The non-apodiform Strisores (potoos, nightjars and allied birds) from the early Eocene London Clay of Walton-on-the-Naze

Mayr, Gerald; Kitchener, Andrew C.

doi:10.1007/s12549-024-00610-9

The non-apodiform Strisores (potoos, nightjars and allied birds) from the early Eocene London Clay of Walton-on-the-Naze

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The non-apodiform Strisores (potoos, nightjars and allied birds) from the early Eocene London Clay of Walton-on-the-Naze

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Abstract

Fossils of the avian clade Strisores (nightjars, swifts and allies) are well represented in some early Eocene localities. These birds were also taxonomically diverse in the British London Clay, but most of the previously described fossils belong to the Apodiformes (swifts and allies), or are represented by very fragmentary specimens. Here we report substantial new material of non-apodiform taxa of the Strisores from Walton-on-the-Naze (Essex, UK) that bears on the diversity and affinities of aerial insectivores from the London Clay. In addition to specimens of the recently described archaeotrogonid Archaeodromus, we report partial skeletons of the enigmatic taxon Palaeopsittacus, as well as two new species of the Fluvioviridavidae. The new Archaeodromus fossils reveal previously unknown osteological details, including features of the skull. The Palaeopsittacus specimens suggest that this poorly known taxon may be the earliest stem group representative of the Nyctibiiformes. The specimens of the Fluvioviridavidae elucidate some aspects of the osteology of these birds and the quadrate shows a morphology that is distinct from all extant Strisores. The non-apodiform Strisores from Walton-on-the-Naze exhibit disparate morphologies, which indicate that there was already significant ecological diversification of these birds by the early Eocene, and the diversity of these aerial insectivores in Walton-on-the-Naze contrasts with the absence of bats in this locality and other sites of the London Clay.

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Introduction

The avian clade Strisores includes swifts and hummingbirds as well as various nocturnal or crepuscular birds, which were long assigned to the paraphyletic taxon “Caprimulgiformes” (Mayr 2002, 2010; Prum et al. 2015; Chen et al. 2019; Kuhl et al. 2021). Current molecular studies suggest that the Caprimulgiformes sensu stricto (nightjars) are the sister taxon of all other Strisores, with a clade formed by the Neotropical Steatornithiformes (oilbirds) and the Nyctibiiformes (potoos) branching next (Prum et al. 2015; Kuhl et al. 2021). According to these sequence-based analyses, the Podargiformes (frogmouths) are the sister taxon of a clade including the Aegotheliformes (owlet nightjars) and apodiform birds.

Despite the fact that many representatives of the Strisores are small birds, the early Cenozoic fossil record of this clade is fairly comprehensive (Mayr 2022a). This is mainly due to their occurrence in Lagerstätten-type sites, the most productive of which is the latest early or earliest middle Eocene locality of Messel in Germany (Mayr 2018). The non-apodiform Strisores from Messel include the extinct Archaeotrogonidae (Hassiavis; Mayr 1998, 2004, 2021), stem group Nyctibiiformes (Paraprefica; Mayr 1999, 2001, 2005a), stem group Podargiformes (Masillapodargus; Mayr 1999, 2001, 2015a), as well as a taxon of uncertain affinities (Palaeopsittacus; Mayr 2003a). None of these taxa are known from fossil sites outside Europe.

From the early Eocene North American Green River Formation, another Lagerstätten-type locality, a stem group representative of the Steatornithiformes was described (Olson 1987). The Green River Formation also yielded a broad-billed and short-legged species, which was described as Fluvioviridavis platyrhamphus by Mayr and Daniels (2001), who assigned it to the monotypic taxon Fluvioviridavidae. Initially, the phylogenetic affinities of Fluvioviridavis were regarded as uncertain, but the taxon was tentatively assigned to the Strisores by Mayr (2009) and subsequently likened to the Podargidae (Nesbitt et al. 2011) and Steatornithidae (Chen et al. 2019). Eurofluvioviridavis robustipes, which was described as a putative European representative of the Fluvioviridavidae from Messel (Mayr 2005b), is now considered to more likely be a representative of the Psittacopasseres (Mayr 2015b; Mayr and Kitchener 2023a).

Various putative representatives of the Strisores were also reported from the early Eocene British London Clay, but most of the formally published fossils belong to the Apodiformes or are of uncertain phylogenetic affinities. Concerning bird fossils, the most productive London Clay locality is Walton-on-the-Naze (Essex), which yielded numerous remains of apodiform and non-apodiform Strisores. Until recently, the majority of these fossils were in the private collection of the late Michael Daniels, which is now curated by the National Museums Scotland.

The apodiform birds from the Daniels collection were described by Mayr and Kitchener (in press), and here we survey the non-apodiform Strisores. In addition to new material of the oldest known archaeotrogon, Archaeodromus anglicus ‒ a species recently described from Walton-on-the-Naze (Mayr 2021) ‒ we describe two new species of the Fluvioviridavidae and revise the affinities of the enigmatic species Palaeopsittacus georgei, which was initially described as a parrot on the basis of a partial skeleton from Walton-on-the-Naze (Harrison 1982a). As we show in the following, the Strisores from Walton-on-the-Naze exhibit very disparate morphologies, which indicate that there was already significant ecological diversification of these birds by the early Eocene.

Material and methods

The studied extant and fossil specimens are deposited in the Natural History Museum, London, UK (NHMUK); the National Museums Scotland, Edinburgh, UK (NMS); the Senckenberg Research Institute Frankfurt, Germany (SMF); the Staatliches Museum für Naturkunde Karlsruhe, Germany (SMNK); and the National Museum of Natural History, Smithsonian Institution, Washington D.C., USA (USNM).

Systematic palaeontology

Aves Linnaeus, 1758

Strisores sensu Mayr (2010)

Archaeotrogonidae Mourer-Chauviré, 1980

Archaeodromus Mayr, 2021

Emended diagnosis: In addition to the features listed by Mayr (2021), Archaeodromus differs from the Eocene-Oligocene Archaeotrogon in that the ulna is distinctly longer than the humerus, and it is distinguished from the latest early or earliest middle Eocene archaeotrogon Hassiavis in that the processus extensorius of the carpometacarpus is very long and markedly protruding.

Archaeodromus anglicus Mayr, 2021

Referred specimens: NMS.Z.2021.40.159 (Fig. 1b; tip of upper beak, jugal bar, left ramus mandibulae within a piece of matrix, a few caudal vertebrae, cranial portion of right scapula, caudal portion of ?right scapula, furcula, fragmentary cranial portion of sternum, proximal portion of left humerus, right ulna, partial left ulna, fragmentary ?left radius, left carpometacarpus, right femur lacking distal end, left os metatarsale I, ungual phalanx; a second tip of the upper beak associated with the specimen belongs to a different species); collected in 1985 by M. Daniels (original collector’s number WN 85500A). NMS.Z.2021.40.160 (Fig. 1c; tip of upper beak, jugal bar, partial ramus mandibulae); collected in 1984 by M. Daniels (original collector’s number WN 84498). NMS.Z.2021.40.161 (Fig. 1d; caudal portion of left ramus mandibulae, jugal bar, fragmentary left quadrate, three partial vertebrae, extremitas sternalis of furcula, right margo costalis of sternum, distal end of right humerus, fragmentary proximal portion of left ulna, radius fragments, proximal portion of left carpometacarpus, both phalanges proximales digitorum majores, fragmentary distal portion of left tarsometatarsus); collected in 1975 by M. Daniels (original collector’s number WN 75078).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation (previously Division A2; Jolley 1996; Rayner et al. 2009; Aldiss 2012), early Eocene (early Ypresian, 54.6‒55 Ma; Collinson et al. 2016).

