Multiple skeletons of Rhynchaeites from the London Clay reveal the osteology of early Eocene ibises (Aves, Threskiornithidae)

Abstract

We describe a new species of Rhynchaeites from the early Eocene London Clay of Walton-on-the-Naze (Essex, UK), which is represented by a partial skeleton comprising a skull and most major postcranial bones. Multiple further partial skeletons are assigned to Rhynchaeites sp. and include skeletal elements that are rarely preserved in Paleogene birds. Rhynchaeites is for the first time included in a phylogenetic analysis, which did not unambiguously resolve its affinities, but provided weak support for a sister group relationship to the Threskiornithidae. If Rhynchaeites is a stem group representative of ibises, its skeletal morphology indicates significant homoplasy in the evolution of Aequornithes, the higher level clade including the Threskiornithidae and most other aquatic or semi-aquatic birds. In particular, Rhynchaeites has much shorter legs than extant ibises and the new fossils show that its palate was schizognathous. Current phylogenies suggest that a desmognathous palate as well as long legs evolved multiple times independently within Aequornithes. Unlike in extant ibises, the tip of the beak of Rhynchaeites lacks a densely pitted surface. We hypothesize that vision played a greater role in the foraging strategy of Rhynchaeites, whereas extant ibises are tactile probers and use their beaks for “remote sensing”.

Introduction

The long-beaked Rhynchaeites messelensis is comparatively abundant in the latest early or earliest middle Eocene oil shale of the fossil site Messel in Germany. The species is the first fossil bird to have been described from this locality (Wittich 1898) and has a confounded taxonomic history. Originally considered to be related to the charadriiform Rostratulidae (painted snipes; Wittich 1898), it was redescribed as a charadriiform bird with “columboid features” by Hoch (1980), who introduced the junior synonym “Plumumida lutetialis” for the species. Peters (1983) identified further material of R. messelensis and proposed affinities to the Threskiornithidae (ibises and spoonbills); additional data on the osteology of R. messelensis were provided by Mayr (2002). A record of Rhynchaeites from the early Eocene Fur Formation in Denmark was reported by Mayr and Bertelli (2011), who also noted that the holotype humerus of the alleged parrot Mopsitta tanta, described by Waterhouse et al. (2008), resembles that of Rhynchaeites.

Molecular analyses congruently place the Threskiornithidae as the sister taxon of a clade including the Ardeidae (herons), Scopidae (hamerkop), Balaenicipitidae (shoebill), and Pelecanidae (pelicans; Prum et al. 2015; Kuhl et al. 2021). All these taxa are part of the Aequornithes, the “waterbird clade”, which also includes the Gaviiformes (loons), Procellariiformes (tubenoses), Sphenisciformes (penguins), Ciconiiformes (storks), and Suliformes (gannets, cormorants, and allies). The extant taxa of the Aequornithes show very different ecomorphologies and include foot- and wing-propelled divers as well as other aquatic and semi-aquatic birds. Many extant ibises are fairly long-legged and predominantly forage in wetlands, but several species are forest-dwelling or occur in arid environments (Matheu and del Hoyo 1992). Rhynchaeites had much shorter legs than its extant relatives and as detailed by Peters (1983) and Mayr (2002), it is distinguished from crown group Threskiornithidae in various plesiomorphic features.

R. messelensis is represented by more than a dozen compression fossils from Messel (Fig. 1a). These vividly illustrate the overall morphology of the species, but allow the recognition of only a limited amount of subtle osteological detail. Here, we report partial skeletons of Rhynchaeites from the early Eocene London Clay of Walton-on-the-Naze (Essex, UK), which reveal previously unknown osteological features of the taxon. With an age of 54.6‒55 million years, these specimens are coeval to the record of Rhynchaeites from the Danish Fur Formation (Mayr and Bertelli 2011) and 6‒7 million years older than the Messel fossils of R. messelensis, which have an age of about 48 million years (Mayr 2022).

Fig. 1

a Skeleton of Rhynchaeites messelensis from the latest early or earliest middle Eocene of Messel in Germany (SMNK.ME 725) with b a detail of the tip of the beak. c Tip of the beak of the extant Threskiornis aethiopicus (SMF 13651); the frame indicates an enlarged detail of the pitted surface, which is absent in Rhynchaeites (see “Discussion”). Scale bar equals 10 mm

The most complete specimen from Walton-on-the-Naze was figured by Mayr (2022: fig. 7.15d) and is here described as a new species. It includes a well-preserved skull, most major postcranial bones, as well as various other three-dimensionally preserved skeletal elements that are rarely preserved in Paleogene birds. Remains of six further individuals of Rhynchaeites add insights into the skeletal morphology of the taxon and reveal previously unknown features in which it differs from crown group Threskiornithidae.

Materials and methods

The osteological terminology follows Baumel and Witmer (1993). The fossils are deposited in the Moler Museum, Mors, Denmark (FU), the Geological Museum of the University of Copenhagen, Denmark (MGUH), the National Museums Scotland, Edinburgh, UK (NMS), the Natural History Museum, London, UK (NHMUK), the Senckenberg Research Institute Frankfurt, Germany (SMF), and the Staatliches Museum für Naturkunde Karlsruhe, Germany (SMNK).

A parsimony analysis was performed based on the emended character matrix of Mayr and Kitchener (2022) (see the Supplementary Information for character descriptions and character matrix). The analysis was run with the heuristic search modus of NONA 2.0 (Goloboff 1993) through the WINCLADA 1.00.08 interface (Nixon 2002), using the commands hold 10,000, mult*1000, hold/10, and max*. Bootstrap support values were calculated with 1000 replicates, ten searches holding ten trees per replicate, and TBR branch swapping without max*. For the primary analysis, the trees were rooted with the Galliformes; a second analysis was run with the Phaethontiformes as outgroup taxon. Three characters were scored as additive. Tree length (L), consistency index (CI), and retention index (RI) were calculated. Ancestral character states were reconstructed under the parsimony criterion with Mesquite 2.71 (Maddison and Maddison 2009) based on tree topologies resulting from recent analyses of molecular data (Prum et al. 2015; Kuhl et al. 2021).

