Here we describe a new basal ornithuromorph bird from the Jiufotang Formation. The new specimen has a V‐shaped furcula with a short hypocleidium, a feature otherwise known only in Schizooura among Mesozoic ornithuromorphs, in which the furcula is typically delicate and U‐shaped (Zhou & Zhang, 2005 ; O'Connor, Gao & Chiappe, 2010 ; Zhou, Zhou & O'Connor, 2012 ; Zhou, O'Connor & Wang, 2014b ). A suit of primitive features hint a basal position for this new taxon, which is corroborated by our phylogenetic analysis. Therefore, the new discovery further advances our understanding about the morphological diversity of early ornithuromorphs.

The Early Cretaceous Jehol Biota in northeastern China is renowned for exquisitely preserved vertebrate fossils (Chang, 2003 ; Zhou, Barrett & Hilton, 2003 ). At least 121 genera and 142 species have been named so far, including fishes, amphibians, turtles, choristoderes, pterosaurs, feathered non‐avian dinosaurs, primitive birds, and mammals, which provide extraordinary information regarding the early evolution of various terrestrial vertebrate groups (Benton et al ., 2008 ; Meng, Wang & Li, 2011 ; Zhou, 2014 ). Over the last 3 decades, thousands of bird fossils have been unearthed from the Jehol Biota and the rate of discovery continues to be high, making the Jehol Biota an important fossil avifauna (Zhou & Zhang, 2006 ; Zhou & Wang, 2010 ; Wang et al ., 2014 ). Representatives spanning nearly the whole spectrum of Mesozoic avian clades have been reported there, containing the long bony tailed Jeholornithiformes, the basal pygostylians Sapeornithiformes and Confuciusornithiformes, Enantiornithes, and Ornithuromorpha (Zhou & Zhang, 2006 ; Zhou, 2014 ). Ornithuromorpha is the clade that includes Neornithes that gives rise to all living birds. The first known appearance datum of this clade is from the Yixian Formation – the middle phase of the Jehol Biota (e.g., Archaeorhynchus and Yixianornis ), whereas the younger Jiufotang Formation has produced more than half the named Jehol ornithuromorphs (e.g., Jianchangornis , Schizooura , and Piscivoravis ), which together recorded over 5 million years of early evolution of this avian group (125‐120 Myr; Pan et al ., 2013 ). The nearly complete and articulated skeletons, some of which additionally preserved feathers, stomach contents, and in rare cases traces of soft tissues such as the crop, have significantly advanced our knowledge about the evolution of morphology and biology of basal ornithuromorphs (Zhou & Martin, 2010 ; Zheng et al ., 2011 , 2013 ; Zhou, Zhou & O'Connor, 2013 , 2014a ; Chiappe et al ., 2014 ; Xu et al ., 2014 ).

Phylogenetic analysis was performed using the modified data matrix of Wang et al . ( 2014 ). Five recently described taxa from the Jehol Biota have been added and scored from their holotypes, Piscivoravis lii (IVPP V17078 ), Iteravis huchzermeyeri (IVPP V18958 ), Tianyuornis cheni (STM7‐53), Parahongshanornis chaoyangensis (PMOL‐AB00161), and Eopengornis martini (STM24‐1); a few characters with ambiguous scorings have been changed based on direct observations of relevant holotype specimens, e.g., Yixianornis and Gansus ; Rahonavis was removed from the data set, giving the fact that recent studies indicate that Rahonavis is an unenlagiine dromaeosaurid rather than a bird (Norell et al ., 2006 ; Turner et al ., 2007 ). The modified data matrix now consists of 61 taxa (58 Mesozoic avian taxa; Dromaeosauridae as the outgroup; Anas platyrhynchos and Gallus gallus as the representatives of Neornithes) and 262 morphological characters, representing the most comprehensive known data set target the phylogeny of Mesozoic birds (see Appendix S1 for character scorings). Phylogenetic analysis was conducted using the TNT software [version 1.1] (Goloboff, Farris & Nixon, 2008 ), with following settings. Space for 10 000 trees was set in the memory as the maxtrees, and all characters were weighted equally. Unconstrained heuristic search starting with Wagner trees were performed. We conducted 1000 replicates of random stepwise addition (branch swapping: tree‐bisection‐reconnection, TBR) and kept ten trees at each step. Branches with minimum branch lengths of zero were collapsed to create polytomies. Bremer and Bootstrap values were calculated as indexes of clade support. Bootstrap analysis was calculated with 1000 replicates with the same setting as in the primary search. Bremer values were calculated using the bremer scripts embedded in TNT.

