Type species.—Musivavis amabilis n. gen. n. sp., only known species, from the Aptian of northeastern China.

Diagnosis.—As for the type species, by monotypy.

Etymology.—The generic name is derived from the Latin “musivum” (mosaic, decoration with small stone tesserae) and “avis” (bird).

Remarks.—As for the type species.

Holotype and only known specimen.—MHGU-3000, a nearly complete and articulated skeleton preserved in a single slab.

Diagnosis.—Musivavis amabilis n. gen. n. sp. can be referred to Enantiornithes by possessing several synapomorphies of this clade, such as the sternum with nearly equal length and width, the “Y”-shaped furcula with a long hypocleideum, the minor metacarpal extending farther distally than the major metacarpal, and the fourth metatarsal relatively thin (Chiappe and Walker, 2002). It can be further referred to the “Bohaiornithidae-grade” group by possessing some of the diagnostic features of this family, such as the subconical teeth with tapered and slightly caudally recurved tips, the sternum with lateral trabecula projected caudolaterally, the omal end of furcular ramus with blunt expansion, and the tapered pygostyle without abrupt distal constriction (M. Wang et al., 2014). It is distinguishable from the bohaiornithids by possessing the following unique combination of features (local autapomorphies denoted by asterisks): the pygostyle abruptly tapers distally (shared with Gretcheniao); the sulcus excavating the epicleideal ramus of the furcula is very deep and extends distally onto the proximal third of the hypocleideum*; a sharp keel runs along the whole ventral surface of the furcula, from the junction of the epicleideal rami to the distal tip of the hypocleideum; the flat cranial margin of the sternum becomes curved near the craniolateral process*; the sternum possesses craniolaterally projected craniolateral processes* (in bohaiornithids, only Zhouornis possesses craniolateral processes, which project laterally); the xiphoid process of the sternum extends nearly to the same level of the distal end of the lateral trabecula and expands as a blunt distal end (shared with Bohaiornis); and the first alular phalanx bears a transversally expanded proximal end followed by a gracile shaft, resulting in a concave proximomedial corner of the bone*.

Occurrence.—The holotype was recovered from the Lower Cretaceous Jiufotang Formation (Aptian) at Shangheshou locality, Chaoyang City, Liaoning Province, China.

Description.—The skull is mainly preserved in ventrolateral view (Fig. 3). The right premaxilla is preserved in lateral view. It gradually tapers rostrally and slightly expands dorsoventrally in the middle part, differing from the robust premaxilla of the bohaiornithids (M. Wang et al., 2014) and the elongated premaxilla of the longipterygids (O'Connor and Chiappe, 2011), but similar in overall proportions to those of the Cathayornithidae (Wang and Liu, 2016). The frontal process of the right premaxilla is elongate and projects caudodorsally, but does not extend to the orbit, comparable with the condition in bohaiornithids (M. Wang et al., 2014; Hu et al., 2020). As in other bohaiornithid-like taxa, the maxilla is robust, with a straight ventral margin and gently expanded dorsal margin. The nasal is too broken to determine the exact anatomical features. The right lacrimal is preserved in dorsolateral view and appears to be “T”-shaped. The rostral ramus is oriented rostroventrally, but the exact length cannot be determined because it is overlapped by the right maxilla. The caudal ramus projects caudodorsally and the ventral ramus extends caudoventrally. The dorsal margin is concave at the middle part, as reported in Pengornis, Longusunguis, and Parabohaiornis, but different from the straight margin of Pterygornis (M. Wang et al., 2016). The frontal is rostrocaudally elongate, narrow rostrally, and domed caudally, as in other enantiornithines (e.g., Protopteryx, Parapengornis, Bohaiornis, and Shangyang; Zhang and Zhou, 2000; M. Wang et al., 2014; Hu et al., 2015; Wang and Zhou, 2019). The rostral portion of the frontal is rectangular-shaped, with a triangular depression, a feature that is also present in the enantiornithine Piscivorenantiornis inusitatus (Wang and Zhou, 2020) and the ornithuromorph Archaeorhynchus spathula (Zhou et al., 2013). A deep oval fossa is located between the forked rostral end and the expanded dome of the frontal. This fossa, which is natural judging by the smooth margins and surface, has not been documented previously in Mesozoic birds. However, it is premature to conclude this oval fossa is a diagnostic character because the skulls of fossil birds are typically incompletely preserved or exposed in views that prevented observing this region. It probably represents the depression of the olfactory bulb and the articulation with the mesethmoid (Baumel et al., 1993). The ventral margin of the frontal is slightly concave and forms the dorsal margin of the orbit. The dorsal margin of the frontal is robust and strongly expanded dorsally. The sclerotic ring is nearly completely preserved and, as preserved, is composed of 14 thin imbricated elements. The ventral seven sclerotic ossicles are much larger than the dorsal seven ossicles. The square-shaped sclerotic ossicles are plate-like and have rounded margins, similar to those of Longusunguis (Hu et al., 2020). The quadrate is partly exposed in lateral view. No orbital process can be observed due to the overlapping by other bones. The rostral portions of the two strap-like jugals are exposed in lateral view, and disarticulated from the jugal processes of the maxillae. The right parietal is displaced above the skull and close to three cervical vertebrae. This condition proves that the parietal is not fused with the frontal. The palatal elements are mostly overlapped by the cervical vertebrae.