Comparisons with extant Threskiornithidae are based on the following species in the ornithological collection of SMF: Bostrychia hagedash, B. olivacea (only skull), Eudocimus albus, E. ruber, Geronticus calvus, G. eremita, Lophotibis cristata, Platalea ajaja, P. alba, P. leucorodia, P. minor, Plegadis falcinellus, Pl. ridgwayi, Pseudibis papillosa, Theristicus melanopis, Threskiornis aethiopicus, T. melanocephalus, and T. spinicollis.

Systematic paleontology

Class Aves Linnaeus, 1758

Order Pelecaniformes Sharpe, 1891 (sensu Sangster et al. 2022)

Family Threskiornithidae Richmond, 1917

Subfamily Rhynchaeitinae Mayr, 2002

Genus Rhynchaeites Wittich, 1898

Emended diagnosis. (1) Beak long, slender, and schizorhinal, tip slightly decurved, and proximal portion of maxilla dorsoventrally deep; (2) palate schizognathous; (3) at least three thoracic vertebrae co-ossified to form a notarium; (4) unfused thoracic vertebra with large pneumatic openings on the lateral surfaces of its corpus; (5) coracoid with deeply excavated, cup-like cotyla scapularis and short processus acrocoracoideus; (6) sternum with deep incisura medialis and wide trabecula mediana; (7) carpometacarpus with long symphysis metacarpalis distalis; (8) ilium not co-ossified with crista spinosa synsacri; (9) tarsometatarsus shorter than femur.

Rhynchaeites litoralis, sp. nov.

Holotype. NMS.Z.2021.40.28 (Fig. 2; partial skeleton including partial skull with mandible, os basihyale, both quadrates, several vertebrae, wing phalanges and carpal bones, furcula, partial left scapula, both coracoids, sternum, proximal end of right humerus, distal end of left humerus, partial left ulna, left femur, distal end of right tibiotarsus, proximal end of left tibiotarsus, proximal and distal ends of both tarsometatarsi, and some pedal phalanges); collected in 1991 by Michael Daniels (original collector’s number WN 91677).

Fig. 2

Holotype of Rhynchaeites litoralis, sp. nov. from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK (NMS.Z.2021.40.28). a Skull; b caudal end of mandible; c os basihyale; d quadrates; e vertebrae; f wing phalanges and carpal bones; g furcula; h partial left scapula; i right coracoid; j left coracoid; k sternum; l proximal end of right humerus; m distal end of left humerus; n distal end of left ulna; o proximal end of left ulna; p left femur; q distal end of right tibiotarsus; r proximal end of left tibiotarsus; s proximal end of left tarsometatarsus; t distal end of right tarsometatarsus; u distal end of right tarsometatarsus; v pedal phalanges. Scale bar equals 10 mm

Differential diagnosis. Somewhat larger than Rhynchaeites messelensis (Tab. 1); os carpi radiale proximodistally narrower and with better defined notch for tendon of musculus ulnometacarpalis ventralis; tarsometatarsus with proportionally wider distal end, less tapering plantar articular surface of trochlea metatarsi III, and trochlea metatarsi IV with more laterally slanting distal margin. Differs from Mopsitta tanta in that crista bicipitalis of humerus does not form a ventral projection.

Table 1 Comparison of some length measurements (in mm) of Rhynchaeites litoralis, sp. nov., Rhynchaeites sp. from Walton-on-the-Naze, and R. messelensis

Etymology. The species epithet is derived from litoralis (Lat.), littoral, in reference to the coastal location of the type locality.

Type locality and horizon. Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation (previously Division A2; Rayner et al. 2009; Aldiss 2012); early Eocene (early Ypresian, 54.6‒55 million years ago; Collinson et al. 2016).

Measurements of the holotype (in mm). Coracoid length, 37.1 (right). Femur length, 49.0 (left). Humerus proximal width, 20.8 (right); distal width, 13.8 (left). Tibiotarsus distal width, 7.8 (right). Tarsometatarsus proximal width, 9.1 (left); distal width, 9.1 (right), 9.2 (left).

Taxonomic remarks. The fossil material from Walton-on-the-Naze in the Daniels collection includes various other specimens that belong to Rhynchaeites (Fig. 3), and some of these are distinctly smaller than the holotype of Rhynchaeites litoralis. In extant Threskiornithidae, female individuals can be considerably smaller than male ones and R. litoralis may also have been sexually dimorphic in size, with the smaller specimens representing females of the species.

Fig. 3

Specimens of Rhynchaeites sp. from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK. a NMS.Z.2021.40.29 (left and partial right coracoid, partial scapulae, furcula, cranial portion of sternum, wing phalanges and carpal bones, proximal end of left humerus, distal end of right humerus, left carpometacarpus, proximal end of right carpometacarpus, praeacetabular portion of right ilium, distal end of left tibiotarsus, proximal and distal ends of left tarsometatarsus). b NMS.Z.2021.40.31 (left coracoid, cranial extremity of left scapula, left humerus lacking distal end). c NMS.Z.2021.40.30 (ossified cartilago cricoidea, right pterygoid, several vertebrae, left coracoid, omal extremity of right coracoid, partial furcula, distal end of left humerus, distal end of right ulna, pedal phalanges). d NMS.Z.2021.40.32 (omal extremity of right coracoid, proximal and distal portions of left humerus, proximal and distal portions of right ulna, distal end of left ulna, carpal bones, proximal end of left femur, distal end of left tarsometatarsus, pedal phalanges). Scale bars equal 10 mm

However, the holotype of R. litoralis also differs from other Rhynchaeites specimens from Walton-on-the-Naze, in which a humerus is preserved, in that the crista bicipitalis does not form a ventral projection. Some differences in the shape of the coracoid (see below) may also indicate the presence of more than one species of Rhynchaeites in the material from Walton-on-the-Naze. Therefore, we prefer to only refer the holotype to the new species and classify all other specimens as Rhynchaeites sp.

