvioviridavis, as well as the Halcyornithidae and Messelasturidae) is characterized by stocky tarsometatarsi and abbreviated proximal phalanges of the fore toes, whereas most representatives of the “near-passerine” morphotype (Morsoravidae, Zygodactylidae) features elongate tarsometatarsi, a comparatively short humerus, and long and slender toes with unabbreviated phalanges.

All current analyses support a clade including the Passeriformes, Zygodactylidae, Psittacopedidae, and Morsoravidae (Mayr and Kitchener 2023a, 2023b, 2023c; Ksepka et al. 2025), but the interrelationships of most taxa of the “parrot-like” morphotype are elusive. The present study revisits the phylogenetic affinities of early Eocene zygodactyl birds and identifies potential apomorphies of some major clades.

Material and Methods

Phylogenetic analyses were performed on the basis of the emended and revised character matrix of Mayr and Kitchener (2023c; see File S1 for character descriptions and and File S2 for the character matrix). Four taxa and eight characters were newly added. The early Oligocene taxon Eocuculus, which was likened to Pumiliornis by Mayr (2008), was excluded, because it is uncertain whether the referred specimen (Mayr 2008) – on which most scorings of the wing and pectoral girdle elements in previous analyses (e.g., Mayr and Kitchener 2023a, 2023b, 2023c) are based – belongs to the taxon.

The analyses were run with the heuristic search modus of PAUP*4.0a169 (Swofford 2002). The primary analysis is based on the unconstrained data set. In a subsequent run, the analysis was constrained to the tree topology of Kuhl et al. (2021) as a molecular backbone phylogeny. Bootstrap support values were calculated with 200 replicates and the stepwise addition search. The trees were rooted with the anseriform Anhimidae. Three characters were scored as ordered. Tree length (L), consistency index (CI), and retention index (RI) were calculated.

The figured fossils are deposited in H.N.B. Garhwal University, Department of Geology, Uttarakhand, India (GU/RSR/VAS); National Museums Scotland, Edinburgh, United Kingdom (NMS); Senckenberg Research Institute Frankfurt, Germany (SMF); Staatliches Museum für Naturkunde Karlsruhe, Germany (SMNK); Bayerische Staatssammlung für Paläontologie und Geologie, Munich, Germany (SNSB-BSPG); and Université Claude Bernard, Lyon, France (UCBL).

Results of the Phylogenetic Analyses and Systematic Paleontology

The unconstrained analysis resulted in 1530 most parsimonious trees (L = 352; CI = 0.32; RI = 0.68), the strict consensus tree of which is shown in Figure 2A. The constrained analysis yielded 2152 most parsimonious trees (L = 367; CI = 0.31; RI = 0.69), the strict and majority rule consensus trees of which are shown in Figures 2Band C. In all phylogenies, there was no bootstrap support (> 50%) for most nodes including fossil taxa, with one notable exception noted further below.

Figure 2.

Results of the phylogenetic analyses; fossil taxa are denoted by a dagger. Strict consensus tree (A) of 1530 most parsimonious trees (L = 352; CI = 0.32; RI = 0.68) resulting from the unconstrained analysis; bootstrap support values >50% are indicated below the internodes. Strict (B) and majority rule (C) consensus trees of 2152 most parsimonious trees (L = 367; CI = 0.31; RI = 0.69) from the analysis that was constrained to a molecular backbone phylogeny; bootstrap support values >50% are indicated below the internodes, values above the internodes indicate the percentages, in which the node was retained. Alternative phylogenetic hypothesis (D) that is not supported by the analyses, in which it is assumed that a furrow between the trochlea accessoria and the main body of the trochlea metatarsi IV evolved only once in the Psittacopasseres, so that the Psittacidae, Psittacomimidae, and Parapasseres form a clade (see text). Newly introduced clade names are indicated in bold face. The colored areas highlight the Psittacopasseres (olive), Zygodactylidae (turquoise), Psittacomimidae fam. nov. (purple), Morsoravidae (brown), and Vastanavidae (yellow).

