Yangavis confucii, Wang & Zhou, 2019

YANGAVIS CONFUCII SP. NOV.

Holotype: IVPP V18929 View Materials , a nearly complete and articulated specimen preserved on a single slab ( Figs 1, 2; Table 1).

Locality and horizon: Sihedang locality, near Lingyuan City, Liaoning Province, northeastern China; Lower Cretaceous, Yixian Formation (Aptian).

Etymology: The specific name is derived from Confucius, intended to show its close affinity with other confuciusornithids.

Diagnosis: A confuciusornithid bird distinguishable from other confuciusornithids by the following features (autapomorphies denoted by asterisks): maxilla with a rectangular dorsal process that bears a rostroventral–caudodorsally oriented ridge laterally*; lateral facets of centra of thoracic vertebrae without broad fossa present in C. sanctus ; elongate forelimb with a (humerus + ulna)/(femur + tibiotarsus) length ratio 1.19 (compared with 0.91–1.08 in C. sanctus , 1.06 in C. dui , 0.93 in C. hengdaoziensis and 0.96 in E. zhengi ); major digit claw not reduced as in other confuciusornithids; metatarsal II with a dorsal tubercle on its lateral surface (centrally located in C. sanctus ); hallux more than half the length of pedal digit II (less than half in C. sanctus and E. zhengi ); hallucal claw as large as that of pedal digits II and IV (smallest in other confuciusornithids); and pedal digit III longer than tarsometatarsus (in contrast to other confuciusornithids).

ANATOMICAL DESCRIPTION

SKULL

The skull is preserved in the left lateral view, and most of the elements remain in articulation ( Fig. 3A, B). The rostral ends of the premaxillae, although broken, exhibit their ventral surfaces, showing that the premaxillae are fused rostrally, as in other confuciusornithids ( Chiappe et al., 1999; Wang et al., 2018). Only the left frontal process of the premaxilla is visible, which obscures whether the frontal processes are fused with each other medially. As in C. sanctus ( Elzanowski et al., 2018) , the frontal processes are long and project beyond the rostral ends of the frontals. The maxillary process is short, approximately one-quarter the length of the frontal process. The maxilla is triradiate, with a short dorsal process and long premaxillary and jugal processes. The premaxillary process extends slightly beyond the rostral end of the external naris. That process fits into a longitudinal depression along the ventral margin of the maxillary process of the premaxilla laterally, together forming the premaxilla–maxilla complex. In contrast, the two elements articulate via an oblique suture in C. sanctus ( Elzanowski et al., 2018) . The premaxillary process gently expands its dorsoventral height caudally and reaches its highest point at the level of the dorsal process. The jugal process is approximately half the length of the premaxillary process and tapers caudally to a pointed end. The dorsal process is robust and rectangular in the lateral view, with expanded dorsal margin. It projects dorsally without the caudal direction seen in Archaeopteryx , S. chaoyangensis and more crownward birds ( O’Connor & Chiappe, 2011; Wang & Hu, 2017). The dorsal process contacts the frontal process of the premaxilla ventrally and thus excludes the nasal from the caudal margin of the external naris. However, this configuration is likely to be the result of preservation, because the antorbital elements appear to be pushed forward ( Fig. 3B). A rostroventral–caudodorsally oriented ridge extends along the lateral surface of the dorsal process, dividing it into a rostrodorsal and a caudoventral depression ( Fig. 3B). The rostrodorsal depression is larger but less concave than the caudoventral depression. The morphology of the maxillary dorsal process in IVPP V18929 View Materials differs greatly from that of other confuciusornithids. In C. sanctus , the dorsal process is trapezoidal in the lateral view ( Wang et al., 2018), with the caudal margin rapidly sloping rostrodorsally from the point halfway down to the conjunction with the maxillary body ( Fig. 3C, D). In C. dui , the dorsal process is triangular in lateral view ( Hou et al., 1999), whereas it is rostrocaudally compressed in E. zhengi ( Zhang et al., 2008) . The rostral portion of the jugal is laminar and forms a concave facet to receive the caudal end of the maxilla laterally. Distally, the jugal becomes rod like. Given that it is overlain by other elements, it is unclear how far the jugal extends rostrally and caudally. The nasal appears to be rhombic and bears a short maxillary process. A premaxillary process is absent, as in C. sanctus and C. dui (e.g. IVPP V12352 View Materials , V13172 View Materials , W.M. personal observation; Hou et al., 1999). In contrast, the premaxillary process of the nasal is well developed in other non-ornithothoracine birds, including S. chaoyangensis and Archaeopteryx ( Mayr et al., 2005; Gao et al., 2012). A fragment placed ventral to the left nasal is tentatively interpreted to be the lacrimal; whereas in C. sanctus , the lacrimal is crescentic and perforated by a fenestra ( Li, 2010; Elzanowski et al., 2018; contrary to Chiappe et al. 1999). The postorbital region is severely crushed, leaving little anatomical information.

