In this description we primarily highlight morphological information from the new specimen (DNHM D2945/6) that either supplements or contradicts the description of the holotype (Zhou & Zhang, 2005). Anatomical nomenclature mainly follows Baumel & Witmer (1993); certain structures not cited therein follow Howard (1929). While the Latin terminology used by Baumel & Witmer (1993) is retained for muscles and ligaments, osteological structures are described using the English equivalents of the Latin terms.

The entire skeleton of DNHM D2945/6 is preserved and visible in two slabs; the bones are broken between the two, leaving clear voids. DNHM D2945 preserves the bones of the skull, most of the axial skeleton, right radius, major digit and tibiotarsus, and both feet (Figs. 1 and 2). DNHM D2946 preserves most of the bones of the wings and thoracic girdle, both femora, the left tibiotarsus, and the distal ends of the pubes (Figs. 3 and 4).

The mandibular symphysial ossification (predentary bone of Zhou & Martin, 2011 ) is visible as a void in DNHM D2945. The fragmentary remains of two to four teeth are preserved set in the dentary. This confirms the presence of teeth in the dentary of hongshanornithids, which was hypothesized by O’Connor, Gao & Chiappe (2010) . Crowns cannot be discerned, but the dentary teeth appear to be smaller than those preserved in the maxilla. The dentary is caudally unforked and unfused to the surangular, as in other basal ornithuromorphs.

At least four uncinate processes can be discerned, including one that is completely preserved (DNHM D2946). These ossifications are long (extending across two ribs) with broad bases and tapered outlines ( Fig. 6 ). The uncinate processes appear not to be fused to the ribs, as in the holotype. Several ventral ribs are preserved in articulation with the sternum (DNHM D2946). The proximal ends of the thoracic ribs are very robust, the expanded proximal portion abruptly narrows towards the much thinner shaft ( Fig. 6 ).

The synsacrum is incomplete and broken between the two slabs (dorsally exposed in DNHM D2946 and ventrally exposed in DNHM D2945) ( Figs. 1 – 4 ). A number of well-differentiated costal processes project from the side of the synsacrum—these processes have expanded distal ends for their attachment to the ilium. Judging by the number of costal processes articulated to the ilium, the synsacrum was composed of no less than nine vertebrae (the synsacral count cannot be determined in either the holotype of Hongshanornis longicresta or Longicrusavis houi ), which is comparable to other basal ornithuromorphs and more than is typical of the more primitive enantiornithines ( Chiappe, 1996 ). The ventral surface of the synsacrum (anterior half preserved in DNHM D2945) is smooth, lacking a distinct groove (e.g., Archaeorhynchus spathula , Patagopteryx deferrariisi ). Dorsally, the portion of synsacrum preserved in slab DNHM D2946—corresponding largely to the postacetabular portion—is longitudinally scarred by a pair of shallow grooves.

The cervical vertebrae are poorly preserved, revealing little more than the remnants of low neural spines. The anterior dorsal vertebrae (visible in ventral view in DNHM D2945) bear compressed centra lacking ventral processes; the posterior dorsals have thicker centra. The articulations are amphyplatan or amphicoelic and the elements are not fused in a notarium. The lateral surfaces of the dorsal vertebrae are excavated by a broad fossa, as in other basal ornithuromorphs (e.g., Yanornis martini, Longicrusavis houi ).