Both analyses supported a clade including the taxa Morsoravis, Sororavis, Consoravis, and Pumiliornis, that is, a monophyletic Morsoravidae sensu Mayr and Kitchener (2023b). All analyses, furthermore, yielded a clade formed by Parapsittacopes and Psittacomimus to the exclusion of Psittacopes, as well as a clade including Vastanavis, Avolatavis, and Eurofluvioviridavis. Notably, even the unconstrained analysis recovered a sister group relationship between the Psittaciformes and Passeriformes, although monophyly of the Psittacopasseres has not yet been established with unambiguous apomorphies that are not also found in other avian clades (see also Chen et al. 2025).

Both analyses found a clade formed by Psittacopes, Parapsittacopes, Psittacomimus, and the Morsoravidae, Zygodactylidae, and Passeriformes. They also congruently supported a clade including most Halcyornithidae and the Messelasturidae, even though in the unconstrained analysis, the halcyornithid-like taxa Scopsoides and Serudaptus were recovered in a polytomy together with “typical” halcyornithids, messelsturids, and vastanavids.

The unconstrained analysis supported a sister group relationship between the Morsoravidae and Parapasseres, whereas the majority rule consensus tree of the constrained analysis recovered a clade formed by the Parapasseres and a clade including Parapsittacopes and Psittacomimus. In concordance with an analysis by Ksepka et al. (2025), the unconstrained analysis found zygodactylids as recognized by Mayr and Kitchener (2023a) to be non-monophyletic and successive sister taxa of passerines; in the majority rule consensus tree of the constrained analysis, the affinities of zygodactylids are unresolved.

The position of Psittacopes differed in the analyses. In the unconstrained analysis it was found to be the sister taxon of the clade (Parapasseres + Morsoravidae), whereas it was recovered as the sister taxon of the Morsoravidae in the majority rule consensus tree of the constrained analysis.

The analyses likewise did not unambiguously resolve the higher-level affinities of the Vastanavidae and the clade (Halcyornithidae + Messelasturidae). The unconstrained analysis resulted in a clade comprising these three taxa, which was shown to be the sister taxon of a clade including the Psittacomimidae, Morsoravidae, and Parapasseres. The majority rule consensus tree of the constrained analysis, by contrast, supported a position of the Vastanavidae outside Psittacopasseres and even Eufalconimorphae (the clade formed by the Psittacopasseres and Falconiformes). Both analyses did not support monophyly of the Halcyornithidae as currently recognized, because Serudaptus was recovered outside a clade formed by other halcyornithids and messelasturids in the unconstrained analysis, whereas the majority rule consensus tree of the constrained analysis found the Messelasturidae to be within the Halcyornithidae.

The taxon Eofringillirostrum was recovered as a stem group psittaciform in the unconstrained analysis, whereas it resulted in an unresolved polytomy in the constrained analysis.

Because all analyses supported a clade formed by Parapsittacopes and Psittacomimus to the exclusion of Psittacopes, a new family-level taxon is introduced (see also Ksepka et al. 2025 for the proposal to name this clade). It is diagnosed as follows:

Psittacomimidae fam. nov.

https://zoobank.org/C98CC5DF-E047-4497-82EA-500A00DE7CF4

Type genus.

Psittacomimus Mayr & Kitchener, 2023

Differential diagnosis.

Characterized by the combination of (1) a short and wide beak with large nostrils and a slightly decurved tip; (2) a quadrate with a pneumatized caudal surface of the processus oticus (Fig. 3K−N), (3) a mandible with dorsoventrally low rami and a very short symphysis, (4) pleurocoelous thoracic vertebrae; (5) a coracoid with an incisura nervi supracoracoidei, (6) a scapula with a very long and pointed acromion, (7) a humerus with a small but well-defined tuberculum supracondylare dorsale (Fig. 4H−J), (8) a carpometacarpus with a curved os metacarpale minus and a wide spatium intermetacarpale, and (9) a comparatively short tarsometatarsus with (12) a large trochlea accessoria, which is separated from the main trochlea by a furrow (Fig. 5X). The new taxon is distinguished from the Zygodactylidae in characters (1), (5), and (9); it differs from the Psittacopedidae in character (2).

