Ypresicolius sandcoleiformis n. gen., n. sp.

urn:lsid:zoobank.org:act: E4D6D9D0-A823-486A-B368-FC5415D24C94

TYPE MATERIAL. — Holotype. United Kingdom • 1 specimen (vertebrae, partial right coracoid, extremitas sternalis of left coracoid, nearly complete right scapula, fragments of the furcula, cranial portion of sternum, distal portion of both radii, right os carpi ulnare, largely complete pelvis in piece of matrix, distal portions of both femora, distal portion of right tarsometatarsus, pedal phalanges of both feet; seeFig. 1A); Essex, Walton-on-the-Naze; Walton Member of the London Clay Formation (previously Division A2); early Eocene (early Ypresian); 1985; M. Daniels leg.; NMS.Z.2021.40.181 (original collector’s number: WN 85512).

DIAGNOSIS. — As for genus.

ETYMOLOGY. — The species epithet refers to the similarity of the new species to taxa of the Sandcoleidae.

TYPE 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 million years ago [Ma]; Collinson et al. 2016).

REFERRED SPECIMEN. — NMS.Z.2021.40.182 (Fig. 1B; proximal portion of right femur, proximal half of right tibiotarsus, proximal and distal ends of right tarsometatarsus, pedal phalanges); collected in 1975 by M. Daniels (original collector’s number WN 75076).

MEASUREMENTS (in mm). — NMS.Z.2021.40.181: right coracoid, length as preserved, 23.3; right scapula, length as preserved, 31.4; left femur, distal width, 7.0. NMS.Z.2021.40.182: right tarsometatarsus, proximal width, 5.9; distal width, 5.8.

DESCRIPTION AND COMPARISONS

NMS.Z.2021.40.181 preserves several vertebrae, but only the third and fourth cervical vertebrae (Fig. 2 A-C) show distinctive morphologies. The third cervical vertebra (Fig. 2A, B) is of similar proportions to that of extant mousebirds (Fig. 2D, E), but has smaller lateral foramina (formed by an osseous strut between the zygapophyses cranialis et caudalis) than the third cervical vertebra of crown group Coliiformes. In contrast to crown group Coliiformes (Fig. 2F), the fourth cervical vertebra (Fig. 2C) also has well-developed lateral foramina, whereas in extant mousebirds there is an osseous bridge from the midsection of the vertebral corpus to the processus costalis; furthermore, the lacuna interzygapophysialis of Ypresicolius n. gen. is more deeply incised in crown group Coliiformes. The morphology of the cervical vertebrae of most other stem group Coliiformes is poorly known, but in Masillacolius from the latest early or earliest middle Eocene of Messel in Germany the fourth cervical vertebra resembles that of crown group Coliiformes (Mayr 2015).

The coracoid (Fig. 2L, M) is similar to that of Anneavis and Eoglaucidium, and has a shorter processus procoracoideus than the coracoid of Sandcoleus. The cotyla scapularis is concave. The shaft of the bone is pierced by a foramen nervi supracoracoidei. The angulus medialis of the right coracoid is more strongly projected than that of the left one. The medial margin of the extremitas sternalis forms a distinct bulge. The extremitas sternalis features a long processus lateralis.

The scapula (Fig. 2G) bears a prominent acromion with a broadly rounded tip; unlike in Eoglaucidium (Fig. 2H, I), it is bipartite and forms a medial process.

NMS.Z.2021.40.181 includes the extremitas sternalis of the furcula as well as one extremitas omalis. An apophysis furculae is not developed, and the omal extremity is simple and has a tapering end.

Only the cranial portion of the sternum is preserved in NMS.Z.2021.40.181 (Fig. 3A). The bone exhibits long processus craniolaterales, which are much better developed than in Sandcoleus. The spina externa is blade-like, but proportionally shorter than in extant mousebirds (Fig. 3D). There are four processus costales.

The os carpi ulnare (Fig. 3G) resembles that of crown group Coliiformes (Fig. 3H) in that crus longum and crus breve are widely separated and of similar length; there is a projection caudal to the crus breve.

