2.1. Generalities

The holotype (MN 4726-V) of Caupedactylus ybaka gen. et sp. nov. consists of an almost complete skull, lower jaw, complete right scapula and coracoid, proximal part of the left coracoid, sternum, right humerus, proximal portion of first phalanx of the right wing finger, and some incomplete rib elements (Figs 1–10). All bones were found in one calcareous nodule in several pieces. Parts of the middle region of the jaws, occiput, posterior portion of the mandibular rami and most of the dorsal region of the skull composed of a cranial crest are not preserved and were lost, possibly during collection of the specimen. The close association between all elements, however, demonstrates that they pertain to one individual.

Figure 6

Caupedactylus ybaka gen. et sp. nov., holotype (MN 4726-V), mandibular symphysis in (A) dorsal and (B) ventral views. Abbreviations: dcr=dentary crest; dfo=dentary fossa; sys=symphyseal shelf. Scale bar=100 mm.

Figure 7

Caupedactylus ybaka gen. et sp. nov., holotype (MN 4726-V), Detail of the ventral side of the mandibular symphysis showing the dentary fossa. Note the reticulate pattern formed by bony struts. Scale bar=50 mm.

Figure 8

Caupedactylus ybaka gen. et sp. nov., holotype (MN 4726-V), sternum in ventral view. Scale bar=50 mm.

Figure 9

Caupedactylus ybaka gen. et sp. nov., holotype (MN 4726-V): scapulocoracoid in (A) anterior and (B)õposterior views; right humerus in (C) dorsal, (D) right lateral and (E) ventral views. Abbreviations: cor=coracoid; fopn=foramen pneumaticum; glfo=glenoid fossa; sca=scapula; scp=supracondylar process; tucor=coracoidal tuberculum. Scale bar=100 mm.

Figure 10

Caupedactylus ybaka gen. et sp. nov., holotype (MN 4726-V), proximal articulation of the right wing phalanx in (A) dorsal and (B) ventral views. Abbreviations: fopn=foramen pneumaticum. Scale bars=30 mm.

Similar to the pterosaur material from the Romualdo Formation, the specimen is exceptionally well preserved, showing almost no distortion. The preparation was carried out using formic acid, following a common procedure very often successfully used in tetrapods, including pterosaurs (e.g. Kellner Reference Kellner2004a). Almost all elements were freed from the matrix, but in some parts, pieces of limestone were left to avoid weakening the skull, particularly around the posterior premaxillary crest (Fig. 1). On the left lateral cranial surface, particularly anterior to the nasoantorbital fenestra, the bone is very fragile and was partially decomposed before final burial, resulting in small voids filled with limestone. In some places, this limestone was attacked by the acid leaving rounded depressions (Fig. 2A). The predominant colour of the bones is yellow, with dark spots, but the postero-lower part of the skull and the mandibular rami display a predominantly reddish tone. The reason for this remarkable colour distinction is unknown; one possibility is differential exposure time as parts of the specimen may have been buried quicker than others, which remained exposed for a longer time period before final burial.

Several bones, such as the scapula and coracoid, the extensor tendon process of the first wing finger phalanx and cranial elements, are fused indicating that the holotype and only known specimen of Caupedactylus ybaka is an adult animal (Figs 2, 9, 10).

2.2. Dermal skull roof

The skull is divided into five pieces, two forming the rostral end and three the posterior portion, leaving a small gap in between (Figs 1, 2). Nonetheless, based on the lateral surfaces of the upper jaw and because this material exhibits almost no distortion, the total length of the skull can be estimated as being approximately 460 mm and 400 mm, measured from the tip of the premaxillae (pm) to the posterior margin of the squamosal (sq) and the length between the tip of the premaxillae and the quadrate, respectively.