Measurements (maximum length, in mm): NMS.Z.2021.40.159: right ulna, 30.0; left carpometacarpus, 17.3.

Taxonomic remarks: Mayr (2021) noted that the humerus of Archaeodromus anglicus (Fig. 2q, r) shows a resemblance to that of the holotype of Eocolius walkeri (NHMUK A 6205) from Walton-on-the-Naze (Fig. 2o, p). Other bones of the E. walkeri holotype are clearly distinguishable from the corresponding elements of A. anglicus, and unlike that of A. anglicus, the coracoid of E. walkeri exhibits a foramen nervi supracoracoidei (note that in the list of differentiating features in Mayr 2021: 2051 the taxa Eocolius and Archaeodromus are interchanged). E. walkeri was initially described as a species of mousebird (Coliiformes; Dyke and Waterhouse 2001), but coliiform affinities were questioned by Ksepka and Clarke (2009). The humerus of Archaeodromus differs from that of Eocolius in several features, such as a narrower and more elongated tuberculum dorsale, a tuberculum supracondylare ventrale that reaches as far proximally as the condylus dorsalis (it does not extend to the proximal end of the condylus dorsalis in E. walkeri), and a blunter processus flexorius (Fig. 2o‒r). However, these features are rather minor and there exists a possibility that the holotype of E. walkeri is a composite of different individuals; if this can be established in future studies, we suggest to then designate the coracoid as the lectotype of E. walkeri in order to avoid future taxonomic confusion.

Description and comparisons: NMS.Z.2021.40.159 includes two tips of upper beaks; the one we identified as belonging to Archaeodromus corresponds to partial beaks associated with other Archaeodromus specimens. The most complete of these, NMS.Z.2021.40.160 (Fig. 2a, b), closely resembles the upper beak of Hassiavis (see Mayr 2004) and shows the nostrils to have been very large. There is a mediolateral constriction just caudal to the tip, and the ventral surface of the rostrum maxillare forms a deep, trough-like fossa (Fig. 2b). The internarial bar is wide. The portions of the jugal bar that are preserved in NMS.Z.2021.40.159 and NMS.Z.2021.40.160 are straight.

The quadrate of Archaeodromus was described by Mayr (2021; in the caption of figure 2 of this publication, the images depicting the quadrates of species of the Aegothelidae and Caprimulgidae are confounded). The most distinctive feature of the bone is a dorsally facing cotyla quadratojugalis.

The caudal end of the mandible was also described by Mayr (2021), and in dorsoventral view its caudal margin forms a distinct convexity. NMS.Z.2021.40.159 for the first time preserves the mandibular ramus in its entire length and shows it to be markedly sigmoidally curved in lateral view (Fig. 2e, g, h). Notably, the mandible lacks the mediolateral widening and inflation of the mid-section of the ramus at the intraramal hinge, which characterises the mandible of the Caprimulgiformes sensu stricto (Bühler 1970; Fig. 2f) and Nyctibiiformes.

The holotype of A. anglicus (Fig. 1a) includes coracoids and scapulae, but the furcula is not preserved. The bone is complete in NMS.Z.2021.40.159 and has fairly wide, strap-like scapi clavicularum (Fig. 2i). The extremitas omalis is simple, with a straight tip; unlike in members of crown group Caprimulgiformes (Fig. 2j) it lacks a facies articularis acrocoracoidei.

The sternum, of which the cranial portion is preserved in NMS.Z.2021.40.159 (Fig. 2k‒m), has a strongly vaulted corpus. As in Hassiavis laticauda from Messel (Mayr 1998), the spina externa is deeply bifurcated. The sulci articulares coracoidei are well separated, dorsoventrally narrow and mediolaterally extensive. Five processus costales can be counted in NMS.Z.2021.40.159. The processus craniolateralis is long and pointed.

The humerus of Archaeodromus anglicus was described by Mayr (2021). Even though none of the Archaeodromus specimens preserves a complete humerus and ulna together, a comparison of these bones from the holotype and NMS.Z.2021.40.159 shows that ‒ unlike in Archaeotrogon ‒ the ulna was distinctly longer than the humerus (Fig. 2q, r, v, w). The proximal end of the ulna was described by Mayr (2021), who noted similarities to the Caprimulgiformes. The distal end of the bone is preserved in NMS.Z.2021.40.159 and corresponds well to the distal ulna of Archaeotrogon (Mourer-Chauviré 1980: fig. 4n, o). As in the latter, the proximal margin of the incisura tendinosa forms a pronounced tubercle.

The carpometacarpus (Fig. 2z‒bb) is not preserved in the Archaeodromus anglicus holotype. As in Archaeotrogon, the bone has a very long and strongly projecting processus extensorius, but unlike in Archaeotrogon (Fig. 2cc) it does not taper into a sharply pointed, spur-like tip. The processus extensorius is longer and more pronounced in NMS.Z.2021.40.159 than in NMS.Z.2021.40.161 and the tip exhibits an exostosis, which may indicate a sexual dimorphism in the size and shape of this process, with NMS.Z.2021.40.159 being a male individual. In the taxon Archaeotrogon, there also exists variability in the development of the spur (compare Mourer-Chauviré 1980: figs. 3g and 4p). The spatium intermetacarpale is narrower than in Archaeotrogon.

The phalanx proximalis digiti majoris was described by Mayr (2021) and bears a small processus internus indicis.

The femur is not preserved in the holotype. In NMS.Z.2021.40.159 (Fig. 2ff) it lacks the distal end, but judging from the preserved portion, it was a relatively long bone and had similar proportions to the femur of the Caprimulgiformes and Aegotheliformes (the femur of other extant non-apodiform Strisores is stouter).

A fragment of the dorsomedial section of the distal portion of the tarsometatarsus is preserved in NMS.Z.2021.40.161 (Fig. 3g‒i). The trochlea metatarsi II is plantarly deflected, but unlike in Archaeotrogon it does not form a marked medioplantar projection.

The os metatarsale I (NMS.Z.2021.40.159; Fig. 2hh) is characterised by a distinct notch in the distal margin of the trochlea. The processus articularis tarsometatarsalis is not as wide and stout as in the species of extant Caprimulgiformes. NMS.Z.2021.40.159 also includes an ungual phalanx with a weakly developed tuberculum flexorium (Fig. 2ii).

?Apodiformes Peters, 1940

?Primapus Harrison and Walker, 1975

cf. Primapus lacki Harrison and Walker, 1975

Referred specimen: NMS.Z.2021.40.162 (Fig. 3a‒f; caudal portion and fragmentary ramus of left mandible, left tarsometatarsus); collected in 1985 by M. Daniels (original collector’s number WN 85504).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation, early Eocene (early Ypresian).

Measurements (maximum length, in mm): Tarsometatarsus, 9.2.

