As in the other chaoyangopterid specimens, the specimen represents an immature individual, inferred from the following ontogenetic characters: complete disarticulation of the skull, neural arch and centrum unfused in dorsal vertebrae, sacrum unfused and not fused to pelvis, scapula and coracoids unfused, epiphysis present and not fused with the distal end of the humerus, syncarpals unfused, extensor tendon process not fused with the first wing phalanx, tibiotarsus unfused, and fibrous bone texture in limb bones [36, 37]. Although not as osteologically mature as the Chaoyangopterus specimens, both considered sub-adults, it is roughly the same size with a wingspan estimate of 1.53 m (calculated by adding the lengths of the humerus, ulna/radius, wing metacarpal, and four wing phalanges on both sides [37]). The holotype and referred specimen of Chaoyangopterus have wingspan estimates of 1.71 and 1.45 m, respectively [9, 19]. The holotype of Jidapterus edentus is therefore considered to be a juvenile individual.

The mandible is exposed mostly in ventral view ( Fig 2C and 2D ). As the largest elements of the mandible, the dentaries are preserved in both sides, whereas the other mandibular elements are not identifiable. As in Chaoyangopterus, the mandible is low and anteriorly pointed with an elongated symphysis. This symphysis is 110.7 mm long, about 53.8% of the mandible length. Along the symphysis is a low ventral keel, but this is possibly a taphonomic artifact. At the level of the posterior end of the symphysis, the mandible reaches the maximum depth. Posterior to the symphysis, the dentary reduces in depth. The lateral surface of the mandible is smooth without visible foramina.

A large and plate-like element is identified as a part of the braincase (Figs 1 and 4A ) delimiting a large and rounded cerebral region. It is sculptured by many shallow and strongly curved ridges, especially on its posterolateral corners. Similar structures occur in Chaoyangopterus and Tapejara but appear to be relatively smoother [ 19 , 41 ].

A possible basioccipital ( Fig 3 ) is located between the atlas-axis complex and the fourth mid-cervical vertebra. It is exposed in anterior view and is 10 mm long by 7.7 mm wide. The basioccipital is outlined by the convex dorsal and ventral margins, and two relatively straight lateral margins. The ventral margin is narrow, about half the width of the dorsal margin. Dorsally, the basioccipital is expanded bilaterally to form a rounded dorsal margin, where the occipital condyle is likely obscured. On the anterior surface, the basioccipital bears a smooth and longitudinal oriented depression, forming the ventral floor of the braincase. Laterally, the basioccipital is sculptured by slightly depressed and rugose troughs that are likely the articular facets for the exoccipital.

A fragmentary quadrate ( Fig 3 ) is preserved near the right scapula. It preserves the ventral portion containing the craniomandibular joint and the pterygoid process. Along the ventral margin, the quadrate is about 14.2 mm in width with its lateral half occupied by the craniomandibular joint. The craniomandibular joint is helical and slightly extended more medially than laterally. Medially, the pterygoid process is lamina-like, and positioned more dorsally relative to the craniomandibular joint. The pterygoid process is poorly ossified at its articular facet for the pterygoid.

Near the right fourth wing phalanx, a lamina-like element is identified as a pterygoid ( Fig 1 ). It is broad laterally, sharply pointed anteriorly, and thickened along the putative medial edge. This medial edge is nearly straight and more ventrally extended than the lateral edge. The identified pterygoid is 56 mm in length.

The jugal is displaced from its original position (Figs 1 and 3 ). It is triradiate with a long and slender anterior process, broader lacrimal process, and dorsoposteriorly inclined postorbital process. Below the postorbital process of the jugal, the posterior end of the jugal is damaged along the ventral margin and may include the contact with the quadratojugal. The anterior process points anteriorly; posteriorly, the process is thickened and confluent with the dorsal process. The lateral surface of the anterior process is rugose. The dorsal process is possibly broken off at its dorsal tip. The rest of it is a broad and short base with the posterior margin nearly vertical with respect to the anterior process, and forms a right angle with the postorbital process. In Chaoyangopterus (PMOL-AR00076) [ 19 ], it is much more acute with a 60 degrees angle. The postorbital process is much broader and shorter than the anterior process. The postorbital process is strongly dorsoposteriorly inclined and nearly parallel to the anterior process. Between the dorsal and postorbital processes, the jugal is constricted dorsoventrally, differing from the deep condition in other azhdarchoids. Ventrally, the main body of the jugal is slightly curved ventrally and gradually deeper posteriorly.