The orbit is piriform, with the dorsal margin rounded and wide, and the ventral portion narrow. Similar to tapejarids and azhdarchids, this opening is located lower than the dorsal rim of the nasoantorbital fenestra (Kellner & Langston Reference Kellner and Langston1996; Cai & Wei Reference Cai and Wei1994; Unwin & Lü Reference Unwin and Lü1997; Wang & Zhou Reference Wang and Zhou2002; Kellner & Campos Reference Kellner and Campos2007; Andres & Ji Reference Andres and Ji2008). There is no suborbital opening such as the one observed in dsungaripterids (Young Reference Young1973; Wellnhofer Reference Wellnhofer1991).

The external naris and antorbital fenestra are confluent, forming a nasoantorbital fenestra (Fig. 1). As in other tapejarids, this is by far the largest cranial opening (estimated length=239 mm; height=143 mm), about 52% of the skull length (pm-sq). The posterior margin is straight and there is no extension dorsal to the orbit as observed in the Chaoyangopteridae (Lü et al. Reference Lü, Unwin, Xu and Zhang2008).

The upper temporal fenestra is round and directed dorsally with most of the lateral surface broken away. The lower temporal fenestra is incomplete, lacking the dorsal portion. However, the latter is clearly reduced, displaying the slit-like condition observed in other tapejarids (Kellner Reference Kellner2004b). Several foraminae are observed on the lateral surface of the skull and are followed by shallow anteroposteriorly oriented grooves (Figs 1, 2A).

The premaxilla is elongated and fused with the maxilla. The tip of the premaxillae is not preserved, but the rostrum (measured from the tip of the premaxilla to the anterior margin of the nasoantorbital fenestra) is estimated to be 180 mm long (preserved portion=165 mm), occupying about 39% of the cranial length (pm-sq). The rostrum is long but does not attain the extreme condition observed in some other pterosaur species like Pteranodon, Dawndraco and the archaeopterodactyloid Ctenochasma (Wellnhofer Reference Wellnhofer1991; Bennett Reference Bennett2001; Kellner Reference Kellner2010). The rostral end of the premaxilla–maxilla is downturned as seen in Tupandactylus, Tapejara, and Europejara, although more gently curved (Kellner Reference Kellner2004b; Vullo et al. Reference Vullo, Marugán-Lobón, Kellner, Buscalioni, Gomez, Fuente and Moratalla2012). In this respect, Caupedactylus differs from Thalassodromeus and Tupuxuara, where the rostrum is directed anteriorly without any noticeable curvature (Kellner & Campos Reference Kellner and Campos2002, 2007).

The rostral value (RV), which is the rostral length (ros-l=180 mm) divided by the height of the anteriormost point of the external naris or nasoantorbital fenestra (aen-h=24·7 mm; see Kellner Reference Kellner2010), is 7·29.

The most conspicuous feature of the premaxilla is the sagittal crest. This structure is very thin and fragile, as observed in other tapejarids (Kellner & Campos Reference Kellner and Campos2002). As in Thalassodromeus, this crest has several grooves on its lateral surface (Fig. 2A) that are consistent with their interpretation as the impressions of blood vessels (Kellner & Campos Reference Kellner and Campos2002). In lateral view, the dorsal margin of the premaxillary crest anterior to the nasoantorbital fenestra is straighter, becoming more horizontal towards the anterior tip (Fig. 2A). The thickness of the crest in this region is less than 0·5 mm and posteriorly, this cranial structure is difficult to trace because no distinction with the frontal crest can be seen: apparently this extends beyond the occipital region (Fig. 1). In the tapejarine pterosaurs Tapejara, Tupandactylus, and Sinopterus, this cranial crest is expanded anteriorly and decreases in height posteriorly (Wang & Zhou Reference Wang and Zhou2002; Kellner Reference Kellner2004b), while the reverse is seen in the Thalassodrominae (Kellner & Campos Reference Kellner and Campos2002; Kellner Reference Kellner2004b). Caupedactylus ybaka differs from both these conditions because this cranial structure is expanded both anteriorly and posteriorly. However, whether or not this cranial structure is developed posteriorly to the same extent as in thalassodromines cannot be established, as its posterior and dorsal margins are not preserved.