The mandible is robust. As in C. sanctus ( Fig. 3C, D), the ventral margin of the dentary slopes ventrally and forms a convexity immediately rostral to the rostral mandibular fenestra ( Fig. 3A, B). As in other confuciusornithids ( Chiappe et al., 1999; Li, 2010; Wang et al., 2018), the dentary is bifurcated caudally. A third caudal process of the dentary was considered present in C. sanctus (mediodorsal process in Elzanowski et al., 2018), which runs medial to the surangular; however, we could not confirm this structure in all the specimens of C. sanctus available for study. The dorsal rami of both sides are obscured by other elements caudally, preventing the assessment of its length. In other confuciusornithids, the dorsal ramus is shorter than the ventral ramus ( Fig. 3C). The ventral ramus is thick and extends to the caudal end of the rostral mandibular fenestra. The surangular, articular and angular are fused with one another, forming the dorsal, caudal and ventral boundaries of the rostral mandibular fenestra. The surangular bears a ventral process, but the exact shape of the ventral process is unknown owing to preservation. In C. sanctus , the ventral process is triangular ( Fig. 3D; Elzanowski et al., 2018), but this structure is absent in C. dui . As in C. sanctus and E. zhengi ( Zhang et al., 2008; Elzanowski et al., 2018), the surangular is perforated by a round caudal mandibular fenestra caudally. The right splenial is exposed in the medial view. Rostrally, the bone terminates well caudal to the rostral end of the dentary and extends caudally to the level of the caudal mandibular fenestra. The splenial overlies the angular medially. The ventral margin of the splenial is straight and remains in line with the ventral margin of the angular, making the angular barely visible in medial view. A dorsal process projects from the rostral half of the splenial that divides the latter into rostral and caudal rami. Owing to the overlap by the left dentary, the shape of the dorsal process is unclear. The caudal ramus is more than twice as long as the rostral ramus. Elzanowski et al. (2018) described the splenial of C. sanctus as a three-pronged shape, with two rostral rami and one caudal ramus, which is readily different from that of Archaeopteryx . Among other confuciusornithids, the splenial can be observed only in E. zhengi , where the splenial, although not fully exposed, appears to bear a dorsal process ( Zhang et al., 2008).

AXIAL SKELETON

The atlas and axis cannot be identified. Six articulated cervical vertebrae are recognized ( Fig. 1). Ten cervical vertebrae (including the atlas and axis) are estimated in C. sanctus on the basis of complete specimens ( Li, 2010); therefore, it is likely that these six cervical vertebrae represent the third to the eighth cervical vertebrae, given their proximity to the skull. In contrast, C. dui is estimated to have fewer than eight cervical vertebrae ( Hou et al., 1999). The costal and transverse processes are fused to enclose the transverse foramen. The costal processes terminate at the midpoint of the centra. The ventral surface of the centrum is keeled. A carotid process is lacking.