Figure 3.

Supraorbital processes (A−D) and quadrates (E−O) of early Eocene “near-passerine” and “parrot-like” zygodactyl birds and early Eocene Strigiformes and Falconiformes (Masillaraptoridae). A Parapsittacopes bergdahli (Psittacomimidae fam. nov.; holotype, SMF Av 653), right side (mirrored). B Cyrilavis cf. colburnorum (Halcyornithidae; NMS.Z.2021.40.67), left side. C Ypresiglaux michaeldanielsi (Strigiformes; NMS.Z.2021.40.26), right side (mirrored). D Danielsraptor phorusrhacoides (Masillaraptoridae; NMS.Z.2021.40.12), left side. E, F ?Pulchrapollia sp. (Halcyornithidae; NMS.Z.2021.40.66), right quadrate in lateral (E) and caudal (F) view; the arrow denotes a detail of the condylus medialis. G, H Tynskya waltonensis (Messelasturidae; SMF Av 652), left quadrate (mirrored) in lateral (G) and caudal (H) view; the arrow denotes a detail of the condylus medialis. I Psittacopes lepidus (Psittacopedidae; holotype, SMF-ME 1279), left quadrate in caudal view; surrounding bones and matrix were digitally brightened. J Morsoravis sedilis (Morsoravidae; holotype, MGUH 28930), left quadrate in caudolateral view; surrounding bones and matrix were digitally brightened. K, L P. bergdahli (SMF Av 653), right quadrate in lateral (K) and caudal (L) view; the arrow denotes a detail of the processus oticus. M, N Psittacomimus eos (Psittacomimidae fam. nov.; holotype, NMS.Z.2021.40.38) left quadrate (mirrored) in lateral (M) and caudal (N) view; the arrow denotes a detail of the processus oticus. O, P Primozygodactylus cf. danielsi (NMS.2021.40.51), right quadrate in lateral (O) and caudal (P) view; the arrow denotes a detail of the processus oticus. Q Procnias nudicollis (Cotingidae, Passeriformes), tip of processus oticus of right quadrate in caudal view. Abbreviations: arf, articular facet; cpo, capitulum oticum, cps, capitulum squamosum; pnf, pneumatic foramina. The scale bars equal 5 mm.

Figure 4.