As noted, the maxilla of Caupedactylus ybaka is fused with the premaxillae and it is not clear to what extend this bone participates in the anterior margin of the nasoantorbital fenestra. However, posteriorly it forms a thin bar that is fused with the jugal to form the ventral margin of the nasoantorbital fenestra. A groove is also present and most likely represents the limits between the maxilla and the jugal. This maxillary-jugal bar is thinner in Caupedactylus than in Thalassodromeus (Kellner & Campos Reference Kellner and Campos2002). Dorsally, the maxillae form the anteroventral margin of the nasoantorbital fenestra, forming a depressed area and an elongated sagittal groove (Fig. 3D).

Only a small part of the nasal is preserved (Fig. 1). This bone comprises the dorsoposterior corner of the nasoantorbital fenestra where it is overlain by the frontal and there is apparently no contact between this bone and the premaxilla. In all tapejarids, the premaxilla-nasal contact is reduced or does not exist, at least on the lateral side of the skull.

The frontal shapes most of the skull roof and extends markedly dorsally forming a frontal crest (Fig. 1). Most of the lateral surface of this element is embedded in limestone where struts of bone can be observed, clearly indicating that this element was in the process of decomposition before final burial. Thus, the limits between the frontal and the premaxillary crest cannot be established with certainty.

The parietal is fused with the frontal. A marked ridge separates the smooth surface of the internal wall dorsal to the upper temporal fenestra and the parietal crest. The latter forms the base of the posterior portion of the cranial crest comprised of the frontal (and premaxilla; Fig. 1).

Only the right lacrimal is preserved: this bone, fused with the jugal, is not fenestrated as in Tapejara wellnhoferi and Sinopterus dongi and bears a lateral process that encloses a large, rounded lacrimal foramen (Fig. 1).

As characteristic of derived pterosaurs, the jugal is a triradiate element that participates laterally in the margins of the orbit, the nasoantorbital and the lower temporal fenestra. On the palatal surface of the skull, this bone forms the lateral margin of the subtemporal, the secondary subtemporal and the postpalatine fenestrae. It bears a thin lacrimal process that is inclined posteriorly around 115°, more than is the case in Thalassodromeus, Tupuxuara and Tapejara (Kellner Reference Kellner2004b) but not to the same degree as in Istiodactylus (Howse et al. Reference Howse, Milner, Martill, Martill and Naish2001). Medially, this bone has a depression that starts at the jugal postorbital process and hollows out the bone for most of its length until the beginning of the jugal maxillary process. A foramen is present on the base of the lacrimal process (Figs 4C, 5A), also on the medial surface.

The quadratojugal is fused with the jugal and the quadrate, forming the ventral margin of the lower temporal fenestra (Fig. 5B). On the right lateral surface, this bone has an unusually thickened bone cortex in a rugose surface that is perforated by small foraminae (Fig. 4A, B). This feature is interpreted to be a pathology and might have been the result of an infection that affected the left jugal–quadratojugal–quadrate region of the skull. It is unknown if this pathology contributed to the death of the animal.

As common in pterosaurs, the quadrate is an elongate bone and has a broad articulation surface with two developed condyles that form a helical jaw joint. It is inclined posteriorly at about 150°, higher than in other tapejarids (Tupuxuara leonardii ∼130°; Thalassodromeus sethi ∼125°; Tapejara wellnhoferi ∼140°; Tupandactylus imperator ∼145°). A sigmoid sharp crest runs from the mediodorsal to the posteroventral surface of this bone (Fig. 4A, B), and the medial flanges are reduced and perforated by several foraminae of different sizes and shapes (Figs 4C, 5B). On the anterior quadrate surface, close to the articulation, an oval pneumatic foramen pierces this bone (Fig. 4C). The position of the articulation between skull and lower jaw is situated under the anterior margin of the orbit (Fig. 1).

Only fragments of the squamosal are preserved, mainly constituting the otic process (Fig. 1). This region is thicker and stronger than Tapejara wellnhoferi but thinner than Tupuxuara leonardii.