The thoracic series is nearly complete, with 11 thoracic vertebrae recognized ( Fig. 4). The lateral surfaces of the centra show no signs of deep suboval excavations of the kind reported in C. sanctus and C. hengdaoziensis ( Chiappe et al., 1999; Li, 2010). In E. zhengi , the thoracic centra are weakly, laterally excavated ( Zhang et al., 2008). The centra are ~180% as long as they are high. The transverse processes are weakly directed caudally, and they are longer than the centra in dorsoventral height. A synsacrum is completely fused with seven incorporated synscral vertebrae, reminiscent of other confuciusornithids. The caudal articular facet of the centrum of the last synsacral vertebra is flat. Only three free caudal vertebrae are recognized. They are preserved close to the caudal end of the synsacrum and thus are interpreted as the three cranial caudal vertebrae. The transverse processes of the second and third caudal vertebrae are directed laterally, and they are longer than the mediolateral width of the associated centrum. The cranial articular facet is nearly flat. A pygostyle is not preserved.

PECTORAL GIRDLE

As in other confuciusornithids ( Chiappe et al., 1999), the coracoid and scapula are fused into a scapulocoracoid ( Fig. 4), a feature otherwise only known in Chongmingia zhengi among Mesozoic birds ( Wang et al., 2016). The long axes of these two bones form an acute angle, as in more advanced birds ( Olson & Feduccia, 1979), in contrast to the right angle observed in C. sanctus , C. hengdaoziensis and C. zhengi ( Chiappe et al., 1999; Wang et al., 2016). The length ratio of the coracoid/scapula is ~0.5. The left and right coracoids are exposed in the medial and lateral views, respectively. The proximal third of the shaft of the coracoid is dorsoventrally thick, but progressively thins distally. Unlike C. sanctus ( Chiappe et al., 1999) , an acrocoracoid process is developed ( Fig. 4B). The acrocoracoid process lies dorsal and cranial to the glenoid facet, as in S. chaoyangensis ( Zhou & Zhang, 2003a) . This glenoid facet is situated more dorsally than the acrocoracoid process in Archaeopteryx and some non-avian paravians, such as Sinornithosaurus ( Xu et al., 1999; Mayr et al., 2007). The proximal margin between the acrocoracoid process and the glenoid facet is concave. The scapula measures ~70% the length of the humerus. The left glenoid facet is well exposed laterally, showing that the scapular contribution is greater than that of the coracoid and that the former is more concave ( Fig. 4A). The glenoid facet of the scapula is craniocaudally longer than transversely wide. The furcula is preserved in caudal view. As the typical condition of confuciusornithids ( Chiappe et al., 1999), the bone is robust and boomerang shaped, without a hypocleidium. The clavicular ramus is dorsoventrally compressed, and its cross-section is suboval. In contrast, the ramus bears a caudal furrow in C. sanctus and C. hengdaoziensis , making the cross-section resemble ‘∞’ ( Chiappe et al., 1999; Wang et al., 2018).

The sternum is exposed ventrally ( Fig. 4). A suture line runs along the entire length of the sternal body, indicating that the left and right plates were incompletely fused medially at the time of death. The ventral surface of the sternum is essentially flat. In contrast, C. sanctus bears a faint ridge on the ventral surface that probably represents an incipient carina ( Chiappe et al., 1999). The absence of this structure probably results from ontogenetic variation, given that the carina ossifies relatively late in development ( Parker, 1868). As in C. sanctus ( Zhang et al., 2008) , the cranial margin bears a central notch. A pair of lateral processes is developed, but their morphology is obscured by preservation. The sternum forms a V-shaped caudal margin that is notched centrally, a feature absent in other confuciusornithids ( Zhang et al., 2008; Li, 2010). However, that notch could be ascribed to the incomplete fusion between the sternal plates distally. Six dorsal ribs of the right side are preserved in association with their thoracic vertebrae. Uncinate processes are preserved in articulation, although not in a state of fusion, with the dorsal ribs. The uncinate processes form an acute angle with the dorsal ribs and cross at least one subsequent rib. Several short, slender elements preserved near the right femur are interpreted as the gastralia. They are tapered at both ends.