Remarks: The tarsometatarsus of NMS.Z.2021.40.162 (Fig. 3b‒f) was mentioned by Mayr (2021: 2061, 2022a: 99), who ‒ based on an initial identification by the collector M. Daniels ‒ likened it to Archaeodromus. It is associated with the caudal end of a mandible (Fig. 3a) that closely resembles the mandible of Archaeodromus anglicus, but is distinctly smaller (Fig. 3j).

The tarsometatarsus of NMS.Z.2021.40.162 is likewise much smaller than the expected length of the bone in A. anglicus. Because the tarsometatarsus of archaeotrogons is only slightly shorter than the carpometacarpus (Mourer-Chauviré 1980; Mayr 2021), the tarsometatarsus of A. anglicus is likely to have had a length of about 15‒16 mm. With a length of only 9.2 mm, the tarsometatarsus of NMS.Z.2021.40.162 is even shorter than the tarsometatarsus of the smallest named archaeotrogon, Hassiavis laticauda from the latest early or earliest middle Eocene of Messel, which has a length of 11.0‒~13.5 mm (Mayr 1998, 2004, 2021).

The tarsometatarsus resembles that of Archaeotrogon in its overall proportions, but differs in several osteological details. This is particularly true for the distinctive morphology of the hypotarsus, which is block-like and lacks canals or sulci (Fig. 3f), whereas the hypotarsus of Archaeotrogon has a centrally situated canal (Mourer-Chauviré 1980).

We now consider it most likely that NMS.Z.2021.40.162 belongs to the similar-sized apodiform species Primapus lacki, of which remains were found in Walton-on-the-Naze (Mayr and Kitchener in press). Even though the caudal portion of the mandible shows a resemblance to that of Archaeodromus, it also resembles the mandible of members of the apodiform Cypseloidinae (Apodidae; the mandible of other crown group Apodiformes has a less strongly convex caudal margin; Fig. 3k‒m).

?Nyctibiiformes Yuri et al., 2013

Palaeopsittacus Harrison, 1982a

Emended diagnosis: The taxon is characterised by a very small and rostrocaudally narrow caudal portion of the mandible, which has a processus medialis with a broadly rounded tip; the scapula bears a long acromion; the coracoid has a rounded processus acrocoracoideus and exhibits a foramen nervi supracoracoidei; the tarsometatarsus is abbreviated and the hypotarsus has two canals; the feet were anisodactyl.

Palaeopsittacus georgei Harrison, 1982a

Referred specimens: NMS.Z.2021.40.163 (Fig. 4b; caudal portion of right ramus mandibulae, fragment of synsacrum, fragmentary distal end of left femur, distal portion of right tibiotarsus, left tarsometatarsus, left os metatarsale I, one complete and one partial pedal phalanx, one ungual phalanx); collected in 1991 by M. Daniels (original collector’s number WN 91682). NMS.Z.2021.40.164 (Fig. 4c; partial right coracoid and left tarsometatarsus of a juvenile individual); collected in 1994 by M. Daniels (original collector’s number WN 94838A).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation, early Eocene (early Ypresian).

Measurements (in mm): NMS.Z.2021.40.163: tarsometatarsus, maximum length, 15.5; width of proximal end, 5.0; tibiotarsus, width of distal end, 4.5. NMS.Z.2021.40.164: right coracoid, maximum length, 20.2; left tarsometatarsus, maximum length, 15.0.

Remarks: Some bones of NMS.Z.2021.40.163 and NMS.Z.2021.40.164 were figured and briefly described by Mayr and Daniels (1998). NMS.Z.2021.40.164 is a juvenile individual, as indicated by the incompletely ossified metatarsals (the foramen vasculare distale is not closed) and the rough and “unfinished” texture of the bone surfaces.

Description and comparisons: NMS.Z.2021.40.163 includes the caudal end of the mandible (Fig. 5a, b), which resembles the caudal mandible of the Caprimulgiformes and Nyctibiiformes (Fig. 5c, d) in being small and rostrocaudally narrow. The processus medialis has a broadly rounded and rostrocaudally wide tip that bears a notch in its midsection. The trough-like cotyla medialis is long and narrow. The cotyla lateralis is shallow and not well delimited; a cotyla caudalis is absent. The ramus immediately rostral to the articular end is mediolaterally wide and dorsoventrally shallow.

The coracoid (Fig. 5f‒j) has a rounded processus acrocoracoideus with a weakly developed facies articularis clavicularis. The foramen nervi supracoracoidei of NMS.Z.2021.40.164 is somewhat larger than in the holotype and situated farther away from the cotyla scapularis, which we consider to be due to the fact that this specimen represents a juvenile individual (as indicated by the rough, “unfinished” bone surfaces). The processus procoracoideus, which is broken in the holotype, has a subtriangular shape in NMS.Z.2021.40.164.

The distal end of the tibiotarsus (Fig. 5n, o) agrees with that of the P. georgei holotype (Fig. 5q) in that the condyli are proximodistally low and widely spaced. The deep sulcus extensorius is medially situated and the pons supratendineus is proximodistally narrow. The condylus lateralis protrudes less cranially than the condylus medialis (see Mayr and Daniels 1998: text-fig. 2C).

The tarsometatarsus (Fig. 5r‒w) closely corresponds to that of the Messel fossil of Palaeopsittacus (Fig. 5x). The hypotarsus exhibits two canals, presumably for the tendons of musculus flexor digitorum longus and musculus flexor hallucis longus. As far as comparisons are possible, it closely corresponds to the fragmentary hypotarsus of the P. georgei holotype as figured by Harrison (1982a: fig. 6a). In extant Strisores two hypotarsal canals only occur in the Nyctibiiformes and Podargiformes (Mayr 2016). The trochlea metatarsi II is mediolaterally wide and reaches farther distally than the trochlea metatarsi IV. The trochlea metatarsi IV forms a plantar flange.

The os metatarsale I has a wide processus articularis tarsometatarsalis. The pedal phalanges have unusually widened distal ends and closely resemble the pedal phalanges of the stem group leptosomiform Plesiocathartes insolitipes from Walton-on-the-Naze (Mayr and Kitchener 2023b: fig. 3a). The ungual phalanx features a low tuberculum flexorium.

Fluvioviridavidae Mayr and Daniels, 2001

Remarks: The fossils described below can be unambiguously referred to the taxon Fluvioviridavis Mayr and Daniels, 2001, with which they share a number of characteristic features, including a long, wide, and dorsoventrally shallow mandible; a coracoid with a deeply excavated, cup-like cotyla scapularis and a foramen nervi supracoracoidei; a furcula with broad and strap-like scapi clavicularum; and a humerus with a dorsally prominent crista deltopectoralis and a small but well-delimited tuberculum supracondylare dorsale.

Fluvioviridavis Mayr and Daniels, 2001

Fluvioviridavis nazensis, sp. nov.

Holotype: NMS.Z.2021.40.165 (Fig. 6a; partial left coracoid, extremitas sternalis of right coracoid, left scapula, furcula, partial sternum, right humerus, distal end of left humerus, proximal portion of right ulna, distal portion of left ulna, left radius, distal half of right radius, partial right carpometacarpus, distal end of left carpometacarpus, right phalanx proximalis digiti majoris); collected in 1988 by M. Daniels (original collector’s number WN 88588).

