Holotype.

IVPP (Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China) V725, partial articulated skull composed of frontals, braincase, parietal, squamosals, postorbitals, left prefrontal, and nasals. Young [17] included in IVPP V725 various axial and appendicular elements that may or may not correspond to individuals other than the braincase exemplar. These bones include left and right partial distal nasal processes, a series of cervical, dorsal, and caudal vertebrae, a complete sacrum, a left scapula and coracoid, left and right sternal plates, left and right humeri, left ilium, right and partial left ischia, partial left pubis, right and partial left femora, partial left and right fibulae, and distal left tibia ([17]: text figs 15—18, 21—23, 26—28, 30, and 31 respectively).

Paratype.

IVPP V818, partial articulated cranium composed of frontals, braincase, parietal, squamosals, postorbitals, and left prefrontal ([17]:fig. 4).

Referred material.

IVPP V723, predentary, and left and right dentaries; V727, left and right humeri; V728, sacrum, left and right humeri, right ilium, right ischium, right pubis, and left femur; V729, sacrum and left humerus; V768, isolated dentary tooth crowns; V829, articulated dorsal fragment composed of rhomboid, distal nasal processes and dorsal region of premaxillae; and V830, partial left and nearly complete right jugal; also, IVPP field numbers K24, partial predentary; K63, predentary; K28, left quadrate and right maxilla; K45, left maxilla; K63, left and right dentaries; K68, nearly complete left quadrate; K69, left ulna; K70, right coracoid and scapula; K82, right ulna; K86, left radius; K93, left maxilla; K96, right coracoid and scapulae; K97, right surangular; K107, left rostral region of premaxilla; K141, predentary; K149, left surangular; K150, left surangular; K155, dentary tooth; K156, several metacarpal and manual phalanges from various individuals; and K197, left and partial right dentaries. Additional material, referred to Tanius by Young [17], may or may not belong to this species, and we consider Tanius chingkankouensis Young, 1958 a nomen dubium pending further revision.

Occurrence.

Lower Campanian [33] Jingangkou Formation, Wangshi Group, near the village of Chingkankou, 20 km south of the city of Laiyang, Shandong Province, eastern China [17,22].

Emended diagnosis.

Lambeosaurine hadrosaurid possessing the following autapomorphies: oral margin of premaxilla with widely arcuate rostroventral corner and thick oral margin laterally broader than rostral-most extent of circumnarial fossa; cirumnarial fossa of premaxilla obliquely divided longitudinally by two rostrolaterally-oriented ridges; tall lobate supraorbital crest composed of nasals and premaxillae that projects dorsally and slightly caudally a distance greater than the height of skull along quadrate; nasals fused sagittally to form elongate tubular process rising dorsally from skull roof; nasals form block-like intracranial process rostral to braincase; nasal articulates with frontal by clipping between tongue-like ventral process and overhanging ectocranial process of frontal; distal end of nasal expanded rostrocaudally into rhomboid process; medial processes of premaxillae at summit of cranial crest, inserted between rhomboid processes of nasals; mediolaterally expanded caudal half of medial processes of premaxillae at summit of cranial crest, with lateral surface containing rhomboid facet for articulation with distal nasal process; medial process of premaxilla with hook-like intracranial process curving caudoventrally and slightly medially, subdividing common median chamber; prefrontal with ascending rostral process that projects dorsally while twisting medially to face and overlap proximal region of tubular process of nasal and part of lateral process of premaxilla; prefrontal with flange extending laterally from ventral region of lacrimal process to ventral region of ascending process, articulating with lateral process of premaxilla to form swelling on lateral preorbital region of skull; lateral and ventral surfaces of prefrontal bearing deep dorsoventrally-oriented grooves; and supratemporal fenestra wider mediolaterally than rostrocaudally. Additionally, T. spinorhinus differs from all other hadrosaurids, except Pararhabdodon isonensis, in possessing maxilla with elevation of jugal facet such that ventralmost extent is well above the level of the ectopterygoid ridge, with acute embayment ventral to jugal process between jugal facet and ectopterygoid ridge; it differs from P. isonensis in having a rostrocaudally narrower rostrodorsal region of the maxilla.