Superficially, Rhynchaeites litoralis resembles Paraortygoides radagasti from the London Clay of Walton-on-the-Naze, which was described as a stem group galliform by Dyke and Gulas (2002). P. radagasti is only known from the fragmentary bones preserved in the holotype, and these differ from Rhynchaeites in, e.g., the presence of large pleurocoels on the corpus of the thoracic vertebrae (in Rhynchaeites there are large pneumatic openings; see Mayr 2021a for the difference between these structures), the less well-delimited tuberculum coracoideum of the scapula, the narrower processus extensorius of the carpometacarpus, the narrower crus longum of the os carpi ulnare, and the further distally reaching trochlea metatarsi II.

Zoobank registration. The new species is registered under urn:lsid:zoobank.org:act:E48FD2B7-B004-463D-A034-77EC49BE33D.

Description and comparisons. The skeletal morphology of Rhynchaeites was described in detail by previous authors (Hoch 1980; Peters 1983; Mayr 2002; Mayr and Bertelli 2011), and in the following, we focus on previously unknown osteological features. Most importantly, NMS.Z.2021.40.28 includes a partial, three-dimensionally preserved skull (Fig. 4). Although a large part of the beak is missing, the remaining portion indicates that it was schizorhinal as in extant ibises, with the slit-like caudal margins of the long nostrils extending well beyond the nasofrontal hinge (ibises are the only representatives of the Aequornithes with schizorhinal beaks). A well-delimited ovate fossa on the dorsal skull surface in the area of the nasofrontal hinge (Fig. 4c) is likely to be a pathological feature. There is a large fenestrated fossa on the ventral surface of the maxillary bone; this fossa is less extensive in extant ibises. The caudal sections of the maxillaries are dorsoventrally deep as they are in crown group Threskiornithidae (Peters 1983), but unlike in extant ibises, the opposing bars formed by the maxillaries are widely separated. This indicates that Rhynchaeites had a schizognathous palate; by contrast, in extant Threskiornithidae the palate is desmognathous, that is, the processus maxillopalatini are co-ossified and the ventral surface of the upper beak forms an osseous surface (Peters (1983) assumed that the palate of R. messelensis is also desmognathous, but the corresponding part of the skull is not well exposed in any of the specimens from Messel). The left palatine (os palatinum) is broken and displaced in the skull of the holotype; its shape appears to have been similar to that of crown group Threskiornithidae, even though the poor preservation prevents the recognition of osteological details. The lacrimal is well-developed and has a long descending process, which appears to have reached the jugal bar; in extant Threskiornithidae, the development of the lacrimal is variable, with the bone having a long descending process in Threskiornis, whereas this process is shorter in Lophotibis, Eudocimus, Bostrychia, and Theristicus. The interorbital section of the frontals is wide. Even though the neurocranium is somewhat crushed, its caudal portion appears to have been proportionally shorter than in crown group Threskiornithidae; an equally short neurocranium is also found in the Messel specimens of R. messelensis (Fig. 1a). The skull has an unusually angled caudal portion, which results in a sharp kink in the caudodorsal portion of the orbital rim (Fig. 4a). Not least because the caudodorsal portion of the neurocranium exhibits a transverse breakage line, it is obvious to interpret this unusual morphology as a taphonomic artefact, but we note that a similar shape also occurs in multiple Rhynchaeites specimens from Messel, which fossilized under different diagenetic conditions. The shapes of the short processus postorbitalis and that of the processus zygomaticus are similar to the corresponding processes of crown group Threskiornithidae, but both show rather unspecific morphologies. The temporal fossae are weakly developed. Basipterygoid processes cannot be discerned, but the basicranial area of the skull is not well preserved and the presence of a facies articularis basipterygoidea on the pterygoid (see below) suggests that at least very low basipterygoid processes were present. The otic region resembles that of extant Threskiornithidae. As in the latter, the recessus tympanicus dorsalis is situated between the cotyla quadratica squamosi and the cotyla quadratica otica. A well-developed pila otica is absent. Furthermore, as in extant Threskiornithidae, but unlike in many other taxa of the Aequornithes (Mayr 2020), there is a recessus columellae, which contains the fenestrae vestibuli et cochleae. The foramen n. maxillomandibularis is proportionally smaller than in extant ibises. The processus paroccipitales are not as pronounced as in crown group Threskiornithidae. The foramen nervi vagi is clearly visible on the right side of the skull, as is the foramen n. glossopharyngealis (Fig. 4h). Unlike in crown group Threskiornithidae, there are no fonticuli occipitales on the caudal surface of the neurocranium. However, the foramen for the arteria ophthalmica externa, also on the caudal surface of the neurocranium, is well developed.

Fig. 4

Rhynchaeites litoralis, sp. nov. from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK, skull of the holotype (NMS.Z.2021.40.28; a, c, g, h) in comparison to that of extant Threskiornithidae (b, d, i). a, c, g, h R. litoralis, skull in lateral (a), dorsal (c), and ventral (g, h) view (photos in g and h were taken under different lighting conditions). b, d, i Skull of the extant Theristicus melanopis (SMF 6126) in lateral (b), dorsal (d), and ventral (i) view. e R. litoralis, right side of the neurocranium in rostrolateral view; the arrow denotes an enlarged detail of the otic region. f R. litoralis, detail of the right lacrimal. j R. litoralis, skull in caudal view. k R. litoralis, detail of the left otic region. aoe foramen for arteria ophthalmica externa, coc condylus occipitalis, cqo cotyla quadratica otica, cqs cotyla quadratic squamosi, fco fenestra cochleae, fmg foramen magnum, fng foramen n. glossopharyngealis, fnv foramen n. vagi, fos fenestrated fossa on ventral surface of maxillary, fto fonticulus occipitalis, fve fenestra vestibuli, lac lacrimal, lbr lateral bars of upper beak, mdb mandible, max, maxillary, mxm foramen n. maxillomandibularis, mxp processus maxillopalatinus, pal palatine, ppa processus paroccipitalis, ppo processus postorbitalis, psm processus suprameaticus, ptf possibly pathologic fossa, pzg processus zygomaticus, rcc recessus columellae, rtd recessus tympanicus dorsalis. Scale bars equal 10 mm

The quadrate (Fig. 5a‒h) corresponds well to that of crown group Threskiornithidae (Fig. 5i‒l) in its shape. As in the latter, the processus orbitalis is long and dorsoventrally deep. The shape of the processus oticus also closely resembles that of extant Threskiornithidae, with its tip having a roof-shaped outline; the capitulum oticum and capitulum squamosum are closely adjacent and separated by a very narrow incisura intercapitularis. Pneumatic foramina are absent. As in extant Threskiornithidae, the condylus medialis has a concave ventrolateral surface, with this concavity being dorsally bordered by a lip-like rim.