A large basal ornithuromorph bird that can be distinguished from other ornithuromorphs by the unique combination of the following features: V‐shaped furcula with a short hypocleidium; sternum bearing elongated lateral trabecula with fan‐shaped distal expansion; broad cranial margin of sternum with obtuse angle defined by coracoidal sulci of 113°; manual claws nearly straight; proximal ends of metatarsals II–IV coplanar; metatarsal IV robust (autapomorphy); hallucal ungual reduced; and intermebral index (humerus + ulna + carpometacarpus/femur + tibiotarsus + tarsometatarsus) of 1.21.

Anatomical description

Axial skeleton The cervical and thoracic series are poorly preserved, but two caudal thoracic vertebrae, preserved close to the left lateral trabecula of the sternum, provide anatomical details (Fig. 3). Both vertebrae are exposed in left lateral view, revealing that the lateral surfaces of the centra are nearly flat, without the broad and deep fossa reported in other basal ornithuromorphs, e.g., Jianchangornis, Schizooura, and Iteravis (Zhou, Zhang & Li, 2009; Zhou et al., 2012, 2014b). Visible from the cranial one of the two vertebrae, the parapophysis is cranially located as in other ornithuromorphs (Chiappe, 2002; Zhou et al., 2014a), rather than centrally located as in enantiornithines (Chiappe & Walker, 2002); the spinous process is well developed, approaching the height of the centrum; the postzygapophysis projects caudally beyond the centrum by a distance that is about half the centrum length, considerably longer than in Jianchangornis, Iteravis, and Patagopteryx (Chiappe, 2002; Zhou et al., 2009, 2014b). Figure 3 Open in figure viewer PowerPoint Photograph and interpretative line drawing of the pectoral girdle and sternum of the holotype of Bellulia rectusunguis gen. et sp. nov., IVPP V17970. Abbreviations: clp, craniolateral process of the sternum; lt, lateral trabecula; po, procoracoid process; sf, supracoracoidal never foramen; pa, parapophysis; zp, zyphoid process of the sternum; other abbreviations follow in Figure 2. The arrow indicates the ventral groove extending to the supracoracoidal nerve foramen. Scale bar, 10 mm. The synsacrum is estimated to be composed of eight to ten fully fused vertebrae, but the count is equivocal due to the poorly preserved cranial end (Fig. 4). Well developed transverse lamina is present between the transverse processes of the caudalmost five sacral vertebrae, but it is unclear among the cranial half. Among the caudalmost five sacral vertebrae, the transverse processes of the cranial three vertebrae are laterally directed; the fourth vertebra has the longest transverse processes which are caudolaterally directed; the transverse processes of the caudalmost vertebra are caudolaterally directed but to a less degree, and contact that of the preceding vertebra. Figure 4 Open in figure viewer PowerPoint Pelvis and hindlimb of the holotype of Bellulia rectusunguis gen. et sp. nov., IVPP V17970. (A) Photograph, and (B) line drawing of pelvis and femur; (C) close‐up of the proximal end of the right tibiotarsus; (D) photograph, and (E) line drawing of the left tarsometatarsus. Abbreviations: ac, acetabulum; an, antitrochanter; ccn, cranial cnemial crest; fh, femoral head; lcn, lateral cnemial crest; mt I–IV, metatarsal I–IV; pv, proximal vascular foramen; tc, trochanter crest; tu, tubercle; other abbreviations follow in Figure 2. The arrow in (D) indicates the tubercle for insertion of the M. cranialis tibialis. Scale bar, 10 mm. Only three free caudal vertebrae are visible, followed by a pygostyle (Fig. 4). The caudal vertebrae are exposed in left lateral view. The postzygapophyses are absent, and the lateral surfaces of centra are nearly flat. Two isolated elements preserved ventral to the caudal vertebrae are interpreted as the haemal arches (Baumel & Witmer, 1993), which are elongated and parallel to the preserved caudal series. The caudal tip of the pygostyle is missing and the preserved part is triangular in lateral view; the centra are fully fused, and the neural spines suggest that about five vertebrae form the pygostyle. Six dorsal ribs are visible, and two of them remain in articulation with the sternal ribs. No uncinate process is preserved, probably a preservational bias given the small size of this bone and the fact that it is unfused with the dorsal rib in basal birds (Chiappe et al., 1999; Wang et al., 2014).