Nasal.

Young figured and briefly described a fragmentary specimen, IVPP V829, as part of the nasal and premaxilla from the crest of an indeterminate lambeosaurine ([17]: text fig. 40). This referral was later supported by Weishampel and Horner [23]. According to Young [17], the specimen was found in the same gully where the type and hypodigm of Tsintaosaurus spinorhinus were excavated. Overall, the fragment is subrectangular in lateral profile and mediolaterally compressed, containing the sagittal plane of the skull (Figure 1). On the best-preserved right side two elements are recognizable: a rhomboid mediolaterally compressed fragment and a surrounding bony mass (Figure 1B). We concur with Young [17] in that the rhomboid element is the nasal and the surrounding bone the premaxilla, and we further suggest that both elements belong to Tsintaosaurus spinorhinus. This identification is based on the fact that the rhomboid profile of the nasal in IVPP V829 (Figures 1B and 2C) is very similar to the fan-shaped and rostrocaudally expanded lateral profile of the distal process of the IVPP V725 nasal (Figure 2A and B). The rostroventral margin of the distal nasal process of IVPP V725 appears straight in lateral profile (Figure 2A); however, this straight profile results from breakage of the process. Close examination of the ventral surface of this margin reveals a flat fractured surface where bone is missing (Figure 3B, white rectangle). Thus, we interpret the distal nasal process of IVPP V725 as the incomplete caudodorsal half of the rhomboid structure preserved in IVPP V829.

In IVPP V829 the distal process of the nasal fits tightly into a deep fossa on the lateral surface of the premaxilla (Figure 2D). The margins of the fossa mirror the rhomboid outline of the distal nasal process. One of the margins of the distal process is gently convex, whereas the other border is slightly sinuous. Given that the caudal margin of the distal process in IVPP V725 is gently convex, we consider the convex margin in the distal nasal process of IVPP V829 to correspond to the caudal margin and the gently sinuous border as the rostral edge of the bone. The rhomboid distal nasal process and its articulation with the premaxilla are well preserved on the right side of IVPP V829, but heavily eroded on the left side (where it is uncertain whether fragments of the nasal process remain; Figure 1D).

In addition to the type skull and IVPP V829, there are two other bone fragments morphologically consistent with the rhomboid nasal process and, thus, referable to T. spinorhinus. Both of them bear the IVPP V725 catalogue number; however, they must be part of specimens other than the type because, as shown above, the latter already bears both distal nasal processes. One of them (Figure 2F) preserves slightly more than the rostral half of the left distal process. The other piece is very fragmentary and appears to represent only the caudal half of a right distal process. Other putative nasals tentatively referred to T. spinorhinus by Young ([17]: text fig. 42) are less certain: IVPP K140 is almost certainly a first ceratobranchial, and IVPP K119 may be as well.

In Tsintaosaurus spinorhinus each rhomboid process of the nasal diverges laterally from the sagittal plane of the tubular process (Figure 3B). Thus, the elongate spike-like structure formed by the fused tubular processes of nasals (which are hollow according to Young [17]: text fig. 2) opens distally to accommodate the intervening premaxillae; each rhomboid process articulates on the lateral surface of each premaxilla as seen in Figure 1B. The separation between the rhomboid processes is present in IVPP V725: near the base of these processes, the nasals bifurcate and each element deviates laterally from the sagittal plane of the skull (Figure 3B). At first glance, this separation may appear too narrow in IVPP V725 to allow insertion of the premaxillae in between the rhomboid processes of the nasals at the sagittal plane of the skull. However, the right nasal process of IVPP V725 is cracked and post-depositionally displaced medially so that it appears closer to the left nasal than it was prior to deformation (Figure 3B, arrow).