Humeri (A−J) and coracoids (K−T) of early Eocene “near-passerine” and “parrot-like” zygodactyl birds and extant Psittacopasseres. A the extant Nestor notabilis (Psittaciformes, Strigopidae), left side. B Vastanavis sp. (Vastanavidae; GU/RSR/VAS 1803), left side; coated with ammonium chloride. C Tynskya brevitarsus (Messelasturidae; holotype, NMS.Z.2021.40.78), left side. D Tynskya eocaena (Messelasturidae; holotype, SNSB-BSPG 1997 I 6), right side (mirrored); surrounding bones and matrix were digitally removed. E ?Pulchrapollia sp. (Halcyornithidae; NMS.Z.2021.40.66). F Halcyornithidae, gen. et sp. indet (NMS.Z.2021.40.71), right side (mirrored). G Sororavis solitaria (Morsoravidae; holotype, NMS.Z.2021.40.75), digitally combined proximal portion of right humerus (mirrored) and distal end of left humerus (separated by dashed line); the dotted line denotes the reconstructed shape of the proximal end. H Parapsittacopes bergdahli (Psittacomimidae fam. nov.; NMS.Z.2021.40.43), left side. I Primozygodactylus cf. danielsi (Zygodactylidae; NMS.2021.40.49), right side (mirrored); the dotted line denotes the reconstructed shape of the proximal end. J Primoscens carolinae (Zygodactylidae; holotype, NMS.2021.40.54), right humerus (mirrored); the dotted line denotes the reconstructed shape of the proximal end. K Corvus frugilegus (Corvidae, Passeriformes), left side. L Vastanavis sp. (GU/RSR/VAS 1254), left side; coated with ammonium chloride. M N. notabilis, left side. N Quercypsitta ivani (Quercypsittidae; UCBL FSL 367080), left side; coated with ammonium chloride. O ?Pulchrapollia eximia (Halcyornithidae; holotype, NMS.Z.2021.40.64), left side. P Tynskya cf. waltonensis (NMS.Z.2021.40.72), right side (mirrored). Q S. solitaria (NMS.Z.2021.40.75), right side (mirrored). R Psittacomimus eos (Psittacomimidae fam. nov.; holotype, NMS. Z.2021.40.38), left side. S ?Psittacopes occidentalis (Psittacopedidae; holotype, NMS.Z.2021.40.44), left side. T P. carolinae (holotype, NMS.2021.40.54), left side. U the extant Myiarchus tyrannulus (Tyrannidae, Passeriformes), right side (mirrored). The dotted lines in B, C, E, and G−J demark the fossa musculi brachialis. Abbreviations: csc, cotyla scapularis; fmb, fossa musculi brachialis; fns, foramen nervi supracoracoidei; pca, processus acrocoracoideus; pcl, processus lateralis; psd, processus supracondylaris dorsalis; tsd, tuberculum supracondylare dorsale. The scale bars equal 5 mm

Figure 5.

Tarsometatarsi of early Eocene “near-passerine” (A−H, Q−T) and “parrot-like” (I−P, V−Y) zygodactyl birds, as well as extant Psittaciformes (U) and Passeriformes (Z). A−P proximal (upper row) and distal (lower row) view of the tarsometatarsus of A, B Vastanavis sp. (Vastanavidae, GU/RSR/VAS 1809), right side, coated with ammonium chloride; the dotted line indicates the reconstructed shape of the broken trochlea accessoria; C, D Avolatavis europaea (Vastanavidae; holotype, NMS.Z.2021.40.76), right side; E, F Tynskya crassitarsus (Messelasturidae; NMS.Z.2021.40.74), right side; G, H an undetermined halcyornithid (NMS.Z.2021.40.69) right side; I, J Sororavis solitaria (Morsoravidae; holotype, NMS.Z.2021.40.75), right side; K Psittacomimus eos (Psittacomimidae fam. nov.; holotype, NMS.Z.2021.40.38), right side; L P. eos (NMS.Z.2021.40.39), left side (mirrored); M, N Primoscens carolinae (Zygodactylidae; holotype, NMS.2021.40.54), left side (mirrored). O, P Primozygodactylus cf. danielsi (Zygodactylidae; NMS.2021.40.47), right side; Q−Z dorsal (left) and plantar (right) view of the tarsometatarsus of Q Vastanavis sp. (GU/RSR/VAS 1809), right side; R A. europaea (holotype, NMS.Z.2021.40.76), right side; S Tynskya crassitarsus (NMS.Z.2021.40.74), right side; T an undetermined halcyornithid (NMS.Z.2021.40.69), right side; U Nestor notabilis (Psittaciformes, Strigopidae), right side; V S. solitaria (NMS.Z.2021.40.75), right side; W P. eos (NMS.Z.2021.40.39), left side (mirrored); X P. carolinae (holotype, NMS.2021.40.54), left side (mirrored). Y P. cf. danielsi (NMS.2021.40.47), right side; Z Corvus frugilegus (Passeriformes, Corvidae), right side. The dashed lines in A, C, E, and G indicate the slanted dorsolateral margin of the proximal tarsometatarsus. The arrows in W−Y denote enlarge details of the distal end of the bone. Abbreviations: acc, trochlea accessoria; fdl, hypotarsal sulcus/canal for tendon of musculus flexor digitorum longus; fhl, hypotarsal sulcus for tendon of musculus flexor hallucis longus; fur, furrow separating trochlea accessoria from main body of trochlea metatarsi IV; fvp, foramina vascularia proximalia; prj, dorsal projection; sul, sulcus formed by dorsally open canalis interosseus distalis; ttc, tuberositas musculi tibialis cranialis. The scale bars equal 5 mm.