Differential diagnosis: The new species differs from Fluvioviridavis platyrhamphus in its smaller size (humerus length ~40.3 mm versus 49.5/50.1 mm in F. platyrhamphus [Mayr and Daniels 2001]); a coracoid with a mediolaterally shorter facies articularis sternalis that does not reach onto the processus lateralis (compare Figs. 8a and 8b); a scapula with a mediolaterally narrower caudal portion (compare Figs. 8k and 8l, m); an ulna with a longer olecranon; and a bifenestrated and craniocaudally narrower phalanx proximalis digiti majoris (compare Figs. 8hh and 8ii).

Etymology: The species epithet refers to the type locality.

Type locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation, early Eocene (early Ypresian).

Referred specimens: NMS.Z.2021.40.166 (Fig. 6b; both coracoids, extremitates craniales of both scapulae, furcula); collected in 1979 by M. Daniels (original collector’s number WN 79241B). NMS.Z.2021.40.167 (Fig. 6c; extremitas cranialis of left scapula, distal end of left humerus, proximal end of right ulna, right carpometacarpus, os carpi ulnare, phalanx distalis digiti majoris); collected in 1993 by M. Daniels (original collector’s number WN 93776).

Measurements (maximum length, in mm; [in brackets the dimensions of the holotype of F. platyrhamphus; after Mayr and Daniels 2001]): NMS.Z.2021.40.165: left coracoid, length as preserved, 17.8; left scapula, 27.5; furcula, 17.0; right humerus, length as preserved, 37.8; estimated total length, ~40.3 [49.5/50.1]; radius, 43.5; right carpometacarpus, 23.5 [27.5/27.7]; phalanx proximalis digiti majoris, 12.3. NMS.Z.2021.40.166: right coracoid, 19.2. NMS.Z.2021.40.167: right carpometacarpus, 23.8.

Taxonomic remarks: Mayr (2005b) described a putative European representative of the Fluvioviridavidae from Messel as Eurofluvioviridavis robustipes. This species agrees with Fluvioviridavis platyrhamphus in the shapes of the skull, wing, and pectoral girdle bones, but has much more robust feet (Mayr 2005b). However, Eurofluvioviridavis is now regarded to be more closely related to the taxon Avolatavis, which is considered to be a representative of the Psittacopasseres (Mayr 2015b; Mayr and Kitchener 2023a). Avolatavis tenens, the type species of the taxon Avolatavis, is based on leg bones from the North American Green River Formation (Ksepka and Clarke 2012). Another representative of Avolatavis, A. europaeus, was recently identified in Walton-on-the-Naze, but is likewise only represented by leg bones (Mayr and Kitchener 2023a). This raises the possibility that some of the Fluvioviridavis-like specimens described below, which lack leg bones, actually belong to A. europaeus. Based on the relative proportions of the major limb bones of E. robustipes, this association is here considered unlikely, because the tibiotarsus would then be about as long as the ulna, whereas the bone is distinctly shorter than the ulna in E. robustipes (the tibiotarsus of the A. europaeus holotype measures 43.6 mm; judging from the length of the radius, the ulna of the holotype of Fluvioviridavis nazensis, sp. nov. had a length of about 45‒47 mm). Wing and pectoral girdle bones of Avolatavis are unknown, but F. nazensis differs from E. robustipes in that the humerus exhibits a tuberculum supracondylare dorsale that is situated far proximally (compare Figs. 8x and 8y, z) and in that the crista deltopectoralis of the humerus is proportionally shorter (compare Figs. 8s, t and 8w).

Description and comparisons: The holotype was figured by Mayr and Daniels (2001), who briefly commented on some features seen in the specimen. The coracoid (Fig. 8b‒d) shows an overall resemblance to the coracoid of Fluvioviridavis platyrhamphus (Fig. 8a). The cotyla scapularis is deeply excavated and cup-shaped, and a foramen nervi supracoracoidei is present. The processus lateralis is long.

The scapula (Fig. 8k) has a short acromion and a very prominent tuberculum coracoideum. The caudal portion of the shaft is strongly angled and mediolaterally narrower than in the F. platyrhamphus holotype (Fig. 8l, m).

The furcula (Fig. 8h, i) has wide scapi clavicularum and a dorsoventrally deep extremitas sternalis with a well-developed ridge-like apophysis furculae. The omal extremity bears a small but well-defined facies articularis acrocoracoidei. The furcula of NMS.Z.2021.40.166 exhibits a pathological morphology, with an irregularly widened shaft (Fig. 8i), which either indicates a healed fracture or a bone deformation owing to an infection.

The sternum of the holotype (NMS.Z.2021.40.165) is situated in a piece of matrix and is largely complete except for the damaged caudal margin (Fig. 8n‒p); the detached left margo costalis is preserved in a separate piece of matrix. Unlike in the taxa of extant Strisores the spina externa is well-developed and blade-like. The corpus of the bone is strongly vaulted and lacks pneumatic openings in its cranial portion. The processus craniolaterales are situated relatively far caudally. The remaining parts of the caudal margin of the bone indicate the absence of deep incisions.

The humerus (Fig. 8s, t) is an elongate bone with a fairly straight shaft and a comparatively small proximal end. The tuberculum dorsale is small. The crista deltopectoralis is proportionally shorter than in the species of crown group Podargiformes. The centrally situated fossa musculi brachialis is mediolaterally narrow and proximodistally long. There is a small but well-defined tuberculum supracondylare dorsale, which is also present on the counter slab of the holotype of F. platyrhamphus (Fig. 8u; Mayr and Daniels 2001 erroneously considered this tubercle, which is not visible on the main slab of the F. platyrhamphus holotype, to be absent). The condylus dorsalis is narrow, the condylus ventralis globular. The tuberculum supracondylare ventrale is proximodistally long.

The cotyla dorsalis of the ulna reaches distally well beyond the cotyla ventralis (Fig. 8bb). The olecranon is well developed, and as noted by Mayr and Daniels (2001) it is longer than in F. platyrhamphus. The tuberculum carpale, on the distal end of the bone, is small (Fig. 8aa).

The carpometacarpus (Fig. 8cc, dd) closely resembles that of the F. platyrhamphus holotype (Fig. 8gg). The bone has a fairly wide spatium intermetacarpale.

The os carpi ulnare (Fig. 6c) has an only moderately long crus longum. Unlike in F. platyrhamphus, the phalanx proximalis digiti majoris (Fig. 8hh) is bifenestrated. A small processus internus indicis is present.

Fluvioviridavis michaeldanielsi, sp. nov.

Holotype: NMS.Z.2021.40.168 (Fig. 6d; partial left ramus mandibulae, partial right quadrate, left and partial right coracoid, furcula, partial sternum, partial left and right carpometacarpi, left phalanx proximalis digiti majoris); collected in 2002 by M. Daniels (original collector’s number WN 02048A).

Differential diagnosis: Differs from Fluvioviridavis nazensis, sp. nov. in that the coracoid is more robustly built, with a wider shaft and a facies articularis clavicularis that forms a sternally directed, bulge-like projection (Fig. 8f); the processus procoracoideus is somewhat wider; the extremitas sternalis of the furcula is proportionally narrower and the apophysis furculae smaller (compare Figs. 8g and 8h); the spina externa of the sternum is proportionally longer and projects much farther cranially than the apex carinae (compare Figs. 8n and 8r); the tip of the processus extensorius of the carpometacarpus is more cranially projected (more proximocranially in F. nazensis; compare Figs. 8cc and 8ee).

