A new species of Gryposaurus (Dinosauria: Hadrosauridae) from the late Campanian Kaiparowits Formation, southern Utah, USA

Authors


E-mail: tgates@umnh.utah.edu

Abstract

A new species of the hadrosaurine hadrosaurid Gryposaurus was discovered in the late Campanian Kaiparowits Formation of southern Utah. Gryposaurus monumentensis, sp. nov. is distinguished from other Gryposaurus species by possessing a more robust skull, enlarged clover-shaped prongs on the predentary oral margin, an anteroposteriorly narrow infratemporal fenestra, and other autapomorphies plausibly associated with feeding adaptations. The derived morphology revealed in G. monumentensis necessitates revision of the generic diagnosis of Gryposaurus, including the addition of synapomorphies that further aid in distinguishing this taxon from Kritosaurus. A revised phylogenetic analysis places Gryposaurus within a monophyletic clade that includes Brachylophosaurus and Maiasaura. Gryposaurus monumentensis represents the most southern example of Gryposaurus, and underlines the remarkable diversification and long duration of this genus. Based on the phylogenetic, geographical, and stratigraphic evidence at hand, Gryposaurus was the most diverse genus within Hadrosaurinae; it also possessed one of the largest geographical and stratigraphic distributions, spanning more than five million years of the Campanian, and ranging from Alberta in the north to Utah (and possibly Texas) in the south. © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society, 2007, 151, 351–376.

INTRODUCTION

Gryposaurus is a hadrosaurine (noncrested or solid-crested) hadrosaurid distinguished by its deep skull and prominent nasal arch. The type species, Gryposaurus notabilis, was described by Lambe (1914) from the Dinosaur Park Formation of Alberta, Canada. Four years earlier, Barnum Brown described a partial skull – missing the premaxillae and nasals – from the Kirtland Formation of New Mexico, which he named Kritosaurus navajovius (Brown, 1910). Both taxa possess a deep skull with large dentaries and predentaries and infratemporal fenestrae larger than the orbit. On the basis of these similarities, Parks (1919) considered Gryposaurus to be a junior synonym of Kritosaurus, and described the species K. incurvimanus, also from the Dinosaur Park Formation. Subsequently, Lull & Wright (1942) assigned G. notabilis to the genus Kritosaurus, designated Gryposaurus as a nomen dubium, and claimed that all hadrosaurids in which the infratemporal fenestra exceeds the orbit in area belong to the former genus. However, more recently, Horner (1992) outlined multiple distinctions between the two genera, including the shape of the frontal–nasal contact and morphology of the nasal ornamentation. In addition, Horner resurrected Gryposaurus, created the new combination of G. incurvimanus, and described yet another species, G. latidens, characterized by relatively wider dentary teeth than in other species.

The Kaiparowits Formation is an 800-m-thick package of rocks deposited between 76.1 and 74.0 Mya on a wet, humid alluvial plain setting with paludal and riparian environments (Roberts, Deino & Chan, 2005; Roberts, 2007). Further, Roberts et al. (2005) demonstrated that the Kaiparowits is temporally equivalent with many other fossiliferous formations within the Western Interior Basin – most significantly for this discussion, the Dinosaur Park Formation, where G. notabilis and G. incurvimanus co-occur. The Kaiparowits Basin Project (KBP), launched in 2000 by the University of Utah, is a multi-institution endeavour that has focused on the unravelling of the macrovertebrate fossil record preserved within the Kaiparowits Formation. To date, KBP has met with considerable success (Sampson et al., 2004), resulting in the discovery of several new species, including a new oviraptorosaur (Zanno & Sampson, 2005), a chasmosaurine ceratopsid (Smith et al., 2004), and a hadrosaurid (this paper).

Hadrosaurid fossils are extremely abundant in the Kaiparowits Formation. Yet, the only taxon described to date from this unit is the lambeosaurine (crested hadrosaurid) Parasaurolophus cf. cyrtocristatus (Weishampel & Jenson, 1979; Sullivan & Williamson, 1999; Gates, 2004), known from multiple partial skulls and a partial postcranium (T. A. Gates; E. K. Lund; M. A. Getty; J. I. Kirkland; A. L. Titus; D. Deblieux; C. A. Boyd & S. D. Sampson, unpubl. data). Previous hadrosaurine materials collected from the Kaiparowits Formation have generally been placed into Kritosaurus, which has become a waste-basket taxon for most nondiagnostic hadrosaurine materials collected in the southern region of the Western Interior Basin. However, in August 2004, personnel from the Raymond M. Alf Museum of Palaeontology excavated a mostly complete hadrosaurine skull from the Kaiparowits Formation. This skull and additional specimens collected by field crews from the Utah Museum of Natural History are clearly referable to Gryposaurus, and represent yet another new species (Fig. 1). This paper describes the new taxon, revises the generic diagnosis of Gryposaurus, and discusses the phylogenetic, palaeobiological, and biogeographical significance of this dinosaur. In addition, biostratigraphic patterns are revealed in the Kaiparowits Formation through comparison of the temporal span of an unidentified, undescribed specimen of Gryposaurus in the base of the formation with the temporal occurrences of the new Gryposaurus taxon described herein.

Figure 1.

Lateral view of type specimen of Gryposaurus monumentensis gen. et sp. nov. skull, RAM 6797: A, photograph of left side of skull; B, line drawing of skull based on photograph in (A) and showing slight reconstruction. Ar, articular; D, dentary; F, frontal; J, jugal; La, lacrimal; Mx, maxilla; Na, nasal; Pd, predentary; Pf, prefrontal; Pm, premaxilla; Po, postorbital; Qj, quadratojugal; Qu, quadrate; Sq, squamosal; Su, surangular. Scale bar, 10 cm.

Institutional abbreviations

AMNH, American Museum of Natural History, New York, NY, USA; CMN, Canadian Museum of Nature, Ottawa, ON, Canada; OTM, Old Trail Museum, Choteau, MT, USA; RAM, Raymond M. Alf Museum of Palaeontology, Claremont, CA, USA; ROM, Royal Ontario Museum, Toronto, ON, Canada; TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, AB, Canada; UMNH, Utah Museum of Natural History, University of Utah, Salt Lake City, UT, USA.

SYSTEMATIC PALAEONTOLOGY

DinosauriaOwen, 1842
OrnithischiaSeeley, 1887
HadrosauridaeCope, 1869
HadrosaurinaeCope, 1869
GryposaurusLambe, 1914

Etymology:  Derived from the Latin for ‘hooked beak’, intended by Lambe to reference the arch on the dorsal surface of the nasal, resembling a gryphin, and the Greek ‘saurus’ for lizard.

Revised diagnosis:  Hadrosaurine hadrosaurids possessing the following autapomorphies: dorsolateral flare on the medial margin of the premaxillary lateral process; unexcavated nasal arch located anterior to the orbits; and sigmoidal nasofrontal suture characterized by a small median process of the nasal inserting between the midline of the frontals. Members of the genus also possess the following unique combination of characters: dorsoventrally deep skull; anterior process of nasal terminates along the dorsal margin of the external naris; ventral nasal process comprising approximately 25% of the ventral margin of the external nares; abbreviated circumnarial depression, extending posteriorly only as far as the lacrimal; jugal with large offset posteroventral flange, postorbital process at or near 90°, and narrow posterior process; and frontal contribution to the orbital rim.

Gryposaurus notabilisLambe, 1914

Revised diagnosis:  Hadrosaurids of the genus Gryposaurus with the following unique combination of characters: sigmoidal lateroventral margin of the maxilla; large nasal arch located near the orbits and rising above the level of the frontals; narrow ‘U’-shaped posterior margin of the external naris; squamosal posterodorsal margin well above the level of the skull roof; infratemporal fenestra larger than the orbit; and rounded mandibular foramen. The skull of G. notabilis is generally more robust than that of G. latidens and G. incurvimanus, but less so than that of G. monumentensis.

Holotype: CMN 2278, complete articulated skull.

Gryposaurus incurvimanusParks, 1919

Revised diagnosis:  Hadrosaurids of the genus Gryposaurus possessing: dorsal premaxillary process that is more concave posteriorly than in other species of the genus; nasal arch smaller than in other species and positioned well anterior to the orbit and rising to a level below the frontals; jugal with a small spur on the posterior margin of the posteroventral flange; and slightly excavated ventral surface of the nasal hump. In addition, G. incurvimanus possesses the following unique combination of characters: nearly straight lateroventral margin of the maxilla; infratemporal fenestra larger than the orbit; and broad ‘U’-shaped posterior margin of the external naris. Gryposaurus incurvimanus is significantly more gracile than either G. notabilis or G. monumentensis. It has a smaller premaxillary lip than the other species, as well as premaxillae that are angled more steeply than in other species. Gryposaurus incurvimanus also possesses an anteroposteriorly shortened skull relative to the maximum skull depth.

Holotype: ROM 764, mostly complete skull and complete postcranium.

Gryposaurus latidensHorner, 1992

Diagnosis (after Horner, 1992): Dentary teeth shorter and wider than in other species of the genus. In addition, G. latidens possesses the following unique combination of characters: arched lateroventral margin on the maxilla; nasal arch positioned near the orbits and rising above the level of the frontals; and broad ‘U’-shaped posterior margin of the external naris. The humerus of G. latidens is larger and more robust than that of G. incurvimanus.

Holotype: AMNH 5465, partial skull and nearly complete skeleton.

Gryposaurus monumentensissp. nov.

Etymology:  In reference to the Grand Staircase-Escalante National Monument in southern Utah, where all referred materials of this species have been recovered.

Diagnosis:  Hadrosaurids of the genus Gryposaurus possessing: anterodorsal process of maxilla seen through the external nares; subparallel anterior and posterior margins of the infratemporal fenestra; anteroposterior width of the infratemporal fenestra approximately half of the width of the orbit; predentary with large, pronged denticles; anterior portion of the dentary sharply downturned; surangular with distinct dorsal process on the median ridge of surangular; and ovoid mandibular foramen. The following suite of characters is also present: sigmoidal lateroventral margin of the maxilla; large nasal arch located near the orbits rising above the level of the frontals; narrow ‘U’-shaped posterior margin of the external naris; and squamosal raised well above the level of the skull roof. Gryposaurus monumentensis is significantly more robust than any other species of Gryposaurus. It is most similar to G. notabilis, but differs in having more steeply angled premaxillae, mediolaterally wider dentary, and generally more prominent jugal tubercle.

Holotype: RAM 6797, largely articulated skull lacking most of the right side of the skull (Figs 1, 3, 5A, 7A, B, 9, 11, 12, 13A, 14, 15).

Figure 3.

Lateral view of Gryposaurus monumentensis gen. et sp. nov. left premaxilla, RAM 6797. en, external nares; MX, maxilla; or, oral margin rugosities; pmd, dorsal process; pmf, premaxillary foramen; pml, lateral process; pmlf, premaxilla lateral process flare; pmlp, premaxillary lip; pms, premaxillary shelf. Scale bar, 5 cm.

Figure 5.

Line drawings of the four Gryposaurus species in lateral view, showing synapomorphies of the genus and other comparative features: A, G. monumentensis gen. et sp. nov. (RAM 6797); B, G. notabilis (ROM 873); C, G. incurvimanus (TMP 80.22.1); D, G. latidens (after Horner, 1992; no scale included). itf, infratemporal fenestra; mf, mandibular foramen; na, nasal arch; pmlf, lateral process flare. Scale bar, 10 cm.

