Saurolophus morrisi, Prieto-Márquez & Wagner, 2013

Prieto-Márquez, Albert & Wagner, Jonathan R., 2013, A new species of saurolophine hadrosaurid dinosaur from the Late Cretaceous of the Pacific coast of North America, Acta Palaeontologica Polonica 58 (2), pp. 255-268 : 257-263

Saurolophus morrisi sp. nov.

Figs. 1–6 , 9B .

Etymology: Named for paleontologist William J. Morris (1923–2000), in recognition of his substantial contributions to our understanding of the functional morphology and evolutionary history of the hadrosaurid dinosaurs of the Pacific coast and Western Interior of North America.

Holotype: LACM / CIT 2852 , a skeleton including fragment of left and most of right premaxilla, both maxillae, right jugal, right quadratojugal, partial right quadrate, right postorbital, paroccipital process of right exoccipital, predentary, right− and posterior fragment of left dentary, partial surangular, angular, and splenial, various cervical, dorsal, and caudal vertebrae, partial right scapula, both ulnae, metatarsal III, and various manual and pedal elements.

Type locality: LACM locality CIT 357, Tumey Hills, San Benito County, California, USA (see Bell and Evans 2010 for further details).

Type horizon: Moreno Formation, lower Maastrichtian.

Referred material.— LACM / CIT 2760 , LACM locality CIT 336 , 36 ° 40'21”N, 120 ° 42'42”E , Panoche Hills , Fresno County, California, USA; Moreno Formation , lower Maastrichtian; fragmentary skull and postcrania consisting of posterior region of skull roof (including partial frontals, parietal, squamosals, prootics, supraoccipital, and fragmentary exoccipitals), possible conjoined distal nasals, both maxillae, nearly complete right quadrate, left− and posterior half of right dentary, partial surangular and angular, various isolated dentary teeth, left coracoid, left scapula missing distal end, left humerus, distal end of right humerus, proximal regions of both ulnae and radii, fragments of both femora, proximal end of the left tibia, left metatarsals III, and various fragmentary manual and pedal elements. These remains appear to represent a single individual approximately 30% smaller than the holotype .

Diagnosis.—Hadrosaurid conforming to diagnosis of Saurolophus (above)¸ with postorbital having ornamentation in form of wide oblique groove on jugal process ( Fig. 1 ) (after Bell and Evans 2010).

Remarks.—In addition to the autapomorphy noted above, Saurolophus morrisi differs from S. osborni and S. angustirostris in two more ambiguous characters. The external narial foramen of Prosaurolophus and Saurolophus is elongate and slit−like, and forms a tightly constricted, almost V−shaped rostroventral terminus. While there is clear evidence that the acute anterior end of the foramen in S. morrisi also possesses a V−shaped rostral margin of the narial foramen, it is not clear that the aperture was in any way slit−like. Taken at face value, the premaxilla and maxilla of LACM/CIT 2852 as preserved suggest a much deeper opening. This may be partly due to diagenetic deformation of the specimen, but it is not clear how any deformation could geometrically account for all of the apparent expansion of the boney naris. We concur with Bell and Evans (2010) that this likely represents the original morphology to some extent. A broad narial foramen is likely ancestral for hadrosaurids, but its absence in Prosaurolophus and lack of information about Kerberosaurus renders interpretation of the polarity of this character in S. morrisi equivocal, and therefore, we have omitted it from the formal diagnosis.

Numerous characters in LACM/CIT 2760, discussed in more detail below, evidence its saurolophine affinities. In addition, when considered all together, these attributes form a combination of characters that support referral of this specimen to Saurolophus . Such character combination includes frontal doming (at least in subadults), long and extensive ectocranial surface of the frontal, substantial anteroventral downwarping of the parietal sagittal crest, intersquamosal joint that excludes the parietal from the occiput, long exoccipital roof above the foramen magnum, subrectangular and anteriorly oriented supratemporal fenestra, relatively high and extensive posterior surface of the squamosal, anteroposteriorly broad anterodorsal region of the maxilla, nearly straight posterior margin of the quadrate, quadratojugal notch of the quadrate being extremely wide and positioned ventral to the mid−length of the quadrate, broad proximal constriction of the scapula, relatively short and robust ulna, dentary with relatively low angle of ventral deflection, and dentition with very reduced or lack of marginal denticles.

Stratigraphic and geographic range.— S. morrisi occurs in lower Maastrichtian strata of central−western California, western North America .