Pectoral girdle Both coracoids are exposed in ventral view, and the right one is better preserved (Fig. 3). The coracoid is strut‐like with concave lateral and medial margins as in other ornithuromorphs. The sternal articulation approaches 60% the omal‐sternal length of the coracoid as in Jianchangornis, shorter in proportional terms than in Hongshanornis, Archaeorhynchus, and Piscivoravis. Proximally, the acrocoracoid is straight. A caudomedially oriented procoracoid process is developed. The process is triangular and tapers medially, whereas it is rectangular with medial expansion in Jianchangornis, Yixianornis, and Yumenornis (Clarke, Zhou & Zhang, 2006; Zhou et al., 2009; Wang et al., 2013). A supracoracoidal nerve foramen penetrates the procoracoid process at the base and opens into a ventral longitudinal groove. The groove extends just proximal to the foramen rather than surpasses it as in Apasavis and enantiornithines (Chiappe & Walker, 2002; Clarke & Norell, 2002; Fig. 3). The supracoracoidal nerve foramen is absent in some Early Cretaceous ornithuromorphs, including Archaeorhynchus, Jianchangornis, and Gansus (You et al., 2006; Zhou et al., 2009, 2013); whereas the foramen is so medially displaced that only forms an incision and is not fully enclosed in Schizooura (Zhou et al., 2012). Distally, the sternal margin is nearly straight. A well developed sternolateral process of the kind present in other basal ornithuromorphs, e.g., Gansus, Tianyuornis, and Yixianornis (Clarke et al., 2006; You et al., 2006; Zheng et al., 2014), is lacking. The right scapula is largely overlain by the sternum, and the left one misses the distal end (Fig. 3). The scapular shaft is sagittally curved, without groove on either the costal or lateral surface. The furcula is largely complete and exposed in ventral view (Fig. 3). The bone is morphologically similar to that of Schizooura: robust, V‐shaped with an interclavicular angle of 53°, strikingly different from the U‐shaped furcula in most other basal ornithuromorphs, e.g., Archaeorhynchus and Jianchangornis (Zhou et al., 2009, 2012, 2013; Fig. 5). The preserved omal end of the left ramus is medially curved and tapered, suggesting that similar acromion process as reported in Archaeorhynchus is likely developed (Zhou et al., 2013). The left ramus is well preserved and seems to have a laterally tapered cross section. Distally, the clavicular rami are fused to form a broad symphysis that extends considerably distally. We propose that a well developed hypocleidium as in Schizooura is likely present based on the following observations (Figs 3, 5): the furcula resembles that of the latter taxon in general morphology, in particularly that the furcula is robust and V‐shaped with a large symphysis tapering and extending distally; although the distal tip is broken, the poorly preserved dorsal fragment extends distally and overlaps the cranial end of the sternum, indicating that a long hypocleidium has been present in complete skeleton. Figure 5 Open in figure viewer PowerPoint Comparative furcular anatomy of the holotype of Bellulia rectusunguis gen. et sp. nov. (IVPP V17970) with selected basal ornithuromorphs. (A) Bellulia rectusunguis; (B) Schizooura lii; (C) Longicrusavis houi; (D) Archaeorhynchus spathula; (E) Gansus yumenensis. Line drawings are not scaled. The sternum is exposed in ventral view, and the imperforate region appears to be hexagonal (Fig. 3). The cranial margin is developed into shallow coracoidal sulci, which define an obtuse angle of 113°, similar to Archaeorhynchus and Schizooura (Zhou et al., 2012, 2013); in contrast, the angle is less than 90° and thus the cranial margin is strongly arched in many other basal ornithuromorphs, e.g., Jianchangornis, Yumenornis, and Iteravis (Zhou et al., 2009, 2013, 2014a; Wang et al., 2013; Fig. 6). The sternum has well developed craniolateral processes and zyphoid process. The craniolateral process projects laterally rather than craniolaterally as in Yixianornis, Yumenornis, and Gansus (Clarke et al., 2006; Li, Wang & Hou, 2011b; Li et al., 2011a; Wang et al., 2013). The zyphoid