Ventral to the rhomboid processes, the nasals of IVPP V725 form their distinctive tubular structure as described by Young [17]. That the nasals might be hollow is not itself unusual; typically among hadrosaurids the internasal articulation only extends for a short distance ventrally, and the paired nasal plates each slope away from the dorsal suture to form an A-shape in transverse cross-section. There is a space enclosed between the two nasal plates, in the ventral vacuity of the A. The nasal plates of Tsintaosaurus spinorhinus are unusual in that they wrap around ventromedially (rostromedially in articulation, due to the dorsal rotation of the nasals) to meet again at the midline in a second internasal articulation at a thick ridge along much of the rostral surface of the tubular process (Figure 3D), forming a broadly O-shaped cross-section.

An alternate interpretation might be that the plates of the nasals are absent, and the tubular process of the nasals represents the upper portion of the A-shape described above. The vacuity in the nasals may then represent a subsequent invagination of the bones. We consider this less likely, but we cannot discount the possibility. In either case, the nasals of Tsintaosaurus spinorhinus are highly derived relative to those of other hadrosaurids.

IVPP V725 also exhibits an unusual apomorphic extension of the nasal bones into the intracranial space, producing a block-like structure that descends rostral to the braincase (Figures 2E and 3A, [17]: text fig. 2). That this structure is composed entirely of the nasals is shown by the Paratype skull, in which the nasals are not preserved and the block-like intracranial process is absent as well (Figure 4). In the holotype, the block tapers dorsally and slightly rostrally between the upturned margins of the frontals and prefrontals into the tubular process, and flares ventrally to span the space between the prefrontals. On the rostral face of the block are paired, dorsoventrally elongate, elliptical excavations bordering a thick, sagittal, dorsoventrally-oriented ridge (Figure 3A). The more complete right depression is emarginated by a thick rim laterally, and extends ventrally deep into the intracranial space on the face of a triangular lateral process. The ventral portion of the thickened septum between the depressions is broken, and it is unclear if it descended as deeply as the lateral process. Each depression extends dorsally up the nasal, and somewhat rostrally following the curve of the bone to a point just above the level of the skull table and just below the midline ridge on the tubular process, where it shallows to a transversely oriented rim. Between the depressions, the midline ridge also shallows and thins dorsally, to disappear at the same level as the depressions.

Premaxilla.

Young [17] referred the nearly complete rostral region of a premaxilla, IVPP K107, lacking the caudal extensions of the medial and lateral processes, to Tsintaosaurus spinorhinus (Figure 5). As detailed below, the morphological attributes of the oral margin and narial fossa of this element are unlike those of the premaxillae of any known hadrosaurid, but are similar to those of Saurolophinae. We follow Young [17] in referring this premaxilla to T. spinorhinus because, according to that author, this element was found in fairly close association with the type cranium and hypodigm of T. spinorhinus, all contained within a quarry a few meters long, wide, and deep. Additionally, as described below, the premaxilla exhibits derived characters of Lambeosaurinae not found in any saurolophine.

The preserved rostral region of the premaxilla of Tsintaosaurus spinorhinus is subrectangular and three times longer than wide (Figure 5C). The degree of flaring of the rostral portion of this bone indicates that Tsintaosaurus spinorhinus had a relatively broad premaxillary ‘bill,’ unlike the narrow rostra of other lambeosaurines [8,9,13]. The caudoventral extreme or ‘corner’ of the bill is broadly rounded, as seen in saurolophines Prosaurolophus maximus and Saurolophus spp. [13], not sharply angled as in other lambeosaurines [8,9].