Figure 7.

Maxillae from Gryposaurus monumentensis gen. et sp. nov., specimens RAM 6797 and UMNH VP 13970: A, left maxilla in lateral view with surrounding elements from RAM 6797; B, posterior region of right maxilla from UMNH VP 13970; C, right maxilla from RAM 6797 in lateral view; D, anterior region of left maxilla from UMNH VP 13970. D, dentary; ecr, ectopterygoid ridge; ecs, ectopterygoid shelf; J, jugal; jp, jugal process; La, lacrimal; madp, anterodorsal process; mavp, anteroventral process; mdp, maxilla dorsal process; mf, maxillary foramen; Mx, maxilla; nf, nutrient foramen; palp, palatine process; Pm, premaxilla; ptp, pterygoid process. Scale bar, 5 cm.

Figure 9.

Posterior view of left lacrimal–jugal contact in Gryposaurus monumentensis gen. et sp. nov. (RAM 6797). The left side of the figure is lateral and the right side is medial. Note the interlocking relationship between these elements. J, jugal; La, lacrimal; lf, lacrimal foramen.

Figure 11.

Gryposaurus monumentensis gen. et sp. nov. (RAM 6797) braincase: A, ventral view; B, anterior region in lateral view. bpt, basipterygoid process; Bs, basioccipital; bsmm, basioccipital median mound; Bsp, basisphenoid; bst, basitubera; btr, basisphenoid transverse ridge; CN, cranial nerve; Exo, exoccipital; Ls, laterosphenoid; mvp, median ventral process; Pr, pro-otic; psp, parasphenoid process; Qu, quadrate. Scale bar, 5 cm.

Figure 12.

Gryposaurus monumentensis gen. et sp. nov. (RAM 6797) right quadrate: A, lateral view; B, medial view. mc, mandibular condyle; ptw, pterygoid wing; qb, quadrate buttress; qh, quadrate head; qjn, quadratojugal notch; qs, quadrate shaft. Scale bar, 5 cm.

Figure 13.

Right vomer of Gryposaurus monumentensis gen. et sp. nov. in lateral view: A, RAM 6797; B, UMNH VP 13970. The broken line marks the estimated position of the dorsal margin of the anteroventral excavation. ave, anteroventral excavation; mxa, maxillary articulation; pma, premaxillary articulation; pvl, posteroventral lobe. Scale bar, 5 cm.

Figure 14.

Predentary of RAM 6797 in left anterolateral view. D, dentary; nf, nutrient foramen; pdd, predentary denticles; pdlp, predentary lateral process; pdvp, predentary ventral process. Scale bar, 5 cm.

Figure 15.

Right surangular of Gryposaurus monumentensis gen. et sp. nov. (RAM 6797): A, lateral view; B, dorsal view. ap, articular process; asn, anterior surangular notch; mc, medial concavity; mr, median ridge; msp, median surangular process; qas, quadrate articular surface; scp, surangular coronoid process. Scale bar, 5 cm.

Referred materials:  UMNH VP 13970 (Figs 4, 7C, D, 8, 13B), partial, disarticulated subadult skull that includes both maxillae and dentaries, right nasal, partial quadrate, right jugal, right vomer, right prefrontal; UMNH VP 12265, partial skull and skeleton preserving both maxillae, left dentary, left quadrate, and partial jugal, left humerus, right and left scapulae, right coracoid, multiple ribs, both ilia, pubes, and ischia, most of the dorsal, sacral, and caudal series, including skin impressions on the right side of the vertebral series.

Figure 4.

Lateral view of Gryposaurus monumentensis gen. et sp. nov. left nasal, UMNH VP 13970. en, external naris; na, nasal arch; nap, nasal anterior process; nfs, nasofrontal suture. Scale bar, 5 cm.

Figure 8.

Lateral view of juvenile right jugal from UMNH VP 13970. jap, jugal anterior process; jpp, jugal posterior process; pop, postorbital process; pvf, posteroventral flange. Scale bar, 5 cm.

Horizon:  All known specimens are from the middle unit of the Kaiparowits Formation (Fig. 2; Upper Cretaceous: late Campanian).

Figure 2.

Map showing location of Grand Staircase-Escalante National Monument (GSENM) within Utah.

DESCRIPTION

The G. monumentensis holotype specimen, RAM 6797, was found in an ancient point bar deposit in the middle unit of the Kaiparowits Formation. The left half of the skull sank into soft sediment, remaining completely articulated, and is preserved in relatively pristine condition. The right side of the skull suffered from disarticulation and loss of elements, apparently the result of decay and transport in the river current; evidence of fluvial influence was seen in the right quadrate, which was recovered on top of the right maxilla. The frontals, parietals, right side of the braincase, and posterior nasals were partially eroded prior to excavation, preventing detailed examination of these elements. The nasals, however, do preserve the nasal ornamentation, although not in articulation. A disarticulated subadult specimen referred to G. monumentensis (UMNH VP 13970) preserves many delicate elements, such as the vomers and complete nasals. All elements of the craniofacial skeleton, except the parietals and frontals, are preserved in detail on the type specimen and/or UMNH VP 13970.

Cranium

General:  Overall, the skull of G. monumentensis is extremely robust, much more so than that of any other species of the genus. It is also relatively large and compact – that is, relatively deep dorsoventrally and abbreviated anteroposteriorly. This combination of increased robustness and anteroposterior shortening results in subtle, but nonetheless significant, modifications to much of the craniofacial skeleton when compared with other species of Gryposaurus.

Premaxilla:  The premaxilla of G. monumentensis is robust and steeply angled, giving the anterodorsal region of the skull its characteristic shape (Fig. 1). It contacts the maxilla and lacrimal posterolaterally along its lateral process and the nasal through both the dorsal and lateral processes (Fig. 1). For the purposes of description, this element is divided into three regions: (1) an anterior region; (2) a dorsal process; and (3) a lateral process.

The oral margin is highly rugose (Fig. 3). In addition, a large patch of rugose bone is present on the midline at the anterior limit of the interpremaxillary contact, a feature not observed in any other Gryposaurus species. It is likely that the highly rugose oral margin and large midline rugosity participated in anchoring a similarly robust keratinous rhamphotheca (Morris, 1970). In dorsal view, the oral margin expands laterally to a maximum width at least equal to the width of the skull at the postorbital. Unlike Brachylophosaurus (Prieto-Marquez, 2005), the premaxillary lip is upturned and slightly folded posteriorly (Fig. 3), a condition that is less pronounced in Gryposaurus than in either Prosaurolophus (Horner, 1992) or Edmontosaurus (Lambe, 1920). Posterior to the premaxillary lip, the premaxillary shelf is broad and bulges dorsally as a result of a concave depression on the ventral side of the element. The large premaxillary foramen resides posteroventral to the dorsal process (Fig. 3).

In lateral view, the dorsal process of the premaxilla arches posterodorsally more than in any other hadrosaurine taxon except G. incurvimanus (TMP 80.22.1; the premaxillae are unknown for Kritosaurus). The dorsal process is approximately 75% of the length of the lateral process, continuing almost to the posterior extent of the external nares. It bears a shallow lateral groove to secure the nasal anterior process.

The lateral process of the premaxilla extends posterodorsally to rest on the lacrimal behind the external nares. Unique amongst other hadrosaurid genera, the medial margin of the premaxilla lateral process flares sharply dorsally approximately midway along the ventral margin of the external naris (Fig. 3). In comparison with other Gryposaurus species, the premaxilla of G. monumentensis (RAM 6797) is slightly more robust than that of some specimens of G. notabilis (ROM 873), and significantly more so than that of G. incurvimanus (TMP 80.22.1) and G. latidens (MOR 553s-7-18-91-107). Despite differences in robustness and width, all Gryposaurus species share an upturned premaxillary lip and dorsal flaring of the lateral process.

Nasal:  The nasal is an axe-shaped element in lateral view that contacts anteriorly with the premaxilla, posteroventrally with the premaxilla, lacrimal, and prefrontal, and posteriorly with the frontals (Fig. 1). The anterior process of the nasal flanks the premaxillary dorsal process laterally, terminating prior to the anterior margin of the external nares, a condition shared with the other species of Gryposaurus, Maiasaura (Horner, 1983), Bactrosaurus (Godefroit et al., 1998), Telmatosaurus (Weishampel, Norman & Grigorescu, 1993), and most other basal iguanodontians (Norman, 2004).

The transversely compressed nasal arch is the most distinctive characteristic distinguishing Gryposaurus from all other hadrosaurids (Figs 1, 4). Gryposaurus incurvimanus (TMP 80.22.1) exhibits a smaller arch positioned more anteriorly than that in G. notabilis (CMN 2278, ROM 873), G. latidens (AMNH 5465, MOR 478), and G. monumentensis (RAM 6797, UMNH VP 13970; Fig. 5).

Posteriorly, the nasals contact the frontals in a sigmoidal suture resulting from small, paired processes of the nasals that insert along the midline of slightly divergent frontals (Horner, 1992; Fig. 6). The frontal–nasal contact is not preserved on the type specimen of G. monumentensis. However, the posterior portion of the right nasal in UMNH VP 13970 indicates that this sigmoidal suture was probably present in the Utah taxon (Fig. 4). The characteristic sigmoidal nasofrontal suture differs from the posteriorly concave suture of Brachylophosaurus (Prieto-Marquez, 2005), and the posteriorly lobate suture of Prosaurolophus and Naashoibitosaurus (NMMNH P-16106) that inserts a small frontal process between splayed nasals (Horner, 1992).

Figure 6.

Dorsal view of frontal–nasal suture in Gryposaurus incurvimanus (ROM 764). F, frontal; Na, nasal; nfs, nasofrontal suture; Pa, parietal; Pf, prefrontal; Po, postorbital. Scale bar, 5 cm.

In lateral view, the posteroventral region of the nasal forms the axe head of the element (Figs 1, 4). The circumnarial depression (Hopson, 1975) is expressed as a shallow halo around the external nares. In contrast, the circumnarial depression extends much further posteriorly in several other hadrosaurines, terminating just anterior to the orbits [Anasazisaurus (BYU 12950), Naashoibitosaurus (NMMNH P-16106); Horner, 1992] or even on the skull roof (Prosaurolophus blackfeetensis and Saurolophus angustirostris; Maryanska & Osmólska, 1981; Horner, 1992).

Not surprisingly, the G. monumentensis nasal underwent a shape change during ontogeny, as evidenced by comparison of nasals from the type specimen and the subadult UMNH VP 13970. In general, it appears that this species increased the size and prominence of the nasal arch and decreased the angle of the anterior process.

Maxilla:  The robust maxilla of G. monumentensis can be subdivided into three regions: an anterior region that includes a pair of premaxillary processes; a central region contacting the jugal and lacrimal; and a posterior region that unites the palate with the facial skeleton (Fig. 7). Two large, anteriorly directed processes extend from the anterior-most edge of the maxilla (Fig. 7C, D). The anteroventral process is short and broad in order to support the lateral process of the premaxilla dorsally. The longer anterodorsal process displays a medial expansion comparable in size with that of Naashoibitosaurus (NMMNH P-16106), but larger than that of P. blackfeetensis (Horner, 1992). In G. monumentensis, the anterodorsal process is visible within the external naris when the skull is viewed laterally (Fig. 1). This contrasts with all other species of Gryposaurus, but is similar to Brachylophosaurus (Prieto-Marquez, 2005) and Maiasaura (Horner, 1983).

