The node-based definition of Hadrosauridae used here is “the clade consisting of Telmatosaurus and Parasaurolophus, their most recent common ancestor and all descendants” [19] and [35]. Other authors [11], [29] and [31] have argued for a more restrictive definition of Hadrosauridae: “Lambeosaurinae plus Hadrosaurinae and their most recent common ancestor” (= Euhadrosauria [35]). The stem-based definition of Lambeosaurinae is: “all hadrosaurids more closely related to Parasaurolophus than to Saurolophus” and that of Hadrosaurinae, “all hadrosaurids more closely related to Saurolophus than to Parasaurolophus” [31]. Within lambeosaurines, Corythosaurini are “all taxa more closely related to Corythosaurus than to Parasaurolophus”, and Parasaurolophini “all taxa more closely related to Parasaurolophus than to Corythosaurus” [8]. Finally, Hadrosauroidea can be defined as “all iguanodontians more closely related to Parasaurolophus than to Iguanodon” [31].

BMNH: The Natural History Museum, London, U.K.; MPZ: Museo Paleontológico de la Universidad de Zaragoza, Gobierno de Aragón, Zaragoza, Spain.

From Aren (Areny de Noguera in Catalonian language), the village of Huesca province (Spain) located near the area where the fossils were found.

As for the type and only known species.

Arenysaurus ardevoli.

Arenysaurus ardevoli sp. nov.

MPZ 2008/1, a partial, articulated skull comprising the skull roof and braincase.

In honour of the geologist Lluis Ardèvol (Geoplay, Tremp, Lleida), who discovered the Blasi sites.

Blasi 3 site in Aren (Huesca, Spain); lower part of the Tremp Formation; Late Cretaceous, Late Maastrichtian (upper part of chron 30n) ([20] and [25]; this work) (Fig. 1 and Fig. 2).

Lambeosaurine hadrosaurid characterized by a very prominent frontal dome, more developed than in other adult specimens; nearly vertical prequadratic process of the squamosal and jugal process of the postorbital; deltopectoral crest of the humerus oriented anteriorly. Differs from other lambeosaurines in having a unique combination of characters: short frontal, with a posterior length/with ratio estimated at 0.5; midline ridge of parietal approximately at the level of the postorbital-squamosal bar; parietal not interposed between the squamosals in the occipital surface of the skull; lateral side of squamosal relatively low above the cotyloid cavity.

Cranial: MPZ2008/256, fragmentary right maxilla; MPZ2008/257, fragmentary left maxilla; MPZ2008/258, left dentary with 12 teeth; MPZ2008/259, right surangular; MPZ2008/260-263, four isolated teeth. Postcranial skeleton: MPZ2007/706, MPZ2007/954-955, MPZ2008/264-267, seven cervical vertebrae; MPZ2008/268, dorsal vertebra; MPZ2008/269-270, two dorsal ribs; MPZ2008/271, partial sacrum with ossified tendons; MPZ2004/480, pathological caudal vertebra; MPZ2006/20, 14 articulated caudal vertebrae and chevrons; MPZ2008/272, 313, two caudal vertebrae; MPZ2008/314, 330, haemal archs; MPZ2008/331-332, two ossified tendons; MPZ2008/333a-b, right scapula (two fragments); MPZ2008/334, right coracoid; MPZ2008/336, right humerus; MPZ2008/335, fragmentary right ilium; MPZ 2007/707, right pubis; MPZ 2007/711, right femur; MPZ2008/337 left femur.

The incomplete but finely preserved skull of Arenysaurus ardevoli (MPZ2008/1, holotype) (Fig. 3) consists of prefrontal, frontal, parietal, postorbital, squamosal, supraoccipital, exoccipital-opisthotic complex, prootic, basioccipital, basisphenoid, parasphenoid, laterosphenoid, orbitosphenoid and presphenoid bones. Elements of the rostral region of the facial skeleton and the palatoquadrate complex are missing. The skull is relatively deep dorsoventrally and anteroposteriorly shortened; this results in a roughly quadrangular skull in dorsal view (Fig. 3A). The right side of the skull is better preserved than the left one that suffered from deformation (supratemporal fenestra) and post-depositional breakage (i.e., left paroccipital process, occipital condyle). As preserved, the skull is approximately 250 mm long; the estimated total skull length is of approximately 600 mm.

