Tapejarine tapejarid with the following autapomorphies: anterior end of the premaxillary strongly deflected ventrally (∼142–149°) relative to the ventral margin of the upper jaw; premaxillae with ventral sagittal bony expansion projected inside the nasoantorbital fenestra; rounded depression in the occlusal concavity of the dentary; elongates groove on the anterolateral margin of the quadrate; and marked lateral depression on maxilla ventral to anterior part of the nasoantorbital fenestra. The new species can be further distinguished from other tapejarine pterosaurs by the following combination of characters: ventral margin of the orbit rounded; gap between upper and lower jaws during occlusion wider; and marked depression on ventral side of the pteroid lacking a pneumatic foramen.

CP.V 865, consisting of the anterior portion of a skull, the posterior portion of the lower jaw, the right jugal, vertebrae, ribs and metatarsals; CP.V 867, rostral end of a skull and long bones; CP.V 868, rostral end of a skull, wing elements and other postcranial bones; CP.V 869, incomplete skeleton with a partial vertebral column (posterior cervicals vertebrae, dorsal elements to the first five caudal vertebrae), right humerus, radius and ulna, carpal elements, coracoid, sternum, some wing phalanges, gastralia, pelvic elements and the right femur; CP.V 870, incomplete postcranial elements, with humeri and pectoral girdle; CP.V 871, fused right scapulocoracoid and incomplete long bones; CP.V 872, partial skeleton including a fragmentary skull, lower jaw, right humerus, radius, ulna, carpals, cervical vertebrae and other long bones; CP.V 873, rostral end of a skull and manual phalanges; CP.V 999, partial skull; CP.V 1001, one incomplete skull with elongated premaxillary crest and lower jaw, and postcranial elements of at least three individuals, wing bones (with humeri), cervical vertebrae and pelvic elements; CP.V 1003, incomplete skull and the rostral tip of the lower jaw; CP.V 1004, rostral end of a skull; CP.V 1005, incomplete skull with an elongated premaxillary crest and a complete lower jaw; CP.V 1006, partial skull with anterior rostral end missing, with large premaxillary crest and several postcranial bones; CP.V 1023, anterior portion of a skull and several postcranial elements; CP.V 1024, skull and several postcranial bones of at least three small individuals; CP.V 1025, isolated femur; CP.V 1026, isolated femur; CP.V 1450, several small individuals (at least 14); CP.V 2003, skull and lower jaw associated with wing bones; UEPG/DEGEO/MP-4151, two skulls on one slab and postcranial elements; and UEPG/DEGEO/MP-4152, a rostrum and several postcranial elements ( Figures 2 , 4 – 8 ). For referred specimens, see List S1 in File S1 .

Description and Comparisons

Several anatomical features show that Caiuajara belongs to the toothless pterodactyloid clade Tapejaridae [13], such as a premaxillary crest from the anterior rostral end extended above the occipital region, nasoantorbital fenestra elongated comprising more than 40% of the cranial length (Figure 3), and a well-developed tubercle on anterior surface of the coracoid. It further has all synapomorphies of the Tapejarinae, such as the downturned anterior part of the rostrum [12], [13], [15]–[17], with the inclination varying from about 138° to 150°; most species average 142°. The orbit is piriform, with the ventral margin more rounded than in other tapejarids [15], [18]. The nasoantorbital fenestra anterior margin is wide, similar to other tapejarines [15], [16], [19] but differing from the narrower condition of thalassodromines [12], [20].

The premaxilla is perforated by a large number of foraminae on the lateral and palatal surface, similar to Tapejara, suggesting that the beak was covered by a horny covering analogous to the rhamphoteca in birds. A developed premaxillary sagittal crest is present in the smallest and the largest individuals (Figure 4), casting doubts on previous interpretations that the presence and absence of cranial crests might be sexually dimorphic [21]. The anterior part of the crest is very high similar to Tupandactylus [13], [22], but differs by being more expanded. The occipital portion of the crest, formed by the supraoccipital and parietal, is dorsally curved, differing from the much longer and straighter structure found in Tupandactylus [22]. Starting close to the anterior margin of the nasoantorbital fenestra, the premaxilla has a sagittal bony expansion that extends posteriorly, where it merges with the lateral margin of this opening (Figure 3). This structure, whose function is unknown, is present in all specimens, from the smallest to the largest, and has not been reported in any other pterosaur before (Figure 4). As in Tapejara, Caiuajara shows a deep concavity in the palate restricted to the anterior part, followed by a posterior convexity. Differing from all other tapejarids where the occlusal surface can be observed, some specimens of Caiuajara have a faint longitudinal crest inside the most concave portion of the palate that does not form a palatal ridge as in some thalassodromines [12] and some pteranodontoids [17], [23]. As in other tapejarines, the upper jaw shows a small lateral expansion close to the anterior margin of the nasoantorbital fenestra. The occlusal surface of the dentary also displays a deep concavity as in Tapejara, but Caiuajara differs by showing a distinctive rounded depression (Figure 5). The dentary shows the typical tapejarine step-like dorsal margin and a blunt dentary sagittal crest that is more developed in larger individuals, similar to Tapejara [18] and Europejara [24], but differing from the Chinese tapejarines [15], [16]. There is no helical jaw joint, differing from Caupedactylus [25].