Appendicular skeleton

The furcula is delicate and U-shaped as in many other basal ornithuromorpha (e.g., Clarke, Zhou & Zhang, 2006; You et al., 2006; Zhou, Zhang & Li, 2009; Zhou, Zhou & O’Connor, 2013b). The interclavicular angle is estimated to be approximately 45°(Fig. 6). The rami are transversely compressed proximally, becoming more dorsoventrally compressed and wider towards the symphysis. The caudal surface of the bone exhibits a distinct trough running along the distal half of the rami and converging towards the symphysial region. The proximal compression and caudal groove of the furcula of DNHM D2946 are comparable to that reported by Li, Wang & Hou (2011) as diagnostic characters of Parahongshanornis chaoyangensis. The presence of these conditions in DNHM D2946 indicates that such characters are unlikely to be diagnostic of the latter taxon. A cross section of the furcula in slab DNHM D2945 indicates the bone may have been hollow. No long hypocleidium like that reported in the holotype of Hongshanornis longicresta (IVPP V14533) is visible but the symphysial region of DNHM D2945/46 is covered by the distal portion of the left coracoid, thus making it unclear if a hypocleidium was present or not. The similarity between the furcula of DNHM D2945/46 and that of Longicrusavis houi and Parahongshanornis chaoyangensis, taxa lacking a long hypocleidium and possessing only a small tubercle at the symphysis (O’Connor, Gao & Chiappe, 2010; Li, Wang & Hou, 2011), suggests that the furcula of hongshanornithids possibly lacked a well-developed hypocleidium.

The sternum, primarily preserved in DNHM D2946 in dorsal view, has a slightly rounded cranial margin (Fig. 6). Distally on the left side of the sternum, a bony bar with a terminal expansion is preserved. Damage makes it difficult to interpret this region of the sternum. One alternative is that this bar corresponds to a slightly displaced, lateral trabecula with an expanded distal end; the lateral trabecula of the sternum of various basal ornithuromorphs (e.g., Archaeorhynchus spathula, Jianchangornis microdonta, Yixianornis grabaui, Yanornis martini) is distally expanded in varying degrees (Zhou, Zhou & O’Connor, 2013b). Alternatively, it may represent the lateral margin of a sternal fenestra, such as those present in the basal ornithuromorphs Songlingornis linghensis, Yixianornis grabaui, and Yanornis martini (Clarke, Zhou & Zhang, 2006; Zhou, Zhou & O’Connor, 2013b), a feature apparently absent in the sternum of the holotype of Hongshanornis longicresta. Zhou & Zhang (2005) described the sternum of the latter with lateral processes lacking a distal expansion; however the sternum of this specimen is too poorly preserved to confidently support the absence of such expansion.

The strut-like coracoids articulate adjacent to each other on the cranial margin of the sternum, nearly touching each other if not slightly overlapping (Fig. 6). There is a depression (possibly corresponding to the impression of the m. sternocoracoidei of modern birds) on the dorsal surface of the sternal half of these bones (also present in Jehol ornithuromorphs such Jianchangornis microdonta and Yixianornis grabaui). This feature in ornithuromorphs is not as pronounced as the dorsal fossa that excavates the coracoids of many Late Cretaceous enantiornithines (Chiappe & Walker, 2002). The proximal ends of the coracoids are poorly preserved so that a procoracoid process cannot be identified—the presence of this process in Hongshanornis longicresta is not as clear as was suggested by Zhou & Zhang (2005) and remains equivocal in this taxon. However, a procoracoid process is known in almost every other Early Cretaceous ornithuromorph (e.g., Jianchangornis microdonta, Yixianornis grabaui, Yanornis martini, Gansus yumenensis) (Zhou & Zhang, 2001; Clarke, Zhou & Zhang, 2006; You et al., 2006; Zhou, Zhou & O’Connor, 2012). The body of the coracoid exhibits no evidence of either a supracoracoid nerve foramen or medial notch (incisura n. supracoracoidei). Both the lateral and medial borders of this bone are clearly concave, indicating that the convex lateral margin described by Zhou & Zhang (2005) for the poorly preserved holotype is incorrect. A lateral process is present; its squared off morphology is consistent with that of other Early Cretaceous ornithuromorphs (e.g., Ambiortus dementjevi, Yixianornis grabaui, Gansus yumenensis) (Clarke, Zhou & Zhang, 2006; Kurochkin, 1982; You et al., 2006).