A low ridge extends anteroposteriorly along the lateral surface of the premaxillary process (Figs 1 and 2 ). Anteriorly, it is rounded and positioned close to the ventral margin of the premaxillary process. Distally, the ridge becomes confluent with the inner margin of the premaxillary process, forming the dorsal margin of the nasoantorbital fenestra. The nasoantorbital fenestra appears to be large, although its posterior border is not preserved. In contrast, its anterior corner is clearly bounded by the premaxilla-maxilla complex. Ventral to this fenestra, the maxilla is incompletely preserved with a damaged posterior end. It is slender with an estimated length of 77 mm. The palatal aspect of the maxilla is partially exposed, and separated posteriorly from the main part of the maxilla by a longitudinal trough, possibly representing the palatine fenestra.

The rostrum is 22.6 mm high at the anterior margin of the nasoantorbital fenestra. The rostral index (RI), the ratio of the prenarial rostrum length to the height at the anterior margin of the nasoantorbital fenestra [ 38 ], is 5.1. This is significantly larger than 3.7 in Shenzhoupterus [ 5 ], but within the range of 4.8–5.3 in Chaoyangopterus (4.8 in IVPP V13397, estimated from Fig 2f in Wang and Zhou [ 14 ]; 5.3 in PMOL-AR00076). The premaxillary process is well preserved forming the dorsal rim of the nasoantorbital fenestra, but its posterior end is damaged and isolated from other elements. This process is slender, elongate, and gradually tapering posteriorly, with a length of 122 mm along the nasoantorbital fenestra. The premaxillary process is slightly longer than the ventral margin of the rostrum, different from the relatively shorter condition in the azhdarchid Zhejiangopterus [ 39 , 40 ]. The process is nearly straight and angled with the ventral margin about 27 degrees.

The rostrum is well preserved (Figs 1 and 2 ) comprising the premaxilla and maxilla, the suture of which is not visible. It is low, tapering, and elongate with a length of 115.7 mm. This rostrum is similar to that of azhdarchids and chaoyangopterids and much longer than the abbreviated rostra of tapejarids and thalassodromines. The ventral margin of the rostrum is straight as in thalassodromines and azhdarchids, but different from tapejarids that show an anteriorly downturned condition. However, in Shenzhoupterus, the ventral margin of the rostrum is slightly concave, associated with a convex condition of the lower jaw [ 5 ]. A low ridge extends longitudinally and forms the labial margin of the rostrum. Along the labial margin is a shallow longitudinal trough that is dorsally bordered by another low ridge, suggesting that a keratinous sheath was present in life. This structure is unknown in other chaoyangopterids, and possibly diagnostic for Jidapterus. The rostrum has a smooth and straight dorsal profile that becomes deflected dorsally at its preserved posterior fifth portion. The curvature is highlighted posteriorly by the upturned premaxillary process above the nasoantorbital fenestra. Among azhdarchoids, this dorsal concavity is characteristic for chaoyangopterids [ 5 , 9 , 19 ]. The prenarial rostrum appears to have a straight dorsal margin as in other azhdarchoids, except for tapejarids whose rostrum is dorsally accentuated by a premaxillary crest.