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Description

Cranial morphology

Neurocranium: The frontal forms the central region of the skull roof, anterior to the supratemporal fenestrae ( Fig. 2A ). The ectocranial surface is extensive, being at least more than 80% longer than it is wide, as it occurs in other saurolophines ( Prieto−Márquez 2010b). The frontals of LACM/CIT 2760 exhibit an anterior, crescentic, and elevated sagittal structure that extends dorsal to the plane of the skull roof ( Figs. 2A 1 , 3 ). The morphology of this structure is consistent with the abraded base of the posterodorsal process of the frontals that underlies the nasal in the crest of Saurolophus osborni and S. angustirostris (Bell 2010, 2011). We can find no other explanation for this structure. The base of the crest seems somewhat less robust that that in the latter species; a more robust posterodorsal process, and possibly a larger crest, may be an apomorphy of Saurolophus osborni and S. angustirostris . However, given the poor preservation of the specimen, we are reluctant to include this in the diagnosis. Notably, posterior to the buttress the ectocranial surface of the frontals forms a dome−like convexity centered around the sagittal plane of the skull. Although this upward doming of the frontals is characteristic of Lambeosaurinae ( Horner et al. 2004), it is also present in juveniles of Saurolophus angustirostris ( Bell 2011) .

The hourglass−shaped parietal contributes to the medial and anterior margins of the supratemporal fenestra. As in all non−lambeosaurine hadrosauroids ( Prieto−Márquez 2008: fig. F.30), the supratemporal fenstra of LACM/CIT 2760 (deformed on the right side, but well preserved on the left half of the skull roof) is subrectangular and its long axis is anteroposteriorly oriented ( Fig. 2A ). Ventrally, the parietal articulates with the laterosphenoid, prootic, opisthotic−exoccipital, and supraoccipital. Its anterior region is mediolaterally expanded into two anterolateral processes, which meet the frontals anteriorly and likely the postorbitals laterally. Although minimized by dorsoventral crashing of the specimen, in lateral view the sagittal crest displays a concave profile and slopes anteroventrally forming an obtuse angle with the frontals, as in lambeosaurines ( Horner et al. 2004) and species of Saurolophus (Bell 2010; Prieto−Márquez 2010b).

At the anteroventral, median region of the braincase lays the orbitosphenoid ( Fig. 2B ). This bone is slightly convex ventrally and its external surface faces ventrolaterally. The median, posteroventral margin of the orbitosphenoid forms the dorsal border of the foramen for the optic nerve.

The laterosphenoid contributes to the laterodorsal wall of the braincase, between the orbitosphenoid and the prootic ( Fig. 2B ). The posterodorsal region is concave anterodorsally and extends posteriorly to meet the prootic. The dorsal margin of the posterodorsal region of the laterosphenoid probably contacts the parietal. The concave surface of this region of the latersophenoid is continuous anterolaterally with the postorbital process. This process projects perpendicularly from the long axis of the skull, becoming anteroposteriorly narrower laterally.

The prootic occupies a median position in the lateral wall of the braincase below the parietal ( Fig. 2 ). It probably joins the laterosphenoid anteriorly, the opistothic−exoccipital posteriorly, and the parietal dorsally.

The opisthotic−exoccipital complex contributes to the posterolateral wall of the braincase, lateral and dorsal to the foramen magnum. The posteromedial region of the opistothic−exocciptal that would give raise to the proximal portion of the paroccipital process is only partially preserved, attaching to the posterior surface of the squamosal. The dorsomedian region of the opistothic−exoccipital underlies the supraoccipital. Notably, the exoccipital roof above the foramen magnum is anteroposteriorly long ( Fig. 2B ), a derived condition shared with Kritosaurus navajovius , Edmontosaurus spp. , Saurolophus spp. , Prosaurolophus maximus , the Sabinas hadrosaurine described by Kirkland et al. (2006), and Shantungosaurus giganteus ( Prieto−Marquez 2008) .

The supraoccipital occupies a median position in the braincase ( Fig. 1A, B ), inset on the posterodorsal region of the occiput between the squamosals and the opistothic−exoccipitals. Little details of its morphology may be appreciated in LACM/CIT 2760, aside from the fact that the ventral surface of the posterior region of the supraoccipital is resting on the dorsal surface of the opisthotic−exoccipital shelf.