The hollow supracranial crest commonly seen in lambeosaurines is not preserved in MPZ2008/1. However, its presence can be inferred from the posterior half of the craniofacial skeleton, which is highly modified to accommodate such a structure (e.g., shortened frontals and parietals, deeply excavated frontal platform; subvertical prefrontal flange), as typically in lambeosaurines [19] and [26].

The prefrontal participates in the formation of the platform that supports the nasal articulation. This participation is smaller than in the Asian lambeosaurines Amurosaurus and Jaxartosaurus [16] and is rather comparable to the condition observed in corythosaurins [9]. The prefrontal joins with the anterior process of the postorbital to exclude the frontal from the dorsal orbital rim. The medial margin of the prefrontal forms a subvertical flange that extends posteriorly over the dorsal surface of the frontal and above the postorbital joint in lateral view (Fig. 3C).

The frontals are very short, wider than long (length approximately half the maximum width). They are not completely fused together and meet in the midline to form a very prominent median dome, more developed than in other adult lambeosaurines [8] and [17]. Anterolaterally, the frontal joins with the prefrontal to form together an excavated and steeply sloping rostral platform. This structure is strongly grooved to provide support for the base of the hollow crest. A median cleft is present between the anterior processes of the frontals, as is characteristic of lambeosaurines but Parasaurolophus [9]. The rostral platform is narrower than the posterior part of the bone, as in Amurosaurus and Jaxartasaurus and in contrast to adult corythosaurins [16] and [17]. This platform does not extend posteriorly above the rostral part of the parietal and of the supratemporal fenestra, unlike Charonosaurus and Parasaurolophus [13] and [17]. The dorsolateral surface of the frontals is slightly depressed but it does not form cup-shaped depression areas as in Sahaliyania [17].

The hourglass-shaped parietals are short anteroposteriorly, with a proximal width that is greater than the length. They articulate with the frontals along a transverse, highly interdigitate suture mediated by a short process into the interfrontal joint (or interparietal process; [19]). The midline ridge is only modestly downwarped and its dorsal margin is approximately at the level with the skull roof and not below the level of the postorbital-squamosal bar, unlike Amurosaurus and derived lambeosaurines [8] and [17]. The sagittal crest is relatively short and narrows posteriorly. The parietal is apparently excluded from the occipital surface of the skull, as typically in Parasaurolophini [13]. The supratemporal fenestrae are small and subovoid, slightly longer anteroposteriorly than wide. They more closely resemble those of Jaxartosaurus [16].

The dorsal surface of the postorbital is nearly flat; it does not form a dorsal promontorium as in parasaurolophins (probably related to the backward extension of the supracranial crest [13]). The postorbital-squamosal joint reaches a point at mid-length of the supratemporal fenestra. In dorsal view, the transverse width of the skull in the postorbital region is broad, with a width roughly maintained from the orbital region to the squamosal corner (Fig. 3A).

The right squamosal is robust and convex dorsally. The lateral bar is short and low above the cotyloid cavity; the median ramus is lower than the paroccipital process, in contrast to all known lambeosaurines except Tsintaosaurus and Jaxartosaurus [8], [16], [17] and [36]. The prequadratic (precotyloid) process is short, only slightly longer than the anteroposterior width of the quadrate cotylus, and nearly vertical (Fig. 3C). The squamosals are well exposed in occipital view and extend medially above the supraoccipital in the midline; they have an extensive contact with each other (intersquamosal joint).

The supraoccipital is excluded from the foramen magnum (Fig. 3B). Its ventral margin is horizontal, with a ridge developed along the supraoccipital-exoccipital suture. The posterior surface of the supraoccipital is inclined steeply forward at approximately 45°. The supraoccipital-exoccipital shelf is poorly developed and does not roof the occiput above the foramen magnum.

The exoccipital and opisthotic are completely fused together. Only the right paroccipital process is complete. It is long and slender, hook-like and lateroventrally directed. The paroccipital process descends to the level of the ventral surface of the basioccipital.

The occipital condyle is damaged. The left alar process of the basipterygoid is small and asymmetrical. The basipterygoid processes are elongate and project ventrolaterally; they diverge from the base of the basisphenoid at an angle of 60° from each other. The cultriform process of the parasphenoid is extended anteriorly, with an unexpanded distal tip. This process is approximately as long as the width across the basipterygoid processes.