The sacrals are not fused in the sacrum. Three sacrals can be identified. Two are articulated and located next to the right femoral head (Figs 1 and 5 ). They are exposed in ventral view and have a smooth surface. Their shape is subtrapezoidal with a broad anterior margin that gradually tapers to a narrower posterior margin, but the anterior sacral is subequal with the posterior one in length. In the anterior sacral, the transverse processes are positioned more anteriorly. Laterally, the transverse process is well expanded posteriorly to contact with the associated transverse process of the posterior sacral, enclosing a small fenestra between the two. A similar structure is slightly reduced in the posterior sacral. More laterally, the transverse processes have a poorly ossified articular facet for the ilium. One isolated sacral is preserved near the lower jaws ( Fig 1 ). Its neural arch is exposed posterodorsally. Two transverse processes extend posterolaterally, setting an acute angle of about 45 degrees with the axial column. The prezygapophyses are oriented anterolaterally with nearly vertical articular facets. The neural spine is stout. The neural canal is exposed posteriorly. It is small and enclosed by the neural arch. Postzygapophyses form the posterior margin of the neural arch, just slightly posterior to the oblique transverse processes.

Seven ribs are identifiable, packed tightly and overlapped with each other. A possible posterior cervical rib is present with a characteristic robust capitulum and tuberculum. The tuberculum is stout, and the capitulum is more slender with a long neck well separated from the tuberculum. In the dorsal ribs, the tuberculum is reduced and confluent with shaft. In contrast, the capitulum is strongly reduced in size with a slender neck.

A neural arch of a dorsal vertebra is well preserved in anterodorsal view and located among the ribs ( Fig 1 ). It is about 16 mm high, and 21.4 mm wide across the transverse processes. The thin neural spine forms about half height of the neural arch. The transverse processes extend laterally to support the diapophyses. A distinct ridge develops from the ventral margin of the neural arch to the ventral side of the transverse process. Anteroventral to the neural spine, prezygapophyses are confluent medially. Another dorsal vertebra next to the right tibia shows the left lateral surface of the neural arch ( Fig 1 ), but the neural spine and transverse process are broken. The neural spine points dorsally and measures about 4.5 mm at its base. The prezygapophyses extend slightly beyond the centrum anteriorly. They are larger than the postzygapophyses and positioned more dorsally than the latter. The postzygapophyses are small and positioned near the posterior margin of the centrum. The transverse processes are located more ventrally than the zygapophyses. Below the level of the transverse processes, the neural arch has pneumatic foramina and constricted anteroposteriorly. The constriction is stronger posteriorly than anteriorly forming a deep notch under the postzygapophyses. Therefore, the contact of the neural arch and centrum is much shorter than the centrum.

The dorsal vertebrae are disarticulated but clustered together in the specimen. In most of the preserved dorsals, the neural arch and centrum are unfused. At least ten dorsals can be identified, all comparable in size with an average length of 9 mm and width of 7.4 mm. The dorsal centra are smooth in texture, slightly constricted in the middle, and procoelous. Dorsally, a longitudinal groove forms the ventral rim of the neural canal. One dorsal vertebra is exposed in anterior view, located between the mandible and the right tibia ( Fig 1 ). Its centrum is round and slightly depressed centrally, forming a crescentic anterior cotyle. Dorsally, the centrum is conjoined with the neural arch and encloses the neural canal. This neural canal is large in size with a diameter of about 4.5 mm. The minimum width of the dorsal vertebra is at this contact. More dorsally, the neural arch is slightly expanded bilaterally. Up to the prezygapophyses, the neural arch is as wide as the centrum.

The eighth cervical is the first of the two posterior cervicals found in pterosaurs. It measures 18.5 mm anteroposteriorly in length, much shorter than the mid-cervicals. The centrum is exposed with the anterior cotyle and posterior condyle visible. The ventral margin of the centrum is concave. Dorsally, a longitudinal depression is present along the base of the neural arch. The prezygapophyses are positioned ventrally and extend beyond the centrum. The postzygapophyses extend slightly beyond the condyle. A tubercle is developed on the postzygapophyses. The neural spine is well developed, but its height is uncertain because its dorsal tip is broken.