PELVIC GIRDLE

The pelvic bones, ilium, ischium and pubis, are poorly preserved ( Fig. 1). The medial surface of the proximal shaft of the ischium is concave, with the ventral margin flaring medially. The iliac peduncle of the ischium is narrower than the pubic peduncle. The ischium bears a large proximodorsal process. The pubes contact caudally to form a symphysis along the caudal third of their length.

FORELIMB

The forelimb is relatively long with respect to the hindlimb, with a (humerus + ulna)/(femur + tibiotarsus) ratio of 1.19, compared with 0.93 for C. hengdaoziensis , 0.96 for E. zhengi , 1.06 for C. dui and 0.91–1.08 for C. sanctus (Supporting Information, Table S1). The humeri are robust and exposed cranially. As in C. sanctus , C. dui and C. hengdaoziensis , the proximal end expands dorsoventrally, with a large deltopectoral crest. The crest is triangular, projects dorsally and extends for one-third of the humeral shaft. Its maximal width exceeds the mid-shaft width of the humerus. The deltopectoral crest is perforated by an oval fenestra, a feature present in all confuciusornithids except E. zhengi ( Chiappe et al., 1999; Zhang et al., 2008). As in C. sanctus and C. hengdaoziensis , the cranial surface of the deltopectoral crest bears a ridge along its proximodorsal margin ( Wang et al., 2018). A bicipital crest is absent. The distal ends of the humeri are crushed, leaving little information, except that the distal margin is perpendicular to the long axis of the shaft. The ulna is shorter than the humerus, as in other confuciusornithids and Archaeopteryx ( Mayr et al., 2007; Wang, 2014), but the former is longer in S. chaoyangensis and J. prima amongst non-ornithothoracine birds ( Zhou & Zhang, 2003b). The ulna is nearly straight. The olecranon is poorly developed. The dorsal cotyle is weakly concave and slopes dorsally ( Fig. 5A, B). A bicipital tubercle is lacking. No traces of quill knobs for the attachment of the secondary remiges are identified. The radius is straight, with a radius/ulna mid-shaft width ratio of ~0.61. The distal end of the radius is expanded.

The major and minor metacarpals are fused proximally with the semilunate carpal ( Fig. 5A, B). As in other non-ornithothoracine birds (Wang et al., 2017), the alular metacarpal is unfused with the major metacarpal. The alular metacarpal is rectangular and measures approximately one-third the length of the major metacarpal, proportionally equal to C. sanctus but shorter than in C. hengdaoziensis (length ratio ~0.5). The proximal margin is concave and the medial margin straight, indicating that an extensor process is absent. The major metacarpal is straight and is the most robust of the three metacarpals. The minor metacarpal terminates proximal to the distal end of the major metacarpal. The former is about half the width of the latter. The minor metacarpal is gently bowed caudally, forming an intermetacarpal space that is narrower than the minor metacarpal.

The manual digit formula is 2-3-4 ( Fig. 5B). All the unguals are associated with horny sheaths, rendering the claws strongly curved, with the distal tip surpassing the ventral level of the proximal end of the ungual. The unguals are excavated by lateral neurovascular sulci and bear well-developed flexor processes. As in other confuciusornithids, the proximal phalanx of the alular digit projects further distally than the major metacarpal, a primitive feature shared with Archaeopteryx . The opposite is true in J. prima , S. chaoyangensis and more crownward birds ( Wang et al., 2016). As is typical of confuciusornithids ( Fig. 5C–E), the ungual of the alular digit is larger than in other digits. The proximal and intermediate phalanges are subequal in length in the major digit. The intermediate phalanx of the major digit is bowed, a feature characteristic of confuciusornithids ( Fig. 5; Wang et al., 2018). IVPP V18929 View Materials has a normal-sized claw on the major digit compared with other confuciusornithids. The most distinguishable feature of confuciusornithids is the presence of a progressively reduced major digit claw ( Chiappe et al., 1999; Zhang et al., 2008, 2009; Wang et al., 2018); the ungual is wedge shaped in lateral view, without a flexor process and the lateral neurovascular sulci, and is less than half the length of the ungual of the minor digit ( Fig. 5C–E). However, in IVPP V18929 View Materials the ungual of the major digit is only slightly smaller than that of the minor digit. Moreover, it retains the flexor process and the neurovascular sulci. As in other confuciusornithids, the proximal phalanx of the minor digit is the shortest manual phalanx.