The oral margin, the relatively flat area dorsal and medial to the occlusal margin of the bone, is remarkably broad throughout its lateral extent, being nearly half as broad as the entire bone. The broad lateral oral margin of T. spinorhinus contrasts with the relatively thinner oral margin of the premaxilla of other lambeosaurines such as species of Corythosaurus [31,34], Lambeosaurus [8,35], Parasaurolophus [36,37], and Hypacrosaurus [38,39], and is more like that seen in saurolophines like Edmontosaurus [40] and Brachylophosaurus [41]. Rostrally, the convex oral margin of T. spinorhinus narrows abruptly to less than half of the maximum breadth that the oral margin achieves laterally; similar thinning of the oral margin rostromedially is observed in other lambeosaurines. The oral margin is clearly ‘reflected,’ strongly emarginated by the circumnarial fossa rostrally as in Saurolophinae, and this morphology extends along the lateral oral margin where it becomes less pronounced. The reflected oral margin is continuous with the folded margin of the caudolateral process of the premaxilla laterally (see below).

A faint emargination extends from the rostral, reflected oral margin across the rostrolateral oral margin of the premaxilla, and appears to represent the dorsalmost extent of the ramphotheca, forming the dorsal border of the beak attachment surface (Figure 5B, C, F). This ridge is apparently continuous with the dorsal extent of the oral margin medially, and gradually descends ventrally as it wraps around the bill, closely approaching the occlusal margin of the bone at the ventrolateral end of the premaxillary rostrum. Ventrally, the rostral margin of the occlusal surface of the premaxilla exhibits a series of six large, evenly spaced, subtriangular denticles (Figure 5C, E).

The medial sagittal lamina, or prenarial septum (Figure 5B, C), of the premaxilla is continuous along the length of the fragmentary premaxilla and is not perforated by the apertura ossis nasi, indicating that the latter structure has been dorsally displaced, as in other Lambeosaurinae. Folds of the premaxilla extend from the dorsal and ventral margins of the bone to enclose the circumnarial fossa and form a secondary enclosure of the nasal vestibulum, here termed the pseudonaris, surrounded by the premaxilla. The pseudonaris (Figure 5B) is a neomorphic structure unknown outside of Lambeosaurinae, in which folds of the premaxilla enclose the distal portion of the nasal vestibulum, while the true apertura ossis nasi that conducts the vestibulum into the intracranial space proper is enclosed within the crest proper [42].

The lateral fold of the premaxilla is quite wide mediolaterally and remains broad as it curves rostrally to become the reflected oral margin (Figure 5B, C, F), unlike in other lambeosaurines where it thins abruptly before becoming the oral margin at the caudolateral ‘corner’ of the premaxillary bill. The dorsal fold is evidenced by a damaged ridge of bone in the caudal quarter of the element cutting across the accessory fossae of the circumnarial fossa (Figure 5D, F). Both folds converge posteriorly, but if they met they did so beyond the preserved limits of the bone. The pseudonaris they formed was thus elongate, as in Corythosaurus spp. [34] and not abbreviated and lacriform as in Parasaurolophus walkeri [36] and Hypacrosaurus altispinus [39].

Unlike the smooth and featureless circumnarial fossa typically present in other lambeosaurines [39], that of Tsintaosaurus spinorhinus is compartmented into three accessory fossae separated by two oblique ridges (Figure 5D). These ridges are inclined lateroventrally and their long axes are oriented rostrolaterally. The accessory fossae that are bounded by these ridges are narrow and deep. The rostral fossa is lacriform and entirely confined within the preserved margins of the bone. The median and caudal fossae are elongate and extend caudally into the narial passage, where the median fossa may have been truncated against the medial fold. Although morphologically somewhat different, compartmentalization of the narial fossa is commonly present in saurolophine hadrosaurids, including Gryposaurus [43,44], Prosaurolophus [45,46], Edmontosaurus [47,48], and Brachylophosaurus [49,50]. There is no indication in T. spinorhinus of the premaxillary foramen typical of saurolophines [13] and at least some non-hadrosaurid iguanodontians [51], but there are two regions of damage to the thin bone between the medial and lateral ridges that may have been perforated in life.