process is triangular with a cranially pointed lateral tip; this process is absent in Schizooura and Jianchangornis (Zhou et al., 2009, 2012). The costal margin between the craniolateral and zyphoid processes is concave, as opposite to the straight (e.g., Yanornis) or convex (e.g., Piscivoravis) condition in some basal ornithuromorphs (Zhou & Zhang, 2001; Zhou et al., 2014a). Caudally, a pair of lateral trabeculae is developed, which extend further caudally than the xiphoid process. The lateral trabecula is elongated and approaches half the entire length of the sternum (cranial margin to xiphoid process), making the caudal margin of the sternum deeply notched, as in Archaeorhynchus and Jiuquanornis (Wang et al., 2013; Zhou et al., 2013). However, in many other Early Cretaceous ornithuromorphs, e.g., Yixianornis, Yanornis, Gansus, and Jianchangornis, the lateral trabecula is short and thus the notch is caudally restricted (Clarke et al., 2006; Li et al., 2011a; Liu et al., 2014; Fig. 6). The lateral trabecula remains parallel to the longitudinal axis of the sternum and bears a fan‐shaped distal expansion. The xiphial region is incomplete and only preserves the part adjacent to the base of the sternal carina; it seems that the presence of fenestra is unlikely, but complete specimen is in need to confirm this reconstruction. The base of the carina is preserved, revealing that the carina extends along most length of the sternum, but terminates just short of the cranial margin as in Schizooura (Zhou et al., 2012); whereas, the carina reaches the cranial margin in many other basal ornithuromorphs, e.g., Archaeorhynchus, Gansus, and Jianchangornis (Zhou et al., 2009, 2013; Li et al., 2011a; Liu et al., 2014). Figure 6 Open in figure viewer PowerPoint Comparative sternal anatomy of the holotype of Bellulia rectusunguis gen. et sp. nov. (IVPP V17970) with selected basal ornithuromorphs. (A) Bellulia rectusunguis; (B) Archaeorhynchus spathula (modified from Zhou et al., 2013); (C) Jianchangornis microdonta; (D) Jiuquanornis niui; (E) Hongshanornis longicresta (modified from Zhou et al., 2013); (F) Yixianornis grabaui (modified from Zhou et al., 2013); (G) Gansus yumenensis; (H) Piscivoravis lii (modified from Zhou et al., 2014a); (I) Iteravis huchzermeyeri (modified from Zhou et al., 2014b); and (J) Yanornis martini (modified from Zhou et al., 2013). Line drawings are not scaled.

Thoracic limb The forelimb is longer than the hindlimb, with an intermebral index of 1.21, much greater than that of other Early Cretaceous ornithuromorphs (ratio = 0.9–1.1), but smaller than that of Archaeorhynchus (Table 2). Both humeri are exposed in cranial view (Fig. 1). The deltopectoral crest is large and projects dorsally, approximately 1.5× as wide as the humeral shaft, exceeding in proportional terms the crests of some other basal ornithuromorphs, e.g., Archaeorhynchus and Jianchangornis. Although a comparable large crest has been reported in Schizooura, the dorsodistal corner of the crest projects distally, making the distal margin concave, a unique condition among Cretaceous ornithuromorphs (Zhou et al., 2012). The massive crest extends well proximally, rendering the proximally margin of the humerus less vaulted than in Jianchangornis and Schizooura. The bicipital crest weakly protrudes cranially and has a pit‐shaped fossa on the craniodistal surface, as in Schizooura, Jianchangornis, and Apasavis (Clarke & Norell, 2002; Zhou et al., 2009, 2012). The distal ends of humeri are abraded heavily, leaving few features recognizable. Table 2. Lengths (mm) of forelimb and hindlimb of the holotype of Bellulia rectusunguis gen. et sp. nov. (IVPP V17970) in comparison with other basal ornithuromorphs Taxon Bellulia Schizooura (IVPP V16861) Iteravis (IVPP V18958) Gansus (GSGM‐05‐CM‐014) Yixianornis (IVPP V12631) Yanornis (IVPP V12558) Archaeorhynchus (IVPP V17075) Piscivoravis (IVPP V17078) Hongshanornis (IVPP V14533) Humerus 69.6 56.4 51.4 47.8 49.4 80.0 52.2 74.0 26.0 Ulna 78.2 65.9 54.7 51.1 50.1 82.0 58.1 77.0 24.4 Carpometacarpus 39.3 30.0 22.9 23.7 28.1 25.0 26.3 34.4 11.8 Femur 53.0 45.5 