Fig. 5

Cranial bones and mandible of Rhynchaeites from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK. a‒d Left quadrate of R. litoralis (holotype; NMS.Z.2021.40.28) in lateral (a), medial (b), caudal (c), and ventral (d) view. e‒h Right quadrate of R. litoralis (NMS.Z.2021.40.28) in ventral (e), lateral (f), medial (g), and caudal (h) view. i‒l Right quadrate of the extant Geronticus eremita (SMF 13341) in lateral (i), medial (j), caudal (k), and ventral (l) view. m‒o Right pterygoid of Rhynchaeites sp. (NMS.Z.2021.40.30) in lateral (m), dorsal (n), and ventral (o) view. p, q Right pterygoid of Eudocimus ruber (SMF 4733) in dorsal (p) and ventral (q) view. r, s Right pterygoid of Rostratula benghalensis (Rostratulidae, Charadriiformes; SMF 13408) in lateral (r) and ventral (s) view. t‒v Caudal end of right ramus mandibulae of R. litoralis (NMS.Z.2021.40.28) in dorsal (t), ventral (u), and lateral (v) view. x‒cc, Caudal end of right ramus mandibulae (dorsal view) of x, y Bostrychia hagedash (SMF 13644), z, aa G. eremita (SMF 13341), and bb, cc Platalea ajaja (SMF 19605) in dorsal (x, z, bb) and lateral (y, aa, cc) view. dd, ee Os basihyale of R. litoralis (NMS.Z.2021.40.28) in dorsal (dd) and right lateral (ee) view. ff, gg Os basihyale of the extant Theristicus melanopis (SMF 6126) in dorsal (ff) and left lateral (gg) view. hh Ossified cartilago cricoidea of Rhynchaeites sp. (NMS.Z.2021.40.30). ii, jj Cartilago cricoidea of ii T. melanopis (SMF 6126) and jj G. eremita (SMF 21541). arf concave ventrolateral articulation facet of condylus medialis, cdl condylus lateralis, cdm condylus medialis, cdp condylus pterygoideus, cpo capitulum oticum, cps capitulum squamosum, dpr dorsal projection, fab facies articularis basipterygoidea, orb processus orbitalis, pmd processus medialis, pra processus retroarticularis, rhk hook-shaped dorsal projection forms by retroarticular process, ros rostral. Scale bars equal 10 mm; same scale for a‒h

Even though only the caudal portion of the mandible is preserved in NMS.Z.2021.40.28 (Fig. 4g), the remaining parts show that, as in extant ibises, the mandibular rami ran in parallel and were closely adjacent. The articular end bears a long processus retroarticularis (Fig. 5t‒v). Among extant Threskiornithidae, equally long retroarticular processes occur in Bostrychia (Fig. 5x, y), whereas these processes are shorter in other taxa (Plegadis, Theristicus, Geronticus, Eudocimus; Fig. 5z‒cc); in all studied crown group Threskiornithidae except Bostrychia hagedash and B. olivacea the retroarticular process has a distinct hook-like shape in lateral view (Fig. 5aa, cc), whereas it is straight in Rhynchaeites (Fig. 5v). The processus medialis is shorter than in extant ibises and the caudolateral portion of the articular end is less protruding.

Unlike in crown group Threskiornithidae, the basihyal of the hyoid apparatus (Fig. 5dd, ee) is not widened. The ossicle is furthermore dorsoventrally deeper than in extant ibises.

NMS.Z.2021.40.28 includes eight praesacral vertebrae, six of which are largely intact. The fourth cervical vertebra (Fig. 6a) has a pathological morphology in that the left zygapophysis cranialis exhibits a callosity; as in crown group Threskiornithidae, it bears a pair of lateral foramina. The fifth cervical vertebra (Fig. 6b) has a long, ridge-like processus spinosus. Another, fairly long cervical vertebra (Fig. 6c) is presumably from the series of the sixth to ninth praesacral vertebrae; a more caudal cervical vertebra (Fig. 6d) is identified as being from the series of the 12th to 15th praesacral vertebrae. Unlike in crown group Threskiornithidae (Fig. 6k), the thoracic vertebra (Fig. 6f) has large pneumatic openings on the lateral surfaces of its corpus, which within extant Aequornithes only occur in the procellariiform Diomedeidae and in the Sulidae, Balaenicipitidae, and Scopidae (Mayr 2021a).

Fig. 6

Vertebrae of Rhynchaeites from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK, in comparison to those of extant Threskiornithidae. a‒f Six praesacral vertebrae of R. litoralis (holotype; NMS.Z.2021.40.28), which are identified as a the fourth praesacral vertebra, b the fifth praesacral vertebra, c the 6th‒9th praesacral vertebra, d the 11th‒13th praesacral vertebra, e the ?16th praesacral vertebra, and f a thoracic vertebra. g‒j Vertebrae of the extant Geronticus eremita (SMF 21541): g fourth vertebra, h fifth vertebra, i sixth vertebra, j 11th vertebra. k Thoracic vertebra of Theristicus melanopis (SMF 6126). l Five caudal vertebrae of R. litoralis (NMS.Z.2021.40.30). m Caudal vertebra of G. eremita (SMF 21541). n Pygostyle of Rhynchaeites sp. (NMS.Z.2021.40.30). o Pygostyle of G. eremita (SMF 21541). cal callosity, for foramen, pnf pneumatic foramen, psp processus spinosus, pvt processus ventralis, zcd zygapophysis caudalis. Scale bars equal 10 mm; same scale for a‒f and g‒j

