Systematic paleontology

Aves Ornithothoraces Ornithuromorpha Hongshanornithidae

Type genus. Hongshanornis, Zhou and Zhang11.

Archaeornithura meemannae gen. et sp. nov.

Etymology. The generic name is derived from Greek ‘Archae’ and ‘ornithura’, meaning ‘ancient ornithuromorph’. The specific name is in honour of Dr Meemann Chang for her continuous support of the study of the Jehol Biota.

Holotype. An articulated partial skeleton with feathers (STM7-145), housed at the Tianyu Natural History Museum of Shandong (STM), China (Fig. 1).

Figure 1: Holotype of Archaeornithura meemannae gen. et sp. nov., STM7-145. (a) Main slab; (b) counter slab. Anatomical abbreviations: al, alular digit; ba, basicranium; co, coracoid; cv, cervical vertebrae; d I–IV, pedal digit I–IV; fe, femur; fi, fibula; fu, furcula; hu, humerus; ma, major digit; mi, minor digit; pr, primary remiges; pu, pubis; ra, radius; re, rectrices; sc, scapula; st, sternum; ti, tibiotarsus; tm, tarsometatarsus; ul, ulna. Scale bars, 10 mm. Full size image

Paratype. An articulated partial skeleton with feathers (STM7-163; Supplementary Fig. 1).

Locality and horizon. Protopteryx horizon in Sichakou basin, Fengning County, Hebei, northeastern China; Lower Cretaceous Huajiying Formation6,7.

Differential diagnosis. The new taxon is referable to the Hongshanornithidae and can be distinguished from the known hongshanornithids by the following combined features: it differs from Hongshanornis and Longicrusavis, in that the cranial margin of the sternum is strongly vaulted; it is distinguished from Hongshanornis and Parahongshanornis, in that the zyphoid process of the sternum is well developed and squared; it differs from other hongshanornithids except Longicrusavis, in that the alular digit extends further distally than the major metacarpal; it is distinguishable from other hongshanornithids except Parahongshanornis, in that the penultimate phalanx of the major digit is longer than the preceding phalanx; and the new taxon has proportionally shorter femur relative to the tarsometatarsus.

Description

The skull is not well preserved in either specimen (Fig. 1; Supplementary Fig. 1). The cervical vertebrae are poorly preserved in articulation with the skull; 10 vertebrae are preserved including the ring-like axis, although the exact position of the cervicothoracic transition cannot be determined due to poor preservation. The vertebrae are clearly not elongated, being approximately equal in width and length, as in other hongshanornithids (Figs 1 and 2b,c; Supplementary Fig. 1). The thoracic vertebrae are only preserved in the counter slab of STM7-145, covered by the sternum. The synsacrum appears to comprise 9–10 vertebrae, although preservation makes this estimate equivocal. Six free caudals are followed by a poorly preserved pygostyle (Fig. 3e). The transverse processes of the caudal vertebrae are less than the width of the centrum in length and caudolaterally directed, at least in the first free caudal. Two well-preserved uncinate processes are preserved in the counter slab of STM7-163, and they are straight and tapered (Fig. 2a). Although not in articulation with the ribs, we infer they would have crossed at least one rib, reaching and potentially crossing a second. Four sets of gastralia are preserved in articulation in STM7-163 (Supplementary Fig. 2b).

Figure 2: Pectoral girdle and sternum of Archaeornithura meemannae gen. et sp. nov., in comparison with other hongshanornithids. (a) STM7-163, counter slab (zyphoid process of the sternum is outlined by a dash line); (b) STM7-145, main slab; (c) photograph and (d) line drawing of STM7-145, counter slab. Line drawing (not scaled) of furcula and sternum of other hongshanornithids: (e) Hongshanornis longicresta; (f) Longicrusavis houi; (g) Tianyuornis cheni; (h) Parahongshanornis chaoyangensis. Anatomical abbreviations: ca, carina; co, coracoid; cv, cervical vertebrae; fu, furcula; hy, hypocleidium; lt, lateral trabecula of the sternum; ne, supracoracoidal nerve foramen; pc, procoracoid process; sc, scapula; sl, sternocoracoidal process; st, sternum; uc, uncinate process; zp, zyphoid process of the sternum. Scale bars, 5 mm. Full size image