A rounded dorsal process (Fig. 7A) rises steeply from the central region of the maxilla. The structure is broader anteroposteriorly than in other hadrosaurine taxa, being most similar to G. notabilis (CMN 2278). The largest maxillary foramen opens at the base of the dorsal process (Fig. 7A, D). The dorsal process contacts the jugal along a broad, sigmoidal suture (Fig. 7A, C). The large overhanging jugal process is larger than that in Brachylophosaurus (MOR 1071 8-13-98-559) and in other species of Gryposaurus, but is more similar in size to that in P. blackfeetensis (Horner, 1992).

Contact with the palatine, ectopterygoid, and pterygoid (Fig. 7B, C) occurs on the posterior region of the maxilla. As exposed laterally on the holotype, the posterior region is narrow dorsoventrally, because of an expanded ventral tubercle of the jugal (see below). This dorsoventral compression is present, although not as developed, in G. notabilis (ROM 873), whereas the posterior maxilla has a significantly broader exposure in G. incurvimanus (TMP 80.22.1) and G. latidens (Horner, 1992; Fig. 5). A long, broad palatine process and triangular pterygoid process emanate posterior to the dorsal process along the tall median ridge of the posterior region (Fig. 7B, C). The ectopterygoid closely adheres to the broad ectopterygoid shelf, draping over the rounded posterior end of the maxilla (Fig. 7C). The ectopterygoid ridge (Fig. 7B) is not as sharply developed as in P. blackfeetensis (Horner, 1992) or Naashoibitosaurus (NMMNH P-16106). There are approximately 49 tooth positions along a sinusoidal tooth row. The lateroventral margin of the maxilla in G. monumentensis (RAM 6797) is dramatically sigmoidal, as in G. notabilis (ROM 873, CMN 2278), yet differs from the nearly straight condition in G. incurvimanus (TMP 80.22.1, and possibly ROM 764) and the slightly arching condition in G. latidens (Horner, 1992; Fig. 5) and most other hadrosaurine taxa. An arching line of the dental foramina is present on the medial side of the maxilla (Fig. 7C).

Jugal:  The jugal (Fig. 8) contacts the maxilla, the lacrimal, the postorbital, the quadrate, and quadratojugal (Fig. 1). Overall, this element is most similar to that of G. notabilis (ROM 873, CMN 2278; Fig. 5), although it shows distinct similarities with the jugal of Kritosaurus (AMNH 5799, BYU 12950, NMMNH P-16106; Brown, 1910; Lucas et al., 2006).

Unlike the anterior process of lambeosaurines, this feature tends to be notably asymmetrical in hadrosaurines. Specifically, in Gryposaurus, Brachylophosaurus, and Kritosaurus, the ventral half of the jugal head forms a large sweeping sigmoidal curve that contacts the posterior side of the maxilla dorsal process (Fig. 8; Maiasaura has a straight ventral margin; see Horner, 1990). Edmontosaurus regalis (e.g. CMN 2288), P. maximus (e.g. TMP 84-1-1), and Saurolophus osborni (e.g. AMNH 5220) all possess relatively convex ventral margins of the jugal anterior process. In G. monumentensis, the ventral tubercle of the anterior process is more hypertrophied than in any other species of Gryposaurus, and more so than in hadrosaurids generally, extending ventrally almost to the alveolar margin. However, the size and shape of this feature vary significantly in individuals of G. notabilis (e.g. ROM 873 and CMN 2278), and should be used with caution in a phylogenetic context.

The postorbital process ascends at nearly a 90° angle (Fig. 8). Maiasaura, Brachylophosaurus, Kritosaurus, Anasazisaurus, and Naashoibitosaurus share a nearly vertical postorbital process, whereas the process of Prosaurolophus spp. and Saurolophus spp. is slightly angled posteriorly, and steeply angled in Edmontosaurus (Brown, 1910; Lambe, 1920; Horner, 1983, 1992; Prieto-Marquez, 2005; Lucas et al., 2006). In G. monumentensis (RAM 6797 and UMNH VP 13970), and Gryposaurus generally, the posteroventral flange is a moderately sized extension of the jugal (Fig. 8). Gryposaurus incurvimanus (TMP 80.22.1) bears a small spur on the posterior edge of the flange, which is probably an apomorphy of the species. The prominent flange of Brachylophosaurus (Prieto-Marquez, 2005) and Maiasaura (Horner, 1983) is only slightly more exaggerated than that of Gryposaurus, but is quite distinct from the shallow flange of P. blackfeetensis (Horner, 1992).

The posterior process is long and slender, again differing from that of both P. blackfeetensis (Horner, 1992) and Brachylophosaurus (Prieto-Marquez, 2005), which possess long and broad or short and expanding processes, respectively.

In UMNH VP 13970, the posteroventral flange is smaller relative to the total size of the jugal than in RAM 6797 (Fig. 1), suggesting positive allometry within this structure. The postorbital process likewise changes slightly through ontogeny in that the angle of ascent morphs from about 60° in UMNH VP 13970 to nearly vertical in RAM 6797, ultimately decreasing the size of the orbit and increasing the size of the infratemporal fenestra with increasing skull length.

Lacrimal:  As in other hadrosaurids, the lacrimal is a triangular bone that forms the anterior margin of the orbit (Fig. 1). The dorsal surface of the lacrimal is sandwiched between the lateral process of the premaxilla laterally and the nasal medially. The prefrontal contributes to the posterodorsal corner of the lacrimal, covering a small section of the lateral surface. Contact between these two elements is accomplished through a triangular process on the dorsal-most corner of the lacrimal that inserts into a receptive groove on the prefrontal. In posterior aspect, the large, ovoid lacrimal foramen penetrates anteriorly through the body of the element (Fig. 9). Ventral to the lacrimal foramen, the jugal–lacrimal articulation is an ‘S’-shaped facet that tightly bonds the two elements together (Fig. 9). The ventral margin of the lacrimal rests subhorizontally within the lacrimal groove on the maxilla dorsal process, closer in form to Edmontosaurus (Lambe, 1920) and Naashoibitosaurus (NMMNH P-16106) than to Brachylophosaurus (Prieto-Marquez, 2005) or Prosaurolophus (Horner, 1992).

Quadratojugal:  Contacting with the jugal anteriorly and the quadrate posteriorly (Fig. 1), the quadratojugal of G. monumentensis resembles that of other hadrosaurine taxa in that it is subtriangular in general shape. Generally, the quadratojugal of the Utah taxon is most comparable with that of Prosaurolophus (Horner, 1992) and least similar to that of Edmontosaurus (Lambe, 1920). Posteriorly, a cupped region accepts the lower portion of the quadrate lateral wing and overlaps the quadratojugal notch on the quadrate. The posteroventral flange of the quadratojugal appears more pronounced in G. monumentensis than in other taxa.

Prefrontal:  The prefrontal of G. monumentensis UMNH VP 13970 is virtually identical to that of G. latidens (MOR 553M 6-27-9-55), contacting with the lacrimal anteroventrally (Fig. 1), the nasal medially, and the frontal medioposteriorly. It forms the anterodorsal corner of the orbital rim. Two foramina are situated near the junction of the smooth anterolateral margin and the heavily rugose posterolateral margin. A large triangular groove on the anteroventral surface of the prefrontal receives the lacrimal. The medial region bows slightly dorsally and overlaps the nasal. Posteriorly, the prefrontal has a large process that inserts into an anterolateral depression in the frontal.

Frontal:  The morphology of the frontal is conserved across known species of Gryposaurus, being broad, depressed medially, and sculpted anteriorly with a sigmoidal nasofrontal suture (Fig. 6). However, the frontal is not well preserved on any specimens assigned to G. monumentensis, including the holotype. Only the lateral-most portion of the left frontal is preserved in RAM 6797, where it participates in the formation of the orbital rim between the prefrontal and the postorbital (Fig. 1). This trait is present in Gryposaurus, Brachylophosaurus, Telmatosaurus, Bactrosaurus, and, indeed, most iguanodontians (Norman, 1986; Weishampel et al., 1993; Godefroit et al., 1998; Prieto-Marquez, 2005).

Postorbital:  The postorbital possesses three distinct processes (anterior, squamosal, and jugal) contacting the frontal, squamosal, and jugal, respectively (Fig. 1). The squamosal process extends posteriorly, but is relatively shorter in G. monumentensis than in G. notabilis (ROM 873 and CMN 2278) and G. incurvimanus (TMP 80.22.1 and ROM 764). The relative shortening of the squamosal process contributes to the anteroposterior narrowing of the infratemporal fenestra and supratemporal fenestra (Fig. 1; see below). Contact with the parietals occurs on a small area of the postorbital medial to the base of the squamosal process. A large pocket located on the medial side of the postorbital, ventral to the contact facet for the parietal, marks the insertion of the laterosphenoid. The lateral margin (i.e. the posterodorsal orbital rim) is highly rugose along its entire length.

Squamosal:  The squamosal makes up the posterolateral section of the skull roof contacting the quadrate, postorbital, parietal, supraoccipital, and exoccipital (Fig. 1). Anteriorly, the postorbital process of the squamosal terminates near the main body of the postorbital. A deep fossa occurs between the postorbital process and the nearly vertical precotyloid process (Fig. 10). The precotyloid fossa is limited posterodorsally by a pronounced depression, but opens anteroventrally. This feature is present in Naashoibitosaurus (NMMNH P-16106), Kritosaurus (AMNH 5799), Prosaurolophus (e.g. TMP 84.1.1 and Horner, 1992), and Brachylophosaurus (MOR 1071), but is shallower and narrower in the last two taxa. The development of this fossa may be a result, in part, of the size of the precotyloid process, as Gryposaurus has a larger process than Brachylophosaurus and Prosaurolophus– which possess smaller processes and fossae – and Edmontosaurus (e.g. CMN 2288), in which the precotyloid process is small, and does not possess a corresponding fossa. Within Gryposaurus, the fossa appears deeper in G. monumentensis than in G. notabilis (ROM 873) or G. incurvimanus (ROM 764, TMP 80.22.1), but is comparable with that of the G. notabilis holotype (CMN 2278). The quadrate cotylus is squared off in a manner similar to that in Brachylophosaurus (Prieto-Marquez, 2005). The dorsal margin of the cotylus is nearly horizontal and lacks the posterodorsal projection seen in P. blackfeetensis (Horner, 1992). Posterior to the quadrate cotylus, the postcotyloid process abuts the exoccipitals along its posterior side and descends nearly the entire length of the exoccipitals (Fig. 10). The medial region of the squamosals is missing from RAM 6797, although in the Gryposaurus sp. skull, UMNH VP 16669, as well as other species of Gryposaurus, the medial rami are separated from one another along the midline by a small ridge of parietal.

Figure 10.

Left squamosal from Gryposaurus sp. UMNH VP 16669 in lateral view. poc, postcotyloid process; pog, postorbital groove; prc, precotyloid process; prf, precotyloid fossa; qc, quadrate cotylus. Scale bar, 5 cm.

Braincase

General:  The left side of the braincase of RAM 6797 is only partially preserved, lacking the parietals, frontals, supraoccipital, orbitosphenoids, and presphenoids. The following description is based on this partial specimen.