Discussion

The interrelationships of “near-passerine” zygodactyl birds

A clade including Parapsittacopes and Psittacomimus received at least moderate bootstrap support and was recovered in all analyses of the present study as well as the analysis of Ksepka et al. (2025). Parapsittacopes and Psittacomimus were assigned to the Psittacopedidae by Mayr and Kitchener (2023a), but the clade including both taxa is well separated from Psittacopes – the type genus of the Psittacopedidae – in the strict consensus tree of the unconstrained analysis and the majority rule consensus tree of the constrained analysis. The new taxon Psittacomimidae fam. nov. is introduced for this clade.

The tarsometatarsus of psittacomimids is characterized by the separation of the trochlea accessoria from the main body of the trochlea metatarsi IV, which also occurs in the Zygodactylidae and in Psittacopes (as per a referred specimen from the London Clay), but not in the Morsoravidae (Fig. 5). Actually, the distal end of the tarsometatarsus of Psittacomimus is very similar to that of the zygodactylid Primoscens in all major features. In addition, the quadrates of the Psittacomimidae, Primozygodactylus, and the Passeriformes agree in the occurrence of pneumatic foramina in the caudal surface of the processus oticus (Fig. 3), which are absent in Morsoravis, Psittacopes, and the majority of other neornithine birds (the quadrates of Primoscens and Zygodactylus are unknown). The derived morphologies of the distal end of the tarsometatarsus and the quadrate support a sister group relationship between the Psittacomimidae fam. nov. and the Parapasseres, which resulted from the majority rule consensus tree of the constrained analysis.

By contrast, the unconstrained analysis recovered a sister group relationship between the Morsoravidae and the Parapasseres, and five characters were optimized as synapomorphies of the clade including both taxa. However, two of these represent reversals into the plesiomorphic condition (ch. 82: tarsometatarsus without dorsally open sulcus between foramen vasculare distale and incisura intertrochlearis lateralis; ch. 86: tarsometatarsus, trochlea metatarsi III not much wider in mediolateral than in dorsoplantar direction). The derived state of another character (ch. 89: tarsometatarsus, distinct fossa immediately proximal to trochlea metatarsi III, on dorsal surface of bone) is not present in the Passeriformes, and that of a further one (ch. 73: tarsometatarsus, medial portion of cotyla medialis forming a proximally projecting lip) is not, or not clearly, visible in the published images of the morsoravid Consoravis. The fifth character (ch. 48: carpometacarpus, fossa between processus pisiformis and proximal end of os metacarpale minus) occurs in a number of other taxa and is of limited phylogenetic significance.

Therefore, it is here concluded that the majority rule consensus tree of the constrained analysis better reflects the actual phylogeny in recovering a clade (Psittacomimidae fam. nov. + Parapasseres) to the exclusion of Psittacopes and the Morsoravidae. This conforms to the results of the analysis of Ksepka et al. (2025), who also found Eofringillirostrum to be within the clade (Psittacomimidae fam. nov. + Parapasseres). As detailed by Ksepka et al. (2025), the main feature supporting a sister group relationship of Eofringillirostrum to the Parapasseres is the presence of a well-developed processus intermetacarpalis (carpometacarpus). The position of Eofringillirostrum in the unconstrained tree of the present study is supported by seven characters, all of which constitute, however, only weak support for a clade including the fossil taxon and the Psittaciformes. Two characters refer to soft tissue features that are not known in the fossil (ch. 2; ch. 102), one is unknown for Eofringillirostrum (ch. 19: shape of the articular surfaces of the thoracic vertebrae), and another represents a reversal into the plesiomorphic condition (ch. 23: furcula without well-developed apophysis furculae). This leaves three homoplastic characters of limited phylogenetic significance (ch. 9: presence of retroarticular processes; ch. 66: distal end of tibiotarsus mediolaterally wide and craniocaudally compressed, trochlea cartilaginis tibialis shallow; ch. 96 proximal three phalanges of fourth toe abbreviated). Compared to the character evidence resulting from the present analysis, the occurrence of a well-developed processus intermetacarpalis appears to be of greater phylogenetic significance owing to its restricted distribution within Psittacopasseres. Thus, and contrary to the results of the present analysis, Eofringillirostrum probably is the sister taxon of the Parapasseres (Fig. 2D; pro Ksepka et al. 2025, contra Mayr and Kitchener 2023a, 2023b, 2023c).