The cervical vertebrae (Figure 2) are slightly elongated, more so than in pteranodontoids [26], but not to the same degree as in archaeopterodactyloids [17], [23] or azhdarchids [27], [28]. The neural spine is blade-like and the centrum pierced laterally by small pneumatic foraminae. One lateral pneumatic opening occurs on each side of the neural canal on the anterior articulation surface. No notarium is developed. The sacrum is formed by five sacrals. The scapula is longer than coracoid, and where complete, the coracoid shows a developed tubercle on the anterior surface (Figure 6). The sternum is semicircular in shape. The humerus displays an elongated deltopectoral crest that is rectangular and slightly curved medially, particularly at the most posterodistal end, but the crest is not warped as in pteranodontoids [17], [23], [26], [29]. About 35 humeri were identified so far (20 right, 14 left and one unidentifiable) with lengths ranging from 31 mm to 115.6 mm. Overall, the radius is thinner than the ulna, but not to the same degree as observed in istiodactylids and anhanguerids [23]. Distal sincarpals show a rectangular shape. The pteroid clearly articulates with the proximal carpal series, showing a developed ventral depression but no pneumatic foramen (Figure 7). Wing metacarpal IV is similar to that seen in other tapejarids, being proportionally longer relative to other wing elements when compared to anhanguerids (Figure 2), but does not approach the extreme elongation reported in nyctosaurids [23], [26]. The femur is bowed and about the same size as the humerus (Figure 6).

A phylogenetic analysis based on a previous study of tapejarid phylogenetic relationships [24] shows that Caiuajara is a member of the Tapejarinae, falling in a polytomy with other tapejarine tapejarids (Figure 9). If Eopteranodon, a poorly described taxon, and Europejara (unfortuntately very incomplete) are removed, Cauiajara falls in a sister group relationship with Tupandactylus, in a trichotomy with Tapejara and Sinopterus+Huaxiapterus, indicating that the known tapejarines from China form a monophyletic entity. This exercise also shows that much more has to be done to resolve the relationships of the Tapejaridae, particularly the Tapejarinae.

Previous studies of pterosaur ontogeny were based on isolated specimens mostly recovered without stratigraphic control that, despite important contributions [30]–[39], have fostered some controversy, particularly over whether or not the studied specimens represent the same species [40], [41]. Caiuajara is the first case where a pterosaur ontogenetic series is provided based on specimens from a pterosaur bone bed that can be confidently assigned to the same species. The sample also has the advantage of having most elements preserved three-dimensionally and not flattened, avoiding the problems related to change of morphology due to distortion [6], [7], [36]. Regarding postcranial elements, there are few differences from smaller to larger individuals except for size and the tendency for ontogenetically more developed individuals to show more ossified bones, particularly the sternum (Figure 6). The humerus, for example, shows the same proportion in smaller and larger individuals, including the development of the deltopectoral crest that corresponds to about 38–40% of the humerus length. This indicates that the general shape of most postcranial elements is formed at a juvenile stage and does not change significantly, as the animal grows older. The most conspicuous exceptions are the prepubis, with older individuals showing a more developed and larger distal plate, and the coracoid, where ontogenetically more developed individuals display a slightly larger ventral expansion. Furthermore, as reported in other pterodactyloids, the scapula and coracoid are fused in adult individuals but unfused in younger individuals, with the same trend happening in the epiphyses of the humerus and the carpal series [32], [35].

Regarding the skull, the main ontogenetic differences can be found in the rostrum and the cranial crest (Figures 4, 10). Younger individuals display a reduced rostrum that grows, becoming more massive in older individuals. The inclination of the occlusal margin relative to the horizontal plane does not vary significantly, mostly being around 142°. The premaxillary crest, on the contrary, shows marked variation, being reduced and inclined posteriorly for about ∼115° relative to the horizontal plane in small individuals. As the animal grew, the crest got rapidly larger and steeper (up to ∼90°). Similar changes are observed in the dentary crest, which is almost absent in young individuals and gets more developed in older ones (Figures 5, 10).