Cervicals four to eight are articulated and exposed in lateral view ( Fig 1 ). The mid-cervicals of this series are longer than the third cervical, such as the fourth (43.2 mm), the fifth (38.2 mm), the sixth (33.7 mm) and the seventh (32.3 mm) cervicals. There is a sequential decrease in length from the fourth to seventh cervical vertebra, so that the seventh cervical is 75% of the length of the fourth cervical. However in Chaoyangopterus (IVPP V13397, PMOL-AR00076) and Shenzhoupterus (HGM 41HIII-305A), the fourth to seventh cervical vertebrae are almost in the same length [ 5 , 9 , 19 ]. Jidapterus is unusual in having its fourth cervical as its longest vertebra, suggesting that some isolated elongate cervicals identified as fifth cervicals might be fourth cervicals. The neural spines are all damaged, so their heights are uncertain. A tiny foramen is apparently present in the lateral surface of the fourth cervical (Figs 1 and 4C ), which likely represents a nutrient foramen rather than a pneumatic foramen that is larger in tapejarids and thalassodromines [ 41 , 44 ]. Other azhdarchoids are characterized by a pair of pneumatic foramina lateral to the neural canal that pierce the anterior posterior facets of the neural arch, but these aspects are obscured in this specimen and so their presence remains uncertain. The prezygapophyses are more ventrally positioned relative to the postzygapophyses. The prezygapophyses extend anterior the centrum. The postzygapophyses are relatively broad in lateral view and separated from the condyle by a deep ventral notch. The centrum is slightly concave along the ventral margin. The posterior condyle is slightly upturned posteriorly. The fifth and sixth cervicals are damaged. The seventh cervical is the last mid-cervical vertebra in pterosaurs. Its prezygapophyses are relatively reduced anterior to the centrum, although its postzygapophyses are still positioned high relative to the prezygapophyses. Along the ventral margin, there is a small notch anterior to the posterior condyle, which is possibly a taphonomic artifact.

The third cervical is exposed in dorsal view ( Fig 4B ). It is moderately elongate, about 29.8 mm in maximum length and 9.2 mm in minimum width. The ratio of the length versus the width is 3.2, comparable to that of non-azhdarchids (3–4) [ 19 , 42 , 43 ]. Typical for pterosaurs, this cervical is procoelous, a cotyle concave anteriorly and a condyle convex posteriorly. The condyle is well exposed in dorsal view. It is rounded and extends posteriorly well beyond the neural arch. Anteriorly, the cotyle can be identified under the curved margin between the prezygapophyses. As in other pterosaurs, the neural arch is expanded laterally at the prezygapophyses and postzygapophyses. Between these zygapophyses, the neural arch is constricted in width. Of these, the anterior margin is more curved than the posterior margin. The prezygapophyses are horn-shaped and at a low angle with the longitudinal axis of the vertebra. In contrast, the postzygapophyses are at a higher angle to the vertebral axis. The neural spine is damaged along the midline.

The identified atlas-axis complex is unfused. The atlas consists of three disarticulated elements. A tiny and rounded element is articulated with the axis with a curved suture, probably representing the intercentrum and centrum of the atlas. This intercentrum is rounded and convex on its exposed face. The centrum is poorly ossified on its exposed face, concave anteriorly, and convex posteriorly. It is about 4.9 mm wide and 2.2 mm long. A tiny element is also identified as the neural arch of the atlas. This neural arch has a broad plate and a neural spine. Ventrally, the arch has a rugose edge that is possibly the articular facet for the intercentrum/centrum. Its neural spine is flattened. The axis is crushed and cannot be described in much detail. It is triangular in outline with a tall neural spine. Both the right postzygapophysis and postexapophysis are visible.

The cervical series is nearly complete in preservation. A possible atlas-axis complex is preserved but isolated from the rest of the cervical series ( Fig 3 ). Another isolated cervical is preserved near the right humerus, possibly representing the third cervical. The last four mid-cervicals and one posterior cervical are preserved in articulation, representing the cervicals four to eight. The ninth cervical is missing.

Appendicular skeleton.

The right scapula and coracoid are preserved in articulation and exposed in lateral view, but they are not fused together as in osteologically mature pterosaurs. The left scapula and coracoid are also preserved but obscured by other elements. The right scapula is 54.5 mm long. Anteriorly, the scapula and coracoid form the glenoid fossa, but the scapula contributes more to the glenoid fossa than the coracoid. Dorsal to the glenoid fossa, the scapula is straight before curving posteroventrally at its proximal third point. The scapular shaft is near constant in width. Posteriorly, the ventral margin of the shaft becomes constricted and convergent with the dorsal margin, forming a rounded and sloped distal end. The distal end of the scapula is incompletely ossified.