2.3. Palatal region

The palatal region is very well-preserved and was fully freed from the matrix, exposing all bones and openings dorsally and ventrally (Fig. 5). The largest palatal openings are the choanae (length=72 mm), occupying about 18% of the length between quadrate and tip of the premaxilla. No vomers separating them were found, suggesting that they were confluent. Only the posterior region the choanae are partially separated by the medially anteriorly projected processes of the pterygoids. Tupuxuara leonardii (the Iwaki specimen) shows similar medial anterior pterygoid processes but reduced vomers separating the anterior part of the choanae. Apart from these tapejarids, Pteranodon longiceps also displays confluent choanae (Bennett Reference Bennett2001). No detailed information of this region is available for other tapejarids.

The postpalatine fenestra is elongated (main axis=22 mm; maximum width∼3 mm) and slit-like (Fig. 5). A secondary ovoid subtemporal fenestra is also present, followed by an oval subtemporal fenestra. In Tapejara wellnhoferi, Tupuxuara leonardii and Thalassodromeus sethi, the postpalatine fenestra is wider and not slit-like. Additionally, there is another smaller ovoid fenestra positioned posterior to the choanae, closer to the midline of the skull. This opening is termed the pterygoid fenestra (fenestra pterygoidalis) and is formed mostly by the pterygoid, except the medial border which is formed by the ectopterygoid (see below).

The anterior region of the palate is formed by the premaxilla and maxilla, but no limits between those bones is perceptible (Figs 2B, 5). It is not certain if an anterior pneumatic foramen (as the one observed in Tapejara wellnhoferi) is present, since the cortical bone surface is incomplete in this region. Nevertheless, two lateral posterior foramina could be observed with the one on the left larger than the one on the right side (Fig. 3C). No palatal ridge is present, contrary to Tupuxuara and Thalassodromeus (Kellner & Campos Reference Kellner and Campos2007). There is also no expansion of the palate close to the anterior margin of the nasoantorbital fenestra as the one present in Tapejara wellnhoferi, Sinopterus dongi and “Huaxiapterus” corollatus.

There is no clear distinction between the anterior region of the palatine and the maxilla. Laterally, a distinct groove is observed and likely represents the maxillopalatine contact surface. The limits with the pterygoid can be observed and show that the palatine forms the anterior margin of the choana and the medial margin of the postpalatine fenestra. At the broken part of the skull, it is clear that the palate floor is pneumatic until the anterior border of the choanae.

The pterygoid shows a complex morphology and is fused with the surrounding elements, but some grooves indicate the limits of this bone (Fig. 5). From the dorsal view, it displays an anteriorly directed rod-like process that forms most of the lateral margins of the choana. Laterally, this process contacts the palatine and is overlain dorsally by the ectopterygoid. Posteriorly, opposite pterygoids join but are not fused, showing an irregular suture that is visible dorsally and ventrally, and comprise a medial process that invades the choanae (Fig. 5). The pterygoid is fused with the medial process of the quadrate and contacts (but is not fused with) the basisphenoid. It completely surrounds the interpterygoid opening that is reduced and is wider than long. The lateral pterygoid process is developed and completely divides the subtemporal fenestra in two openings, with the anteriorly placed one here being called the secondary subtemporal fenestra. Such a fenestra is also observed in Tupuxuara leonardii and in some non-pterodactyloids (e.g. Rhamphorhynchus; Wellnhofer, Reference Wellnhofer1975). An additional fenestra (the pterygoid fenestra) is located lateral to the medial pterygoid process.

The ectopterygoid shows a complex morphology as well (Fig. 5). It starts posteriorly on the lateral surface of the medial pterygoid process, runs dorsal to the pterygoid anterior process, and fuses with the internal surface of the upper jaw made by the jugal and the maxilla. It forms the lateroposterior margin of the postpalatine fenestra, the anteromedial margin of the secondary subtemporal fenestra and part of the anteromedial margin of the pterygoid fenestra. The ectopterygoid is thinner than in Thalassodromeus sethi and Tupuxuara longicristatus, particularly between the postpalatine and the secondary subtemporal fenestrae.