Facial skeleton.—The maxilla displays a triangular lateral profile ( Fig. 4A, B). The anterodorsal region of the maxilla is anteroposteriorly very broad, unlike the narrower and triangular morphology typically present in lambeosaurines (e.g., Hypacrosaurus altispinus ROM 702). The articular surface for the jugal is anteroposteriorly extensive and the geometry of its ventral margin appears to have accommodated a similarly long and asymmetrical rostral process of the jugal; such jugal morphology is found in saurolophines except Brachylophosaurini (sensu Gates et al. 2011). The summit of the antero−

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dorsal region of the maxilla is positioned slightly anterior to the mid−length of the bone. Given the latter condition, the base of the dorsal process (not preserved) was probably located approximately above the level of the mid−length of the maxilla. The ectopterygoid shelf is horizontally oriented and comprises about 40% of the total length of the maxilla. The lateral emargination of the shelf is dorsoventrally thick and gradually becomes slightly shallower anteriorly. Its ventral margin is very prominent. Medial and dorsal to the ectopterygoid shelf is a relatively large palatine ridge, which extends over the dorsal margin of the medial surface of the posterior third of the maxilla. Posterior to the palatine ridge, near the posterodorsal end of the maxilla, lays the finger−like pterygoid process. This process is mediolaterally compressed and relatively deep, missing the distal end in LACM/CIT 2760.

Only the dorsal region of the main body of the postorbital is preserved, articulated in the skull roof of LACM/CIT 2760 ( Fig. 2 ). The main postorbital body is mediolaterally compressed and triangular. The abraded orbital and infratemporal margins of the postorbital converge ventrally forming an angle of 120 ° to give rise to the jugal ramus.

The dorsal surface of the squamosal, at the posterodorsal region of the skull roof, is relatively extensive ( Fig. 2A ). The medial rami are anteroposteriorly broad and meet medially to exclude the parietal from the sagittal plane of the skull; this condition is typically found in lambeosaurines, as well as in saurolophines Maiasaura peeblesorum , Saurolophus angustirostris ( Bell 2011) , and Shantungosaurus giganteus ( Prieto−Márquez 2010b) . The posterior surfaces of the squamosals of LACM/CIT 2760 substantially increase in depth toward the sagittal plane of the skull, showing steep dorsal margins that converge mediodorsally. Notwithstanding the dorsoventral postdepositional compression experienced by the specimen, the posterior surface of the squamosal is relatively high. This condition is typically found in lambeosaurinaes ( Horner et al. 2004) and is also present in Saurolophus spp. ( Prieto−Márquez 2010b). On the lateral side of the squamosal, the quadrate cotylus is shallow and anteroposteriorly wide. Only the wedge−shaped proximal extent of the precotyloid process is preserved.

The quadrate is missing the proximal end and most of the pterygoid flange ( Fig. 4C). The posterior margin of the bone is relatively straight, displaying only a very gently curvature proximally. Straight to slightly curved quadrates are typically present in saurolophines, in contrast to the strongly curved lambeosaurine quadrates ( Prieto−Márquez 2008: fig. D.76). The quadratojugal notch is very wide; its dorsal margin is slightly longer than the ventral margin, and forms a 23 ° angle with the posterior margin of the quadrate. In saurolophines this angle is always less than 45 ° (in most cases even less than 30 °), whereas in lambeosaurines it is greater than 45 ° ( Prieto−Márquez 2008: fig. D.78). The mid−length of the notch is located well below the mid−length of the quadrate; this condition is also commonly seen in saurolophines, whereas lambeosaurines typically display a quadratojugal notch centered around the mid−length of the quadrate ( Prieto−Márquez 2008: fig. D.77).

Mandible: The left dentary is 350 mm in length. The dorsal margin of the edentulous region and the medial surface of the coronoid process are concealed by rock matrix ( Fig. 4A, B 1 , B 2 ). The anterior region of the ventral margin of the dentary is ventrally deflected forming a 12 ° angle (in medial view) with the tooth row (this angle increases to 19 ° when measured in lateral view). In species of Saurolophus the angle of deflection ranges from 10 ° to 15 ° in adults (measured in medial view; see Prieto−Márquez 2008: fig. C.17). However, in subadults this angle can be as hight as 19 °, as it occurs in the S. angustirostris specimen ZPAL MgD I−159 (measured laterally). In LACM/ CIT 2760 the ventral deflection originates near the rostral end of the dentary. Specifically, the ratio between the distance from the posterior margin of the coronoid process to the origin of the deflection and the distance between the posterior margin of the coronoid and the rostral−most tooth position ( Prieto−Márquez 2008: fig. C.20) is 0.82. The lingual projection of the symphyseal process is moderate, as in most hadrosaurids except Tsintaosaurus and Pararhabdodon ( Prieto−Márquez and Wagner 2009) . The bulging of the ventral margin of the dentary is very well developed in LACM/CIT 2760 and it is located rostral to the base of the coronoid process, a condition shared by species of Edmontosaurus and Saurolophus ( Prieto−Márquez 2010b) . The coronoid process is large in comparison with the dentary ramus and its long axis is strongly tilted rostrally, forming a 69 ° angle with the tooth row. A minimum of 32 tooth families are preserved. There are at least four tooth crowns arranged dorsoventrally within a single alveolus at the middle of the dental battery. The occlusal plane is not exposed.