The coracoids are partially obscured by overlapping bones. The left coracoid is 47.5 mm long, shorter than the scapula. Below the glenoid fossa, the coracoid bears a fan-like flange that extends posteriorly and continues ventrally. The flange is reduced posteriorly along the coracoid shaft and disappears close to the mid-point of the shaft. Similar flanges are also present in other azhdarchoids from China, and they are comparable with the coracoid flange of azhdarchids, and the coracoid tubercle of tapejarids and thalassodromines from Brazil. Posterior to the flange, the coracoid shaft expands bilaterally. A facet is well developed at the coracoidal terminus for articulating with the sternum.

The sternum is preserved in dorsal view (Fig 5). The cristospine is weakly developed as in tapejarids, such as Eopteranodon, Tapejara, and Caiuajara [41, 45, 46]. The sternal plate is subrectangular and wider than long, different from the square or semicircular shape in tapejarids [41, 45, 47]. The sternal plate is thick anteriorly and thin posteriorly. The posterior margin is well preserved medially, but poorly preserved laterally. Along the lateral margins of the sternal plate, crenulations are developed as possible articular facets for the anterior sternocostae. On the sternal plate, a fossa is present near the cristospine, but it does not perforate the plate to form a pneumatic foramen as in Tapejara [41].

The right humerus is preserved separate from the associated ulna and radius. It is exposed in posteroventral view and slightly damaged at the posterior tuberosity and distal end. The left humerus appears to be complete, but it is overlapped partially by the right humerus so that its proximal portion is poorly exposed. The left humerus is 78.6 mm long. The well-developed deltopectoral crest is proximally positioned, extending about 25.4% of the humeral shaft. The crest is elongate, not warped, and slightly curved ventrally with a subrectangular profile. Its straight proximal margin is located at the same proximal level as the humeral head. Anteriorly, the crest is rounded and slightly thickened. It is uncertain whether a pneumatic foramen is present on the ventral surface due to compression. Posterior to the humeral head, however, a possible pneumatic foramen is present on the dorsal surface. The humeral shaft is slightly curved anteriorly and expanded distally. An unfused epiphysis is present and displaced from the left humerus. The main part of the epiphysis is exposed and has a subrectangular shape. It is convex centrally, turning upwards at the proximomedial and distolateral corners. A small element, possibly representing another epiphysis, is situated between the distal end of the right humerus and the ulna.

The ulna and radius are preserved for both sides, but the left side is obscured proximally by the overlapping right side. They are elongate and slender with a length of 112.2 mm for the radius and 113.6 mm for the ulna. The ulna is slightly wider than the radius in diameter. The ulna is expanded proximally and distally, producing a constricted shaft. The proximal end of the ulna is expanded more ventrally, whereas the distal end is expanded more dorsally. The left ulna contacts the radius on its proximal and distal ends. The radius appears to be concave on its proximal end and convex on its distal end.

The carpus is preserved for both sides (Fig 6). The right carpus is visible in dorsal view and articulated with the ulna and radius proximally and metacarpal I and wing metacarpal IV distally. The left carpus is visible in ventral view, but its elements are disarticulated. The carpus is composed of two proximal carpals, two distal carpals, and one preaxial carpal. The two proximal carpals are not fused to form the proximal syncarpal, and the two distal carpals are not fused to form the distal syncarpal of mature pterosaurs. In dorsal view, the radiale is semilunate, and the ulnare is irregular in shape. In ventral view, they are poorly ossified with a rugose surface. In the distal carpal series, the anterior distal carpal is relatively smooth and large. It is distinctly expanded at the anterodistal corner forming a round articular facet. The posterior distal carpal is oval and poorly ossified. Proximally, the preaxial carpal possibly articulates with the anterodistal corner of the anterior distal carpal. A fovea is developed on the preaxial carpal, possibly as the articular facet for sesamoid A in pterosaurs [48]. On the left side, a tiny element is placed against the preaxial carpal, possibly representing the sesamoid A.