Fused exoccipital–opisthotic:  The exoccipital makes up the posterodorsal limit of the skull. Only a portion of the left exoccipital is preserved in the type specimen. It is shown in this specimen to contact the squamosal, basioccipital, and pro-otic through two main regions: the paroccipital process and the exoccipital base. The ventrolaterally hooking, pendant paroccipital processes are relatively elongate, terminating level with the base of the exoccipital, as in Prosaurolophus (Horner, 1992), and shorter than in Brachylophosaurus (MOR 1071). The paroccipital process tightly abuts the postcotyloid process of the squamosal anteriorly. The entire median section of the left exoccipital is missing in RAM 6797, giving no indication of the morphology of this region. Unlike Prosaurolophus (Horner, 1992), the exoccipitals do not make up the ventral border of the foramen magnum, but are instead separated by a thin region of basioccipital. The basioccipital forms a long anteroposteriorly orientated articulation with the ventral processes of the exoccipital base. Although the sutures are obscured in RAM 6797, there seems to be no evidence of the anterior projection of the exoccipital that touches the basisphenoid in Prosaurolophus (Horner, 1992). Foramina for cranial nerves (CN) X, XI, and XII are located on the lateral surface of the exoccipital base, with XII positioned posteriorly and X and XI sharing a more anterior opening. The posterior border of CN VIII is composed of the anterior margin of the exoccipital, which contacts the pro-otic anteriorly.

Basioccipital:  The basioccipital contacts the exoccipitals posterodorsally, the pro-otic dorsally, and the basisphenoid anteriorly. In posterior view, the basioccipital is crescentic and dorsoventrally grooved. The dorsal margin of the crescent represents the base of the foramen magnum. In ventral aspect, the posterior region of the basioccipital is convex, whereas the anterior region is dominated by the laterally positioned paired basitubera (Fig. 11A). They are low, bulbous, and distinctly separated from the posterior half of the basioccipital by a large groove not seen in Brachylophosaurus (MOR 1071). A much smaller mound is present medial to the basitubera along the midline. Overall, this configuration closely resembles that of other hadrosaurines, such as Prosaurolophus (Horner, 1992).

Pro-otic:  The nearly pentagonal pro-otic contacts the supraoccipital dorsally, the exoccipital posteriorly, the basioccipital posteroventrally, and the basisphenoid anteroventrally. The pro-otic contacts the laterosphenoid along its entire anterior border, forming the posterior margin of the foramen for CN V (Fig. 11B). A thin ridge separates CN V foramen from the more ventral CN VII exit. In medial view, the CN VII foramen shares an internal depression with that for CN VIII, although the CN VIII opening occupies a more posteroventral position within this depression than is found in Prosaurolophus (Horner, 1992). The anterior border for the external opening of CN VIII is formed by the pro-otic.

Laterosphenoid:  In anterior view, the laterosphenoid is triradiate, possessing a ventral prong that articulates with the basisphenoid and the opposite laterosphenoid, a dorsomedial prong that contacts the parietal, and a blunt-headed dorsolateral process that inserts into a medial pocket on the postorbital. The orbitosphenoid and frontal are sutured to the laterosphenoid on its dorsal surface. The opening for CN II is prominent on the ventral half of the element. The remainder of the anterior region is not preserved. Posterolaterally, the laterosphenoid creates the anterior border of CN V (Fig. 11B), as in more derived hadrosaurids, whereas the basisphenoid contributes to the ventral border of CN V in at least Iguanodon (Norman, 1980, 1986), Bactrosaurus (Godefroit et al., 1998), and Telmatosaurus (Weishampel et al., 1993). This conformation appears to represent a previously unrecognized synapomorphy of Euhadrosauria (sensuWeishampel et al., 1993) or Hadrosauridae (sensuForster, 1997). A large, anterolaterally orientated depression traces the path of CN V to the lateral edge of this element in G. monumentensis (RAM 6797). A long extension of the laterosphenoid encircles the dorsal margin of CN V, overlapping the pro-otic (Fig. 11B). The extension is long and tapered, terminating well past the apex of the opening for CN V, which differs substantially from the relatively short, broad extension in P. blackfeetensis (Horner, 1992). The morphology of the posterior extension in Brachylophosaurus (MOR 1071) is more similar to that of Gryposaurus, except that it extends further around the foramen for CN V in the former taxon. Ventral to the CN V opening is a raised, slightly concave plateau that resembles a structure seen in Prosaurolophus (Horner, 1992); however, in RAM 6797, it is slightly taller with sharper and more distinct edges.

Basisphenoid:  The basisphenoid contacts the basioccipital posteriorly in a long broad facet (Fig. 11A). In addition, this element contacts the pro-otic posterodorsally, laterosphenoid anterodorsally, and pterygoid ventrally. The basipterygoid processes diverge lateroventrally at approximately 30° from the vertical, approximately equivalent to G. notabilis (ROM 873). The transverse ridge connecting the basipterygoid processes is not as well developed as in Brachylophosaurus. However, a short, stout, anteroposteriorly flattened prong descends vertically from the midline of the transverse ridge (Fig. 11A), a character shared with Brachylophosaurus, cf. Kritosaurus (YPM-PU 16970; Horner, 1992), Edmontosaurus (Lambe, 1920; Horner, 1992), G. notabilis (ROM 873), and G. incurvimanus (TMP 80.22.1) (basisphenoids are unknown from G. latidens). The presence of this character is considered primitive as it is present on at least Camptosaurus, Iguanodon (Norman, 1980: Fig. 5), and Bactrosaurus. The ventral prong may be absent in Prosaurolophus (Horner, 1992). Anteriorly, the parasphenoid process forms the apex of the basisphenoid, as in other hadrosaurids (Fig. 11B).

Palatoquadrate complex

General: RAM 6797 preserves all palatal elements, including the paired vomers.

Quadrate:  The quadrate of G. monumentensis is straight and robust (Fig. 12). The mandibular condyle is composed of a large lateral condyle that contacts the surangular, and a somewhat smaller, dorsomedially positioned condyle that contacts the articular. In lateral view, the quadrate lateral wing expands anteroventrally in a gentle arch, but is interrupted two-thirds along the length of the shaft by a tall quadratojugal notch (Fig. 12). The quadratojugal notch is asymmetrical and subtriangular, with a dorsal margin descending posterodorsally at approximately 60° and a short subhorizontal ventral margin. Brachylophosaurus, on the other hand, possesses a nearly symmetrical quadratojugal notch (Prieto-Marquez, 2005). A prominent ridge common to all hadrosaurines, often referred to as the quadrate buttress, is found on the posterior aspect of the quadrate head (Fig. 12). The quadrate buttress present on G. monumentensis (RAM 6797), G. incurvimanus (TMP 80.22.1), and G. notabilis (CMN 2278) is much larger than that described by Prieto-Marquez (2005) for Brachylophosaurus; however, it should be noted that the size of this feature varies substantially amongst individuals of the same species (e.g. UMNH VP 16667 and UMNH VP 16668). Medially, the rounded pterygoid wing is nearly symmetrical (Fig. 12).

Pterygoid:  The pterygoid, the largest element of the palate, contacts the quadrate posteriorly, the basisphenoid medially, and the ectopterygoid, palatine, and vomer anteriorly. In general, it is quite similar to that described by Horner (1992) for Prosaurolophus, and nearly identical to the pterygoid of G. incurvimanus (Parks, 1920). Therefore, only new information pertaining to the pterygoid is provided here. The posterodorsal quadrate process is tall, rising to nearly the dorsal-most surface of the quadrate. This condition is present in Prosaurolophus and apparently Edmontosaurus (Horner, 1992), but differs from that of G. latidens, which has a shorter dorsal quadrate process (Horner, 1992). The entire palatal arch is preserved; as viewed laterally, it forms a slightly raised sigmoidal contact with the palatine on the pterygoid anterodorsal contact facet. The pterygoid contacts the vomers medial to the palatal arch. The ventral border of the element overlaps the maxilla on the large pterygoid process, producing a more substantial contact than in Prosaurolophus (Horner, 1992) and Brachylophosaurus (Prieto-Marquez, 2005).

Palatine:  The palatine unites the dorsally positioned pterygoid with the maxilla ventrally through two broad articulation facets. The arching pterygoid facet encompasses the entire dorsal surface of the palatine, whereas the ventral maxilla facet is long and straight. Posteriorly, the pterygoid facet is concave dorsomedially, whereas it appears nearly straight in an isolated Brachylophosaurus palatine (MOR 1071-7-16-98-248-S), and is longer than illustrated for P. blackfeetensis (Horner, 1992).

Vomer:  The vomer is rarely preserved in hadrosaurids, making comparisons difficult. The vomer of G. monumentensis is narrow anteriorly where it contacts the premaxilla and maxilla, and dorsoventrally expanded posteriorly, where it meets the pterygoid. The laterally positioned premaxillary and maxillary articular facets are flat and occur near the anterior tip of the element (Fig. 13). There seems to be little variation in the morphology of the anterior vomer in hadrosaurines (Lambe, 1920; Horner, 1992). Posteriorly, the vomer remains subhorizontal on its dorsal edge, whereas it expands ventrally to posteroventrally into a lobe-shaped process (Fig. 13) that most resembles the conformation in Brachylophosaurus (CMN 8893). The morphology of the posterior end differs substantially from cf. Kritosaurus (YPM-PU 16970; Horner, 1992) in that the dorsal margin is smooth edged, not rugose and flared as in the latter, and the ventral region is rounded rather than subrectangular. The vomer of Maiasaura (OTM F138) also varies from that of Gryposaurus in that both the dorsal and ventral margins are expanded in the vertical plane. In addition, the anterior margin of the ventral lobe is excavated in RAM 6797, revealing two notches unobserved in other taxa (Fig. 13A).

Mandibular complex

Predentary:  The predentary is massive, deep, and scoop-shaped (Fig. 14) compared with that of other hadrosaurids. Several authors have noted the close similarities shared in the predentaries of Kritosaurus and Gryposaurus (Lull & Wright, 1942; Kirkland et al., 2006). Other hadrosaurines tend to have shallower and more gracile predentaries, particularly Brachylophosaurus (Prieto-Marquez, 2005), in which the oral margin is straight and squared off anteriorly instead of rounded as in Gryposaurus. The oral margin of the G. monumentensis predentary possesses several large (width, 2 cm; height, 1.5 cm), vertically orientated, anteroposteriorly compressed, clover-shaped denticles (Fig. 14). These denticles are not seen on the predentary of Kritosaurus (AMNH 5799), other species of Gryposaurus, or in any other hadrosaurid species; therefore, they are regarded here as an apomorphy of G. monumentensis. Laterally, a dorsoventrally wide process extends posteriorly, overlapping the lateral side of the dentary. The process bears a depression on its anterior region that accepts the ventrolateral deflection of the premaxillary oral margin. The dorsal and ventral edges of the predentary lateral process terminate subequally, such that the longer ventral margin nearly reaches the tooth row. In ventral view, two wide flat prongs project posteriorly from the ventral midline of the predentary to secure the element to the predentary shelf of the dentary (Fig. 14).