A different region of the premaxilla of Tsintaosaurus spinorhinus is preserved in IVPP V829. As noted above, this is a blocky subrectangular structure that surrounds and articulates laterally with the rhomboid distal process of the nasal (Figures 1 and 2). Given its dorsal position in the skull, indicated by the articulation with the distal process of the nasal, we interpret this fragment of premaxilla as the nearly complete right and partial left sides of the dorsal summit of the supracranial crest of T. spinorhinus. Most of the lateral surface of the premaxilla is excavated for reception of the distal nasal process (Figure 2C). In IVPP V829 the rhomboid nasal process is partially displaced ventrally from its articular position (Figure 2C and D), allowing observation of the morphology of the rostral and caudal margins of the lateral fossa. The contour described by those borders mirrors the rhomboid outline of the distal nasal process, indicating that the process fit tightly against the premaxilla. The rostral and caudal margins of the articular fossa for the nasal rise abruptly and prominently from the surrounding rostral region of the lateral surface of the premaxilla. The rostral margin is relatively broad, gently concave, and slightly recessed relative to the caudal margin and the rostral edge of the articular fossa. Adjacent to the articular fossa for the nasal, the eroded rostral region of the premaxillary fragment (where the bone separated from the rest of the premaxilla) is further compressed mediolaterally (Figure 1E, G), and forms a solid extension of the crest beyond the hollow nasal passages, or ‘cockscomb.’

The ventral surface of IVPP V829 is expanded mediolaterally and strongly concave. This concavity is D-shaped in ventral view and occupies the rostral half of the ventral surface of the bone fragment (Figure 1E, F). It exhibits a pair of narrow vascular grooves that are parallel and longitudinally arranged, one on either side of the sagittal plane of the crest. Dorsal and medial to the ventral concave surface of the premaxillary fragment, there is a large foramen that opens rostromedially (Figure 1D). At the caudal region of the premaxillary fragment, the ventral concavity is continuous with a caudoventrally-directed, hook-like process (Figure 1D, E). Although only the right caudoventral process is preserved, the bilateral symmetry of the premaxillae suggests that most likely a left process was also present. In addition to its hook-like caudoventral curvature, the process is also slightly curved medially. The lateral edge of the process is continuous with and forms a 125° angle with the lateral border of the rostral D-shaped ventral concavity. The ventral surface of the process is slightly concave and smooth. In ventral view, its elongated contour, with the slightly wider caudal end, is that of a narrow paddle. A median space lies in between the hook-like processes and deepens dorsally, adjacent to the caudal end of the D-shaped ventral concavity of the bone. This space is divided by a short longitudinal ridge that is coincident with the sagittal plane of the cranial crest. Caudal to the hook-like process, the ventral surface of the fragment is shallowly concave and evidently formed a small secondary chamber (Figure 1F).

A small, triangular bony fragment partially overlaps the dorsolateral surface of the right hook-like caudoventral process of IVPP V829, just ventral and adjacent to the proximoventral margin of the partially disarticulated rhomboid nasal process (Figure 1C, E). This fragment represents a different element than the nasal and premaxillary fragments described above. In the absence of data indicating otherwise and given its position in IVPP V829, we tentatively suggest that it represents the caudodorsal extreme and only preserved portion of the lateral process of the premaxilla.

Prefrontal.

The prefrontal of Tsintaosaurus spinorhinus is unique in possessing a rostral process that projects significantly dorsally (Figure 6; this bone was identified in IVPP V818 as part of the nasal by Young [17]). This rostral ascending process projects dorsally and twists medially along its mid-length, so that the external surface of its dorsal segment faces laterally (Figure 6A, B). The dorsal extremity is mediolaterally compressed and its narrow dorsal margin is inscribed with short transverse indentations (Figure 6A). When the prefrontal is articulated on the lateral side of the skull, the ascending process buttresses the base of the nasal, as previously noted by Young [17] in IVPP V725 and Buffetaut and Tong-Buffetaut [25] in IVPP V818. Another unique trait of the prefrontal of T. spinorhinus is a thin flange that extends laterally from the rostral margin of the lacrimal process to the proximal region of the ascending process (Figure 6A, B). The caudodorsal external surface of this flange is concave and smooth, and continuous dorsomedially with the proximal region of the ascending process. The medial surface of the main body of the prefrontal shows a deeply indented articular surface for the frontal. Long longitudinal grooves excavate the ventral portion of the ascending process and the main body of the prefrontal (Figure 6D). Caudally, the central body of the prefrontal is continuous with a deep postorbital process.