The pteroids are disarticulated from their associated carpi. The pteroid is long, slender, and rod-like. It is 70.5 mm in length, about 62% of the ulna length. Proximally, the pteroid is curved and expanded. The shaft of the pteroid is nearly straight and pointed distally.

The four metacarpals vary in length but are distally positioned at almost the same level. Metacarpals I–III are slender, whereas wing metacarpal IV is robust and dominates the metacarpus as in other pterosaurs. The metacarpal I is elongate and contacts the carpus, which is unknown in other chaoyangopterids [5, 9]. By contrast, metacarpals II and III are strongly reduced in length–less than one third of the length of the metacarpal I–and fail to contact the carpus proximally. Distally, metacarpals I–III are expanded more dorsoventrally than anteroposteriorly. The wing metacarpal IV is large with a length of 145.3 mm (left) and 150.1 mm (right), 128–132% of the length of the ulna. The wing metacarpal is broad proximally, constricted along the shaft, and abruptly expanded at the distal end. This distal end is a dorsoventrally oriented trochlea forming a roller joint for articulation with the first wing phalanx.

The manual phalangeal formula is 2-3-4-4-x, typical of pterosaurs. The manual digits I–III are preserved in articulation. The distal portion of the right digit III is broken, but its imprint is still identifiable. On the left side, the ungual of digit II is damaged distally; the phalanx III-3 is completely damaged. The relative length of the digits increase from digit I to digit III. The phalanx III-1 is the longest of the manual phalanges, slightly longer than the phalanx I-1 and much longer than phalanx II-1. This condition is different from Chaoyangopterus, in which the phalanx II-1 is subequal with the phalanx III-1 and much longer than the phalanx I-1 [9]. The phalanx II-1 is slightly shorter than the phalanx II-2; in Chaoyangopterus, the condition is reversed [9]. The phalanx III-2 is nubbin-like and the shortest of the manual phalanges (Table 1). Except for the phalanx III-2, the non-ungual phalanges are expanded at their proximal end, reduced distally along the shaft, and slightly expanded at the distal condyle. The distal ends are about half as wide as the proximal ends. Ventrally, a distinct concave margin is present and reduced distally along the shaft. The manual unguals are approximately twice as large as the pedal unguals in size. Proximally, the flexor tubercle is well developed at the proximal margin of the ungual. Distally, the ungual is strongly curved and sharply pointed. The curvature is about 70 degrees along the inner margin, which is more curved than the outer margin.

The four wing phalanges of the digit IV sequentially reduce in size. The first wing phalanges are preserved with the right one exposed in dorsal view and the left one in ventral view. The first wing phalanx is the longest element in the wing. Proximally, the first wing phalanx expands anteroposteriorly to nearly twice the diameter of the shaft. The extensor tendon process is positioned anteriorly on the proximal end of the first wing phalanx, and unfused with the latter. Generally, the extensor tendon process fuses with the first wing phalanx in mature stages of pterosaurs [36, 37]. Therefore, the unfused condition implies an immature stage for this specimen. Posterior to the process, the proximal end of the first wing phalanx bears a curved articulation associated with the trochlea of the wing metacarpal. The proximal portion of the first wing phalanx is smooth on the dorsal side, but a large pneumatic foramen is present on the ventral side, which is not reported in other chaoyangopterids [9, 13, 19]. The shaft is smooth, straight, and relatively constant in diameter. The first wing phalanx bears distal end comparable with the shaft in diameter. The distal ends are rounded and articulated with the slightly curved proximal end of the second wing phalanx.

The second wing phalanges are preserved across one another. Proximally, the left phalanx is partially overlapped by the right ulna, and the right one is partially damaged on its shaft. They articulate with the first phalanx proximally, but their distal ends are disarticulated from the succeeding third phalanx. The second wing phalanx is reduced to about 66–69% the length of the first wing phalanx. Proximally, the second wing phalanx expands more posteriorly than anteriorly. The shaft of the wing phalanx is smooth and nearly straight. Ventrally, a round ridge is weakly developed along the middle portion of the shaft, differing from a well-developed ridge creating T-shaped cross-section in azhdarchids [40, 49, 50]. Distally, the phalanx is slightly expanded.