2.4. Occipital region

The occipital region is incomplete, lacking the portion between the postemporal fenestra until the basis of the basioccipital (Figs 3B, 5B). Therefore no information about the foramen magnum, the exoccipital, opisthotic and occipital condyle is available.

The supraoccipital extends backwards and shows one pneumatic foramen on each side (Fig. 3B). Dorsally, it develops in a sagittal supraoccipital crest that forms the basis of the parietal cranial crest.

There is no observable limit between the preserved part of the basioccipital and the basisphenoid, where they form a broad, thin and laterally expanded plate (Fig. 5B). At the probable limits between those bones, there is a pneumatic foramen. The basisphenoid starts as a laterally expanded plate that has a marked medial depression. Ventral to this depression, it turns into a smaller but separate bony plate that thins ventrally until joining the medial processes of the quadrate and the pterygoid, from which it can be separated by marked sutures. Two lateral longitudinal ridges are preserved that end before the contact with the pterygoid-quadrate (Fig. 5). Anteriorly, the basisphenoid is formed by a meshwork of bones, showing that this bone was highly pneumatic (Fig. 5A). No distinct basipterygoid processes could be identified and this bone does not take part in the interpterygoid fenestra. The basisphenoid separates the posterior cranial fenestrae that are elliptic with the main axis 69 mm long (right side).

Although no detailed information of the braincase is available, it is worth noting that in the midline hold by the calcareous matrix, there is an ossified hollow tube whose function is unknown (Fig. 1A).

2.5. Mandible

The lower jaw is incomplete, lacking the rostral tip, part of the distal end of the mandibular symphysis and the posterior portion of both mandibular rami (Figs 1, 2A, 6). Nonetheless, the length is estimated to be approximately 410 mm.

The dentary is the largest element of the lower jaw, a common condition within pterosaurs (Figs 2A, 6). It is downturned, with the dorsal margin parallel to the ventral margin of the upper jaw and, therefore, leaving no gap as the one present in some tapejarids (e.g. Tapejara wellnhoferi). It lacks the step-like dorsal margin observed in the tapejarines Sinopterus and Tapejara. Dentaries are fused, forming a mandibular symphysis (∼265 mm) that is estimated to form about 65% of the lower jaw. The dorsal surface starts slightly concave and turns gradually more flat. After the highest point, the flattened surface turns into a middle groove flanked by two ridges that ends in a deep symphyseal shelf (Fig. 6A). The surface of the symphyseal shelf is smooth, differing from Thalassodromeus, where a marked grooved bordered by lateral short but strong ridges appears to form an interlocking system with the upper jaw.

A shallow dentary crest is present on the ventral margin that differs from the deep dentary crest of Tupandactylus imperator and Tapejara wellnhoferi (Kellner Reference Kellner2004b; Pinheiro et al. Reference Pinheiro, Fortier, Schultz, Andrade and Bantim2011), being more similar to the condition found in Sinopterus (Wang & Zhou Reference Wang and Zhou2002).

The mandibular symphysis ends in two segments, with the dorsal one corresponding to the most posterior point of the symphyseal shelf, placed posterior relative the ventral segment (Fig. 6). This leaves an empty ventral space, the dentary fossa (Figs 6B, 7). Such a space has also been observed in Thalassodromeus, Tupuxuara leonardii, Tapejara wellnhofer, and Quetzalcoatlus sp.

The ventral margin of the dentary is exceptionally thin anteriorly until the deepest point of the lower jaw (Fig. 6B). From there on, it gradually thickens, forming a slightly convex surface that is separated from the lateral border of the lower jaw by a shallow continuous groove. This groove marks the ventral margin of the mandibular rami.

Only a limited portion of the mandibular rami are preserved. The dorsal margin is blunt while the ventral is thin, forming a sharp blade. Internally, the Meckelian fossa is oval, with the longest axis directed anteroposteriorly.

2.6. Postcranial elements

From the sternum, only the sternal plate is partially preserved and exposed in ventral view, having been dislocated towards the premaxillary crest prior to fossilisation (Fig. 8). No information is available for the cristospine and most of the left lateral surface is missing. The right lateral surface is complete and shows the costal articulation – seven were counted. Some elongated elements, possible incomplete ribs, are positioned close to this bone.