34.4 29.3 41.1 54.5 35.8 56.3 22.0 Tibiotarsus 66.0 61.1 58.4 63.7 52.9 79.8 40.6 71.4 38.0 Tarsometatarsus 34.9 36.2 31.5 40.0 27.3 37.8 20.3 35.0 21.0 Forelimb/Hindlimb 1.21 1.06 1.04 0.92 0.97 1.09 1.58 1.14 0.77 The ulna is slightly longer than the humerus and is bowed proximally (Fig. 7). The proximal end bears a well developed and robust olecranon process, which projects proximally well beyond the ventral cotyla; whereas, the olecranon process is low and weakly developed in other Early Cretaceous birds (Chiappe et al., 1999; Sanz et al., 2002; Zhou & Zhang, 2003; Wang et al., 2014). As in primitive birds (Mayr et al., 2007; Zhang et al., 2013), the ulna lacks quill knobs for the attachment of the secondary remiges. The radius is straight and more than half the width of the ulna. Two rectangular bones distal to the radius and ulna are interpreted as the radiale and the ulnare, respectively, but the poor preservation reveals few anatomical features. Figure 7 Open in figure viewer PowerPoint Photographs and line drawings of forearms of the holotype of Bellulia rectusunguis gen. et sp. nov., IVPP V17970. (A) and (B) the left wing; (C) and (D) the right hand. Abbreviations: al, alular metacarpal; al‐1,2, the first and second phalanx of the alular digit; mm, major metacarpal; mm‐1, 2, 3, the first, second and third phalanx of the major digit; mi, minor metacarpal; mi‐1, the first phalanx of the minor digit; ol, olecranon; other abbreviations follow in Figure 2. Scale bars, 10 mm. Proximally, the alular, major and minor metacarpals are fused with each other and with the semilunate carpal (Fig. 7). The alular metacarpal is rectangular and bears a ginglymoid articular facet for the proximal phalanx. An extensor process is not developed. The carpometacarpus lacks a distinct pisiform process of the kind present in Longicrusavis and Yixianornis (Clarke et al., 2006; O'Connor et al., 2010). The major metacarpal is robust and straight, without an intermetacarpal process as other Early Cretaceous birds (Chiappe et al., 1999; Zhou & Zhang, 2003; Wang, 2014). The minor metacarpal is bowed craniocaudally and forms a wide intermetacarpal space with the major metacarpal. Distally, the minor metacarpal terminates proximally to and fused with the distal end of the major metacarpal. The alular digit extends to the distal end of the major metacarpal, proportionally longer than in Schizooura (Zhou et al., 2012); whereas, this digit extends further distally than the distal end of the major metacarpal in many other Jehol ornithuromorphs, including Jianchangornis and Yixianornis (Clarke et al., 2006; Zhou et al., 2009). As in Jianchangornis (Zhou et al., 2009), the proximal phalanx of the alular digit is longer than the proximal phalanx of the major digit; whereas, this phalanx is shorter in some other Jehol ornithuromorphs, including Archaeorhynchus and Schizooura (Zhou et al., 2012, 2013). The ungual is smaller than that of the major digit; however, the opposite condition is present in basal pygostylians (e.g., Sapeornis and Confuciusornis) and most Early Cretaceous ornithuromorphs, e.g., Archaeorhynchus, Jianchangornis, and Schizooura (Chiappe et al., 1999; Zhou & Zhang, 2003; Zhou et al., 2009; Zhou et al., 2012, 2013). The unguals of the alular and major digits have deep and wide neurovascular sulcus. Unguals of the alular and major digits lack noticeable curvatures and their dorsal margins are nearly straight; the ventral margins are step‐like and constrict after the poorly protruding flexor processes. However, distinctive curvature is present at least in the ungual of the alular digit in other Mesozoic birds (Chiappe et al., 1999; Elzanowski, 2002; Burnham et al., 2011; Wang et al., 2014). The proximal phalanx of the major digit is robust and craniocaudally expanded as in other basal ornithuromorphs except Schizooura (Zhou et al., 2012). The penultimate phalanx is straight and slightly shorter than the preceding one. Only one phalanx is preserved in the minor digit. The phalanx tapers distally and is about two‐thirds the length of the penultimate phalanx of the major digit.