Distinguished from F. platyrhamphus Mayr and Daniels, 2001 in its smaller size (carpometacarpus length 22.5 mm versus 27.5‒27.7 mm in the F. platyrhamphus holotype) and a bifenestrated phalanx proximalis digiti majoris (compare Figs. 6d and 8ii).

Etymology: The species epithet honours the collector of the fossil, the late Michael Daniels (1931-2021).

Type locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation, early Eocene (early Ypresian).

Measurements (maximum length, in mm): Right carpometacarpus, 22.5 [F. platyrhamphus holotype: 27.5/27.7 (Mayr and Daniels 2001)].

Description and comparisons: The holotype includes most of the left half of the mandibular ramus, which shows the mandible to have been very similar to that of Fluvioviridavis platyrhamphus (as exemplified by the specimen referred to this species by Nesbitt et al. 2011; Fig. 7o‒r). As in the latter species the mandibular symphysis (of which only the caudolateral portion is preserved) was shallow and wide. The caudal end of the mandibular ramus is damaged, but it can be discerned that the cotyla lateralis was well developed and rostrocaudally extensive. The fossa next to the cotyla lateralis is not as deeply concave as in extant Podargiformes, so that the interlocking mechanism between the quadrate and mandible that characterises extant frogmouths was not developed. There is a fenestra mandibulae, but owing to the fact that its margins are damaged, this fenestra may appear larger than it actually was; a fenestra mandibulae is also present in a specimen that was referred to F. platyrhamphus by Nesbitt et al. (2011).

The processus orbitalis and capitulum oticum of the quadrate (Fig. 7a‒e) are broken, but otherwise the bone is well preserved. The processus oticus has a broad, medially slanted tip with a wide but shallow incisura intercapitularis; on its lateral face there is a small, lip-like tuberculum subcapitulare, which is absent in the taxa of extant Strisores. The caudal surface of the processus oticus bears a few small pneumatic openings. The processus lateralis is very long and projected; as in most neornithine birds, the cotyla quadratojugalis is facing laterally (it is facing dorsally in Archaeodromus, the Aegotheliformes and Apodiformes, and directed dorsolaterally in the Caprimulgiformes and Nyctibiiformes). The condylus medialis is rostrocaudally short. Unlike in extant Strisores, including the Podargiformes (Fig. 7g‒j) and Steatornithiformes (Fig. 7k‒n), the lateral surface of the condylus medialis bears a distinct, concave articular facet. The condylus pterygoideus is well developed, as is the condylus caudalis. The quadrate of Fluvioviridavis distinctly differs from the quadrates of extant Podargidae (Fig. 7g‒j) and Masillapodargus from the latest early or earliest middle Eocene of Messel (Fig. 7f), in which the bone is mediolaterally wider, the condylus medialis lacks a lateral articular facet, and the processus lateralis is less protruding.

The coracoid is somewhat more massive than that of F. nazensis. Unlike in the latter species the facies articularis clavicularis forms a sternally directed bulge. The furcula (Fig. 8g) has narrower scapi than that of F. nazensis and lacks a well-developed apophysis furculae.

The spina externa of the sternum (Fig. 8q, r) is longer than in F. nazensis, and unlike in the latter species the apex carinae of the sternum does not project as far cranially as the tip of the spina externa (compare Figs. 8n and 8r).

The crista deltopectoralis of the humerus has a subtriangular outline (Fig. 8v). The carpometacarpus resembles that of F. nazensis, but the processus extensorius is somewhat more cranially directed (compare Figs. 8cc and 8ee). The phalanx proximalis digiti majoris is bifenestrated.

Fluvioviridavis, sp.

Referred specimen: NMS.Z.2021.40.169 (Fig. 6e; partial mandible); collected in 1990 by M. Daniels (original collector’s number WN 90663).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation, early Eocene (early Ypresian).

Remarks: This specimen exhibits the characteristic mandibular morphology of Fluvioviridavis, but because the mandible is unknown for F. nazensis, sp. nov. and the distinguishing features of the two new species from Walton-on-the-Naze concern postcranial bones, an assignment to either species is not possible.

Aves indet.

Gen. et sp. indet. A (cf. Parvicuculus Harrison and Walker, 1977)

Referred specimen: NMS.Z.2021.40.170 (Fig. 9a‒f; right tarsometatarsus); collected in 1995 by M. Daniels (original collector’s number WN 95905).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation, early Eocene (early Ypresian).

Measurements (maximum length, in mm): Tarsometatarsus, 10.6 [Parvicuculus minor: 11.7; Fluvioviridavis platyrhamphus: holotype, 13.5; referred specimen, 11.9 (Nesbitt et al. 2011)].

Remarks: This stocky tarsometatarsus shows a resemblance to the tarsometatarsus of Parvicuculus minor from the London Clay of Burnham-on-Crouch (Harrison and Walker 1977; Harrison 1982a, b; Fig. 9g, h), but is somewhat smaller than the P. minor holotype (tarsometatarsus length 10.6 mm versus 11.7 mm [Harrison and Walker 1977]). The fossil from Walton-on-the-Naze is also shorter than a tarsometatarsus from the early Eocene North American Nanjemoy Formation (Fig. 9i‒m), which was compared with Parvicuculus and Fluvioviridavis by Mayr et al. (2022) and has a length of 11.8 mm. In size it corresponds to a tarsometatarsus from the early Eocene of Condé-en-Brie in France, which was identified as Parvicuculus sp. by Mayr and Mourer-Chauviré (2005) and is 10.5 mm long.

Unlike in P. minor and the Parvicuculus fossils from Condé-en-Brie and the Nanjemoy Formation, the impressiones retinaculi extensorii of NMS.Z.2021.40.170 are developed as a marked, dorsally projected ridge. Also unlike in P. minor and the North American fossil, the lateral portion of the tuberositas musculi tibialis cranialis forms a very pronounced, dorsally projected tubercle. The hypotarsus exhibits a deep sulcus, presumably for the tendon of musculus flexor digitorum longus; the lateral hypotarsal portion bears a shallow sulcus, which may have guided the tendon of musculus flexor hallucis longus (Mayr 2016). Unlike in Parvicuculus there is no strongly developed crista medianoplantaris along the plantar surface of the shaft. The fossa metatarsi I has an ovate shape and is distinct but not very deeply excavated. The dorsal opening of the foramen vasculare distale is recessed. The trochlea metatarsi II exhibits a medioplantarly directed projection. The longitudinal axis of the trochlea metatarsi III is medially inclined. There is a marked fossa at the base of the dorsal surface of the trochlea metatarsi III. The trochlea metatarsi IV forms a distinct, plantarly directed flange.

NMS.Z.2021.40.170 is shorter than the tarsometatarsus of the Fluvioviridavis platyrhamphus holotype, which measures 13.5 mm (Mayr and Daniels 2001); with a length of 11.9 mm, the tarsometatarsus of the referred F. platyrhamphus specimen described by Nesbitt et al. (2011) is also somewhat longer. The ratio of the humerus length of the F. nazensis holotype (~40.8 mm) to the tarsometatarsus length of NMS.Z.2021.40.170 (10.6 mm) is ~3.8, which corresponds to the value for the humerus to tarsometatarsus length ratio of 3.7 for F. platyrhamphus holotype (with humerus and tarsometatarsus lengths of 49.5/50.1 mm and 13.5 mm, respectively); the referred specimen described by Nesbitt et al. (2011) does not preserve wing bones. Detailed morphological comparisons are impeded by the poor preservation of the tarsometatarsus of the F. platyrhamphus holotype (Fig. 9z), but NMS.Z.2021.40.170 is somewhat stockier and has a wider proximal end.