The right scapula is preserved, fused with the coracoid forming a scapulocoracoid (Fig. 9A, B). It is an elongated bone (129 mm, measured from the middle part of the glenoid fossa) that is longer than coracoid (113 mm). The proximal surface is elongated and a well-developed, broad tuberculum is present on the lateroposterior margin. Medially, it shows an elongated foramen that also invades the coracoid. The glenoid fossa is large, with the dorsal margin thicker than the ventral. The scapula is rotated relative the coracoid, similar to the condition observed in Anhanguera piscator, Pteranodon, Quetzalcoatlus sp. and Istiodactylus.

Whilst the right coracoid is basically complete, only the proximal portion of the left coracoid is preserved, placed between the premaxillary crest and the sternum (Fig. 1). As a result of its position, the latter was not removed from the calcareous matrix and most of the description presented here is based on the right element. This bone is elongated (length 113 mm), showing the articulation with the sternum dorsoventrally flattened. The articulation surface is fork-like and strongly concave. The posterior margin shows a small ridge, but there is no expansion as observed in other pterosaurs (e.g. Anhanguera). Despite being partially damaged, there is clearly a well-developed tuberculum on the posteroventral margin, but no deep coracoidal flange (Fig. 9A). The blunt coracoidal process (also known a biceps tubercle) is more developed than in Pteranodon (Bennett Reference Bennett2001) but less than in Anhanguera piscator (Kellner & Tomida Reference Kellner and Tomida2000). A rounded pneumatic foramen can be observed between the glenoid fossa and the coracoidal process.

The humerus is complete except for the distal condyles and a small portion of the distal border of the deltopectoral crest (Fig. 9C–E). It is a robust element with a preserved and total estimated length of 160 mm and 175 mm, respectively. The bone cortex is thin, varying between 0·4 mm and 1 mm thick. The head forms the typical pterosaurian saddle-shaped articulation and is bounded anteriorly by a bony ridge. As in other tapejarids, the deltopectoral crest is long, placed proximally and curves ventrally (Fig. 9E). A long muscle scar runs through the posterior surface of the deltopectoral crest and ends on a small knob-like protuberance. There is also a well-developed oval pneumatic foramen on the ventral surface of the humerus, close to the base of the deltopectoral crest, near the humerus head (Fig. 9E). The ulnar crest, separated from the proximal articulation by a marked constriction, is massive and shows a developed proximal ridge that forms a small, semicircular shallow depression. The shaft distal to the deltopectoral crest has a rounded transverse section and becomes gradually wider until reaching the D-shaped configuration of the distal end. On the dorsal surface, two distinct rugosities are observed and likely represent muscle scars. On the ventral margin, a blunt bony ridge is present at the posterior margin, again likely for muscle attachment. The distal articulation lacks the condyles. A developed pneumatic foramen is present on the ventral surface above the distal articulation. Although incomplete, there is a crest-shaped supracondylar process positioned on the anterior margin and directed ventrally. This structure differs from the one reported in Pteranodon, Istiodactylus and Anhanguera piscator (where it is broken and identified as a muscle scar; Kellner & Tomida Reference Kellner and Tomida2000) by being positioned more distally and closer to the distal articulation. There is a concavity between the supracondylar process and the pneumatic foramen.

The sole element of the wing finger is the proximal portion of the first wing phalanx from the right side (Fig. 10). The extensor tendon process is fused to the shaft. A foramen pierces this bone inside the articulation surface for the wing metacarpal, close to the base of the extensor tendon process (Fig. 10A). Such a foramen in this position is found in Tupuxuara longicristatus and Tupuxuara leonardii (Iwaki specimen), but also in Pterandon longiceps (Bennett Reference Bennett2001). On the ventral surface, close to the posterior margin, another pneumatic foramen is found, which is also commonly reported in derived pterodactyloids (Fig. 10B).