Both third wing phalanges are preserved. The right one is partially overlapped by the mandible. This phalanx is about 40% of the length of the first wing phalanx. In most tapejarids, the third wing phalanx is slightly longer than the half of the first one (Table 2). The shaft of the phalanx is gradually constricted towards the distal end. Its proximal third portion is flat, and the remaining portion is rod-like. The distal end is slightly expanded and more condyle-shaped.

The fourth wing phalanx is preserved on both sides. However, the right fourth phalanx is broken and rotated at its midpoint, and its proximal end is hidden by the overlapping tibia. On the left side, the terminal end is broken and slightly displaced from its original position. The fourth wing phalanx is reduced in size and much smaller than other wing phalanges. The phalanx is slender, rod-like, and distinctly expanded at the proximal end with a shallow cotyle-shaped articular facet. Distally, the phalanx is constricted along shaft. The terminal ends are poorly ossified.

The probable left pelvic girdle is preserved in association in medial view (Fig 5). The acetabulum is invisible in medial view and appears to be imperforate, as in other pterosaurs. The ilium is obscured posteriorly by overlapping of the right femur, and the preacetabular process is missing. In contrast, the main body of the ilium is exposed. It starts narrow anteriorly and gradually deepens posteriorly along the iliopubic suture, reaching the maximum depth of about 10.4 mm at the conjoined point of the ilium, pubis, and ischium. Posterior to this conjoined point, the ilium is reduced along the ilioischial suture. The pubis is slightly displaced from its original position. It is a laminar plate with a thick anterodorsal edge bordering the acetabulum. Along the acetabulum, the pubis is thickened and extended anterodorsally, well beyond the anterior margin of the pubic plate, forming a distinct concave anterior margin of the pubis with the latter. This condition is different from a straight or slightly concave anterior margin of the pubis in other azhdarchoids [41, 57, 58]. Ventrally, the pubic plate seems to extend anteroventrally and leaves a distinct space with the ischium ventrally. The pubic plate is subrectangular with parallel anterior and posterior sides and a round ventral margin. The pubic plate is much anteroposteriorly shorter than it is dorsoventrally deep (Fig 5). In contrast, the pubic plate of Tapejara wellnhoferi Kellner 1989 [59] is elongated anteroposteriorly with ventrally divergent anterior and posterior margins. A round obturator foramen is present on the posterior edge of the pubic plate. The foramen is open posteriorly, implying that the ischium contributes to enclose the foramen with the pubis. The left ischium is partially exposed. It articulates with the ilium dorsally and the pubis anterodorsally. Ventrally, the pubis and ischium seem to be separated. A possible separation between the pubis and ischium is also known in Vectidraco daisymorrisae Naish et al. 2013 [57] but absent in other known azhdarchoids [41, 58]. However, this ventral separation is also affected by ontogenetic changes of the pubis and ischium [58, 60]. Another large and lamina-shaped element possibly represents the right ischium, but it is partially obscured by the left ischium.

In contrast, the right pelvic girdle is poorly identified and scattered. The postacetabular process of the right ilium is isolated and located next to the atlas-axis complex, while the right pubis is identified near the left coracoid. The postacetabular process is exposed in lateral view with a smooth surface. It is robust and hatchet-like (Fig 3). Anteriorly, its neck portion is narrow and has parallel margins with a depth of 5.4 mm. This structure is comparable with that of Vectidraco [57], but different from a constricted condition in other azhdarchoids [58]. Posteriorly, as in other azhdarchoids, it is hatchet-like with a pointed dorsal process and a more robust ventral process that measures 17 mm along the lateral margin. The pointed dorsal process is distinctly less than that of other azhdarchoids [41, 58]. The right pubis is perforated by an obturator foramen as the left one (Fig 1).