Both humeri are exposed in caudal view in DNHM D2946 (Fig. 6). The head is prominent—largely projected caudally—and proximally flat. Such design is more reminiscent to that of the humeral head of Patagopteryx deferrariisi and other basal ornithuromorphs (i.e., Archaeorhynchus spathula, Jianchangornis microdonta) and it differs from the domed-head of modern birds (Chiappe, 2002). The proximal third of the humerus appears to be very broad, expanded as in the holotype of Hongshanornis longicresta, Ichthyornis dispar and Ambiortus dementjevi (Kurochkin, 1985; Clarke, 2004) (Fig. 6). The caudal surface is not perforated by a pneumotricipital foramen and the pneumotricipital fossa is minimally developed. A distinct furrow—presumably the capital incisure—separates the ventral margin of the head from the ventral tubercle. Ventral to the latter, on the ventroproximal corner (and the bicipital area) of the bone, there is a shallow circular depression also present in Ichthyornis dispar, possibly corresponding to the attachment site of the m. pectoralis superficialis (Clarke, 2004). The deltopectoral crest is large—extending longitudinally for more than 1/3 the length of the bone (Fig. 6)—and rounded, lacking the cranial deflection of more advanced ornithuromorphs (neornithines). Distally, the margin of the crest gradually diminishes along the dorsal border of the shaft, typical of Cretaceous ornithuromorphs (e.g., Jianchangornis microdonta, Yixianornis grabaui, Archaeorhynchus spathula), as opposed to the rapid step-like constriction of this crest in most basal birds (e.g., Confuciusornis sanctus, Sapeornis chaoyangensis, Rapaxavis pani) and some basal ornithuromorphs (e.g., Schizooura lii, Zhongjianornis yangi).

In caudal view (DNHM D2946) the distal humeri bear no evidence of humerotriciptial or scapulotricipital grooves (Fig. 6). The olecranon fossa is present but poorly developed. A well-developed dorsal supracondylar process is present, clearest on the right humerus (DNHM D2946) as in Longicrusavis houi (O’Connor, Gao & Chiappe, 2010) and Ichthyornis dispar (Clarke, 2004). The flexor process is small and poorly developed; the transversal distal margin is roughly perpendicular from the longitudinal axis of the shaft as in other ornithuromorphs, not angled as in many enantiornithines (Chiappe & Walker, 2002). The cranial surfaces of the humeri are not visible, planted in the matrix.

The radius is straight and roughly half the width of the ulna. The ulna, comparable in length to that of the humerus (Table 1), is exposed in caudal-dorsal view. Remige papillae are absent. The olecranon process is weakly developed. Distally, the ulna’s dorsal condyle is rounded in caudal view. Near its articulation with the radius, it exhibits a small circular depression that may correspond to the radial depression of living birds (Baumel & Witmer, 1993).

The radius and ulna are in articulation with the proximal carpals and the carpometacarpus (Fig. 7). The radiale is in articulation with the radius. The left carpometacarpus, exposed dorsally in DNHM D2946, is completely fused, proximally and distally. Close to the contact between the major (II) and alular (I) metacarpals there is a raised area. Both the major (II) and minor (III) metacarpals are straight (Fig. 7). Proximally, there is a small depression on the minor metacarpal. The proximal end of the intermetacarpal space extends proximal to the level of the distal end of the alular metacarpal (Fig. 7). The alular (I) metacarpal is subrectangular. An extensor process is not absent, only minimally developed in such a way that the proximal end of the alular metacarpal is slightly more expanded than its distal end (Fig. 7), as in some other Cretaceous ornithurmorphs (e.g., Jianchangornis microdonta, Gansus yumenensis, Yixianornis grabaui).

Figure 7: Photograph of the left manus of DNHM D2945/6 in ventral (A; DNHM D2945) and dorsal (B; DNHM D2946) views. Abbreviations: mc I-III, metacarpals I-III; p1-I, phalanx 1 (proximal) of digit I (alular digit); p2-I, phalanx 2 (ungual) of digit I (alular digit); p1-II, phalanx 1 (proximal) of digit II (major digit); p2-II, phalanx 2 (intermediate) of digit II (major digit); p3-II, phalanx 3 (ungual) of digit II (major digit); p1-III, phalanx 1 (proximal) of digit III (minor digit); p2-III, phalanx 2 of digit III (minor digit); ra, radius; sl, semilunate carpal; ul, ulna.