Frontal.

Our observations in IVPP V725 and V818 confirm those of Young [17] and Buffetaut and Tong-Buffetaut [25] regarding the dorsal curvature of the rostral region of the ectocranial surface of the frontals. This stands in contrast to most lambeosaurins, in which the frontal is down-warped as part of the platform supporting the crest. The frontals form a narrow process between the nasals rostrally, as in Jaxartosaurus aralensis [15] and Kazaklambia convincens [52], but in Tsintaosaurus spinorhinus this process clearly ascended to participate in the crest as reconstructed by Young [17].

The Paratype skull, IVPP 818, is missing the nasal, and reveals details of the nasofrontal articulation. The rostral surface of the frontal is deeply embayed along the sagittal plane endocranially, and features a weakly-striated, tongue-like process extending rostrally from the rostrolateral margin, above which is a nearly smooth, concave surface for articulation with the nasals (Figure 4). The ectocranial surface of the frontal forms a shelf that overhangs this articular surface and the tongue-like process, extending rostrodorsally well rostral to the tongue-like process and concealing it in dorsal view (Figure 4). The concave area dorsal to each tongue-like process forms a slot or socket into which the nasal must have fit, effectively clamping it between the tongue-like process ventrally and the ectocranial process dorsally.

The tongue-like processes of the nasofrontal articulation are not continuous across the midline of the skull; they taper into the embayed rostral margin of the bone before reaching the sagittal plane. The overlying ectocranial process is however continuous across the midline of the skull. The articular socket dorsal to the tongue-like process extends laterally and is divided from a dorsoventrally deep beveled facet for the prefrontal by a thick, dorsoventrally oriented ridge. There is a small notch at the base of this ridge where the lacrimal process of the prefrontal apparently clips between the ventral tongue-like process and ectocranial process of the frontal. This ridge, at the edge of the ectocranial process of the frontal, buttresses the caudal margin of the prefrontal for some distance dorsally before the nasal interposes between them.

Internal morphology of the crest.

Because of the limited material of the crest, we are only able to reconstruct some aspects of the internal anatomy of the crest and the path of the nasal passage (Figure 8). Lambeosaurines are characterized by enclosure of the nasal vestibule by the premaxillae [8]. It is unclear if the rostral portion of the vestibule was completely enclosed in Tsintaosaurus spinorhinus, because of the incomplete nature of the premaxilla. However, as noted above, the outgrowths of the premaxilla that define the pseudonaris appear to converge caudally, suggesting that the vestibule was indeed enclosed by bone before reaching the crest. We hypothesize that the posterior portions of the vestibule (within the crest proper) were also enclosed, but lacking a complete premaxilla we cannot be certain.

As noted above, the premaxillary fragment represented in IVPP V829 shows a concave ventral surface. Its hypothetical relatively high position within the crest of Tsintaosaurus spinirhinus, just below the level of the rhomboid distal process of the nasal (Figure 7B), suggests that it probably constitutes the roof of the common median chamber [3]. The common median chamber is an osteologically undivided supraorbital extension of the nasal cavity that has been documented in lambeosaurine genera like Corythosaurus, Lambeosaurus, Hypacrosaurus, and Parasaurolophus [9,34,38,39]. The common median chamber is enclosed by variable contributions of the nasals and premaxillae that form the hollow supracranial crests of these animals [59].