Figure 3: Detail anatomy of Archaeornithura meemannae gen. et sp. nov. (a) Photograph and (b) line drawing of the left wing, STM7-145, counter slab; (c) line drawing of hands of other hongshanornithids (not scaled; from left): Hongshanornis longicresta, Longicrusavis houi, Tianyuornis cheni, Parahongshanornis chaoyangensis; (d) STM7-163, counter slab; (e) STM7-163, main slab; (f) feet, STM7-163, main slab. Anatomical abbreviations: al, alular digit; am, alular metacarpal; ca, caudal vertebrae (six vertebrae counted); del, deltopectoral crest; d I–IV, pedal digit I–IV; hu, humerus; is, ischium; ma, major digit; mam, major metacarpal; mi, minor digit; mim, minor metacarpal; mt I, metatarsal I; pb, pubic boot; pu, pubis; py, pygostyle; ra, radius; sp, supracondylar process; ti, tibiotarsus; tm, tarsometatarsus; ul, ulna. Scale bars, 10 mm (a,f), 5 mm (d,e). Full size image

The scapula is curved, as in other ornithuromorph birds, but the distal end is not well preserved in either specimen (Figs 1 and 2c). The furcula is typically hongshanornithid: delicate, U-shaped, with tapered omal margins and a small tubercle-like hypocleidium as in Tianyuornis and Parahongshanornis12,13,14, which, however, is short and sharply tapered in Hongshanornis (Fig. 2)11. In contrast, a hypocleidium is absent from most other Cretaceous ornithuromorphs, although a short one has been reported in Schizooura15. The furcular symphysis is much thicker craniocaudally compared with the omal rami as in Hongshanornis, whereas the furcula appears to be nearly of equal thickness throughout in Longirostravis and Tianyuornis12,13. The coracoid is narrow along the proximal half, and wide distally with a well-developed sternocoracoidal process (Fig. 2b), as in hongshanornithids and most other ornithuromorphs2,12,13,14. Although poorly preserved, a procoracoid process is visible on the right coracoid in the counter slab of both specimens; it is medially oriented although the distal half appears more cranially directed (Fig. 2b). This process is perforated at the base by a small circular supracoracoidal nerve foramen. The coracoid is preserved in the dorsal view in the counter slab of STM7-163 (Fig. 2b); the acrocoracoid is blunt, the scapular cotyla is deeply concave, and the laterally positioned glenoid is developed as a weak convexity. Distally, the corpus is concave, more so than the condition in Yixianornis but not so deeply as in some Late Cretaceous enantiornithines16. The sternum is not well preserved, but a fragment in STM7-163 indicates the rostral margin is vaulted as in Tianyuornis and Parahongshanornis (Fig. 2d,g,h)13,14, but more pointed than in Longicrusavis and Hongshanornis, in which the cranial margin is broad and parabolic (Fig. 2e,f)2,11,12. A well-developed zyphoid process is present and squared in shape, as in Longicrusavis and Tianyuornis (Fig. 2a,f,g)12,13, whereas the process is absent in Hongshanornis and Parahongshanornis, and the corresponding lateral margin only bulges laterally (Fig. 2e,h)11,14. A second fragment in the same slab indicates that the lateral trabecula was short and narrow as in other hongshanornithids, with a distinct triangular distal expansion similar to that of Tianyuornis but smaller (Supplementary Fig. 2a)13.

The humerus is short and robust with a large rounded deltopectoral crest that extends 44% the length of the humerus and slightly exceeds the width of the shaft, as in other hongshanornithids (Fig. 3a). The distal margin is perpendicular to the shaft. The condyles are well developed and bulbous, and a dorsal supracondylar process is present as in other hongshanornithids (Fig. 3a; Supplementary Fig. 2d)2,12; however, the process has so far been reported only in Ichthyornis among other Mesozoic birds2,17. The ulna is bowed and more robust than the straight radius as in all basal birds. An olecranon process is not developed. The carpometacarpus appears to be fused proximally but not distally in STM7-145. All the metacarpals are straight, and the minor metacarpal is less than half the thickness of the major metacarpal. The alular metacarpal bears a small extensor process that is less than half the width of the articular facet of this metacarpal. The alular digit is long, with half of the ungual extending just beyond the distal margin of the major metacarpal as in Longicrusavis, whereas in other hongshanornithids, only the tip of the ungual slightly surpasses the distal end of the major metacarpal (Fig. 3a,c). The ungual phalanx of the alular digit is larger and more recurved than that of the major digit, as in other hongshanornithids2,11,12,13,14. The first phalanx of the major digit is mediolaterally compressed and caudally expanded, as in other ornithuromorphs. As in Parahongshanornis, the penultimate phalanx is longer than the preceding phalanx, whereas these two phalanges are subequal in length in other hongshanornithids (Table 1). The minor digit only preserves a single reduced wedge-shaped phalanx that tapers distally. A second phalanx, which is extremely reduced in other hongshanornithids2,11,12,13,14, may have being missing due to preservation. Similar to other hongshanornithids, the forelimb is much shorter than the hindlimb, with an intermembral index (humerus+ulna/femur+tibiotarsus) of ∼0.84 (Table 1), whereas the forelimb is typically longer in other Early Cretaceous ornithuromorphs18,19.