The coracoid of Rhynchaeites (Fig. 7a‒h) is very different from that of crown group Threskiornithidae (Fig. 7j). The extremitas omalis is proportionally smaller (in Rhynchaeites it measures about a quarter of the length of the coracoid, whereas it is one third of the coracoid length in extant ibises) and the processus acrocoracoideus has a hooked outline, owing to the fact that the facies articularis clavicularis overhangs the sulcus supracoracoideus; with regard to the proportions of the omal extremity, the early Miocene Gerandibis (De Pietri 2013: Fig. 2a) is intermediate between Rhynchaeites (Fig. 7i) and extant Threskiornithidae. Unlike in extant ibises, the bone lacks a foramen nervi supracoracoidei and there is a deeply excavated, subcircular cotyla scapularis rather than a flat facies articularis scapularis (a cup-shaped cotyla is plesiomorphic for neornithine birds; Mayr 2021b). The processus procoracoideus is narrower in the sterno-omal direction than that of extant ibises, and unlike in extant Threskiornithidae there is no crista procoracoidei. The sternal end of the bone is distinguished from that of extant ibises in that the processus lateralis is shorter and in that the facies interna of the crista articularis sternalis is distinct. As in extant ibises, there are muscular striae on the dorsal surface of the sternal end.

Fig. 7

Coracoid and furcula of Rhynchaeites from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK. a, b partial left (a) and right (b) coracoid of the holotype of Rhynchaeites litoralis, sp. nov. (NMS.Z.2021.40.28) in dorsal view. c, d Left coracoid of Rhynchaeites sp. (NMS.Z.2021.40.30) in dorsal (c) and ventral (d) view. e, f Left coracoid of Rhynchaeites sp. (NMS.Z.2021.40.31) in dorsal (e) and ventral (f) view. g Partial right coracoid of Rhynchaeites sp. (NMS.Z.2021.40.32) in dorsal view. h Left coracoid of NMS.Z.2021.40.29 in dorsal view. i Left coracoid (dorsal view) of R. messelensis from Messel, Germany (SMF-ME 3577); the fossil was coated with ammonium chloride. j Left coracoid (dorsal view) of the extant Theristicus melanopis (SMF 6126) in dorsal view. k, l Furcula of Rhynchaeites sp. (NMS.Z.2021.40.29) in caudal (k) and cranial (l) view. m Furcula of T. melanopis (SMF 6126) in cranial view. acr processus acrocoracoideus, cpc crista procoracoidei, csc cotyla scapularis, fai facies interna of crista articularis sternalis, fns foramen nervi supracoracoidei, ppc processus procoracoideus. Scale bars equal 10 mm

The scapula (Fig. 2h) has a narrow and tapering acromion and a well-developed, hemispherical tuberculum coracoideum. The extremitas sternalis of the U-shaped furcula (Figs. 2g, 7k, l) is dorsoventrally wider than that of extant ibises (Fig. 7m). The extremitas omalis bears a moderately long processus acromialis.

The sternum (Fig. 8) closely resembles that of R. messelensis, which was described by Mayr (2002). The spina externa is blade-like and forms marked dorsal and ventral convexities, whereas the spina externa of extant ibises is dorsoventrally narrow. The sulci articulares coracoidei do not overlap. There are five processes costales. The short and broad processus craniolateralis has a straight tip. The carina sterni is deep and its apex broadly rounded. The incisurae in the caudal portion of the bone are deeper than in extant ibises and the incisura medialis is proportionally wider.

Fig. 8

Sternum of Rhynchaeites litoralis, sp. nov. from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK (holotype, NMS.Z.2021.40.28). a, b Sternum in lateral view, in b the surrounding matrix was digitally removed. c Cranial portion of sternum in craniodorsal view. d Sternum of the extant Theristicus melanopis (SMF 6126) in lateral view. icl incisura lateralis, icm incisura medialis, pcl processus craniolateralis, pcs processus costales, spe spina externa. Scale bars equal 10 mm

Only the proximal and distal ends of the humerus are preserved (Fig. 9a‒d). As in extant ibises, the tuberculum dorsale on the proximal end of the bone is very large. However, contrary to R. messelensis and Mopsitta tanta (Mayr and Bertelli 2011), the crista bicipitalis does not form a ventral projection (Fig. 9). The distal end resembles the distal humerus of extant ibises, even though the processus flexorius is somewhat more ventrally projected. The sulcus scapulotricipitalis is wide and shallow. The distal humerus of Rhynchaeites is clearly distinguished from that of Proplegadis fisheri from the London Clay of the Isle of Sheppey (Fig. 9e), which was described as an ibis by Harrison and Walker (1971), but probably belongs to the phaethontiform Prophaethontidae (Mayr 2015a, 2022), in that the ventrodistal portion is more ventrally prominent and the fossa musculi brachialis less extensive and shallower.

Fig. 9

Humeri of Rhynchaeites from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK, in comparison to the humeri of Mopsitta tanta Waterhouse et al., 2008 and Proplegadis fisheri Harrison and Walker, 1971. a‒d Proximal (a, b) and distal (c, d) ends of right (a, b) and left (c, d) humerus of R. litoralis (holotype, NMS.Z.2021.40.28) in caudal (a, c) and cranial (b, d) view. e Distal left humerus of P. fisheri from the London Clay of the Isle of Sheppey (holotype, NHMUK A 10) in cranial view. f, g Right humerus of M. tanta from the early Eocene Fur Formation (holotype, FU 110⁄139) in caudal (f) and cranial (g) view. h, i Left humerus of Rhynchaeites sp. (NMS.Z.2021.40.31) in cranial (h) and caudal (i) view. flx processus flexorius, prj ventral projection formed by crista bicipitalis, tbd tuberculum dorsale. Scale bars equal 10 mm

The proximal end of the ulna (Fig. 10a) has a large processus cotylaris dorsalis and closely resembles the proximal ulna of extant ibises. The distal end of the bone likewise corresponds well to the distal ulna of crown group Threskiornithidae.