Dentary:  Overall, the dentary of the new taxon is robust and massive, yet otherwise similar to that of other Gryposaurus taxa (Fig. 1). It contacts the predentary anteriorly, the angular and splenial medially, and the surangular posteriorly. In lateral view, the dentary downturns at the anterior end of the tooth row slightly more than in other Gryposaurus species, and in contrast with the relatively straight dentary of Edmontosaurus (Lambe, 1920) and Shantungosaurus (Hu, 1973). Numerous foramina penetrate the lateral side dentary body, including a large anterior dentary foramen. There are over 40 tooth positions in the jaw, with four to five replacement teeth in each alveolus. The coronoid process is faintly inclined anteriorly. Its dorsal margin appears more rounded than that seen in Brachylophosaurus (Prieto-Marquez, 2005), Prosaurolophus (Horner, 1992), or Edmontosaurus (Lambe, 1920). Ontogenetically, the dentary appears to have undergone substantial change, including a major increase in robustness and an alteration of the dorsal surface of the diastema (i.e. the contact surface for the predentary), from dorsally concave in juveniles and subadults (i.e. UMNH VP 13970) to dorsally convex in adult specimens (i.e. RAM 6797).

Surangular:  Five elements contact the surangular: the dentary, splenial, angular, articular, and quadrate. The articular adheres to the posterior border of the surangular. The splenial and angular are positioned dorsally and ventrally, respectively, about a small sagittal ridge along the medial side of the surangular. Laterally, the mandibular condyle of the quadrate rests in a dorsally orientated depression (Fig. 15). A midline sagittal ridge separates the medial and lateral halves of the surangular from each other (Fig. 15B). Midlength along this ridge, an anteroposteriorly broad process ascends anterodorsally (Fig. 15A). The morphology of this process in G. monumentensis differs from the more triangular process seen in Prosaurolophus (Horner, 1992), Brachylophosaurus (Prieto-Marquez, 2005), and Edmontosaurus (Lambe, 1920), in that the posterior region is dorsoventrally reduced and the anterodorsal corner is flared dorsally to form a small flange. The anterior region of the surangular is shaped laterally by a dorsally projecting, mediolaterally compressed, surangular coronoid process. This process is shorter and more robust than that in Brachylophosaurus (Prieto-Marquez, 2005). A small notch, here termed the anterior surangular notch, is present on the mediolateral margin of the element (Fig. 15B). This notch is absent in Brachylophosaurus and P. blackfeetensis (Horner, 1992), but unobservable and unverifiable on any other specimen of Gryposaurus because of contact with the dentary. However, several basal iguanodontians possess a second foramen in the same location on the surangular (Norman, 1998).

The mandibular foramen is composed of the surangular anteriorly and ventrally, the quadrate posteriorly, and the jugal dorsally. In other species of Gryposaurus, the foramen is subcircular, whereas, in G. monumentensis, it is ovoid (Fig. 5; see below).

Postcranial skeleton

The postcranial skeleton referred to G. monumentensis, UMNH VP 12265, is largely unprepared and will not be described here. In general, the skeleton is much more robust than that seen in G. incurvimanus (Parks, 1920) and Brachylophosaurus (Prieto-Marquez, 2007). The type specimen of G. notabilis is the only specimen of this species with a postcranial skeleton, but, unfortunately, offers little comparative material because it is not well preserved.

PHYLOGENETIC ANALYSIS

A phylogenetic analysis was conducted in order to assess the relationships of G. monumentensis within Hadrosaurinae. The matrix for this analysis was based on that presented by Horner, Weishampel & Forster (2004), supplemented with characters from Weishampel et al. (1993) and novel characters resulting from this study (Appendix 1). A total of 120 characters was analysed amongst the 15 taxa using a branch and bound search under ACCTRAN optimization in PAUP version 4.0b10 (Swofford, 2002). Tree manipulation was performed and the character state distribution was analysed using MacClade version 4.06 (Maddison & Maddison, 2000).

Figure 16 shows the strict consensus tree derived from the seven most parsimonious trees. The tree posits two main clades of hadrosaurine hadrosaurids – one clade consisting of Saurolophus + Prosaurolophus + Edmontosaurus and the other comprising Naashoibitosaurus + Gryposaurus + Maiasaura + Brachylophosaurus. Lophorothon rests at the base of the Hadrosaurinae clade (Fig. 16). Gryposaurus falls out within a clade including Maiasaura + Brachylophosaurus, supported by four ambiguous characters (Fig. 16). Character support for this clade is weak owing to a lack of phylogenetic resolution within Hadrosaurinae. Therefore, more data are needed in order to support or refute this grouping. All four species of Gryposaurus form a polytomy supported by five ambiguous characters (Fig. 16). Horner (1992) hypothesized that G. latidens is the most primitive Gryposaurus taxon based on the large size of the dentary teeth. Numerous other authors (Weishampel et al., 1993; Horner et al., 2004; Prieto-Marquez et al., 2006) have also considered this trait as primitive. Horner (1992) further explains that G. latidens possesses traits of both G. incurvimanus and G. notabilis, which makes refinement of the phylogenetic tree more problematic. Gryposaurus monumentensis, on the other hand, is plausibly more closely related to G. notabilis than to the other species. The two taxa share similar general skull morphology, both are more robust than either G. latidens or G. incurvimanus, and both possess the following features: increased size of the premaxillary lip; sigmoidal maxillary lateroventral margin; and pronounced jugal tubercle. Yet, all species of this genus are quite comparable, limiting the resolution of specific relationships.

Figure 16.

Strict consensus of the seven most parsimonious phylogenetic trees demonstrating the relationship of Gryposaurus to other hadrosaurines. A total of 15 taxa and 120 characters was analysed in MacClade version 4.06 (Maddison & Maddison, 2000) and PAUP version 4.0b10 (Swofford, 2002) using the ACCTRAN optimization. Iguanodon and Eolambia were considered as outgroups. Letters on branches denote clades with the character support listed below, and numbers following branch designations refer to the bootstrap values (1000 replicates) associated with that branch. Branch A is supported by characters 1, 29, 35, 37, 59, 64, 76, and 86 (bold, unambiguous characters). Branch B is supported by characters 24, 43, 53, 65, 66, 67, and 79. Branch C is supported by characters 29, 40, 54, and 62. Branch D is supported by characters 12, 14, 31, 42, and 65. Branch E is supported by characters 10, 22, 23, 40, 45, 58, 62, 88, and 100. See Appendix 1 for character descriptions and Appendix 2 for character codings.

The overall tree topology presented here differs in several aspects from the analyses of Horner et al. (2004) and Prieto-Marquez et al. (2006). In the former analysis, Naashoibitosaurus and Saurolophus pair together within a small clade, whereas Prosaurolophus and Edmontosaurus are posited in a larger clade that includes Brachylophosaurus and Gryposaurus. Lophorothon remains basal to the clade containing Gryposaurus in Horner et al. (2004), whereas it is positioned here at the base of Hadrosaurinae. In this study, Gryposaurus is closely aligned with Brachylophosaurus and Maiasaura, by contrast with the results of Horner et al. (2004) and Prieto-Marquez et al. (2006), which both suggest a much closer relationship between Gryposaurus and Prosaurolophus.

DISCUSSION

Taxonomic and evolutionary implications

With the addition of G. monumentensis, Gryposaurus now represents the most taxonomically diverse hadrosaurid genus known. A revised generic diagnosis unites the four species by a suite of synapomorphies, including: (1) dorsolateral flare on the medial side of the premaxillary lateral process (Figs 3, 5); (2) mediolaterally compressed, arched nasal ornamentation; and (3) sigmoidal frontal–nasal suture in which the nasals insert a small process into the frontals (Fig. 6). Previous generic diagnoses of Gryposaurus included only a single unique feature – the nasal arch – together with several other features (e.g. dorsoventrally deep skull and infratemporal fenestra larger than the orbit) that pertain to some other hadrosaurine taxa (i.e. Kritosaurus; Lull & Wright, 1942).

Gryposaurus monumentensis exemplifies the inadequacy of using the infratemporal fenestra in generic diagnoses, as this feature is significantly smaller than the orbit in this taxon (see below). Horner (1992) also addressed the morphology and use of the infratemporal fenestra in the generic diagnoses of Gryposaurus and Kritosaurus. He recognized subtle differences between the taxa, determined that the infratemporal fenestra was not a taxonomically informative character of these hadrosaurines, and ultimately eliminated it from the diagnoses of both genera.

With regard to the taxonomic relationship of Gryposaurus to Kritosaurus– that is, whether the two taxa are synonymous, sister taxa, or convergent in morphology – the present analysis lends additional evidence, but cannot fully resolve this issue, because of morphological uncertainties within Kritosaurus. The two genera are superficially similar in possessing deep skulls with a massive predentary and (for most Gryposaurus species) infratemporal fenestrae that are larger than the orbit. However, detailed anatomical study of the type specimen of Kritosaurus (AMNH 5799) and those taxa that have been referred to this genus [Naashoibitosaurus (sensuHorner, 1992; Horner et al., 2004) and Anasazisaurus (sensuHorner, 1992)] is required before the taxonomy and phylogenetic relationships of these hadrosaurids can be assessed. Nevertheless, following the synonymy of Horner (1992) and Horner et al. (2004), we currently find a clear distinction between Gryposaurus and Kritosaurus based on: (1) the distinct morphology of the maxilla in Anasazisaurus (Lucas et al., 2006); (2) differing morphology of the nasal ornamentation – which is a posteriorly folded, rugose, triangular crest in Anasazisaurus, and a raised rugose region in Naashoibitosaurus (Horner, 1992; Hunt & Lucas, 1993; Lucas et al., 2006); (3) a circumnarial depression extending to near the orbits in Anasazisaurus, much further than that observed in Gryposaurus (Horner, 1992; Lucas et al., 2006; this paper); (4) different nasofrontal sutures – the frontals insert a small process into the nasal in Naashoibitosaurus instead of the opposite condition in Gryposaurus (Horner, 1992); and (5) opposing shapes of the ischia – the shaft is straight in Gryposaurus, whereas referred specimens of Kritosaurus have dorsally curved shafts (Kirkland et al., 2006).

Table 1 presents the ratio of the average orbital area to the infratemporal fenestra area for the three species of Gryposaurus. This reveals that G. monumentensis is the only taxon whose ratio is greater than one (1.13); that is, the orbit is significantly larger than the infratemporal fenestra, further confirming that this character is not a genus-level synapomorphy. Gryposaurus sp. (UMNH VP 16667) has a subequal orbit to infratemporal fenestra area ratio of 0.97, whereas the two G. notabilis specimens and both G. incurvimanus specimens sampled had a maximum value of 0.70, indicating that the orbit is significantly smaller than the infratemporal fenestra. The fact that G. notabilis and G. incurvimanus both possess an orbit to infratemporal fenestra area ratio of 0.70 weakens the hypothesis that these two taxa are part of an ontogenetic series instead of distinct taxa. If they were part of a series, it would be expected that the infratemporal fenestra would be smaller relative to the orbit in G. notabilis than in G. incurvimanus.