An elongate strap of bone, oriented anterodorsally and found adjacent to the posterior end of the dentary, may represent part of the anterior ascending flange of the surangular ( Fig. 4B 1 , B 2 ). Likewise, a small finger−like bony fragment lies above the ventral margin of the posterior region of the medial side of the dentary. This element is probably part of the left angular ( Fig. 4B 1 , B 2 ).

Dentition: The apicobasal height/mediodistal width ratio of the diamond−shaped crowns ( Fig. 4D–F) is relatively low, ranging from 2.3 to slightly over 2.4. These values are only slightly higher than the height/width ratios found in Gryposaurus latidens ( Horner 1992; Prieto−Márquez 2010c) and a large dentary referable to Saurolophus cf. angustirostris (ZPAL MgD−I 162). The enameled lingual sides of tooth

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crowns have a single large and prominent median ridge. This ridge is straight in most teeth and sinuous in a few tooth crowns. Marginal denticles are either very reduced or absent; poor preservation of the specimen does not allow to discriminate between these two possibilities. Notably, all these dental attributes are also present in the dentary teeth of LACM/CIT 2852 ( Bell and Evans 2010). In addition, the reduced or absent denticulation is a condition shared with Saurolophus spp. (e.g., AMNH 5221 and ZPAL MgD−I 162) and Edmontosaurus spp. (e.g., CMN 2289).

Maxillary tooth crowns are almost entirely concealed by matrix. They appear to have a single straight and median ridge, and height/width proportions similar to those of the taller dentary crowns.

Appendicular anatomy

Pectoral girdle: The available coracoids are so poorly preserved and severely eroded that no anatomical details can be discerned ( Fig. 5A, B ), except for its medially concave platelike morphology that is expanded posteriorly to form the glenoid and scapular articular facets, and the subtriangular ventral process.

The scapula shows a wide proximal constriction in proportion to the dorsoventral breadth of the distal blade ( Figs. 5A , 6A ). Scapulae with relatively wide proximal constrictions are characteristic of saurolophine hadrosaurids, unlike the proportionately narrower constrictions present in lambeosaurines ( Prieto−Márquez 2008: fig. H.11). The deltoid ridge is prominent and bounds dorsally a moderately deep deltoid fossa. The pseudoacromion process forms a narrow ledge at the proximodorsal region of the scapula and it is nearly horizontally oriented, a synapomorphy of saurolophines ( Prieto−Márquez 2010b). The dorsal margin displays a gentle convex lateral profile and diverges gradually from the ventral margin toward the distal end of the blade.

Forelimb: The humerus ( Fig. 5B ) is moderately elongate in overall proportions, with a length/width (across the proximolateral margin) ratio of 4.8. The deltopectoral crest comprises slightly more than half of the total length of the humerus. Its laterodistal corner is prominent and its maximum breadth is 1.91 times the minimum diameter of the humeral shaft. Such expansion of the deltopectoral crest is comparable to the greater breadth ratios recorded in saurolophine hadrosaurids and lower than those observed in the more expanded crests of lambeosaurines ( Prieto−Márquez 2008: fig. H.16).

The ulna is robust and moderately elongate ( Figs. 5B , 6B ). Its length/width (dorsoventrally at mid−length) ratio is 8.9. Among hadrosaurids, that value is relatively low and comparable to the low ratios recorded in Saurolophus osborni (e.g., 8.7 in AMNH 5220) and S. angustirostris (e.g., 7.8 in MPC−D 100/706); in all other hadrosaurids, except Gryposaurus latidens (e.g., AMNH 478) and Parasaurolophus walkeri (e.g., ROM 768) the ulna is more than 9 times long than it is deep at mid−length ( Prieto−Márquez 2008: fig. H.19). The olecranon process is massively constructed and dorsoventrally compressed. The lateral and medial flanges are relatively thick.