Fig. 10

Selected postcranial bones of Rhynchaeites from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK, in comparison to extant Threskiornithidae and R. messelensis. a, b Left ulna of R. litoralis (a; holotype, NMS.Z.2021.40.28) and extant Theristicus melanopis (b; SMF 6126) in cranial view. c, d Left carpometacarpus of Rhynchaeites sp. (c; NMS.Z.2021.40.29) and T. melanopis (d; SMF 6126) in ventral view. e‒g Left os carpi radiale (cranial view) of R. litoralis (e; holotype, NMS.Z.2021.40.28) and R. messelensis from Messel, Germany (f, g; SMF-ME 3577, coated with ammonium chloride); in g the surrounding matrix was digitally removed. h, i Left femur of R. litoralis (h; holotype, NMS.Z.2021.40.28) and T. melanopis (i; SMF 6126) in caudal view. j, k Distal end of right tibiotarsus of R. litoralis (j; holotype, NMS.Z.2021.40.28) and T. melanopis (k; SMF 6126) in cranial view. ctr crista trochanteris, pcd processus cotylaris dorsalis, pst pons supratendineus, smd symphysis metacarpalis distalis, tmg tuberculum musculi gastrocnemialis, umv notch for tendon of musculus ulnometacarpalis ventralis. Scale bars equal 10 mm

The os carpi radiale is differently shaped than in R. messelensis, in which it is proximodistally wider and has a less well defined notch for the tendon of musculus ulnometacarpalis ventralis (Fig. 10e‒g).

The femur is not well preserved in the Messel specimens of Rhynchaeites messelensis. The bone is complete in NMS.Z.2021.40.28 and has a prominent crista trochanteris (Fig. 10h). On the distal end of the bone, there is a marked tuberculum musculi gastrocnemialis. With regard to these features, it agrees with the femur of extant ibises, but the resemblances are not diagnostic and similarly shaped femora are also found in other avian groups, such as the gruiform Aramidae (limpkins).

As in Rhynchaeites messelensis and crown group Threskiornithidae, the tibiotarsus has widely spaced condyli and a medially situated sulcus extensorius. The pons supratendineus is proximodistally shorter than in extant ibises (Fig. 10j, k).

The proximal and distal ends of both tarsometatarsi are preserved (Fig. 11a‒f, i‒k). The hypotarsus (Fig. 11k) forms a sulcus for the tendon of musculus flexor digitorum longus. Lateral to this sulcus, there is a platform that bears two shallow sulci, presumably for the tendons of musculus flexor hallucis longus and m. flexor perforatus digiti 2 (Mayr 2015b). Extant Threskiornithidae show two hypotarsal morphologies, with two hypotarsal crests encompassing a deep sulcus for all flexor tendons in Plegadis and Eudocimus (Fig. 11m) and the sulcus for m. flexor hallucis longus being well separated from that for m. flexor digitorum longus in other taxa (Fig. 11l). The latter condition is likely to be plesiomorphic and also occurs in the early Miocene Gerandibis pagana (De Pietri 2013). The foramen vasculare distale on the distal end of the bone is smaller than in Gerandibis and crown group Threskiornithidae; unlike in some extant ibises (e.g., Plegadis, Eudocimus; Olson 1981), the canalis interosseus distalis does not open distal to the dorsal opening of the foramen vasculare distale. The trochleae metatarsorum II et IV are proportionally shorter than in extant Threskiornithidae (Fig. 11g, h). The trochlea metatarsi II is less cylindrical than in extant Threskiornithidae, with a more convex dorsal surface. The plantar articular surface of the trochlea metatarsi III has an asymmetric shape, with a proximodistally longer lateral portion. The trochlea metatarsi IV closely resembles that of extant ibises. Compared with Rhynchaeites messelensis (Fig. 11n), the distal end of the bone is proportionally wider, the trochlea metatarsi III has a less tapering plantar articular surface, and the distal margin of the trochlea metatarsi IV is more laterally slanting.

Fig. 11

Tarsometatarsus and pedal phalanges of Rhynchaeites from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK. a‒f Distal ends of the left (a‒c) and right (d‒f) tarsometatarsus of the holotype of R. litoralis (holotype; NMS.Z.2021.40.28) in dorsal (a, d), plantar (b, e), and distal (c, f) view. g, h Distal ends of left tarsometatarsi of the extant Threskiornis melanocephalus to illustrate size differences between male (g; SMF 18572) and female (h; SMF 18537) individuals. i‒k Proximal ends of the right (i) and left (j, k) tarsometatarsus of the holotype of R. litoralis (NMS.Z.2021.40.28) in plantar (i, j) and proximal (k) view. l, m Proximal end of the left tarsometatarsus of Theristicus melanopis (l, SMF 6126) and Eudocimus ruber (m; SMF 4733). n Tarsometatarsus of Rhynchaeites messelensis in plantar view (SMF-ME 3577; right side, mirrored); the fossil was coated with ammonium chloride. o Pedal phalanges of Rhynchaeites sp. (NMS.Z.2021.40.32); numerals indicate tentative assignment to toes. p Os metatarsale I and two ungual phalanges of Rhynchaeites sp. (NMS.Z.2021.40.30). q Feet of Rhynchaeites sp. from the Fur Formation in Denmark (MGUH 20288). cal pathological callosity, fdl sulcus for tendon of musculus flexor digitorum longus, fhl sulcus for tendon of m. flexor hallucis longus, fp2 sulcus for tendon of m. flexor perforatus digiti 2, fvd foramen vasculare distale, mtI os metatarsale I. Scale bars equal 10 mm

Rhynchaeites sp.

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.2021.40.29 (Fig. 3a; partial skeleton including left and partial right coracoid, partial scapulae, furcula, cranial portion of sternum, proximal end of left humerus, distal end of right humerus, left carpometacarpus, proximal end of right carpometacarpus, praeacetabular portion of right ilium, distal end of left tibiotarsus, proximal and distal ends of left tarsometatarsus), collected in 1979 by M. Daniels (original collector’s number WN 79241); NMS.Z.2021.40.30 (Fig. 3c; partial skeleton including right pterygoid, partial furcula, left coracoid, omal extremity of right coracoid, distal end of left humerus, distal end of right ulna), collected in 1984 by M. Daniels (original collector’s number WN 84493); NMS.Z.2021.40.31 (Fig. 3b; left coracoid, cranial extremity of left scapula, left humerus lacking distal end), collected in 1986 by M. Daniels (original collector’s number WN 86516); NMS.Z.2021.40.32 (Fig. 3d; omal extremity of right coracoid, proximal and distal portions of left humerus, proximal and distal portions of right ulna, distal end of left ulna, proximal end of left femur, distal end of left tarsometatarsus, and several pedal phalanges), collected in 1988 by M. Daniels (original collector’s number WN 88595).