The alular digit bears two phalanges. The second phalanx, a claw, extends slightly past the distal end of the major metacarpal, as described by Zhou & Zhang (2005) for the holotype. The major digit has three phalanges; the proximal phalanx is broad bearing a well-developed, sinusoidal lateral flange (Fig. 7). As in the holotype, the intermediate phalanx is S-shaped (Zhou & Zhang, 2005). The claw of the major digit is much smaller than that of the alular digit. The minor digit bears a single, wedge-shaped phalanx that tapers distally; Longicrusavis houi and the Hongshanornis longicresta holotype bear two phalanges on this digit, the second being extremely reduced (and not an ungual) suggesting that this small phalanx is simply not preserved in DNHM D2945/6.

The ilia are poorly preserved and broken between the two slabs; the left is visible in medial view in DNHM D2945, while portions of the right are preserved in DNHM D2946 in dorsal view. The preacetabular wing has a straight dorsal margin that tapers cranially. The caudal half of the ventral margin of the preacetabular wing defines a broad notch, a condition observed in other Jehol ornithuromorphs (e.g., Archaeorhynchus spathula, Schizooura lii) (Zhou, Zhou & O’Connor, 2012; Zhou, Zhou & O’Connor, 2013b).

A fragment of the pubis is preserved near the midshaft of the right tibiotarsus in DNHM D2946 (Figs. 1–4 and 8). The distal ends of both pubes are in contact and only slightly disarticulated. They are not fused but their flat medial surfaces form a short, expanded symphysis, although a distinct ‘boot’, with a prominent caudal projection, like that of some enantiornithines is absent (Chiappe & Walker, 2002) (Fig. 8). The presence of this distal pubic expansion, identical to that present in Parahongshanornis chaoyangensis, indicates that unlike what was claimed by Li, Wang & Hou (2011), this feature is not diagnostic of the latter species. The cross-section of the shaft of the distal portion of the pubis is oval, with the main axis oriented craniocaudally. No pygostyle or caudal vertebrae are preserved in DNHM D2945/6.

Figure 8: Close up photograph of the right knee (medial view) and pubic symphysis (left laterocaudal view) of DNHM D2946. Note the detail of the gastroliths. Abbreviations: fe, femur; fi, fibula; gl, gastroliths, pu, pubis; ti, tibia. (l) and (r) refer to the left and right pubis, respectively.

The elongate hindlimbs are completely preserved in both slabs as partly bone and partly void (Figs. 1–4). The left femur is preserved in articulation with the left ilium in DNHM D2945—its rounded head is visible through the acetabulum, which is exposed medially. The femoral shaft is only slightly bowed craniocaudally. Its laterodistal end is exposed in DNHM D2946; similar to other Jehol ornithuromorphs (e.g., Yixianornis grabaui), there is minimal development of the fibular trochlea and tibiofibular crest, which are developed in Patagopteryx deferrariisi and ornithurines (Chiappe, 2002; Clarke, 2004).

The tibiotarsus is more than 150% the length of the femur (tibiotarsus: femur = 1.6) (Table 1). The right element is in articulation with the fibula, which is exposed caudally in DNHM D2946. Proximally, the tibiotarsus exhibits a large, well-developed cnemial crest (Fig. 9). This crest, exposed laterally in DNHM D2946, is limited proximally, and projects proximally beyond the proximal articular surface of the tibiotarsus. Its cranial edge develops into an inflated quadrangular prominence that projects laterally (Fig. 9). The morphology of this crest is comparable to that of other Early Cretaceous ornithuromorphs (e.g., Schizooura lii, Yixianornis grabaui) (Clarke, Zhou & Zhang, 2006; Zhou, Zhou & O’Connor, 2012). This cranial prominence is separated by a lateral trough from the proximal articular surface of the tibiotarsus. It is not possible to determine whether the tibiotarsus had one or two cnemial crests; if a cranial cnemial crest were present, it would be embedded in sediment and obstructed by the lateral cnemial crest. In caudal view, the proximal fourth of the tibiotarsus is marked by a robust ridge that slants towards the proximomedial corner of the bone—this feature is not present in Longicrusavis houi, further distinguishing this taxon from Hongshanornis longicresta.