Paraphyly of the Zygodactylidae relative to the Passeriformes – as resulting from the unconstrained analysis of the present study and an analysis by Ksepka et al. (2025) – would constitute the strongest evidence for the hypothesis that separation of the trochlea accessoria from the main body of the trochlea metatarsi IV is a plesiomorphic trait of the Parapasseres. However, the affinities of zygodactylids are unresolved in the majority rule consensus tree of the constrained analysis, and it remains possible that these birds form a clade rather than a “grade” at the base of passerines. Still, the characteristic shape of the trochlea metatarsi IV is likely to be a derived feature uniting the Psittacomimidae fam. nov. and Parapasseres, being secondarily lost in the Passeriformes. With regard to the separation of the trochlea accessoria from the main body of the trochlea metatarsi IV, the Psittacomimidae fam. nov. and Zygodactylidae agree with crown group Psittaciformes, in which the trochlea accessoria is, however, larger and more distally elongated (Fig. 5U). In the Psittacomimidae fam. nov. and Zygodactylidae, the hallux is much weaker than in other early Eocene psittacopasserines (Fig. 6C, D), so that the distinctive morphology of the trochlea metatarsi IV may have been due to a more strongly reversed fourth toe, which – as a grasping adaptation – possibly compensated for the short hallux. The Morsoravidae (including Pumiliornis) are characterized by a very wide first phalanx of the fourth toe, the significance of which is unknown.

Figure 6.

Pedal phalanges and feet of early Eocene (A−F, H−Q) and early Oligocene (G) “near-passerine” and “parrot-like” zygodactyl birds. A Tynskya sp. (Messelasturidae; NMS.Z.2021.40.82). B Avolatavis europaea (Vastanavidae; holotype, NMS.Z.2021.40.76). C Parapsittacopes bergdahli (Psittacomimidae fam. nov.; SMF Av 653). D Primoscens sp. (NMS.2021.40.59). E Psittacopes lepidus (Psittacopedidae) (holotype, SMF-ME 1279); coated with ammonium chloride. F Morsoravis sedilis (Morsoravidae; holotype, MGUH 28930); coated with ammonium chloride. G Zygodactylus luberonensis (Zygodactylidae; SMF Av 519). H P. bergdahli (SMF Av 653). I Cyrilavis olsoni (Halcyornithidae; uncatalogued cast of the holotype in SMF). J Tynskya waltonensis (SMF Av 652). K A. europaea (holotype, NMS.Z.2021.40.76). L Vastanavis sp. (GU/RSR/VAS, nos. 1813). M A. europaea (holotype, NMS.Z.2021.40.76). N Cyrilavis cf. colburnorum (NMS.Z.2021.40.67). O T. waltonensis (SMF Av 652). P Psittacomimus eos (Psittacomimidae fam. nov.; holotype, NMS.Z.2021.40.38). Q Primoscens sp. (NMS.2021.40.59). The specimens in E, F, I, J, L, and O were coated with ammonium chloride. The toes are numbered in A−G. Abbreviations: rdg, ridge; snf, sulcus neurovascularis; tbe, tuberculum extensorium; tbf, tuberculum flexorium. The scale bars equal 5 mm.