Gen. et sp. indet. B

Referred specimen: NMS.Z.2021.40.171 (Fig. 9n‒q; left tarsometatarsus); collected in 1992 by M. Daniels (original collector’s number WN 92723).

Tentatively referred specimens: NMS.Z.2021.40.172 (Fig. 9r‒u; incomplete distal end of left tarsometatarsus, right tarsometatarsus lacking proximal end, several pedal phalanges); collected in 1996 by M. Daniels (original collector’s number WN 96908B). NMS.Z.2021.40.173 (Fig. 9v‒y; distal end of right tarsometatarsus, pedal phalanx); collected in 1992 by M. Daniels (original collector’s number WN 82414).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation, early Eocene (early Ypresian).

Measurements (maximum length, in mm): NMS.Z.2021.40.171: tarsometatarsus, 12.6 [Parvicuculus minor: 11.7; Fluvioviridavis platyrhamphus: holotype, 13.5; referred specimen, 11.9 (Nesbitt et al. 2011)].

Remarks: This tarsometatarsus is somewhat larger and more elongated than the tarsometatarsus of NMS.Z.2021.40.170. It is shorter than the tarsometatarsus of the Fluvioviridavis platyrhamphus holotype, but somewhat longer than the specimen referred to this species by Nesbitt et al. (2011). The ratio of the humerus length of the F. nazensis holotype (~40.8 mm) to the tarsometatarsus length of NMS.Z.2021.40.171 (12.6 mm) is ~3.2, which is less than the ratio for the F. platyrhamphus holotype (3.7; see above).

Unlike in NMS.Z.2021.40.170 (cf. Palaeopsittacus) the impressiones retinaculi extensorii are not developed as a distinct ridge. The tuberositas musculi tibialis cranialis is marked, but unlike in NMS.Z.2021.40.170 it does not form a pronounced tubercle. In proximal view of the bone, the fossa parahypotarsalis medialis is shallower than in NMS.Z.2021.40.170. The hypotarsus bears a sulcus for the tendon of musculus flexor digitorum longus; unlike in NMS.Z.2021.40.170 a second sulcus, presumably for the tendon of musculus flexor hallucis longus, is absent. The trochlea metatarsi II is mediolaterally wider than in NMS.Z.2021.40.170. As in NMS.Z.2021.40.170 the base of the dorsal surface of the trochlea metatarsi III exhibits a marked fossa, which is proximally bordered by a curved bulge; the. The trochlea is not as strongly medially inclined as the trochlea metatarsi III of NMS.Z.2021.40.170. The plantar surface of the trochlea metatarsi IV of NMS.Z.2021.40.171 is damaged, but in the tentatively referred specimen NMS.Z.2021.40.173, it bears a plantarly directed, wing-like flange.

NMS.Z.2021.40.171 shows an overall resemblance to the tarsometatarsus of Quercypodargus olsoni from the late Eocene of the Quercy fissure fillings in France (Mourer-Chauviré 1989). However, apart from being of a much smaller size (with a length of 30.1 mm, the tarsometatarsus of Q. olsoni is more than twice as long), the hypotarsus has only a deep sulcus, whereas there are a canal and a deep sulcus in Quercypodargus.

The tentatively referred specimen NMS.Z.2021.40.172 closely corresponds to NMS.Z.2021.40.171 in size and morphology, but lacks the depression and curved bulge at the base of the trochlea metatarsi III. These features are also absent in the tentatively referred specimen NMS.Z.2021.40.173, which also differs from NMS.Z.2021.40.171 in that the trochlea metatarsi II has a more strongly pronounced medially directed projection. This difference may be because the tarsometatarsus of NMS.Z.2021.40.171 is somewhat worn, which may account for the weak development of the medial projection. The trochlea metatarsi III of NMS.Z.2021.40.173 is somewhat narrower than that of NMS.Z.2021.40.171, and the trochlea metatarsi II reaches slightly farther distally.

Discussion

Ecomorphology and phylogenetic affinities of the Archaeotrogonidae: The Archaeotrogonidae comprise the taxa Archaeodromus, Hassiavis, and Archaeotrogon (Mayr 2022a). Mayr (2021) already noted features of the wing and pectoral girdle bones, in which Archaeodromus is distinguished from Archaeotrogon from the late Eocene and Oligocene of the Quercy fissure fillings in France (Mourer-Chauviré 1980). The new fossils show Archaeodromus to have had a proportionally longer ulna than Archaeotrogon, with the bone exceeding the humerus in length in Archaeodromus, whereas ulna and humerus have about the same length as the humerus in Archaeotrogon. The ulna of Hassiavis from Messel is likewise longer than the humerus (Mayr 1998, 2004, 2021). The processus extensorius of the carpometacarpus of Archaeotrogon forms a sharply pointed spur (Fig. 2cc; Mourer-Chauviré 1980), which ‒ in analogy to similar spurs in extant birds, such as the charadriiform lapwings and jacanas (Rand 1954) ‒ indicates that the wings were used for intraspecific combat. The new specimens show that the tip of the processus extensorius of Archaeodromus is more rounded than in Archaeotrogon, which suggests that combat played a minor role; Hassiavis altogether lacks an enlarged processus extensorius. The different length proportions of ulna and humerus of Archaeodromus and Hassiavis on the one hand and Archaeotrogon on the other may therefore be functionally correlated to differences in combat behaviour, which appears to have become more pronounced in the evolution of the Archaeotrogonidae and was most strongly developed in the late Eocene and Oligocene taxon Archaeotrogon.

Archaeotrogons are the only aerial insectivores with pronounced wing spurs, which among extant birds otherwise almost exclusively occur in species of the Anseriformes and Charadriiformes (Rand 1954; the carpometacarpus of the extinct columbiform taxon Pezophaps has a knob-shaped processus extensorius). Extant birds with wing spurs use these in combats on the ground, and although it is unknown in which situations the spurs were used by archaeotrogons, combats on the ground (e.g. at the nesting sites) are more likely than those in the air. The strongly developed tuberculum dorsale of the humerus, which served for the attachment of musculus supracoracoideus that elevates the wing, indicates well-developed wing flapping capabilities of archaeotrogons, and these may have constituted a preadaptation for the presumed combat behaviour of Archaeotrogon. Various species of extant Caprimulgiformes sensu stricto perform wing clapping in territorial behaviour (Mengel et al. 1972). If archaeotrogons are indeed stem group representatives of the Caprimulgiformes, this distinctive behaviour of nightjars may go back to morphological adaptations for wing flapping in archaeotrogons. In crown group Caprimulgiformes the processus extensorius of the carpometacarpus is of variable length (Fig. 2dd, ee).