The arrangement of the cranial foramina is similar to that seen in hadrosaurids [26] (Fig. 3C). The trigeminal (V) and auditory (VIII) foramina are the largest ones; between them, the lateral wall of the braincase is pierced by a single foramen (for the facial nerve VII). Posterior to the trigeminal foramen and separated from it by a crista, there are two foramina for transmission of the hypoglossal (XII) and of the vagus (X) and glossopharyngeal (IX) nerves, respectively. In the anterolateral part of the braincase, the foramen for the oculomotor (III) and abducens (VI) nerves is larger than the one for the optic (II) nerve.

In addition to the holotype skull, two maxillary fragments (MPZ2008/256-257) have been found in nearly association to it and are here referred to Arenysaurus. The ectopterygoid ridge seems very prominent on the lateral side of the maxilla. A maxillary foramen is present on the dorsal surface of the bone along the maxilla-premaxilla suture, as is common in derived hadrosaurids [17].

Many of the postcranial bones referred to Arenysaurus were found disarticulated but in reasonably close association with the skull. They consist of parts of the axial skeleton, including cervical and dorsal vertebrae with ribs, an incomplete sacrum and a large portion of the caudal series (Fig. 4 in [20]); the appendicular skeleton is represented by bones of the pectoral girdle (right scapula and coracoid), forelimb (right humerus), pelvic girdle (right ilium and pubis) and hindlimb (left and right femora).

Seven isolated postaxial cervical vertebrae and three cervical ribs are known. As typically in hadrosaurids [19], the centra are short (length less than the height of the neural arch) and strongly opisthocoelous. The postzygapophyseal processes are long and dorsally arched; they extend well above the level of the neural canal. The posteriormost cervicals exhibits a low but prominent neural spine. The dorsal region is very incompletely known: only one isolated vertebra and two ribs have been discovered. The dorsal neural spine is elongate, approximately 3.5 times the centrum height. As preserved, the partial sacrum consists of five fused vertebrae. The sacral neural spines are broken but they seem relatively elongate. Indeed, the anteriormost caudal vertebrae have long neural spines that are more than three times the height of the centrum, as in lambeosaurines [19]. About two thirds of the caudal region is preserved, including 14 anterior articulated vertebrae. The chevrons are directed posteroventrally at an angle equivalent to the posterodorsal angle of the long neural spines. Fragmentary ossified tendons have been found in close association with the sacrum.

The right scapula is represented by a proximal fragment and an almost complete blade corresponding to a single specimen (estimated length: 525 mm). It is typically hadrosaurid in having a narrow proximal end, an acromion process that projects horizontally, a notched anteroventral corner, and a restricted articulation for the coracoid [19]. The dorsal and ventral margins of the scapula are slightly curved and divergent; distally, the blade is approximately 60% wider than the proximal neck. As is common in hadrosaurids, the coracoid hook is prominent and pointed anteroventrally, and the biceps tubercle is large and projects laterally [17] and [19]. The incomplete right humerus (preserved length 300 mm) of Arenysaurus differs from that of other lambeosaurines in that the distal width is greater that the maximum width at the level of the deltopectoral crest because the latter is oriented anteriorly (Fig. 5). The anterior projection of the deltopectoral crest of Arenysaurus resembles the condition found in the hadrosaurine Wulagasaurus [17].

The iliac peduncle of the pubis is a prominent, dorsally directed process. The prepubic blade is dorsoventrally expanded and looks nearly symmetrical. As is typical in hadrosaurids, the femur (length 711 mm) has a massive shaft, straight in lateral view, and a developed intercondylar extensor groove that is closed forming a funnel [19].

The braincase of Arenysaurus is fully ossified; the absence of open cranial sutures indicates that the holotype skull MPZ2008/1 is not from a juvenile [7]. The development of the frontal platform, the absence of a prefrontal-frontal “clamp”, the interdigitate suture between the frontals and parietals, and the fusion of the supraoccipital to the skull roof support this interpretation [10], [16] and [19]. On the other hand, the skull probably does not belong to an old individual as there is no complete fusion of the frontals [17]. The nature of most of the sutures together with the size of the skull suggests that it represents an adult individual. In addition to the skull, the transverse processes and neural arches are fused to the centra of the vertebrae, and the sacral centra are fused together, as usually in mature hadrosaurid individuals. On the basis of lambeosaurine skeletal reconstructions [19], the estimated body length of Arenysaurus is about 5 to 6 m.