Table 1 Comparative measurements of Archaeornithura meemannae gen. et sp. nov. and other hongshanornithid taxa. Full size table

The pelvic girdle is not well preserved in either specimen; the ilium is displaced cranially in STM7-145. Visible in the main slab, the cranial margin is convex, while the lateral margin proximal to the acetabulum is concave; the two margins are separated by a weakly developed ventral hook present in most ornithuromorphs and some enantiornithines. The pubes are long and contact along their distal tenth but remain unfused, as in Hongshanornis (Supplementary Fig. 2c). Although incomplete, the distal ends of pubes are well preserved in lateral and medial views in the counter slab of STM7-163, revealing a small distally expanded boot as in Hongshanornis and Parahongshanornis (unclear in Tianyuornis and Longirostravis; Fig. 3d,e)2,14. The ischia appear to be just over half the length of the pubes, straight, narrow and tapered along their distal halves (Fig. 3e), lacking the low dorsal process located mid-corpus and the concave ventral margin present in most other ornithuromorphs, including Yixianornis and Piscivoravis20,21.

The femora are short and fairly robust, approximately equal in length to the tarsometatarsi; in contrast, the femur is considerably longer than the latter in most other basal birds, including Jeholornis, Sapeornis, Confuciusornis, enantiornithines and ornithuromorphs18,19. The tibiotarsus is proportionally shorter than other hongshanornithids relative to the tarsometatarsus. As in Longicrusavis, the fibula is preserved bowing out from the tibiotarsus, not appressed against it, and only appears to extend to the midshaft of the tibiotarsus12. The proximal medial surface bears a shallow excavation, visible on the left side in the counter slab of STM7-163.

The tarsometatarsi are well preserved in the main slab of STM7-163 (Fig. 3f). They are fully fused, although the individual metatarsals can be distinguished. Overall, the foot is very similar to other hongshanornithids2,11,12,13,14. The proximal half of metatarsal III is plantarly displaced relative to metatarsals II and IV, as in all ornithuromorphs. Metatarsal III is the longest and metatarsals II and IV end approximately at the same level. The hallux is small and placed above the trochlea of the other digits as in most ornithuromorphs including other hongshanornithids. The first phalanx is approximately as long as metatarsal I itself; the ungual phalanx is more strongly recurved than that of the other digits. As in other hongshanornithids, the phalanges decease in length distally, digit III is the longest and digit II is substantially shorter than IV.

Both specimens preserve nearly complete plumage (Figs 1 and 4; Supplementary Fig. 1). Six long asymmetrical primary remiges are preserved in the left wing of the main slab in the holotype (Fig. 4a); the second and third feathers are the longest. The primary remiges are overlain by a layer of short feathers, which measure just under half the length of the primary remiges themselves—we interpret these feathers as the dorsal coverts (Fig. 4a,b). They appear to be symmetrical, definitely lacking the strong asymmetry present in the primary remiges in which the leading edge vane is less than one-third the width of the trailing edge vane (Fig. 4a). Portions of a few secondary feathers are also preserved; these appear to be narrower than the primary remiges and symmetrical with rounded distal margins. An alula is preserved in both specimens, composed of at least three feathers in STM7-163, visible where the alular digit is disarticulated on the left side (Fig. 4d). The rounded distal margins of three large symmetrical pennaceous feathers are preserved near the right foot in STM7-145 (Figs 1 and 4b). These feathers are staggered so that each medial feather ends distal to the lateral feather, suggesting that these feathers represent the distal portion of an incomplete fan-shaped array of rectrices, like that present in Hongshanornis (IVPP V14533 and DNHM D2945/6)2.

Figure 4: Plumage of Archaeornithura meemannae gen. et sp. nov. (a) Left wing, STM-7-145, main slab; (b) right wing, STM-7-145, main slab; (c) covert feathers over the skull and neck, STM 7-163, counter slab; (d) alular feathers on the left alular digit, STM7-163, main slab. Abbreviations: af, alular feather; dc, dorsal coverts; pr, primary remiges; re, rectrices. Scale bars, 10 mm (a–c), 5 mm (d). Full size image

Short rachis-less covert feathers are found all over the body, particularly well preserved in STM7-163 (Supplementary Fig. 1). These feathers cover the head, neck, shoulders, extend off the proximal ulna and humerus and line the caudal end of the body (Fig. 4c; Supplementary Fig. 1). These feathers are notably absent from the distal three-quarters of the tibiotarsus in both specimens (Figs 1 and 4a; Supplementary Fig. 1), consistent with the wading habitat inferred for hongshanornithids12.