The hook-like medial processes of the premaxilla appear to divide the common median chamber into two, unequal portions. The larger, rostral chamber is less complete; it was bounded dorsally and anteriorly by the medial process of the premaxilla, and the lateral processes may have covered it at least partially laterally. This was most likely where the osseous nasal ducts from the external naris conducted air into the crest cavity proper. Because of incomplete preservation, we cannot reconstruct the extent of the lateral diverticula, nor their relationship to the common median chamber.

The hook-like processes of the premaxillae form the lateral margins of a substantial midline foramen within the crest passing between the sub-chambers. The smaller caudal chamber is bounded dorsally and caudodorsally by the medial process of the premaxilla, caudally by the nasals, and laterally by the premaxilla. Air presumably passed from the rostral to the caudal chamber through the interconnecting foramen before being directed ventrally into the cranium proper.

We do not believe that the nasal passage passed through the hollow tubular process formed by the nasals (contra [17]). Not only would this be unprecedented in hadrosaurid anatomy, but the cross-sectional area of the tubular process of the nasals is quite small and there is no ventral exit to the tubular process by which the enclosed tube could communicate with the intracranial space.

We reconstruct the nasal passage as passing ventrally down the rostral face of the nasal bones to enter the cranial cavity rostral to the orbits. Typically, a large foramen at the ventral limit of the common median chamber communicates between the latter and the antorbital space [2,42]. This foramen is not preserved in Tsintaosaurus spinorhinus; however, the entrance of the nasal passage into the intracranial space is marked by the two large elliptical depressions on the block-like intracranial process of the nasal described above (Figure 3A). These depressions would have bounded the nasal passage caudally, whereas the premaxilla and possibly the prefrontal would form its rostral and lateral walls. Note that the previously described swelling of the lateral surface of the cranium occurs approximately at this level, and it may be a structural consequence of threading both the dorsal and ventral nasal duct through the otherwise relatively narrow base of the crest. Indeed, it appears that the crest was mediolaterally quite broad rostrodorsal to the orbits.

It may be significant that the entrances of the nasal passage into the intracranial space are apparently paired. The foramen communicating between the crest and the cranial cavity [2,52] is undivided in other lambeosaurines. Our interpretation suggests that, in Tsintaosaurus spinorhinus, airflow was separate on left and right sides of the crest after passing through the common median chamber. The simplest interpretation would be that airflow from each naris was completely separate through the entire crest, and did not mix in the common median chamber. The latter may have therefore been divided by an unossified sagittal septum, possibly extending from the sagittal ridge on the dorsal surface of the common median chamber noted above.

Function of the tubular process of the nasal.

As noted above, we doubt that the nasal tubular process, if indeed it is hollow, transmitted the nasal passages into the intracranial space. We have reconstructed this area as housing a diverticulum of the crest cavity (Figure 8). However, the unusual shape of the nasals warrants further mention. A hollow cylinder is structurally stronger than a solid one, and we suspect that the apomorphic structure of the nasals may be an adaptation to support the weight of the crest. In other lambeosaurines, the crest rests on the dorsal surface of the frontals and prefrontals, and its weight is transferred to the skull roof ventrally via these bones. In Tsintaosaurus spinorhinus, the ectocranial connection to the frontal is limited to edge-on contact. Presumably the intracranial process of the nasals slotted into the peculiar nasofrontal articulation caudally, where reciprocal processes of the nasal would have been ‘clipped’ in between the tongue-like processes and the overlying concave articular surface.

We suspect that the weight of the crest was transferred via the complex socketed arrangement of the rhomboid processes to the tubular process of the nasals, and then ventrally to the prefrontals and the block-like intracranial processes of the nasals, where it was then passed to the skull roof via the tongue-like rostral processes of the frontal. In this regard, it may be significant that the prefrontals do not underlie the crest as part of the frontal platform, but buttress the crest laterally. In other lambeosaurines, the extension of the frontal platform onto the prefrontal (see below) may have permitted greater support of the crest laterally, relieving the nasals of more of the burden of the crest, and explaining the absence of a tubular nasal process in these taxa.