The radius ( Fig. 5B ) is subcylindrical and displays an expanded cup−shaped proximal end. The proximal surface is slightly more expanded mediolaterally than dorsoventrally. The shaft of the radius gently becomes deeper towards its distal end, but less so than the proximal end.

The manus is represented by a possible phalanx II−1 ( Fig. 6B ). This element is dorsoventrally compressed and it is slightly wider proximally than distally, with lateral and medial dorsal margins that are nearly parallel to each other. The phalanx is 2.2 times longer than it is mediolaterally wide at mid−length.

Hindlimb: Only the distal segments of both femora are preserved ( Fig. 5D ). Their morphology does not differ from that in other hadrosaurids: the shaft is straight and ends in two large, mediolaterally compressed and anteroposteriorly expanded condyles. Anteriorly, these distal condyles are fused in the better−preserved right femur, whereas a wide intercondylar groove separates the condyles posteriorly.

The tibia is known from a proximal fragment ( Fig. 5C ). This region of the tibia is anteroposteriorly expanded and mediolaterally compressed, with a convex lateral surface. The cnemial crest extends along the anterior margin of the proximal tibia; however, most of the crest is abraded. Posteriorly, the lateral condyle is massive and more prominent than the posterior condyle, protruding from the proximolateral margin of the tibia. A narrow and deep groove separates the two posterolateral proximal condyles.

In the pes, metatarsal II is solely represented by a proximal fragment. This fragment is mediolaterally compressed and greatly expanded dorsoventrally at the proximal end, having a long and elliptical articular surface. Metatarsal III is composed of a relatively long but robust shaft that expands proximally and distally ( Figs. 5E , 6C ). The proximal articular surface is mediolaterally compressed, with the dorsomedial corner further projected medially than the ventral margin. The metatarsal III of LACM/CIT 2760 is dorsoventrally crashed; consequently, the proximal dorsomedial corner is deformed and unnaturally prominent ( Fig. 5E ). The proximal half of the medial surface of the metatarsal shows a large depression for articulation with metatarsal II. The distal region is dorsoventrally compressed and the distal surface is dorsoventrally convex and mediolaterally concave.

Figures

Fig. 1. Partial right postorbital of a hadrosaurid dinosaur Saurolophus morrisi sp. nov. (LACM/CIT 2852), lower Maastrichtian Moreno Formation of San Benito County, California, USA, showing the autapomorphic ornamentation of its jugal process. Posterior (A) and right lateral (B) views.

Fig. 2. Partial skull roof of a hadrosaurid dinosaur Saurolophus morrisi sp. nov. (LACM/CIT 2760, a subadult), lower Maastrichtian Moreno Formation of Panoche Hills, Fresno County, California, USA. Dorsal (A) and ventral (B) views. Photographs (A1, B1) and interpretative drawings (A2, B2).

Fig. 3. Right posterolateral view of the frontal of a hadrosaurid dinosaur Saurolophus morrisi sp. nov. (LACM/CIT 2760, a subadult), lower Maastrichtian Moreno Formation of Panoche Hills, Fresno County, California, USA, showing the eroded remnant of the buttressing base of the posterodorsal frontal ramus.

Fig. 9. Comparison of the general skull and premaxillary morphology of two hadrosaurid dinosaurs Saurolophus osborni Brown, 1912, holotype AMNH 5220 (A) and Saurolophus morrisi sp. nov., holotype LACM/CIT 2852 (B), highlighting characters shared by these two taxa. Skull in right lateral view (A1, B1), right premaxilla in lateral view (A2, B2).The white inscription on the premaxilla denote the abbreviation for that bone, painted by the curatorial staff back in the early twentieth century.

Fig. 5. Appendicular elements of a hadrosaurid dinosaur Saurolophus morrisi sp. nov. (LACM/CIT 2760, a subadult), lower Maastrichtian Moreno Formation of Panoche Hills, Fresno County, California, USA. A. Partial left scapula and coracoid in lateral view. B. Partially articulated forelimb elements. C. Proximal segment of right tibia in lateral view. D. Distal fragments of femora. E. Right metatarsal III in dorsal view.

Fig. 6. Appendicular elements of a hadrosaurid dinosaur Saurolophus morrisi sp. nov. (LACM/CIT 2852), lower Maastrichtian Moreno Formation of San Benito County, California, USA. A. Partial right scapula in lateral view. B. Right ulna in lateral view and possible manual phalanx II−1 in dorsal view. C. Right metatarsal III in dorsal (C1) and lateral (C2) views.