The tree topologies resulting from the phylogenetic analyses suggest that separation of the trochlea accessoria from the main trochlea metatarsi IV by a furrow evolved independently in the Psittaciformes (including Namapsitta and Quercypsitta) and in the clade formed by the Psittacomimidae fam. nov. and Parapasseres. However, the tarsometatarsus of Psittacomimus is very similar to that of Namapsitta from the middle Eocene of Namibia (Mourer-Chauviré et al. 2015, 2017), as are additional bones known from both taxa. Only the humerus of Namapsitta more closely resembles that of crown group Psittaciformes in that the tuberculum dorsale is proximodistally elongated. If Namapsitta is indeed a stem group psittaciform as assumed by Mourer-Chauviré et al. (2015, 2017) − which essentially depends on the correct referral of the parrot-like humeri to this taxon − the characteristic tarsometatarsus morphology of Namapsitta and Psittacomimus may be plesiomorphic for the Psittacopasseres. In this case, the Morsoravidae, as well as the Halcyornithidae, Messelasturidae, and Vastanavidae, would most likely be outside crown group Psittacopasseres (in these taxa, the trochlea accessoria is not separated from the main body of the trochlea metatarsi IV). This alternative phylogeny (Fig. 2D) is here considered a reasonable hypothesis to be addressed in future studies, and, pending on the correct assignment of Namapsitta to stem group Psittaciformes, it is in greatest accordance to the tarsometatarsus morphologies of the fossil taxa.

The affinities of the Vastanavidae, Halcyornithidae, and Messelasturidae

Even though both analyses did not support monophyly of the Halcyornithidae as currently recognized, the strict consensus tree of the unconstrained analysis and the majority rule consensus tree of the constrained analysis congruently recovered a clade including the Messelasturidae and the halcyornithid taxa Cyrilavis, Pseudasturides, and Pulchrapollia. The new term Messelasturiformes is introduced for the least inclusive clade including Messelastur and Cyrilavis, which is well characterized by (1) long, caudally directed supraorbital processes (in Parapsittacopes, supraorbital processes are also present, but they are much shorter than in halcyornithids and messelasturids; Fig. 3A, B), a quadrate with (2) a strongly asymmetric otic process (the capitulum squamosum is projecting much farther dorsally than the capitulum oticum; Fig. 3F, H) and (3) a condylus medialis with a concave lateral articular facet (which has a restricted distribution among neornithine birds, but also occurs in a somewhat less pronounced form in Parapsittacopes; Mayr 2020, 2025), (4) a humerus with a dorsoventrally extensive fossa musculi brachialis (Fig. 4C, E), (5) a wide and dorsally open canalis interosseus distalis between the trochleae metatarsorum III et IV (a narrow sulcus is also present in Psittacomimus and the referred specimen of Psittacopes from the London Clay; Fig. 5S, T, Mayr and Kitchener 2023a), (6) a very small trochlea metatarsi II (hence, the second toe is much narrower than the other fore toes; Fig. 6A), and (7) a mediolaterally wide and dorsoplantarly narrow trochlea metatarsi III (Fig. 5F, H).

A clade including the Vastanavidae and Messelasturiformes, which resulted from the unconstrained analysis, is supported by derived characters of the tarsometatarsus and pedal phalanges. This includes a previously unrecognized and possibly functionally correlated character complex of the proximal end of the tarsometatarsus, whose medial portion forms a dorsal projection and whose lateral portion is markedly slanted (Fig. 5A, C, E, G). In addition, the foramina vascularia proximalia are asymmetrically positioned, and the lateral foramen is situated farther proximally than the medial one (Fig. 5Q). Whereas the phalanges of the second and third toes are long and slender in the Morsoravidae (Fig. 6F) and most Zygodactylidae (Fig. 6G; except for Primozygodactylus eunjooae, see Mayr and Zelenkov 2009), the first phalanx of the second digit is abbreviated in Avolatavis (Fig. 6K), Eofringillirostrum, the Messelasturidae (Fig. 6J), and the Halcyornithidae (Fig. 6I).