Table 1.  List of skull measurements for Gryposaurus monumentensis, G. incurvimanus, and G. notabilis. Skull depth at the nasal was measured vertically from the dorsal-most surface of the nasal to the base of the dentary. Orbital dimensions were obtained from measuring the longest vertical length and longest width perpendicular to the length. The first infratemporal fenestra (ITF) length was taken along the longest diagonal length of the fenestra, not vertically, with the subsequent measurement taken perpendicular to the initial measurement. In addition, length and width measurements were taken at 45° to the previous measurements. Average orbital and ITF areas were obtained by averaging the area from both sets of measurements. The ITF and orbital ratios were calculated by (smallest measure/largest measure) in each set
 G. monumentensisG. incurvimanusG. notabilisGryposaurus sp.
RAM 6797RTMP 80.22.1ROM 764ROM 873CMN 2278UMNH VP 16667
Max. skull length (MSL) (cm)76.042.575.080.090.0
Skull depth at nasal (NSD) (cm)49.535.045.048.553.3
Max. skull depth (MSD) (cm)54.640.040.051.052.055.9
NSD/MSL0.650.820.600.610.59
MSD/MSL0.720.940.680.650.62
Av. orbit area (AOA) (cm2)342.3166.2188.0290.3246.9425.2
Av. orbit ratio0.760.860.840.830.670.92
Av. ITF area (ITFA) (cm2)301.9215.5275.5416.0326.9439.3
Av. ITF ratio0.370.770.570.670.550.53
AOA/ITFA1.130.700.680.700.670.97

The decrease in the size of the infratemporal fenestra in G. monumentensis appears to have been associated with a substantial restructuring of the elements in the posterior region of the skull. Minimally, this transformation involved shortening of the postorbital and squamosal processes that comprise the supratemporal bar and rotation of the jugal posterior process dorsally. In addition, other probable consequences of this restructuring include slight anteroposterior shortening of the jugal posterior process, anteroposterior shortening of the parietal (this conformation is inferred, as the parietal of G. monumentensis is currently not known), and parallel shortening of the surangular and/or dentary relative to the skull depth (Table 1). The latter modification appears to have been necessary, as decreasing the supratemporal bar decreases the total skull length and thereby necessitates an equivalent decrease in the length of the mandibular complex. Observation of the mandibular foramen in other species of Gryposaurus reveals a subcircular opening that contrasts with the oval foramen present in G. monumentensis (RAM 6797; Fig. 5). Anteroposterior shortening of the surangular, not the dentary, would be necessary in order to achieve the shape change from a subcircular to an oval mandibular foramen.

Overall, the general skull morphology of G. monumentensis most closely resembles that of G. notabilis, although the skull of the former taxon appears boxier than the latter. The skull depth to length ratios presented in Table 1 confirm that the skull shape of G. monumentensis is in fact boxier than that of G. notabilis– that is, the ratios are closer to one in the new Utah taxon. However, this ratio is less than that exhibited by G. incurvimanus, which possesses a quadrangular skull of nearly subequal maximum skull depth to length ratio of 0.94 (Table 1).

Gryposaurus monumentensis can be further distinguished from other species of the genus by the more robust nature of the skull and lower jaws, a feature that is perhaps best revealed in the predentary, dentary, and premaxilla (Figs 1, 3). Most of these unique features of the skull and lower jaws of G. monumentensis are reasonably inferred as adaptations for generating and absorbing heightened bite forces. These features include: (1) a more robust dentary, predentary, and premaxilla; (2) a steeper angled premaxillae and shorter skull length relative to depth, both of which would have permitted a more vertical bite action and increased the bite force of the anterior region of the skull; (3) increased rugosity on the midline of the premaxilla to more securely attach the upper rhamphotheca; and (4) large, pronged denticles of the predentary, plausibly serving as an increased attachment area for the lower rhamphotheca. Whether or not these characters indicate a tougher, more fibrous diet relative to that of closely related species cannot be ascertained from this strictly morphological assessment. However, the evidence is intriguing and might profitably be the focus of a future functional analysis. Nonetheless, given the distinction in predentary shape between Gryposaurus species and other hadrosaurid taxa, members of this genus may have been more capable of processing a wider variety of plant material.

Ontogenetic considerations

Although G. monumentensis occurs later in time and in a different location than G. notabilis and G. incurvimanus, the possibility exists that the specimens currently at hand comprise an ontogenetic series of one species, as opposed to distinct species. The only line of evidence supporting the notion that these taxa are part of a growth series beginning with G. incurvimanus and ending with G. monumentensis is that overall skull gracility increases from G. incurvimanus to G. monumentensis. However, several other pieces of evidence conflict with the ontogeny hypothesis. Hadrosaurids grow continuously throughout their life (Horner, Ricqles & Padian, 2000); therefore, if G. monumentensis is the culmination of the ontogenetic series, the maximum skull length should be longer than that observed in G. notabilis. Table 1 shows that the type specimen of G. notabilis (CMN 2278) is larger than the type of G. monumentensis (RAM 6797), and yet retains the characteristics seen on a slightly smaller G. notabilis specimen (ROM 873, which is 1 cm shorter than RAM 6797). Further, the infratemporal fenestra does not slowly decrease in size throughout the hypothetical growth series, as discussed above. It should be assumed that, during ontogeny, the infratemporal fenestra would not remain the same size for the majority of the hadrosaurid's life and then dramatically decrease in size (including modification of the entire posterior skull). It seems more parsimonious to assume that the difference in size and posterior skull structure seen in G. monumentensis is the result of speciation, not ontogeny. Finally, subadult specimens of G. monumentensis (UMNH VP 13970) that coincide in size with those of G. incurvimanus (TMP 80.22.1) demonstrate that the nasal morphology differs between the two taxa. More specifically, the nasal arch in the subadult G. monumentensis (UMNH VP 13970) is comparatively larger and positioned more posteriorly than that in G. incurvimanus, more similar to the nasal seen in adult specimens of G. notabilis and G. monumentensis. In short, morphological evidence indicates that G. monumentensis is probably a new taxon and not a data point along the ontogenetic growth of one Gryposaurus species.

Biogeography and biostratigraphy

Fossils of G. monumentensis have been found within the middle unit of the Kaiparowits Formation. Hadrosaurine materials recovered from near the base of the formation and a portion of the lower middle unit, including an articulated skull (UMNH VP 16667), clearly pertain to Gryposaurus, but not to G. monumentensis. These materials, which will be the subject of a separate study, more closely resemble those of G. notabilis. The presence of two, plausibly stratigraphically separated species of Gryposaurus in the Kaiparowits Formation (although more specimens are needed to determine how much stratigraphic overlap, if any, is present between the two taxa) is significant, in that it is strongly suggestive of within-lineage faunal turnover from a single geological formation. The turnover proposed in the Kaiparowits is similar to patterns observed in dinosaurian faunas in northern Western Interior Basin strata, such as the Two Medicine Formation (Horner, Varricchio & Goodwin, 1992) and the Dinosaur Park Formation (Ryan & Evans, 2005; Evans et al., 2006). This discovery indicates that approximately 75 Mya, one species of Gryposaurus inhabiting southern Utah was replaced by G. monumentensis. Presently, there is no evidence that this replacement correlates with changing environmental conditions, and clearly not enough of the fossil record of these taxa is known regionally to differentiate between other patterns of replacement, such as habitat tracking or extinction.

Hadrosaurine material collected from the Lower Shale Member of the Aguja Formation in southern Texas may be attributable to yet another new species of Gryposaurus (Kritosaurus of Wagner, 2001; Wagner & Lehman, 2001). This material possesses the characteristic Gryposaurus arched nasal ornamentation, together with other features that are cited by Wagner (2001) as evidence for a close relationship to G. latidens. In addition, Wagner (2001) describes maxillae from the Upper Shale Member of the Aguja that closely resemble Gryposaurus (Kritosaurus of Wagner, 2001) notabilis.

The overall biostratigraphic pattern of Gryposaurus reveals that the genus persisted for at least five million years, significantly longer than any other taxon within Hadrosauridae, with the exception of Edmontosaurus. Gryposaurus latidens first occurs in the lower Two Medicine Formation of Montana (c. 80 Mya), approximately four million years prior to the appearance of the other species of Gryposaurus[but it also occurs throughout the Two Medicine Formation (Varricchio, 1995) at the same time as other Gryposaurus species]. Ryan & Evans (2005) documented the co-occurrence of G. notabilis and G. incurvimanus in the lower third of the Dinosaur Park Formation in Alberta, Canada, at approximately 76 Mya. The specimens of Gryposaurus sp. found in the lower portion of the Kaiparowits Formation occur equivalently in time with G. notabilis and G. incurvimanus in Dinosaur Park, whereas G. monumentensis occurs in somewhat younger sediments dating between 75 and 74 Mya, apparently coeval with P. maximus in the Dinosaur Park Formation (Gates & Evans, 2005).

CONCLUSIONS

Gryposaurus monumentensis, recovered from the middle portion of the Kaiparowits Formation, represents a new hadrosaurine from the late Campanian of southern Utah. It is the fourth Gryposaurus species to be named, making this genus the most diverse within Hadrosauridae. Previous attempts at diagnosing Gryposaurus relied heavily on such characters as the distinguishing nasal arch, the shape of the frontal nasal contact, and a larger infratemporal fenestra relative to the orbit. The latter character is absent in G. monumentensis, which possesses a narrow infratemporal fenestra, nearly half the size of the orbit; thus, this discovery required amendment of the generic diagnosis of Gryposaurus to exclude this feature. In addition, a flaring on the medial side of the premaxillary lateral process was found to be a diagnostic feature of Gryposaurus. Gryposaurus monumentensis is distinguished from the other species of Gryposaurus by the possession of a boxy (i.e. relatively short length to height ratio), more robust skull and lower jaws, a relatively small infratemporal fenestra, the presence of a median flange on the surangular, and large prongs on the oral margin of the predentary. Most of the unique morphologies of G. monumentensis appear to be related not to classically appointed display structures, but rather to extreme overall robustness of the skull (producing a more dramatic difference between congeners than observed in most other hadrosaurid taxa), plausibly related to feeding adaptations.

A phylogenetic analysis of Hadrosaurinae that includes the new taxon resulted in Gryposaurus being united in a clade with Brachylophosaurus and Maiasaura. Gryposaurus is positioned one node higher on the tree than Naashoibitosaurus (Kritosaurus?), suggesting a possible close relationship between these taxa. Although the genera Kritosaurus (Brown, 1910) and Gryposaurus are superficially similar and have often been referred to a single genus, this study supports the conclusion of Horner (1992) that these taxa should be regarded as separate genera.

Biostratigraphically, Gmonumentensis has currently been found only in the middle unit of the Kaiparowits Formation, stratigraphically higher than another, unidentified species of Gryposaurus known from the lower unit and a portion of the middle unit. This pattern suggests that there was faunal turnover of Gryposaurus species in the southern region of the Western Interior Basin during the late Campanian. Specimens representing the four species of Gryposaurus have been excavated from Alberta, Canada, and Montana, Utah, and Texas, USA – spanning over five million years from ∼ 80 to ∼ 74.5 Mya. This extensive geographical range (exceeding 2000 km north to south) is one of the largest amongst late Campanian dinosaur genera.

ACKNOWLEDGEMENTS

We express sincere thanks to the following: Duncan Everhart, who discovered the holotype specimen of G. monumentensis; the students, teachers, and volunteers of the ALF Museum Peccary Society, who excavated the holotype specimen; Don Lofgren for support and unrestricted study of the holotype specimen; Alan Titus for his logistical support, enthusiasm, and keen eye for discovering new specimens; Grand Staircase-Escalante National Monument and the Bureau of Land Management for permits and funding; Mike Getty and the UMNH laboratory volunteers and field crew for excavating the hadrosaurid material from GSENM; Eric Lund, Sharon Walkington, and the volunteers of UMNH for their devoted preparation work; David Evans, Albert Prieto-Marquez, and Jack Horner for enlightening hadrosaurid conversations; Lindsay Zanno for ongoing support and illustrations that greatly improved the manuscript; and Jim Gardner, Jack Horner, Jim Kirkland, Albert Prieto-Marquez, Kevin Seymour, and Kieran Shepherd for access to specimens in collections and photographs. Reviews by Tony Ekdale, Jack Horner, and two anonymous reviewers greatly improved the quality of the manuscript. Additional funding was provided by the Discovery Channel and the Jurassic Foundation.