The two dentaries are not fused at the symphysial area. The rostral end of the dentary tapers rostrodorsally. In its rostral two-thirds, the ventral margin of the dentary is straight, while the caudal one-third is slightly curved and tapered caudoventrally, as in other enantiornithines (Chiappe et al., 2019). A series of nutrient foramina is visible on the rostral part of the lateral surface of the right dentary. The right angular is robust, with tapered rostral end and expanded caudal end, measuring ∼75% the length of the right dentary. The right surangular is preserved in lateral view and is shorter than the right angular, with extremely tapered rostral and caudal ends, and dorsally expanded middle part. The left surangular is articulated with the left dentary. The hyoids are straight and slender.

Teeth are present in the premaxilla, maxilla, and dentary (Fig. 3). As in most enantiornithines, the premaxillae possess four teeth, which are small (< 1 mm) and subconical in shape. The tooth root is rectangular-like in lateral view and nearly the same height as the crown. There is a slight constriction between the root and crown. The crown slightly expands basally and becomes sharp apically with the caudally recurved labial side and the relatively straight lingual side. The premaxillary teeth are most similar in size and morphology to the teeth of Eoenantiornis (Zhou et al., 2005). They strongly differ from the large robust teeth in bohaiornithids, the large recurved and laterally compressed teeth of Longipteryx, and the numerous small and blunt teeth of Pengornis (O'Connor et al., 2013). The interalveolar space is nearly equal to the crown width. Six teeth are visible in the right maxilla. The completely exposed maxillary teeth are much larger than the premaxillary teeth. The dentary teeth are smaller than the premaxillary and maxillary teeth but similar in morphology.

The cervical vertebrae are partially disarticulated (Fig. 4). The anterior four cervical vertebrae are preserved between the skull and the right coracoid. Three cervical vertebrae are preserved above the skull, and two additional cervical vertebrae are preserved near the furcula and the left coracoid. The cervical vertebrae are short and robust, with prominent ventral keels and zygapophyses. Three thoracic vertebrae are visible. These exhibit rectangular centra and distinctly concave posterior inter-central facets, as in most enantiornithines (Chiappe and Walker, 2002). The synsacrum is completely fused and includes at least seven vertebrae, with no more-detailed features that can be observed due to poor preservation (Fig. 5). The free caudal vertebrae are poorly preserved. The pygostyle is preserved in latero-ventral view and abruptly tapers distally (Fig. 5), similar to the shape of the distal end in Gretcheniao (Chiappe et al., 2019), and in contrast to the gently tapered distal end in other bohaiornithids (M. Wang et al., 2014), and different from the more conspicuous constrictions in most enantiornithine pygostyles (e.g., Boluochia, Shanweiniao, and Rapaxavis; Zhou, 1995; O'Connor et al., 2009, 2011). The exact length of the pygostyle cannot be determined due to the broken proximal end. Two thoracic ribs are preserved next to the left humerus, which extend dorsally, defining an obtuse angle with the corresponding straight shaft (Fig. 6), differing from the acute angle in Parabohaiornis (M. Wang et al., 2014). Six sternal ribs are preserved adjacent to the right side of the sternum. The middle three sternal ribs are longer and more recurved than the cranial and caudal sternal ribs. Six sternal ribs are also preserved close to the left side of the sternum, but are partially overlapped by the left scapula or are broken.