An assignment of Archaeodromus to the Strisores is unambiguous and supported by a derived morphology of the quadrate, as well as various other derived features shared with particular subclades, especially the Caprimulgiformes and Aegotheliformes (Mayr 2021). However, the phylogenetic affinities of archaeotrogons within the Strisores are still elusive and controversially resolved in current analyses. An analysis by Mayr (2021), which was constrained to a molecular backbone phylogeny, resulted in a sister group relationship between the Archaeotrogonidae and Caprimulgiformes (nightjars), whereas an analysis of the morphological data alone did not resolve the affinities of archaeotrogons. Analyses by Chen et al. (2019) supported an assignment of the Archaeotrogonidae to the Aegotheliformes. However, a position of archaeotrogons within Daedalornithes, the clade formed by the Aegotheliformes and Apodiformes (Mayr 2002; Sangster et al. 2022), conflicts with the absence of a foramen nervi supracoracoidei (coracoid), the presence of which is likely to be an apomorphy of the Daedalornithes. The morphology of the proximal end of the ulna of archaeotrogons is also unlike that of the Aegotheliformes and Apodiformes, in which the tuberculum ligamenti collaterale ventrale is more pronounced (Mayr 2003b: Fig. A1).

Still, we acknowledge that archaeotrogons exhibit several features, which may indicate affinities to taxa of the Daedalornithes. An unusual derived characteristic of archaeotrogons is the bifurcated spina externa of the sternum, which resembles the spina externa of the Aegotheliformes (Chen et al. 2019). The beak of Archaeodromus appears to have been similar to that of Hassiavis, which closely corresponds to that of the Aegotheliformes (Mayr 2004). However, these similarities are of a rather general nature, and it is to be expected that the derived beak morphology of the Caprimulgiformes, which are highly specialised aerial insectivores that use their widely open beaks as “insect nets”, was absent in early stem group representatives of this clade. If the above-described Archaeodromus-like mandible fragment and the associated tarsometatarsus (NMS.Z.2021.40.162) belong to the apodiform taxon Primapus, as we hypothesise, the proximal end of the mandible of archaeotrogons also shows a strong resemblance to that of early apodiform birds.

In case close affinities between archaeotrogons and the Daedalornithes are corroborated in future studies, the fossil birds are likely to be outside crown group Daedalornithes. At present, we maintain an identification of archaeotrogons as stem group representatives of the Caprimulgiformes sensu stricto (nightjars).

Affinities of Palaeopsittacus: Harrison (1982a) assigned P. georgei to the Psittaciformes, but this classification was rebutted by Mayr and Daniels (1998), who ‒ based on specimens from the Daniels collection ‒ showed that the species had anisodactyl feet. Mayr (2003a) described a postcranial skeleton of Palaeopsittacus cf. georgei from the latest early or earliest middle Eocene of Messel in Germany and noted similarities to the Podargiformes.

In particular, the tarsometatarsus of Palaeopsittacus georgei resembles that of Quercypodargus olsoni from the late Eocene of the Quercy fissure fillings in France, which was described as a stem group podargiform by Mourer-Chauviré (1989). In the same publication the putative stem group nyctibiiform Euronyctibius kurochkini was reported from the late Eocene of the Quercy fissure fillings (Mourer-Chauviré 1989, 2013). Because Qu. olsoni is only known from the tarsometatarsus, which is unknown for E. kurochkini, Mayr (2017) raised the possibility that Quercypodargus and Euronyctibius may represent the same taxon. However, the known fossils of Qu. olsoni and E. kurochkini stem from different localities of the Quercy fissure fillings, and Euronyctibius was recently assigned to the Steatornithiformes (Mourer-Chauviré 2013).

The previously unknown morphology of the caudal portion of the mandible of Palaeopsittacus corroborates a position of the taxon within the Strisores. However, instead of being close to the Podargiformes, it shows a strong resemblance to the caudal end of the mandible of the Nyctibiiformes and Caprimulgiformes in being unusually small, rostrocaudally narrow, and having a broadly rounded processus medialis (see description and Fig. 5a‒d). A similar mandibular morphology to that of Palaeopsittacus, the Nyctibiiformes, and the Caprimulgiformes does not occur in other avian taxa, but because the Nyctibiiformes and Caprimulgiformes are not recovered as closely related in current sequence-based analyses (e.g. Prum et al. 2015; Kuhl et al. 2021), it seems to have evolved at least twice independently. Palaeopsittacus otherwise shows no derived similarities to the Caprimulgiformes, but as in the Nyctibiiformes and Podargiformes and unlike in other Strisores (Mayr 2016), its tarsometatarsus exhibits two hypotarsal canals for the tendons of musculus flexor digitorum longus and musculus flexor hallucis longus. The distal end of the tibiotarsus has proximodistally low and widely spaced condyli, as does the distal tibiotarsus of crown group Nyctibiiformes, in which the condylus lateralis forms a distinctive lateral projection that is absent in Palaeopsittacus (Fig. 5p). A classification into the Nyctibiiformes is furthermore supported by the short tarsometatarsus of Palaeopsittacus, which is more abbreviated than that of other Strisores except for the Nyctibiiformes and Steatornithiformes.

Crown group Nyctibiiformes are characterised by a grotesquely abbreviated tarsometatarsus (Fig. 5z). The tarsometatarsus of the stem group nyctibiiform Paraprefica from Messel (Fig. 5y; Mayr 1999, 2001, 2005a) is likewise extremely short, whereas the tarsometatarsus of Palaeopsittacus is abbreviated but proportionally longer than in Paraprefica and extant Nyctibiiformes. If Palaeopsittacus is a stem group representative of the Nyctibiiformes, we therefore consider it most likely that it is the earliest and phylogenetically most basal one. Palaeopsittacus and Paraprefica coexisted in Messel, but the London Clay fossils of P. georgei predate the Paraprefica fossils from Messel by about seven million years.

However, we note that nyctibiiform affinities of Palaeopsittacus are tentative and need to be critically assessed once further fossil material is available. Unlike in crown group Nyctibiiformes, the coracoid of Palaeopsittacus exhibits a foramen nervi supracoracoidei (Fig. 5f‒i, k), and the proximal end of the ulna of the P. georgei holotype is distinguished from that of crown group Nyctibiiformes in a smaller cotyla ventralis and larger cotyla dorsalis (Fig. 5l, m).

The distribution of extant Nyctibiiformes is restricted to the tropical areas of South and Central America. As detailed by Mayr (2022b), the putative stem group roller Ueenkekcoracias tambussiae from the early Eocene of Argentina (Degrange et al. 2021) shows a close resemblance to Palaeopsittacus. If affinities to the latter taxon are confirmed in future studies, U. tambussiae is potentially the oldest New World representative of stem group Nyctibiiformes.

Affinities and taxonomic status of the Fluvioviridavidae: As noted in the introduction, Fluvioviridavis resulted as a stem group representative of the Podargiformes in an analysis of Nesbitt et al. (2011) and as a stem group representative of the Steatornithiformes in analyses by Chen et al. (2019). These analyses only included taxa of the Strisores and therefore could not assess the placement of Fluvioviridavis into that clade.

In the analysis of Nesbitt et al. (2011) the assignment of Fluvioviridavis to the Podargiformes was mainly based on cranial characters. If compared with extant Strisores, the beak and palate morphologies of Fluvioviridavis indeed most closely correspond to those of the Podargidae. However, similar beaks evolved in a number of only distantly related avian taxa, such as Balaeniceps (Balaenicipitidae), the taxon Cochlearius (Ardeidae), and some broadbills (Passeriformes, Eurylaimidae); in Balaeniceps and Cochlearius the morphology of the palate also resembles that of the Podargidae. As detailed by Mayr (2015a), Fluvioviridavis is clearly distinguished from Masillapodargus, a presumptive stem group representative of the Podargiformes from Messel. Profound differences concern the morphologies of the coracoid and humerus (Mayr 2015a), and the quadrate of Fluvioviridavis is also distinguished from that of Masillapodargus and crown group Podargiformes (this study).