Mayr et al. (2010, 2013) and Ksepka and Clarke (2012) detailed that the major postcranial bones of Vastanavis and Avolatavis, respectively, show an overall resemblance to those of the Quercypsittidae. As in Quercypsitta (Fig. 4N), the coracoid of Vastanavis (Fig. 4L) has a large and broadly rounded processus acrocoracoideus and a deeply concave cotyla scapularis; the latter also occurs in the Messelasturiformes, whereas those “near-passerine” taxa of which the coracoid is known have a shallow facies articularis scapularis, and the processus acrocoracoideus is proportionally smaller and more “hook-shaped” in halcyornithids, messelasturids, and “near-passerine” taxa (Fig. 4Q−T).

The Messelasturiformes are less similar to the Quercypsittidae, and their psittacopasserine affinities have not yet been unambiguously established. The Messelasturidae were previously likened to the Strigiformes (Mayr 2000b, 2005a), and it is possible that messelasturids and halcyornithids – rather than vastanavids – are outside Australaves. Both taxa agree with the Strigiformes and Falconiformes in various features, such as the long supraorbital processes (which are present in stem group Strigiformes, but were reduced in the crown group taxa; Fig. 3C), the asymmetric processus oticus of the quadrate (only Falconiformes), and the presence of a foramen nervi supracoracoidei (Fig. 4O, P; which was, however, also reported for the vastanavid Eurofluvioviridavis; Mayr 2005b). Messelasturids have a sharply hooked beak, and the ungual phalanges of Avolatavis, Eurofluvioviridavis, the Messelasturidae, Serudaptus, and Eofringillirostrum are also at least superficially raptor-like in their shape and lack a laterally open neurovascular sulcus (Fig. 6L−O). As in the Strigiformes and most Accipitriformes, the ungual phalanx of the third toe of the Messelasturidae and Avolatavis bears a longitudinal ridge (Fig. 6M; Mayr 2011b, 2021). However, the flexor tubercles are less developed than in most raptorial birds that use their feet to subdue prey, and it is the extensor tubercle, which is prominent in messelasturids (Fig. 6O).

If the Falconiformes are the extant sister taxon of the Psittacopasseres, the stem species of the latter may have been raptorial and the similarities of the Messelasturiformes to the Falconiformes probably are be plesiomorphic. However, further data on critical aspects of the skeleton are needed for a robust placement of messelasturiforms and vastanavids, and in particular more information on the skull morphology of the Vastanavidae need to be gathered. Moreover, a monophyletic Eufalconimorphae is not recovered in all sequence-based analyses (Stiller et al. 2024: extended data, fig. 4), in which case the raptor-like features of the Messelasturiformes would have to be judged differently.

The remarkable diversity of early Eocene fossils assigned to the Psittacopasseres stands in contrast to the morphologies of early Eocene stem group representatives of the Afroaves (the clade including all telluravian taxa other than those assigned to the Australaves), which are more uniform and similar to those of the crown group taxa (Mayr 2022). The osteological heterogeneity of early Eocene zygodactyl birds may have been due to a radiation into disparate ecological niches, which was facilitated by their zygodactyl feet and vacant ecological niches for arboreal birds in the earliest Cenozoic. Alternatively, it may indicate that some fossils were erroneously assigned to the Psittacopasseres and convergently evolved superficially “parrot-like” features, which especially has to be taken into account for the Messelasturiformes.

Acknowledgements

Sven Tränkner is thanked for taking some of the photographs (others are by the author).

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Supplementary material

Supplementary material 1

Files S1, S2

Author: Mayr G (2026)

Data type: .zip

Explanation notes: File S1. Description of characters used in phylogenetic analysis [docx file]. — File S2. Character matrix used in the phylogenetic analysis. The anseriform Anhimidae were specified as the outgroup taxon; extinct taxa are indicated by a dagger. The matrix is based on Mayr & Kitchener (2023c), newly added taxa and characters, as well as revised character scorings, are highlighted in bold [docx file].

This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.

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