Both coracoids are preserved in dorsomedial view and are strut-like (Figs. 6, 7). The supracoracoid nerve foramen is visible in the right coracoid. The foramen is placed inside a distinct fossa excavating the medial margin of the coracoid. The acrocoracoid process is well developed and rectangular, with the long axis aligned with the long axis of the coracoid shaft. The coracoid bears a prominent tubercle forming the convex scapular facet, a feature present in the majority of enantiornithines (Chiappe and Walker, 2002; M. Wang et al., 2014). The scapular facet prominently projects medially. CT scans reveal the sternal margin of the coracoid, which is relatively concave and slightly expanded mediolaterally (Fig. 7; Supplementary data 3, 4): the ratio of sternal margin width to coracoid shaft length is 44%. This condition is similar to most bohaiornithids but different from that of Gretcheniao, in which the sternal margin is less expanded mediolaterally (Fig. 8) (M. Wang et al., 2014; Chiappe et al., 2019; Hu et al., 2020). The scapula is straight and long, nearly 87% the length of the humerus. The acromial process is well developed and aligned to the long axis of the scapular shaft. The glenoid fossa is prominently concave and oriented laterally. The scapular morphology is similar to those of most bohaiornithids, Eoenantiornis, and Pterygornis (M. Wang et al., 2016), but different from the sagittally curved scapula of Zhouornis, Fortunguavis, and Dunhuangia (M. Wang et al., 2015), and from the laterally projected acromial process of Gretcheniao (Chiappe et al., 2019). A shallow elliptical fossa excavates longitudinally the distal half of the lateral surface of the scapula. CT scan reveals that the furcula is Y-shaped with a prominent hypocleideum, nearly 70% the length of the furcular ramus (Fig. 7; Supplementary data 5, 6). The hypocleideum is comparable in extent to other enantiornithines, being shorter than the furcular ramus, and gradually tapered distally. The epicleideal rami are relatively straight, with expanded and robust omal ends, forming an angle of ∼45° at the junction, a value which is the same as that of Gretcheniao and Zhouornis, and smaller than that of Shenqiornis (50°), Sulcavis (60°), Bohaiornis (60°), and Parabohaiornis (60°). The expanded omal ends of the furcular rami are also consistent with those in Bohaiornithidae, but different from the tapered omal ends of Gretcheniao and most enantiornithines (Chiappe et al., 2019). The right furcular ramus, which is exposed in visceral view, shows a deep fusiform sulcus running along the whole ramus and extending onto the proximal third of the hypocleideum (Fig. 7; Supplementary data 5, 6). In other enantiornithines, the sulcus is either shallower or less expanded distally, not reaching the hypocleidum (e.g., Parapengornis; Hu et al., 2015). The sulci on both furcular rami are clearly visible in the CT scans (Fig. 7; Supplementary data 5, 6). A sharp keel runs along the ventral surface of the furcula, from the junction of the epicleideal rami to the distal tip of the hypocleideum (Supplementary data 5, 6).