The assignment of Fluvioviridavis to the Steatornithiformes by Chen et al. (2019) was only weakly supported by the lack of a facies articularis acrocoracoidea of the furcula. However, and as detailed above, the furcula of Fluvioviridavis actually exhibits a small facies articularis acrocoracoidea. Therefore, we consider both podargiform and steatornithiform affinities of Fluvioviridavis to be only weakly supported.

Instead, the new specimens show Fluvioviridavis to be distinguished from all extant Strisores in a number of features, including the presence of a concave articular facet on the lateral surface of the condylus medialis of the quadrate, a coracoid with a very long processus lateralis, and a sternum with a long, blade-like spina externa. Among extant Neornithes, a concave articular facet on the lateral surface of the condylus medialis of the quadrate occurs in taxa of the Charadriiformes, Podicipediformes, Phoenicopteriformes, Aequornithes (the clade including most extant aquatic and semi-aquatic birds), Gruiformes, Accipitriformes (Cathartidae), and Falconiformes (Mayr and Clarke 2003). This articular facet is absent in all extant representatives of the Strisores, but we cannot exclude the possibility that its presence in Fluvioviridavis represents an autapomorphy of the taxon (rather than being plesiomorphic for the Strisores as a whole).

The above-described Fluvioviridavis fossils from Walton-on-the-Naze lack remains of the leg bones. We identified two different types of isolated tarsometatarsi in the Daniels collection that are candidate elements for Fluvioviridavis, and one of these potentially raises some taxonomic issues. As detailed above, the bone assigned to this tarsometatarsus type, NMS.Z.2021.40.170, shows a resemblance to the tarsometatarsus of Parvicuculus minor from the London Clay of Burnham-on-Crouch (Harrison and Walker 1977; Harrison 1982a, b). Based on its overall proportions and the presence of a plantar projection of the trochlea metatarsi IV, the tarsometatarsus of Parvicuculus was likened to that of the Podargiformes by Mayr and Mourer-Chauviré (2005). Mayr et al. (2022) noted a resemblance between the Parvicuculus and Fluvioviridavis tarsometatarsi and hinted at the possibility that the taxon Fluvioviridavidae Mayr and Daniels, 2001 is a junior synonym of the Parvicuculidae Harrison, 1982b. Owing to a lack of overlap in the known bones, this possibility currently cannot be substantiated or disproven, but even in case these two family-level taxa turn out to be synonymous, the tarsometatarsus of the Fluvioviridavis platyrhamphus holotype is slenderer than that of P. minutus, and the proportions of the bone clearly differentiate the taxa Fluvioviridavis Mayr and Daniels, 2001 and Parvicuculus Harrison and Walker, 1977.

We note that the second tarsometatarsus type, which is represented by NMS.Z.2021.40.171 and two tentatively referred specimens, is less similar to the tarsometatarsus of Parvicuculus. This tarsometatarsus type is more abundantly represented in the Daniels collection, which may indicate that it is the one belonging to Fluvioviridavis (of which several specimens were identified). If so, it would falsify a synonymy of the Fluvioviridavidae and Parvicuculidae. However, and as detailed above, this second tarsometatarsus type is proportionally somewhat longer than the tarsometatarsus of the Fluvioviridavis platyrhamphus holotype and further material is needed for a definitive assignment of one of these tarsometatarsus types to Fluvioviridavis.

The diversity of the non-apodiform Strisores from Walton-on-the-Naze: The non-apodiform Strisores from Walton-on-the-Naze show distinct differences in their beaks, which indicate disparate foraging strategies. Whereas the beak of Archaeodromus appears to have been most similar to that of owlet-nightjars or swifts, that of Palaeopsittacus may have been more potoo-like, whereas Fluvioviridavis had an at least superficially “frogmouth-like” beak.

Differences in the tuberculum dorsale of the humerus (which is proximodistally long in Archaeodromus, but small in Palaeopsittacus and Fluvioviridavis), and the absence (Archaeodromus) or presence (Palaeopsittacus, Fluvioviridavis) of the foramen nervi supracoracoidei of the coracoid suggest different flight characteristics of these taxa. As detailed above, the large tuberculum dorsale of archaeotrogons indicates flapping flight capabilities of these birds, and we consider it likely that there exists a functional correlation between the development of this tubercle and the presence or absence of the foramen nervi supracoracoidei, which is usually absent in birds that are capable of sustained flapping flight (GM, pers. obs.).

In general, the avifauna of the London Clay is very similar to those of the North American Green River Formation and Messel (Mayr 2022a). A notable absence in the London Clay concerns stem group representatives of the Steatornithiformes, which were reported from the early Eocene of North America (Olson 1987, 1999). The absence of Paraprefica in Walton-on-the-Naze is also remarkable, because this taxon is comparatively abundant in Messel, where it co-occurs with Palaeopsittacus.

The high diversity of the Strisores from Walton-on-the-Naze indicates ample food resources for aerial insectivores and contrasts with the absence of bats (Chiroptera) in Walton-on-the-Naze. Bats were found in the somewhat younger fossil sites of Messel (Germany) and the Green River Formation (Wyoming, USA), and in Messel they are extremely abundant. In part, these differences in the abundance of bats may be due to the fact that the London Clay was deposited in a marine environment, but the different relative abundances of taxa of the Strisores and Chiroptera in the London Clay and other fossil sites still needs to be explored in more detail.

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Acknowledgements

We thank S. Tränkner for taking some of the photographs (additional images are by GM; the photos of the Parvicuculus tarsometatarsus from the Nanjemoy Formation were provided by P. Scofield and V. De Pietri). Comments from Antoine Louchart and an anonymous reviewer improved the manuscript.

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Ornithological Section, Senckenberg Research Institute and Natural History Museum Frankfurt, Senckenberganlage 25, 60325, Frankfurt am Main, Germany
Gerald Mayr
Department of Natural Sciences, National Museums Scotland, Chambers Street, Edinburgh, EH1 1JF, UK
Andrew C. Kitchener
School of Geosciences, University of Edinburgh, Drummond Street, Edinburgh, EH8 9XP, UK
Andrew C. Kitchener

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Mayr, G., Kitchener, A.C. The non-apodiform Strisores (potoos, nightjars and allied birds) from the early Eocene London Clay of Walton-on-the-Naze. Palaeobio Palaeoenv (2024). https://doi.org/10.1007/s12549-024-00610-9

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Received: 06 February 2024
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DOI: https://doi.org/10.1007/s12549-024-00610-9

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The non-apodiform Strisores (potoos, nightjars and allied birds) from the early Eocene London Clay of Walton-on-the-Naze

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The non-apodiform Strisores (potoos, nightjars and allied birds) from the early Eocene London Clay of Walton-on-the-Naze

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Similar content being viewed by others

The “Diahot Tooth” is a Miocene rhinocerotid fossil brought by humans to New Caledonia

New Quaternary mammals support regional endemism in western Hispaniola

The Quaternary lions of Ukraine and a trend of decreasing size in Panthera spelaea

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