Appendices

APPENDIX 1

The character descriptions listed below are those presented by Horner et al. (2004), unless otherwise noted. Numerous characters have been removed from the original matrix that referred solely to lambeosaurine taxa.

  • 1Number of tooth positions in maxillary and dentary tooth rows: 30 or fewer (0); 34–40 (1); 42–45 (2); 47 or more (3).
  • 2Number of replacement teeth per tooth position: one or two (0); three or more (1).
  • 3Number of functional teeth per tooth position: one (0); at least two, and often three, teeth in the vertical series contribute to the occlusal surface (1).
  • 4Maxillary tooth crown length to width ratio at centre of tooth row: broad relative to length, ratio less than 2.4 : 1 (0); elongate and lanceolate, ratio at least 2.5 : 1 (1).
  • 5Dentary tooth crown length to width proportions at centre of tooth row: broad, ratio of 2.9 : 1 or less (0); elongate, ratio of 3.2–3.8 : 1 (1).
  • 6Dentary teeth, angle between crown and root: greater than 130° (0); less than 130° (1). (Adapted from Weishampel et al., 1993: character 31.)
  • 7Dentary teeth, ornamentation on lingual surface: numerous subsidiary ridges present (0); only one or two subsidiary ridges present, located mesial and distal to primary carina (1); loss of all but primary carina (2).
  • 8Maxillary teeth, ornamentation on labial surface: subsidiary ridges present (0); loss of all but primary carina (1).
  • 9Teeth, position of apex: offset to either mesial or distal side, tooth curved distally (0); central, tooth straight and nearly symmetrical (1).
  • 10Dentary, length of diastema between first dentary tooth and predentary: short, no more than width of four or five teeth (0); long, equal to approximately one-fifth to one-quarter of length of tooth row (1); extremely long, equal to approximately one-third of tooth row (2).
  • 11Dentary tooth row, posterior extent of tooth row relative to apex of coronoid process: tooth row terminates even with or anterior to apex (0); tooth row terminates posterior to apex (1).
  • 12Dentary, orientation of dentary anterior to tooth row: moderately downturned, dorsal margin of predentary rests above ventral margin of dentary body (0); strikingly downturned, dorsal margin of anterior dentary extends below the ventral margin of dentary body, premaxillary bill margin extends well below level of maxillary tooth row (1). (Adapted from Horner et al., 2004: character 11.)
  • 13Dentary tooth row, shape in occlusal view: bowed lingually, curves in towards coronoid process (0); straight (1).
  • 14Predentary shape: deep and robust (0); gracile and shovel-shaped (1). (Adapted from Horner et al., 2004: character 13.)
  • 15Predentary shape: arcuate anterior margin, neurovascular foramina large and located near midline of predentary body, dorsally directed spike-like denticles on anterior margin that fit into slots on underside of premaxilla (0); straight to gently rounded anterior margin, numerous nutrient foramina across entire anterior margin, loss of spike-like denticles and gain of rounded, triangular denticles that project anteriorly and fit into a continuous transverse slot on underside of premaxilla (1).
  • 16Predentary triturating surface, orientation: horizontal, oral margin of premaxilla rests on dorsal predentary (0); canted dorsolaterally to form a nearly vertical surface, oral margin of premaxilla broadly overlaps lateral surface of predentary (1).
  • 17Angular size: large, deep, exposed in lateral view below the surangular (0); small, dorsoventrally narrow, exposed only in medial view (1).
  • 18Coronoid bone: present (0); absent (1).
  • 19Coronoid process configuration: apex only slightly expanded anteriorly, surangular large and forms much of posterior margin of coronoid process (0); dentary forms nearly all of greatly anteroposteriorly expanded apex, surangular reduced to thin sliver along posterior margin and does not reach to the distal end of the coronoid process (1).
  • 20Posterior extent of the posteroventral dentary: ends even with or anterior to apex of coronoid process (0); posteriorly expanded to terminate well behind the coronoid process (1).
  • 21Surangular foramen: present (0); absent (1).
  • 22Premaxilla, width at oral margin: narrow, expanded laterally to less than twice width at narrowest point (postoral constriction), margin orientated nearly vertically (0); expanded transversely to more than twice postoral width but not more than interorbital width, margin flared laterally into a more horizontal orientation (1); further expanded transversely to width subequal to that across jugal arches (2).
  • 23Premaxilla, undercut (‘reflected’) rim around oral margin: absent (0); present (1).
  • 24Premaxillary anterior bill margin shape: horseshoe-shaped, forms a continuous semicircle that curves smoothly to postoral constriction (0); broadly arcuate across anterior margin, constricts abruptly behind the oral margin (1).
  • 25Premaxillary foramen ventral to anterior margin of external nares that opens onto the palate: absent (0); present (1).
  • 26Premaxilla, accessory foramen entering premaxilla in outer narial fossa, located anterior to premaxillary foramen: absent (0); present, empties into common chamber with premaxillary foramen, then onto the palate (1).
  • 27Premaxillae, oral margin with a ‘double layer’ morphology consisting of an external denticle-bearing layer seen externally and an internal palatal layer of thickened bone set back slightly from the oral margin and separated from the denticulate layer by a deep sulcus bearing vascular foramina: absent (0); present (1).
  • 28Premaxilla, outer (accessory) narial fossa anterior to circumnarial fossa: absent (0); present, separated from circumnarial fossa by a strong ridge (1).
  • 29Premaxilla-nasal: shape between anterior-most point of premaxilla and posterior region of external nares: slightly arched (0); highly arched (1); slightly concave to horizontal (2) (new).
  • 30Premaxillary posterior processes (PM1, PM2) and construction of nasal passages: posterodorsal premaxillary process short, posterodorsal and posteroventral processes do not meet posterior to external nares, nasal passages not enclosed ventrally, anterior nasal passage roofed by the nasal, external nares exposed in lateral view (0); posteroventral and posterodorsal processes elongate and join behind external opening of narial passages to exclude nasals, nasal passages completely enclosed by tubular premaxillae, left nasal passage divided from right passage, external nares not exposed in lateral view (1).
  • 31Premaxilla, lateral process flares dorsally into external naris: absent (0); present (1) (new).
  • 32External nares length to basal skull length ratio: 20% or less (0); 30% or more (1).
  • 33External nares, composition of posterior-most apex: formed entirely by nasal (0); formed equally by nasal (dorsally) and premaxilla (ventrally) (1); formed only by premaxilla (2). (Adapted from Horner et al., 2004: character 29.)
  • 34External naris: posterior ventral margin composed of premaxilla (0); posterior margin composed of nasal up to 25% of total naris length (1) (new).
  • 35External naris: anterodorsal corner composed of only premaxilla dorsal process (0); composed of both premaxilla dorsal process and nasal anterior process (1); entire naris formed by premaxilla (2) (new).
  • 36Circumnarial fossa, posterior margin: absent (0); present (1).
  • 37Circumnarial fossa, posterior margin morphology: absent (0); present, lightly incised into nasals and premaxilla, often poorly demarcated (1); present, well demarcated, deeply incised, and usually invaginated (2).
  • 38Nasals and anterodorsal premaxilla in adults: flat, restricted to area anterior to braincase, cavum nasi small (0); premaxilla extended posteriorly and nasals retracted posteriorly to lie over braincase in adults resulting in a convoluted, complex narial passage and hollow crest, cavum nasi enlarged (1).
  • 39Solid nasal crest over snout or braincase (does not house a portion of the nasal passage): absent (0); present (1).
  • 40Solid nasal crest, association with posterior margin of circumnarial fossa: absent (0); solid crest present but circumnarial fossa does not excavate side of crest, fossa terminates anterior to solid crest (1); solid crest present, excavated laterally by circumnarial fossa (2).
  • 41Solid nasal crest, composition: absent (0); solid crest present, composed of nasals (1). (Adapted from Horner et al., 2004: character 40.)
  • 42Solid nasal crest, creating a mediolaterally compressed arch anterior to orbits: absent (0); present (1) (new).
  • 43External nares, shape of posterior margin: lunate (0); ‘V’-shaped (1).
  • 44Maxilla, anterodorsal process: has a separate anterior process that extends medial to the posteroventral process of premaxilla to form part of medial floor of external naris (0); anterior process absent, anterodorsal margin of maxilla forms a sloping shelf that underlies the premaxilla (1).
  • 45Maxilla, medial anterodorsal process: not seen in lateral view of external nares (0); projects into external nares in lateral view (1) (new).
  • 46Antorbital fenestra, external opening: present (0); absent (1).
  • 47Maxillary foramen, location: opens on anterolateral body of maxilla, exposed in lateral view (0); opens on dorsal maxilla along maxilla–premaxilla suture (1).
  • 48Maxilla–lacrimal contact: present (0); lost or covered as a result of jugal–premaxilla contact (1).
  • 49Maxilla–jugal contact: restricted to finger-like jugal process on posterior margin of maxilla (0); jugal process of maxilla reduced to a short projection but retaining a distinct facet (1); jugal process of maxilla lost, anterior jugal has an extensive vertical contact with maxilla anterior to orbit (2).
  • 50Maxilla, location of apex in lateral exposure: well posterior to centre (0); at or anterior to centre (1).
  • 51Maxilla, shape of apex in lateral exposure: low and gently rounded (0); tall and sharply peaked (1).
  • 52Maxilla, shape of ‘lateroventral margin’: straight to nearly straight (0); sigmoidal (1); arched (2) (new).
  • 53Prefrontal shape at anterodorsal orbit rim: prefrontal lies flush with surrounding elements (0); prefrontal flares dorsolaterally to form a thin, everted, wing-like rim around anterodorsal orbit margin (1).
  • 54Prefrontal, shape: smoothly curved laterally (0); anteriorly broad with square anteromedial corner (1).
  • 55Ectopterygoid–jugal contact: present (0); absent, palatine–jugal contact enhanced (1).
  • 56Jugal, expansion of anterior end below lacrimal: dorsoventrally narrow, forms little of the anterior orbital rim (0); expanded dorsoventrally in front of orbit, lacrimal pushed dorsally to lie completely above the level of the maxilla, jugal forms lower portion of orbital rim (1).
  • 57Jugal, shape of anterior end: with distinct anteriorly pointed process fitting between the maxilla and lacrimal (0); point truncated, smoothly rounded anterior margin (1).
  • 58Jugal, anteriorly pointed process: absent (0); present, process restricted to dorsal portion of jugal, anterior jugal appears asymmetrical (1); present, process centred on anterior jugal, anterior jugal appears symmetrically triangular in shape (2).
  • 59Jugal, ventral margin of anterior process: straight to slightly curved (0); sigmoidal (1); convex (2) (new).
  • 60Jugal, postorbital process at or nearly at 90° to the body of jugal (0); highly inclined posteriorly (1) (new).
  • 61Jugal, development of posteroventral flange: absent, jugal expands gradually below infratemporal fenestra to meet the quadratojugal–quadrate (0); present, jugal dorsoventrally constricted beneath infratemporal fenestra to set off flange anterior to constriction (1).
  • 62Jugal flange size, ratio of depth of jugal at constriction below infratemporal fenestra to length of free ventral flange on jugal: small, 0.70–0.90 (0); prominent, well offset from body of jugal, 0.55–0.66 (1).
  • 63Shallow posterior jugal process: relatively deep posterior region of jugal posterior process (0); relatively shallow posterior process (1) (Weishampel et al., 1993: character 17).
  • 64Scalloped ventral margin of posterior process of jugal: posterior process straight to slightly convex (0); posterior process distinctly concave (1) (Weishampel et al., 1993: character 18).
  • 65Frontonasal suture at midline of skull: no distinct processes (0); nasals insert small process into frontals (1); frontals insert small process between nasals (2); frontals rise posterodorsally to support nasal (3) (new).
  • 66Frontal at orbit margin: forms part of margin (0); excluded by prefrontal–postorbital contact (1).
  • 67Frontals, upward doming over braincase in adults: absent (0); present (1).
  • 68Supraorbital articulation: freely articulate on orbit rim (0); fused to orbit rim or absent (1).
  • 69Quadrates, shape of mandibular condyle: mediolaterally broad, lateral and medial condyles subequal in size (0); lateral condyle expanded anteroposteriorly so that condyles appear subtriangular in distal view, lateral condyle longer than medial one (1).
  • 70Paraquadratic foramen, present (0); absent (1) (Weishampel et al., 1993: character 21).
  • 71Quadratojugal notch: ventral margin of notch extends dorsally to form an acute and well-defined opening (0); well-defined notch absent, reduced to a poorly defined embayment of quadrate (1).
  • 72Paroccipital process and accompanying squamosal, orientation: straight and ventrally directed (0); curved anteriorly (1).
  • 73Squamosals on skull roof, separation: widely separated (0); squamosals approach midline, separated by narrow band of parietal (1); squamosals have broad contact with each other (2).
  • 74Squamosal, shape of posteroventral surface: shallowly exposed in posterior view (0); forms a deep, near-vertical, well-exposed face in posterior view (1).
  • 75Supraoccipital, inclination: posterior surface nearly vertical (0); posterior surface inclined steeply forward at approximately 45° (1).
  • 76Supraoccipital, ventral margin: bowed or expanded ventrally along midline (0); horizontal, strong ridge developed along supraoccipital–exoccipital suture (1).
  • 77Supraoccipital–exoccipital contact: straight suture that meets squamosal (0); ventrolateral corner of supraoccipital inset into exoccipital so that supraoccipital is ‘locked’ between exoccipitals (1).
  • 78Basisphenoid, transverse ridge: present between basipterygoid processes (0); absent (1) (new).
  • 79Basisphenoid, ventral process on transverse ridge: present, descends from transverse ridge, ventrally or anteriorly inclined (0); absent (1) (new).
  • 80Cranial nerve V: composed of pro-otic, laterosphenoid, and basisphenoid (0); composed of only pro-otic and laterosphenoid (1) (new).
  • 81Transverse width of the cranium in the postorbital region in dorsal view: broad, width maintained from orbit to quadrate head (0); distinctly narrowed at quadrate heads (1).
  • 82Occiput shape in posterior view: square (0); triangular, narrow dorsally, distal quadrates splay distinctly laterally (1).
  • 83Parietal, midline ridge: straight to slightly downwarped along length (0); strongly downwarped, dorsal margin bends below the level of the postorbital–squamosal bar (1).
  • 84Parietal crest, length: long, posterior parietal narrows quickly to form the crest, crest more than half the length of the supratemporal fenestrae (0); short, parietal crest narrows gradually posteriorly, crest less than half the length of the supratemporal fenestrae (1).
  • 85Infratemporal fenestra, acute angle between postorbital bar and jugular bar: absent (0); present (1).
  • 86Infratemporal fenestra: posterior margin of postorbital straight to gently curving onto supratemporal bar (0); distinct oblique angle between posterodorsal margin and supratemporal bar, creating a boxy infratemporal fenestra (1) (new).
  • 87Cervical vertebrae, number: 11 or fewer (0); 12–15 (1).
  • 88Cervical centrum axial length: long (0); shortened so that axial length of centrum is less than height of neural arch (1).
  • 89Cervicals, shape of zygapophyseal peduncles on arches: low (0); elevated, extend well above the level of the neural canal, zygapophyses long and dorsally arched (1).
  • 90Sacral vertebrae, number: seven or less (0); eight or more (1).
  • 91Dorsal (posterior) and sacral neural spines: short, less than three times centrum height (0); elongate, more than three times centrum height (1).
  • 92Coracoid size: large, coracoid to scapula length more than 0.2, length of articular surface greater than length of glenoid (0); coracoid reduced in length relative to scapula, glenoid longer than articulation (1).
  • 93Coracoid, shape of cranial margin: straight or convex, biceps tubercle small (0); concave, large, laterally projecting biceps tubercle (1).
  • 94Coracoid, cranioventral process: short (0); long, extends well below the glenoid (1).
  • 95Scapula, shape of proximal end: dorsoventrally deep, acromion process directed dorsally, articulation extensive (0); dorsoventrally narrow (no wider than distal scapula), acromion process projects horizontally, cranioventral corner notched, articulation restricted (1).
  • 96Scapula, orientation of borders of distal blade: divergent (0); subparallel to one another (1).
  • 97Scapula, shape of distal end: asymmetrical, either dorsal or ventral border longer than the other (0); symmetrical, dorsal and ventral border terminate at same point (1).
  • 98Deltopectoral crest: short, much less than half the length of the humerus, narrows noticeably distally (0); extends at least to midshaft or longer, distally broad (1).
  • 99Humeral distal condyles: mediolaterally broad, flare moderately from shaft of humerus (0); compressed mediolaterally, flare little from shaft of humerus (1).
  • 100Antebrachium length: humerus subequal to or longer than radius (0); radius longer than humerus (1).
  • 101Carpus: all elements present (ulnare, radiale, intermedium, distal carpals) and fused, metacarpal I fused onto carpus and divergent from rest of manus (0); reduced to two small, unfused carpals (1).
  • 102Manus, digit 1: metacarpal and one phalanx present (0); entire digit absent (1).
  • 103Metacarpal III, relative position of proximal end: aligned with those of metacarpals II and IV (0); offset distally relative to metacarpals II and IV (1).
  • 104Metacarpal, shape: short and robust, width at midshaft to length ratio of 0.2 or greater (0); slender and elongate, width at midshaft to length ratio of 0.15 or less (1).
  • 105Penultimate phalanges of digits II and III, shape: rectangular, lateral sides subequal in length (0); wedge-shaped, medial side significantly shorter than lateral side (1).
  • 106Ilium, size of supracetabular process: small, projects only as a lateral swelling (0); large, broadly overhangs the lateral side of the ilium and usually extends at least halfway down the side of the ilium (1).
  • 107Ilium–pubis articulation: large iliac contribution, pubic peduncle of ilium long, iliac peduncle of pubis small (0); pubic process of ilium short with restricted articular surface, prominent dorsally directed iliac peduncle of pubis (1).
  • 108Ilium, postacetabular process shape: tapers posteriorly to nearly a point, wide brevis shelf (0); rectangular, no brevis shelf (1).
  • 109Ilium, postacetabular process size relative to total length of ilium: less than 40% (0); more than 40% (1).
  • 110Pubis, distal width of prepubic process: dorsoventrally expanded to no more than twice the depth of the proximal shaft (0); expanded to more than twice the depth of the proximal shaft (1).
  • 111Pubis, length of prepubic process constriction: long, dorsoventral expansion restricted to distal process (0); shaft short, dorsoventral expansion begins at base of process (1).
  • 112Pubis, obturator foramen: closed or partially closed ventrally by tubercle arising from pubic shaft (0); fully open, tubercle absent (1).
  • 113Ischium, shape of shaft in lateral view: strongly curved downward (0); nearly straight (1).
  • 114Ischium, shape of distal end: small knob-like foot (0); large and pendant foot (1).
  • 115Ilium, shape of dorsal margin: nearly straight (0); distinctly depressed over supracetabular process and dorsally bowed over base of preacetabular process (1).
  • 116Femur, development of intercondylar extensor groove: moderately deep, groove fully open (0); deep, edges of groove meet or nearly meet cranially to enclose an extensor tunnel (1).
  • 117Tarsus, distal tarsals 2 and 3: present (0); absent (1).
  • 118Metatarsal I, length: short, thin splint (0); absent (1).
  • 119Pes, distal phalanges of pedal digits II to IV: axially shortened to disc-like elements with width at least three times length (0); greatly shortened, width at least four times length (1).
  • 120Pes, shape of unguals: taper evenly distally, claw-like (0); dorsoventrally flattened and broadened, hoof-like (1).