HINDLIMB

The femur is ~80% the length of the tibiotarsus. The femoral head is situated on a short neck. The proximal tarsals and the tibia are fully fused, forming a true tibiotarsus ( Fig. 6A). No cnemial crest is developed. The fibular crest is low and extends approximately one-third of the distance from the proximal end. In C. sanctus ( Chiappe et al., 1999) , an oval tubercle is present on the cranial surface of the distal tibiotarsus, but this structure is absent in IVPP V18929 View Materials ( Fig. 6A). Owing to preservation, the presence of this tubercle cannot be verified in C. dui , C. hengdaoziensis and E. zhengi . The medial condyle is transversely wider than the lateral condyle, but their proximal margins end at the same level. The left fibula is exposed in the lateral view. The bone is broad proximally and rapidly narrows distally to a splint ( Fig. 6B). The fibula bears a craniolaterally directed tubercle for the insertion of m. iliofibularis at the level approximately one-quarter of the distance from the proximal end of the tibiotarsus. In contrast, that tubercle is laterally directed in the basal ornithuromorphs Patagopteryx deferrariisi and Vorona berivotrensis ( Forster et al., 1996) , but caudally or caudolaterally oriented in Ornithurae ( Chiappe, 1996). The distal end of the fibula is missing, precluding assessment of its total length.

Metatarsals II–IV are fused proximally with the distal tarsals, but remain separated distally ( Fig. 6), a primitive feature in non-ornithuromorph birds ( Wang & Zhou, 2017b). A short, delicate bone preserved lateral to metatarsal IV is most likely to be the fifth metatarsal. The proximal articular facet of the tarsometatarsus is essentially flat. A short slit exists between the proximal ends of metatarsals III and IV ( Fig. 6B), resembling the condition in C. sanctus , which has been considered to be the proximal vascular foramen ( Chiappe et al., 1999). Metatarsals III and IV are of subequal width, and metatarsal II is thinner. Metatarsal III is the longest, followed by metatarsal IV and then metatarsal II. As in C. sanctus , metatarsal II bears a dorsal tubercle proximally, which is most probably for the insertion of m. cranialis tibialis ( Baumel & Witmer, 1993). However, the tubercle is laterally positioned and contacts metatarsal III ( Fig. 6A), rather than being centrally located as in C. sanctus ( Chiappe et al., 1999) . Furthermore, a tubercle is weakly developed at the same level on the dorsal surface of metatarsal III in C. sanctus but is lacking in IVPP V18929 View Materials . Metatarsal I is poorly preserved, with few anatomical features visible.

The pedal digit formula is 2-3-4-5. The feet of IVPP V18929 View Materials are readily distinguishable from other confuciusornithids both proportionally and morphologically ( Fig. 6). First, digit I is more than half the length of digit II, as in C. hengdaoziensis , but the opposite is true in C. sanctus , C. dui and E. zhengi . Second, digit III is longer than the tarsometatarsus, but the latter is longer in all other confuciusornithids. Third, the claw of digit I is as large as that of digits II and IV, but it is the smallest in other confuciusornithids ( Wang et al., 2018; Fig. 6). Digit III is the longest of the pedal digits, followed by digit IV and then digit II. The penultimate phalanx of digit II is the longest non-ungual pedal phalanx. In digit III, the non-ungual phalanges decrease in length distally.