Pelvic girdle The pelvis is mainly exposed in ventral view, except the left ilium and ischium in lateral view (Fig. 4). The pelvic elements are fused with each other at the level of the acetabulum, and the ilia are fused with the synsacrum. Both ilia are poorly preserved. A large but abraded antitrochanter is located on the caudodorsal margin of the acetabulum. The left ischium only preserves the proximal part. The bone tapers distally and bears concave ventral margin. Along the preserved distal end of the ischium, a longitudinal ridge is developed, close to the dorsal margin, but to what degree it extends proximally is unclear due to abrasion. Similar structure has also been reported in Yixianornis, Gansus, and Iteravis, which extends proximally nearly to the level of the iliac peduncle (Clarke et al., 2006; You et al., 2006; Zhou et al., 2014b). In some Early Cretaceous ornithuromorphs (e.g., Yixianornis, Chaoyangia, and Gansus), the ischium has a prominent triangular dorsal process projecting off the middle of the dorsal margin (Clarke et al., 2006; O'Connor & Zhou, 2013); however, the corresponding area is obscured by the left femur, preventing identification of this structure. The pubes are poorly preserved, but it is evident that an obturator process is absent.

Hindlimb The left femur is exposed in cranial view and the right one is poorly preserved (Fig. 4). The femur is robust and measures about 80% the length of the tibiotarsus, proportionally longer than in many other early ornithuromorphs, e.g., Yanornis, Iteravis, and Gansus (Table 2). Proximally, the trochanter crest projects proximally beyond the level of the femoral head, as in Iteravis and Chaoyangia (O'Connor & Zhou, 2013; Zhou et al., 2014b). The femoral head is separated from the shaft by a thick neck. The craniomedial surface of the femoral head is abraded, preventing recognition of fossa for the capital ligament, which is widespread among Late Cretaceous ornithuromorphs, including Apasavis, Vorona, Ichthyornis, and Hesperornis (Clarke & Norell, 2002; Forster et al., 2002; Galton & Martin, 2002; Clarke, 2004); however, preservation prevents identification of this feature among other Early Cretaceous ornithuromorphs. The tibiotarsus is completely fused (Fig. 1). Proximally, two cnemial crests are developed, visible on the right tibiotarsus (Fig. 4c): the lateral cnemial crest is well developed and appressed against the medial surface of the proximal end of the fibula; and the cranial cnemial crest is a low ridge situated on the cranial surface of the tibiotarsus, possibly diminished by postmortem crushing. The lateral cnemial crest projects slightly proximally, rendering the proximal articular facet of tibiotarsus inclining medially; however, extensive proximal projection as in Gansus is lacking (You et al., 2006; Liu et al., 2014). The fibular crest extends along the proximal one‐third of the tibiotarsus. An extensor sulcus is not developed. Distally, the medial and lateral condyles are heavily abraded. The fibula is more than one‐half the length of the tibiotarsus and falls to reach the proximal tarsals distally as in most other ornithuromorphs (Forster et al., 1996; Chiappe, 2002; Clarke et al., 2006). The bone expands proximally and tapers distally along the proximal half shaft, making the distal half thin and delicate. Both tarsometatarsi are preserved in dorsal views. As in other ornithuromorphs, the tarsometatarsus is fully fused (Fig. 4). Metatarsal V is not visible. The cranial margin of the proximal articular facet is straight, and the intercotylar prominence is lacking. Proximally, metatarsals II–IV are placed in the same transverse plane as in Jianchangornis and Schizooura (Zhou et al., 2009, 2012), contrasting with most other basal ornithuromorphs, in which the proximal end of metatarsal III is plantarly displaced with respect to that of metatarsals II and IV (Chiappe, 2002; Clarke & Norell, 2002; Clarke et al., 2006; Zhou et al., 2014b). A proximal vascular foramen perforates the tarsometatarsus between metatarsals III and IV. Mediodistal to the proximal vascular foramen is a low tubercle for insertion of the M. cranialis tibialis, which is situated on the dorsolateral margin of the metatarsal III (visible on both tarsometatarsi; Fig. 4d). Although varying in its position (dorsomedial, center, or dorsolateral), this tubercle is placed on metatarsal II among other ornithuromorphs, e.g., Longicrusavis, Yanornis, and Schizooura (O'Connor et al., 2010; Zhou et al., 2012). Metatarsal III is the longest among major metatarsals; metatarsal IV is slightly shorter, but projects further distally than the middle of the metatarsal III trochlea, proportionally longer than in some other Early Cretaceous ornithuromorphs, e.g., Yixianornis, Schizooura, and Jianchangornis; metatarsal II is the shortest, falling to reach the proximal margin of the metatarsal III trochlea. Metatarsal II is the narrowest; whereas, metatarsal IV is robust and the proximal three fourths of its shaft is wider than that of metatarsal III, a feature unknown amongst Cretaceous birds, in which metatarsals II–IV are either subequal in width (e.g., Sapeornithiformes, Confuciusornithiformes, and ornithuromorphs Archaeorhynchus and Schizooura) or metatarsal IV is thinner, e.g., Enantiornithes and ornithuromorph Hollanda (Chiappe et al., 1999; Chiappe & Walker, 2002; Zhou & Zhang, 2003; Bell et al., 2010; Zhou et al., 2012, 2013). The robust metatarsal IV resembles the condition in troodontids among paravians (Xu & Wang, 2000; Xu et al., 2002; Zanno et al., 2011), but the relatively narrow metatarsal III in Bellulia is not resultant from being pinched by the proximal ends of metatarsals II and IV as in troodontids (Arctometatarsalian). Therefore, we propose that the robust metatarsal IV represents an autapomorphy of Bellulia. The dorsal surface of metatarsal IV is strongly convex along its proximal three‐fourths with respect to metatarsals II and III, delimitating laterally the flat dorsal surfaces of the latter two bones. Distally, the dorsal convexity of metatarsal IV diminishes, whereas metatarsal III projects dorsally slightly. As in Jianchangornis, the distal trochleae of metatarsals II–IV are nearly coplanar; in Schizooura, metatarsal II is weakly plantarly displaced (Zhou et al., 2012); whereas, metatarsals II and IV are both plantarly displaced in some other basal ornithuromorphs (Clarke & Norell, 2002; You et al., 2006; Zhou et al., 2014a, b). The trochlea of metatarsal II is narrower than that of metatarsal IV, and the metatarsal III trochlea is the widest. Metatarsal I is nearly straight in laterocaudal view and tapers proximally. The left pedal digits are well preserved and remain in partial articulation (Fig. 4). Digit III is the longest, followed by digit IV and then digit II, but all digits are shorter than the tarsometatarsus. Within digits II–IV, the phalanges decrease in length proximodistally. The non‐ungual phalanges are spool‐shaped and stout with deep pits for the attachment of the collateral ligaments. The unguals are weakly curved as in other Early Cretaceous ornithuromorphs, and have deep neurovascular sulci on lateral surfaces. As in Piscivoravis and Schizooura (Zhou et al., 2012, 2014a), the ungual of digit I is the most reduced, contrary to many other ornithuromorphs, in which the smallest ungual belongs to digit IV, e.g., Yixianornis, Longicrusavis, Gansus, and Iteravis (Clarke et al., 2006; O'Connor et al., 2010; Liu et al., 2014; Zhou et al., 2014b). The ungual of digit III is the largest, followed by that of digit II. The proximal phalanx of digit II is the longest in the foot, while the proximal phalanx of digit III is the most robust. The proximal phalanx of digit IV is much longer than the distal three non‐ungual phalanges, which are subequal in length.