APPENDIX 2

Taxon character state matrix

 5101520253035404550
Igaunodon00000100000000000000000000000000000000000000000000
Eolambia100?001110000????00??1010000000??????0???0?0???21
Brachylophosaurus21110121121011111111120110111001011110111000110?21
Corythosaurus1111101111111111111111010010?10?2?2001000?01?11121
Edmontosaurus spp.31110121121011111111121111112001001120000000010?21
Gryposaurus notabilis21110121111110111111111110111011010110121100110021
G. incurvimanus2111012111111???1111111110111011010110111100010021
G. latidens211?0?2111111????111111110111011010110111100010021
G. monumentensis21110121111110111111111110111011010110111100110021
Lophorothon??10?01?1??????????????????????????1101210????????
Maiasaura2111012112101111111112011?100000000110111000110021
Naashoibitosaurus2?????211????????????????????00?0??110121010??0?21
Prosaurolophus spp.2111012111101111111111101?112001001120121010010021
Saurolophus spp.31110121111011101111110000112000101120121010010021
Telmatosaurus0101101100101???00011100?00000000000000000000?0?21
 556065707580859095100
Igaunodon00000000000001000000000000000000??0000000000000000
Eolambia??000????????0?0?0??0??00??0000?000????0???????110
Brachylophosaurus00001102011111000111111111100111001010110111110111
Corythosaurus1200111012100001111111211?11??11111111111111110111
Edmontosaurus spp.00011101121000000111111111100111001011110111110110
Gryposaurus notabilis0101110101111110011111?1101001110011??????????????
G. incurvimanus0001110101111110001111?110100111001111110111100110
G. latidens000?11010110111???????????????????????????????????
G. monumentensis01011101011111100011111110100111??11?11?0111100110
Lophorothon??00100????????0???1???1??1??10????0?????????????
Maiasaura001111020111110011111121101???11001110110111100111
Naashoibitosaurus0200110101101121?1111?111?1???1?0011???????????11?
Prosaurolophus spp.01101101121000211111111111101111001011110111110110
Saurolophus spp.001011011210003111111121111??011001011110111100110
Telmatosaurus00??1002000?010001111?0000???00?0001?11?0??110101?
 105110115120      
Igaunodon00000000000000000000      
Eolambia?????0000???110?????      
Brachylophosaurus11111111010110111111      
Corythosaurus11111111110111111111      
Edmontosaurus spp.11111111110110111111      
Gryposaurus notabilis????????????????????      
G. incurvimanus11111111110110111111      
G. latidens????????????????????      
G. monumentensis?????1111101101?????      
Lophorothon????????????10???1?1      
Maiasaura11111111110110111111      
Naashoibitosaurus????????????????????      
Prosaurolophus spp.11111111110110111111      
Saurolophus spp.11111111000110111111      
Telmatosaurus???????????????1????      

Ancillary