Measurements (in mm). NMS.Z.2021.40.29: carpometacarpus length, 40.8. NMS.Z.2021.40.30: coracoid length, 36.0, humerus distal width, 13.0. NMS.Z.2021.40.31: coracoid length, 35.7.

Description and comparisons. The material includes some skeletal elements not preserved in the holotype. Other bones are distinguished from those of the holotype in osteological features, and the following description focuses on previously unknown osteological feature of Rhynchaeites and differences between the above specimens and the holotype of R. litoralis.

The pterygoid (Fig. 5m‒o) is preserved in NMS.Z.2021.40.30. The bone markedly widens rostrally, whereas the pterygoid of extant Threskiornithidae is more rod-shaped and has a narrower rostral portion (Fig. 5p, q). On the caudal end, there is a dorsal projection, which is absent in extant ibises, but occurs in some Charadriiformes (e.g., Rostratula benghalensis, Fig. 5r, s). Furthermore, unlike in extant Threskiornithidae, the pterygoid of Rhynchaeites exhibits a facies articularis basipterygoidea.

Specimen NMS.Z.2021.40.30 also includes the ossified cartilago cricoidea of the larynx, which has an aliform shape, owing to the fact that the caudal margin forms a deep caudal incision with a pair of central clefts (Fig. 5hh). The shape of this bony sheet is variable in extant ibises, being aliform in some taxa (e.g., Geronticus; Fig. 5jj) and with a straight caudal margin in others (e.g., Theristicus; Fig. 5ii).

Five caudal vertebrae and the pygostyle are preserved in NMS.Z.2021.40.30 (Fig. 6l, n). Two of the caudal vertebrae have well-developed zygapophysis caudales, which do not occur in extant ibises (Fig. 6m). The pygostyle is small; as in extant Threskiornithidae (Fig. 6o), it is co-ossified with the caudalmost caudal vertebra, which has a long processus ventralis.

The coracoids show some variation with regard to the width of the processus procoracoideus, which is narrower in NMS.Z.2021.40.30 than in NMS.Z.2021.40.32 (Fig. 7). These differences may be due to individual variation, but possibly they indicate that the material involves more than one species (unfortunately, the processus procoracoideus is not preserved in the holotype of R. litoralis).

The humerus is not completely preserved in any of the specimens. The bone is most complete in NMS.Z.2021.40.31 and corresponds well to the humerus of Mopsitta tanta from the Danish Fur Formation (Fig. 9f‒i). Unlike in the holotype of R. litoralis, the crista bicipitalis forms a ventral projection (Fig. 9).

A complete carpometacarpus is preserved in NMS.Z.2021.40.30 (Fig. 10c). As in R. messelensis, but unlike in crown group Threskiornithidae, the bone is craniocaudally narrow and has a very long distal symphysis.

Of the pelvis, the praeacetabular portion of the right ilium is present in NMS.Z.2021.40.29 (Fig. 3a). Unlike in extant Threskiornithidae, it appears to have not been co-ossified with the synsacrum (in extant ibises, the crista iliaca dorsalis is furthermore tightly fused with the crista spinosa synsacri).

The os metatarsale I (Fig. 11p) has a long processus articularis tarsometatarsalis. The complete set of non-ungual phalanges is preserved in NMS.Z.2021.40.32 (Fig. 11o) and the length proportions of the phalanges correspond to those of R. messelensis and a Rhynchaeites specimen from the Danish Fur Formation (Fig. 11q). The digits are proportionally shorter and stouter than those of extant ibises. The ungual phalanges are very short, weakly curved, and have a low tuberculum flexorium.

Results of the phylogenetic analysis

Our analysis of the character matrix in the Supplementary Information, using Galliformes as an outgroup taxon, resulted in two most parsimonious trees (Fig. S1a; L = 257; CI = 0.43; RI = 0.63). The analysis did not support threskiornithid affinities of Rhynchaeites, but recovered it as the sister taxon of a clade including the Phaethontiformes, Procellariiformes, Sphenisciformes, and Gaviiformes. This placement did not receive bootstrap support, but concerning the extant taxa, the analysis did not recover various clades that are well-supported in current molecular analyses (e.g., Prum et al. 2015; Kuhl et al. 2021).

Because two critical derived characters shared by Rhynchaeites and crown group Threskiornithidae, retroarticular processes and a notarium, are also present in the Galliformes, the outgroup taxon used in the primary analysis, we ran a second analyses, in which Galliformes were excluded and Phaethontiformes were specified as the outgroup taxon. This analysis resulted in 14 most parsimonious trees (Fig. S1b; L = 245; CI = 0.46; RI = 0.66). In the consensus tree, Rhynchaeites and crown group Threskiornithidae were recovered in a basal polytomy. In four of the individual trees, a sister group relationship between Rhynchaeites and crown group Threskiornithidae was obtained, with Rhynchaeites being placed in variable positions in the remaining trees. However, most nodes did not receive support in the bootstrap analysis.

Discussion

Rhynchaeites is clearly distinguished from crown group Threskiornithidae in a number of presumably plesiomorphic aspects, including a much shorter tarsometatarsus and a different morphology of the coracoid, but most other bones correspond well to those of extant ibises in overall morphology. Still, a sister group relationship between Rhynchaeites and crown group Threskiornithidae was obtained in only a small subset of the trees resulting from our analysis, and the character evidence for threskiornithid affinities of Rhynchaeites is far from strong.