The sternal plate, exposed in dorsal (visceral) view, is short and broad, with the width and length subequal (Fig. 6), as in other members of Enantiornithes (Chiappe and Walker, 2002). The cranial margin is flat at midline and curved near the craniolateral process. In other bohaiornithid-like taxa, craniolateral processes are present in Zhouornis (Zhang et al., 2013), but absent in Bohaiornis, Parabohaiornis, and Longusunguis (M. Wang et al., 2014). Furthermore, the process extends craniolaterally and describes a nearly right angle, different from the laterally projected craniolateral process of Zhouornis, but similar to that of Pterygornis and Shangyang (M. Wang et al., 2016; Wang and Zhou, 2019). Craniolateral processes are hypothesized to ossify from separate centers and develop late in certain taxa (e.g., Rapaxavis and Concornis; M. Wang et al., 2015), which suggests that Musivavis n. gen. was not juvenile at the time of death. There is a very small notch on the lateral margin, similar to the condition in Longusunguis and Zhouornis, but different from the smooth lateral margin in Bohaiornis and Parabohaiornis (Hu et al., 2020). The lateral trabecula is oriented caudolaterally and expands into a triangular process distally that expands more medially (resulting in a more acuminate medial corner) than laterally, a morphology consistent with most bohaiornithids (except for Longusunguis) and several Cathayornithidae (e.g., Cathayornis, Eocathayornis, and Houornis) (M. Wang et al., 2015). As in Bohaiornis, Eocathayornis, Shanweiniao, and Longirostravis, the xiphoid process extends nearly to the same level as the distal end of lateral trabecula and slightly expands transversely to be a blunt distal end (Zhang et al., 2013). The xiphoid process projects farther caudally than the lateral trabecula and flares slightly mediolaterally at the distal end in Parabohaiornis (M. Wang et al., 2014). In other enantiornithines, the xiphoid process is tapered distally (e.g., Cathayornis, Houornis, Eoenantiornis, Longipteryx, and Rapaxavis; Zhang et al., 2013; M. Wang et al., 2015). As in Bohaiornis, Pterygornis, Cathayornis, and Eoenantiornis, the intermediate trabecula is short and oriented mediocaudally, contrary to the laterocaudally oriented intermediate trabecula in Parabohaiornis (M. Wang et al., 2014) (Fig. 9).

The left forelimb is nearly complete and articulated (Fig. 10). It is nearly the same length as the hindlimb ([humerus + ulna + carpometacarpus]/[femur + tibiotarsus + metatarsal III] = 1.0). The intermembral index is similar to those of Gretcheniao (1.1) and Bohaiornithidae (0.9–1.1) (Chiappe et al., 2019; Hu et al., 2020), a smaller value than that of other enantiornithine groups (e.g., >1.3 in Pengornithidae and ∼1.2 in Longipterygidae) (X.-R. Wang et al., 2015; Hu et al., 2020).

The humerus is slightly S-shaped and shorter than the ulna and radius. Proximally, the deltopectoral crest expands dorsally and decreases caudally at the first one-third of the humeral shaft. A prominent excavation runs along the dorsal margin of the deltopectoral crest. The bicipital crest is robust and projects anteroventrally, forming a deep notch with the humeral shaft (Fig. 10). The bicipital crest is not excavated by any fossa or foramen; in contrast, a large and deep fossa excavates the bicipital crest in Gretcheniao and Linyiornis (Chiappe et al., 2019; Y. Wang et al., 2016). Distally, the dorsal condyle is broad and projects almost perpendicular to the long axis of the humeral shaft. The ventral condyle is not as developed as the dorsal condyle, only slightly extended beyond the distal end of the humerus. A deep and wide notch separates the dorsal and ventral condyles. The ulna is almost the same length as the radius, but considerably more robust and wider than the latter. The proximal part of the ulna is slightly curved, while the distal part is straight. The radius is relatively gracile and straight. The distal end of the radius seems slightly expanded. The carpals are not fused with the metacarpals. The semilunate carpal is preserved between the ulna and the minor metacarpal. The ulnare overlaps the radiale, preventing a detailed description of size and morphology of the latter.

As in other bohaiornithid-like taxa, the alular metacarpal is sub-rectangular in shape and ∼29% the length of the major metacarpal (M. Wang et al., 2014; Chiappe et al., 2019). The proximal ends of the major and minor metacarpals are completely fused together (Fig. 10). The major metacarpal, the most robust element in the metacarpus, is straight and uniformly wide. The minor metacarpal is nearly straight and extends farther distally than the major metacarpal, as in all enantiornithines (Chiappe and Walker, 2002). The relatively straight minor metacarpal and the narrow intermetacarpal space are similar to the condition in Longipteryx, Gretcheniao, and Bohaiornithidae, but different from other enantiornithines, which bear a more curved minor metacarpal forming the lateral margin of a relatively expanded intermetacarpal space (e.g., Hebeiornis and Eoenantiornis; M. Wang et al., 2014; Chiappe et al., 2019; Hu et al., 2020).

