Sororavis solitarius Mayr & Kitchener 2023, sp. nov.
Creators: name: Mayr, Gerald
name: Kitchener, Andrew C.
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Aves
Anseriformes
Anhimidae
Sororavis
Sororavis solitarius

description: Sororavis solitarius sp. nov.
Figs. 1–3.
LSID Zoobank: http://zoobank.org/ urn:lsid:zoobank.org:act: A979CC 23-3409-4CDD-B28E-2FEA52691A86.
Etymology: From Latin solitaries, single, solitary, in reference to the fact that only one specimen of this species has been identified among the numerous birds from Walton-on-the Naze in the Daniels collection.
Holotype: NMS.Z.2021.40.75 (Fig. 1; tip of upper beak; fragments of mandibular rami; both coracoids; partial furcula; partial sternum; right humerus lacking distal end; proximal and distal ends of left humerus; proximal end of right ulna; left tibiotarsus lacking proximal end; proximal end of left tarsometatarsus; right tarsometatarsus; four pedal phalanges), collected in 1986 by M. Daniels (original collector’s number WN 86530).
Type locality: Walton-on-the-Naze, Essex, UK.
Type horizon: Walton Member of the London Clay Formation (previously Division A2; Rayner et al. 2009; Aldiss 2012); lower Eocene lower Ypresian, 54.6‒55 million years old; Collinson et al. 2016).
Diagnosis.—As for genus. The new species is of similar size to Morsoravis sedilis (tarsometatarsus length 14.3 mm vs. ~13.0 mm in M. sedilis; Bertelli et al. 2010) and distinctly larger than Pumiliornis tessellatus (tarsometatarsus length 8.9‒9.9 mm; Mayr 1999).
Measurements (maximum length, in mm).—Right coracoid, 11.0; right humerus, length as preserved, 14.0, estimated total length, ~15‒16; left tibiotarsus, length as preserved, 16.5, estimated total length, ~23‒25; right tarsometatarsus, 14.3.
Description.—The tip of the upper beak (Figs. 1A 1, 2B 1) is narrow and pointed, and its shape indicates similar proportions to the beak of Morsoravis sedilis (Fig. 2A 1) and extant thrushes (Turdidae, Passeriformes). The fragments of the mandibular rami show that the lower jaw was long and dorsoventrally low as it is in M. sedilis.
The coracoid (Figs. 1A 3 ‒A 5, 2B 2) resembles the corresponding bone of the Psittacopedidae (Fig. 2D) and Zygodactylidae (Fig. 2C) as well as that of crown group passerines (Fig. 2E). The extremitas omalis has a hook-like shape and the facies articularis scapularis is shallow. The processus procoracoideus is broken, but judging from the larger portion preserved in the right coracoid, it appears to have been fairly long. A foramen nervi supracoracoidei is absent. The extremitas sternalis is comparatively narrow, with a short processus lateralis. The medial margin bears an incipient notch.
Only a fragment of the extremitas sternalis of the furcula is preserved (Fig. 1A 7), which indicates that the bone was U-shaped and had slender shafts. The apophysis furculae is broken, but appears to have been small.
The specimen includes the cranial portion of the sternum Fig. 1A 8, A 9), which has a long and mediolaterally wide spina externa that is only incipiently bifurcated. The apex carinae shows little cranial projection.
The humerus (Figs. 1A 10 ‒A 15, 2B 3) resembles that of the Psittacopedidae and Zygodactylidae (Fig. 2G, H; Mayr and Kitchener in press). As in the latter, the fossa pneumotricipitalis lacks pneumatic openings, which is likely to be a plesiomorphic characteristic and functionally correlated with the occurrence of pleurocoels on the thoracic vertebrae (Mayr 2021). The tuberculum dorsale is small. The crista deltopectoralis is broken, but the remaining portions indicate that it was proximodistally short. The sulcus scapulotricipitalis is very shallow. The condylus ventralis is more elongated and less globose than in the Psittacopedidae and Zygodactylidae; unlike in the latter two taxa, the dorsal margin of the bone does not form a well-defined tuberculum/processus supracondylaris dorsalis. With regard to other features, such as the size and position of the fossa musculi brachialis and tuberculum supracondylare ventrale as well as the shape of the processus flexorius, the distal end of the bone resembles the distal humerus of psittacopedids and zygodactylids (Fig. 2G, H).
The holotype includes the proximal end of the ulna Fig. 1A 16), which shows a fairly undiagnostic morphology, with an olecranon of moderate size, a rather small cotyla dorsalis, and an average-sized cotyla ventralis with a circular outline.