Derived features shared with extant Threskiornithidae include (1) a long, decurved, and schizorhinal bill, (2) a dorsoventrally deep caudal portion of the maxillaries, (3) a mandible with a long processus retroarticularis, (4) a humerus with a large tuberculum dorsale, and (5) a notarium (co-ossified thoracic vertebrae; Peters 1983). Characters (1), (2), and (5) were listed as evidence for threskiornithid affinities of Rhynchaeites by Peters (1983). However, all three also occur in the gruiform Aramidae (limpkins), from which Rhynchaeites is (among others) distinguished in the absence of a pila otica with a trabeculate surface (otic region, see Mayr 2020), the presence of long retroarticular processes on the mandible, and the morphologies of the distal end of the tibiotarsus and the hypotarsus (Mayr 2015b). Furthermore, a dorsoventrally deep caudal portion of the maxillaries is found in most extant taxa of the Aequornithes except for the Gaviiformes. On the other hand, we could not identify osteological features that argue for a better supported alternative classification of Rhynchaeites, so that we maintain its current classification.

If Rhynchaeites is a stem group representative of the Threskiornithidae, its plesiomorphic skeletal morphology indicates a great amount of homoplasy in the evolution of the Aequornithes, with characteristic derived features having evolved multiple times independently. This is exemplified by the palate, which is desmognathous in extant ibises, that is, the lateral bars of the upper beak are co-ossified by fusion of the processus maxillopalatini. Peters (1983) considered the palate of Rhynchaeites to be desmognathous, but the skull of the holotype of Rhynchaeites litoralis, sp. nov. shows the maxillaries to be widely separated, so that the palate is schizognathous. Amongst extant representatives of the Aequornithes, a schizognathous palate occurs in the Gaviiformes, Sphenisciformes, and Procellariiformes, whereas all other extant taxa of the Aequornithes (that is, the Pelecanimorphae sensu Sangster et al. 2022) have a desmognathous palate. A schizognathous palate is also found in most early diverging neognathous birds, and its occurrence in Rhynchaeites—as an early stem group representative of ibises—suggests that it is plesiomorphic for the Threskiornithidae. With Rhynchaeites included as a stem group representative of the Threskiornithidae, our ancestral state reconstruction indicates a fourfold origin of a desmognathous palate in the Pelecanimorphae, in: (1) the Ciconiiformes, (2) the Suliformes, (3) crown group Threskiornithidae, and (4) the clade including the Ardeidae, Scopidae, Balaenicipitidae, and Pelecanidae (Fig. 12a).

Fig. 12

Ancestral state reconstructions for some morphological traits of the Aequornithes under the assumption that Rhynchaeites is a stem group representative of the Threskiornithidae: a desmognathous palate, b long legs, c foramen nervi supracoracoidei (coracoid), and d wide basihyal bone (hyoid apparatus); concerning the extant taxa, the tree topology is based on the analyses of Prum et al. (2015) and Kuhl et al. (2021)

The differences in palatal morphology suggest disparate feeding ecologies of Rhynchaeites and crown group ibises. The beak of extant ibises has a densely pitted tip (Fig. 1c), which houses mechanoreceptors and enables the birds to use their beaks for “remote sensing” of prey hidden in substrate (Cunningham et al. 2010). The bill tip of Rhynchaeites lacks this pitted surface (Fig. 1b; Mayr 2002). Extant ibises are tactile probers (Cunningham et al. 2010; Martin and Portugal 2011) and, because of the absence of a pitted bill tip, we hypothesize that vision played a greater role in the foraging strategy of Rhynchaeites (an extant species with a beak without a pitted tip similar to that of Rhynchaeites is the Ibisbill [Ibidorhynchidae], which pecks insects from the surface of slow-moving mountain rivers or probes for aquatic invertebrates and fishes; Knystautas 1996). Different feeding strategies to extant ibises are also suggested by the plesiomorphic shape of the basihyal bone of the hyoid apparatus of Rhynchaeites.

The tarsometatarsus of Rhynchaeites (Fig. 11n) is much shorter than that of extant ibises. If the fossil taxon is a stem group representative of the Threskiornithidae, a long and decurved beak originated in the evolution of ibises before lengthening of the legs occurred. As detailed by Peters (1983), the short tarsometatarsus and toes indicate that Rhynchaeites had galliform- or columbiform-like locomotory characteristics and was more terrestrial than most extant Threskiornithidae. The long legs and toes of extant ibises are best interpreted as adaptations for foraging in aquatic environments, that is, shallow shores and wetlands with soft or swampy ground (even though, as noted in the introduction, some species occur in arid areas). The divergent hindlimb proportions of Rhynchaeites and extant ibises as well as the lack of sensory pits on the tip of the beak of Rhynchaeites indicate disparate feeding ecologies. Ancestral state reconstruction suggests that long legs evolved five times in the Aequornithes: in the Ciconiidae, in crown group Threskiornithidae, and in the Ardeidae, Scopidae, and Balaenicipitidae (Fig. 12b). Considerable homoplasy also exists concerning other skeletal features, such as the foramen nervi supracoracoidei of the coracoid (Fig. 12c) and the basihyal bone of the hyoid apparatus (Fig. 12d).

Rhynchaeites also differs from extant Threskiornithidae in the morphology of the sternum. For the depth of the incisions in the caudal margin of the sternum, extant Charadriiformes suggest that a functional correlation with the length of the legs may exist, because short-legged species of the Scolopacidae (e.g., Scolopax spp.) and Thinocoridae have deep incisions resembling those of Rhynchaeites, whereas the sternum of long-legged taxa (e.g., Recurvirostridae) is more similar to that of extant Threskiornithidae and has shallower incisions in the caudal margin. Functional constraints may be due to different centers of gravity and locomotory demands, but in the absence of studies exploring possible correlations between the morphologies of the pectoral girdle and hindlimb elements, any evolutionary considerations remain speculative.

If the short-legged, terrestrial habitus of Rhynchaeites is plesiomorphic for the Aequornithes, an aquatic ecomorphology evolved four times independently in aequornithine birds, in the Gaviiformes, the clade Procellariiformes + Sphenisciformes, the Suliformes, and the Pelecanidae (see also Mayr and Kitchener 2022). The stem species of these taxa probably had a very different morphology to their highly specialized extant members, and the fact that aquatic stem group representatives of the Gaviiformes and Sphenisciformes already occurred in the early Eocene (Gaviiformes; Mayr and Kitchener 2022) and mid-Paleocene (Sphenisciformes; Mayr 2022) suggests that the Aequornithes had probably already diverged in the Late Cretaceous.