Both hind limbs are nearly complete with the femora, tibiae and fibulae, and pedes preserved. The femora are preserved side by side (Figs 1 and 5), partially overlapped by the sternum and the skull. The right femur is exposed in posterior view, whereas the left one is exposed in medial view. Proximally, the femoral head is condyle-shaped and separated from the shaft by a distinct neck. As in other pterodactyloids, the femoral head extends dorsomedially and is located on the dorsomedial side, rather than the medial side of the shaft. In posterior view, the greater trochanter forms a round corner of the proximal shaft. In addition, the shaft has a pneumatic foramen near its proximal margin. The femoral shaft is slightly curved. The distal articulation is formed by two condyles (Fig 4D). Both condyles are comparable in size, but the medial condyle extends more caudally than the lateral one. They are widely separate with a distinct intercondylar sulcus.

The tibia is 148.1 mm long, the longest element in the hindlimb, but distinctly shorter than the first wing phalanx (Table 1). The similar condition is known in Chaoyangopterus (PMOL-AP00076; [19]); whereas, in the relatively larger holotype of Chaoyangopterus (IVPP V13397), the tibia is the longest of the long bones [9]. This difference in Chaoyangopterus is possibly due to the ontogenetic variance. Proximally, the tibia is broad and gradually constricts along the shaft towards the distal end. The distal end of the tibia is not fused with proximal tarsals as in osteologically mature pterosaurs [36, 37].

The fibula is reduced to a slender and rod-like element. The left fibula is obscured by the overlapping right femur and tibia. The right fibula is well exposed with a length of 61.2 mm. Its proximal end is not positioned at the level of the tibia proximal end, possibly due to preservation. However, the similar condition seems to be present on the left side.

The tarsus is preserved on the left side. Its elements are slightly displaced from their original position, although they are still preserved between the tibia and pes. The tarsus is composed of two proximal tarsals and two distal tarsals. The proximal tarsals are not fused with the tibia. They are large and semilunate, which functions as a roller joint to articulate with the distal tarsals. Lateral distal tarsal is subrectangular and located distally against metatarsals III–V. The medial distal tarsal is irregular in shape with a tubercle-like process on its proximomedial corner. It is preserved against metatarsals I–II distally, the lateral distal tarsal laterally, and the proximal tarsals proximally.

The pes is preserved in articulation on both sides (Fig 7). The metatarsus is tightly bound. Metatarsals I–IV are all long and slender. Metatarsals I and II are subequal in length, whereas metatarsals III and IV are successively shorter. In Chaoyangopterus, however, metatarsal I is shorter than metatarsal II [8]. Metatarsals I and IV appear to be wider than metatarsals II and III in diameter. Metatarsal V is strongly reduced in size, less than one third the length of the other metatarsals (Table 1). Proximally, metatarsal V is expanded with a broad articular facet for the lateral distal tarsal. Distally, the metatarsal V is strongly reduced with pointed but rounded distal end.

The pedal formula is 2-3-4-5-1. The relative length of the digits increases sequentially from digit I to IV. Most of the non-ungual phalanges are elongate, slender, and nearly straight, except for the phalanges III-2, IV-2, and IV-3, which are well reduced and nubbin-like as in other monofenestratan pterosaurs [61]. Proximal phalanges are relatively longer than other pedal phalanges, except for phalanx II-1 that is shorter than II-2. The longest pedal phalanx is phalanx IV-1, which is only slightly longer than the phalanx I-1. Digit V has a single nubbin-like phalanx, typical of pterodactyloids. The pedal unguals are exposed in lateral view and are much smaller than those of the manus. The unguals are laterally compressed, slightly curved, and sharply pointed. Proximally, the flexor tubercle is weakly developed at the base, different from the well-developed tubercles in “Nemicolopterus”, Sinopterus, and Tapejara [52, 62, 63]. Distally, the ungual curves slightly (inner and outer curvatures are 64 and 57.6 degrees, respectively) and points sharply, differing from tightly curved unguals in tapejarids and possibly in thalassodromines (e.g. Sinopterus,”Nemicolopterus”, and Tapejara) [3, 52, 62–65]. The ungual of pedal digit I is slightly larger relative to those of digits II–IV.