The holotype includes a nearly complete pelvis in a piece of matrix, which is exposed in ventrolateral view (Fig. 4H). The alae praeacetabulares ilii are mediolaterally wider than in crown group Coliiformes. There is a rudimentary osseous sheet between the ilium and the pubis, which is well developed in crown group Coliiformes (Fig. 4I). As in Sandcoleus, but unlike in Chascacocolius (Houde & Olson 1992: fig. 11) and extant Coliidae, a spina dorsolateralis ilii is absent.

The proximal end of the femur is preserved in NMS.Z.2021.40.182(Fig.4A,B), whereas NMS.Z.2021.40.181 includes the distal end of the bone (Fig.4G). The caput femoris is globose. The distal end shares a characteristic derived morphology with extant Coliiformes in that the lateral portion of the distal end, that is, the area around the tuberculum musculi gastrocnemialis lateralis, forms a marked and proximodistally extensive prominence (Fig. 4F, G).

The tibiotarsus (Fig. 1B) is too poorly preserved for a meaningful description.

The tarsometatarsus (Fig. 5 A-F) closely resembles that of Sandcoleus, Anneavis (Fig. 5 I-L), Eobucco, and Eoglaucidium (Fig. 5 T-W) in its overall shape. The lateral foramen vasculare proximale is situated much farther proximally than the medial one. The tuberositas musculi tibialis cranialis is in the medial portion of the bone. The medial margin of the proximal end forms two small pointed projections (Fig. 5A). The hypotarsus (NMS.Z.2021.40.182; Fig. 5F) has a canal and a medially situated deep sulcus. This configuration closely corresponds to the hypotarsus of crown group Coliiformes (Fig. 5P), in which the canal is for the tendon of musculus flexor hallucis longus and the sulcus for that of m. flexor digitorum longus (the hypotarsus of coliiform birds was incorrectly scored in earlier analyses [Mayr & Mourer-Chauviré 2004; Ksepka & Clarke 2009, 2010], in which the large canal of crown group Coliiformes was erroneously identified as being for musculus flexor digitorum longus [see Mayr 2013, 2016 for its correct identity as a guiding structure for musculus flexor hallucis longus]). The hypotarsus of Sandcoleus and Anneavis has two canals for the aforementioned tendons as well as a sulcus for the tendons of the flexor muscles of the second toe. In Eoglaucidium, there also appear to have been two canals for m. flexor hallucis longus and m. flexor digitorum longus, but these are not fully closed. In plantar view, the hypotarsus of NMS.Z.2021.40.182 (Fig. 5B) is proximodistally longer than in Eoglaucidium (Fig. 5V). The distal end of the bone closely corresponds to the distal tarsometatarsus of Sandcoleus, Anneavis, Eobucco, and Eoglaucidium. The fossa metatarsi I is located on the medial surface of the shaft (Fig. 5D). The trochlea metatarsi II bears a ridge-like plantar flange. The trochlea metatarsi III is mediolaterally wide and exhibits a deep trochlear furrow. The trochlea metatarsi IV forms a large, flange-like accessory trochlea, which is less medially inflected than in Eobucco (compare Fig. 5E with Feduccia and Martin 1976: fig. 6d and Houde & Olson 1992: fig. 16).

Both specimens include pedal phalanges (Figs. 1, 6). The os metatarsale I (NMS.Z.2021.40.182) has a short processus articularis tarsometatarsalis. As in other sandcoleids, but unlike in crown group Coliiformes, the first phalanx of the second toe (Fig. 6F) is shortened. The first and second phalanges of the third toe (Fig. 6G, H) are also abbreviated and the first phalanx has a wide proximal end. The first and second phalanges of the fourth toe (Fig. 6I, J) are likewise shortened, and the first phalanx is very stout. The ungual phalanges (Fig. 6E) have the “raptor-like” morphology seen in sandcoleids, with a marked sulcus neurovascularis and a prominent tuberculum flexorium. As in the Strigiformes, Falconiformes, and some Accipitriformes, there are lateral foramina next to the tuberculum flexorium.