The phalangeal formula of the manus is thus 2-3-1-X-X, consistent with that of Cathayornithidae, Eoenantiornis, Gretctheniao, and Bohaiornithidae (Zhou et al., 2005; O'Con-nor and Dyke, 2010; Chiappe et al., 2019; Hu et al., 2020). As in most enantiornithines, but unlike the bohaiornithids, Protopteryx, Longipteryx, Eoenantiornis, and Monoenantiornis (Zhou et al., 2005; Hu and O'Connor, 2017; Hu et al., 2020), the alular digit terminates at two-thirds of the major metacarpal shaft. The shaft of the first phalanx of the alular digit is straight and slender. The proximal end of the phalanx expands transversely, resulting in a prominent medial corner, which describes a gentle concavity with the medial margin of the phalanx shaft. The alular ungual phalanx is smaller than the major ungual phalanx and not as curved as the latter. In the bohaiornithids, the alular ungual phalanx is larger than or equal to the major ungual phalanx (M. Wang et al., 2014; Hu et al., 2020). The first phalanx of the major manual digit is straight and as wide as the major metacarpal, which is also the longest phalanx. The second phalanx of the major manual digit is almost the same length of the first phalanx of the alular manual digit, but much wider and more robust than the latter. Only the first phalanx of the minor manual digit is preserved.

The pelvic elements are not well preserved. Only the proximal end of the ilium and the distal end of the ischium are visible, but few anatomical characteristics can be determined. CT scan reveals that the pubis is long, curved at the middle portion, and bears a distal boot expanded caudodorsally (Fig. 5). The femur is straight and 82% the length of the tibiotarsus, which is similar to most enantiornithines, but less than the pengornithids (where it is >90% tibiotarsus length; Hu and O'Connor, 2017). The femoral head is prominent and projects almost perpendicular to the long axis of the femoral shaft. Distally, the lateral and medial condyles are well developed and separated by a deep notch. The tibiotarsus is long and robust, nearly the same in width as the femur. Only the proximal ends of both fibulae are preserved, and it is difficult to determine the exact length and the extent of the distal end.

The distal tarsals are fused to the proximal ends of metatarsals II–IV (Fig. 11). Metatarsal I is short (21% the length of metatarsal III) and terminates as a rounded articular facet at the proximal facet of the trochlea of metatarsal II. Metatarsals II–IV are completely fused proximally. Metatarsal III is the longest and metatarsal II is slightly shorter than metatarsal III and metatarsal IV. As in most enantiornithines, metatarsal IV is much thinner than metatarsals II–III. The pedal phalangeal formula is 2-3-4-5-X. The first pedal digit is reversed compared with other digits. The second pedal digit is the most robust toe, as in other bohaiornithid-grade taxa (M. Wang et al., 2014), while the third is the longest. All phalanges have well-developed proximal flexor processes. All pedal digits possess sharp and moderately recurved ungual phalanges. Each pedal ungual phalanx bears prominent neurovascular sulci and shows a distinct horny claw. The fourth pedal ungual phalanx is smaller than the other three pedal digit ungual phalanges, a condition approaching that in bohaiornithids (M. Wang et al., 2014). The first pedal ungual phalanx is the most robust ungual phalanx and nearly equal in length to the second and third ungual phalanges, a condition which is different from that in other bohaiornithids, where the third ungual phalanx is the longest, recalling the condition in pengornithids (M. Wang et al., 2014; Hu et al., 2015, 2020).

Etymology.—The specific name is derived from the Latin “amabilis” (lovely or beautiful), referring to the exquisite preservation of the holotype.

Remarks.—Musivavis amabilis n. gen. n. sp. is much smaller in size than currently known bohaiornithids and pengornithids, and differs from the comparably sized cathayornithids (e.g., Cathayornis, Hebeiornis, Sinornis) in the following features: omal tips of furcula transversely expanded; sternum with craniolateral processes and expanded, triangular-shaped distal end of the lateral trabecula (both features absent in Hebeiornis); deltopectoral crest less prominent relative to humerus shaft (more developed in Cathayornis and Sinornis); minor metacarpal not strongly bowed posteriorly, resulting in a slit-like intermetacarpal space (distinctly bowed in Cathayornis and Sinornis, resulting in a larger intermetacarpal space); ungual phalanx of major digit more curved than the alular ungual phalanx (less curved in Hebeiornis); distal end of metatarsal II subequal in width to distal end of metatarsal III (wider in Hebeiornis and Sinornis); poorly developed flexor tubercles in the pedal ungual phalanges (more prominent in Hebeiornis and Sinornis).