Figure 9: Close up photograph of the left knee (lateral view) of DNHM D2946. Abbreviations: cn, cnemial crest; fc, fibular condyle; fe, femur; fi, fibula; ld, lateral articular facet of tibia; md, medial articular facet of tibia; ti, tibia.

Distally, the tibia is largely fused to the distal tarsals, however the suture of an ample ascending process of the astragalus remains visible on both the left (DNHM D2946) and right (DNHM D2945) tibiotarsi. The distal condyles are preserved in cranial view in DNHM D2945. The lateral condyle is larger than the medial condyle, as in other basal ornithuromorphs (Chiappe, 1996; Zhou & Zhang, 2006), and separated by a wide intercondylar groove (incisura intercondylaris). The cranial surface of the distal end is scarred by a deep extensor sulcus, which ends near a raised area just proximal to the intercondylar groove—a supratendinal bridge like that of more advanced ornithurines is not developed. The lateral epicondyle is minimally developed and there is only a slightly developed, crescent-like lateral epicondylar depression. Caudally, neither the lateral nor medial crests of the cartilaginous trochlea of the tibia (trochlea cartilaginous tibialis) are developed. These crests are slightly developed in Yixianornis grabaui and well developed in Apsaravis ukhaana (Norell & Clark, 2001; Clarke, Zhou & Zhang, 2006). The fibula is very slender (Figs. 8 and 9)—its distal end does not seem to extend beyond the midpoint of the tibiotarsus.

Both tarsometatarsi are well-exposed in cranial view in DNHM D2945. As reported in the holotype of Hongshanornis longicresta (Zhou & Zhang, 2005), metatarsals II–IV are completely fused to one another. The intercotylar prominence is at best minimally developed. The proximal cotyla are slightly concave—the lateral one is slightly more distally placed than the medial one. Metatarsal III is plantarly displaced so that proximally, metatarsal II and metatarsal IV form ridges defining a recess excavating the central portion of the tarsometatarsus. Such morphology is consistent with that of other basal ornithuromorphs (e.g., Yanornis martini, Yixianornis grabaui, Gansus yumenensis, Ichthyornis dispar). Inside this proximocentral recess there is a foramen located between metatarsals III and IV, and medial to it, a tubercle on metatarsal II (possibly corresponding to the m. tibialis cranialis tuberosity of modern birds). Metatarsal III is the longest; metatarsal IV is slightly shorter, followed by the even shorter metatarsal II (Fig. 10). Metatarsal I is robust and fairly straight, with a concave medial margin. A distal vascular foramen is located between metatarsals III and IV as in Longicrusavis houi; the distal margin of the foramen is raised. The foramen seems to penetrate the bone at an oblique angle—from cranial to plantar surfaces. All trochleae appear to be ginglymous.

Figure 10: Close up photograph of the feet and rectrices of DNHM D2945. Abbreviations: mt, metatarsals; re, rectrices; I-IV, digits I-IV.

Digit III is the longest (Table 1); digit II is substantially shorter than IV (Fig. 10). Digit I is short and slender. All the pedal phalanges are long and slender and decrease in length distally. The phalanges of the second and third digits are approximately 2/3 the length of the preceding phalanx. The phalanges of digit IV are subequal, but still slightly decrease in length distally. The ungual phalanges bear distinct flexor tubercles. The morphology of the distal tarsometatarsus and the proportions of the pedal phalanges are consistent with cursorial function (Hopson, 2001), as in other Early Cretaceous ornithuromorphs.