The tibiotarsus (Figs. 1A 17, A 18, 2B 4), whose proximal end is lacking, is a very long and slender bone, and its shaft is proportionally narrower than in Morsoravis. The distal end of the bone closely resembles the distal tibiotarsus of Morsoravis sedilis (Fig. 2A 2). The pons supratendineus, which is absent in Pumiliornis (Fig. 2F 2), is proximodistally long. Proximal to the condylus dorsalis and lateral to the distal end of the sulcus extensorius, there is an embossment of subtriangular outline. The condylus medialis is somewhat less tall proximodistally and less wide mediolaterally than the condylus lateralis.
The tarsometatarsus (Figs. 1A 19 ‒A 24, 3A 1 ‒A 3) is proportionally longer and narrower than the corresponding bone of Morsoravis sedilis (Fig. 3B) and Pumiliornis tessellatus (Fig. 3C). The shaft is very f lat in dorsoplantar direction. As in P. tessellatus, the proximal margin of the bone is not oriented perpendicular to its longitudinal axis. Furthermore as in M. sedilis and P. tessellatus, the medial portion of the cotyla medialis forms a proximally projecting lip (Fig. 3A 1); medial to it there is a small medial embossment (Fig. 1A 23). The hypotarsus exhibits two sulci for the tendons of musculus flexor digitorum longus and musculus f lexor hallucis longus, respectively. The small foramina vascularia proximalia are on the same level and are widely separated. The large tuberositas musculi tibialis cranialis is situated at the medial margin of the bone and contributes to a medial bulge of the shaft; in P. tessellatus, there is an equally large and medially situated tuberositas musculi tibialis cranialis (this feature is not visible in the M. sedilis holotype, but a large tuberositas musculi tibialis cranialis was present on the proximal end of the right tarsometatarsus; this bone was figured by Kristoffersen 2002, but it broke off subsequently and is now lost). The plantar surface of the shaft is essentially flat and lacks a crista medianoplantaris. The distal end of the bone closely resembles the tarsometatarsus of M. sedilis. The foramen vasculare distale is small and there is no sulcus between this foramen and the incisura intertrochlearis lateralis. The fossa metatarsi I is situated on the plantar surface of the bone, near to its medial margin. In distal view, the trochleae form an arch, with both the trochlea metatarsi II and the trochlea metatarsi IV being plantarly deflected. The trochlea metatarsi II is large and of subcylindrical shape; it is plantarly deflected and bears a marked trochlear furrow that extends onto its medial surface (Fig. 3A 3). In the holotype and only known specimen of M. sedilis this furrow on the medial surface of the trochlea metatarsi II is clearly visible (Fig. 3B 2, the plantar tarsometatarsus surface is not exposed). The distal end of the tarsometatarsus is poorly preserved in the known specimens of Pumiliornis tessellatus, but the trochlea metatarsi II appears to be of similar shape to that of S. solitarius and is much larger than in psittacopedids (Fig. 3D 2). On the dorsal surface of the distal tarsometatarsus of S. solitarius, there is a depression immediately proximal to the trochlea metatarsi III, which is also present in P. tessellatus (the corresponding area of the tarsometatarsus is damaged in the M. sedilis holotype). The trochlea metatarsi III is not mediolaterally widened and has a well-developed trochlear furrow. The trochlea metatarsi IV is plantarly deflected and forms a wing-like flange of similar shape to that in extant Leptosomiformes and Strigiformes, which indicates at least semizygodactyl feet. As in Morsoravis and Pumiliornis, the distal margin of the trochlea metatarsi IV is laterally slanted, forming an angle of about 45° relative to the longitudinal axis of the tarsometatarsus.
The holotype includes four pedal phalanges, which we identify as the first phalanx of the third toe and the second to fourth phalanges of the fourth toe (Fig. 1A 27, A 28). The first phalanx of the third toe has a distinctive morphology in that the trochlea has very long plantar rims. The proximal phalanges of the fourth toe are not greatly abbreviated.
Stratigraphic and geographic range.— Type locality and horizon only.
description: Published as part of Mayr, Gerald & Kitchener, Andrew C., 2023, A new fossil from the London Clay documents the convergent origin of a " mousebird-like " tarsometatarsus in an early Eocene near-passerine bird, pp. 1-11 in Acta Palaeontologica Polonica 68 (1) on pages 4-6, DOI: 10.4202/app.01049.2022, http://zenodo.org/record/10626780