Saurolophus

A photograph of the panel mount of the holotype of S. osborni, from Barnum Brown, 1913

Two species are regarded as valid today: the type species S. osborni, and S. angustirostris. S. osborni (Brown, 1912) is known from a skull and skeleton, two other complete skulls, and skull fragments. S. angustirostris (Rozhdestvensky, 1952) is known from at least 15 specimens.[7] It differs from S. osborni by some details of the skull, as well as in the pattern of scales found in skin impressions. The Mongolian species had a longer skull (by 20%) and the front of the snout (the premaxillary bones) were more upwardly directed.[8] S. angustirostris also had a distinctive row of rectangular scales along the midline of the back and tail, known as 'midline feature-scales'; these are not currently preserved in S. osborni. In S. angustirostris, the scales on the tail flank were arranged in vertical patterns, which may have corresponded to striped coloration in life. This area was covered in radial scale patterns in S. osborni, possibly indicating a more mottled or spotted coloration.[9]

Skulls in Moscow Paleontological Museum

S. kryschtofovici (Riabinin, 1930) is not considered valid; either it is regarded as a dubious name,[10][7] or as a synonym of S. angustirostris[6] (although the name antedates S. angustirostris).[11]

Until a 2011 reevaluation of the species by Phil R. Bell, S. angustirostris was not well-described. No autapomorphies, unique derived traits, had been established distinguishing it from S. osborni. Bell found in a publication earlier in the year that the two previous studies of S. angustirostris, by Rozhdestvensky in 1952, and Maryanska and Osmolska in 1981, do not provide a comprehensive enough description to compare the species with S. osborni.[8]

In 1939–40, two partial skeletons were found in the late Maastrichtian age Moreno Formation of California. These specimens were referred to cf. Saurolophus sp. In 2010, one of the skulls was instead assigned to Edmontosaurus.[12] A 2013 study placed the two specimens in a new species, S. morrisi.[11] In 2014, the species was reassigned to a new genus, Augustynolophus.[13]

Diagram with labelled skulls of S. angustirostris (A) and S. osborni (B)

The most distinctive feature of Saurolophus is its cranial crest, which is present in young individuals, but is smaller. It is long and spike-like and projects upward and backward at about a 45° angle, starting from over the eyes. This crest is often described as solid, but appears to be solid only at the point, with internal chambers that may have had a respiratory and/or heat-regulation function.[16] The unique crest of Saurolophus is made up almost completely by the nasal bones, and in S. angustirostris it is solid. In adult specimens the crests are a rounded triangular shape in cross section. The crest protrudes past the edge of the skull backwards. Thin processes from the frontals and prefrontals extend along the underside of the crest, probably to strengthen it. At the end of the crest is a swelling of the nasal, which is often termed differently.[8]

Restoration of S. angustirostris

Restoration of S. osborni

The holotype of S. angustirostris is a skull and postcrania, so the cranium of the species is well-described. Bell et al. re-evaluated the entire species in a 2011 publication with Acta Palaeontologica Polonica. Their description found the skull to be generalized among hadrosaurines, and are much larger than any skulls of S. osborni. The most unusual feature for a hadrosaurine is the long, protruding, solid crest that extends upwards diagonally from the back of the skull roof. Unlike lambeosaurines, the crests are made up completely of the nasal bone. The premaxilla bones make up almost 50% of the entire skull length, and both sides are filled with small holes. Only in adult individuals has the front of the premaxillary contact been fused. Longer than the premaxilla, the nasal bones are the longest in the skull. They make up the entire length of the crest, and are never preserved as fused.[8]

As a hadrosaurid, Saurolophus would have been a bipedal/quadrupedal herbivore, eating a variety of plants. Its skull permitted a grinding motion analogous to chewing, and its teeth were continually replacing and packed into dental batteries that contained hundreds of teeth, only a relative handful of which were in use at any time. Plant material would have been cropped by its broad beak, and held in the jaws by a cheek-like organ. Its feeding range would have extended from the ground to about 4 m (13 ft) above.[7]

Skull of S. angustirostris

The distinctive spike-like crest of Saurolophus has been interpreted in multiple ways, and could have had multiple functions. Brown compared it to the crest of a chameleon, and suggested it could provide an area for muscle attachment and a connection point for a nonbody back frill like that seen in the basilisk lizard. Peter Dodson interpreted similar features in other duckbills as having use in sexual identification.[20] Maryańska and Osmólska, noting the hollow base, suggested that the crest increased the surface area of the respiratory cavity, and helped in thermoregulation.[16] James Hopson supported a function as a visual signal, and further mentioned the possibility that the inflatable skin flaps over the nostrils could have acted as resonators and additional visual signals.[21] This idea has been picked up by authors of popular dinosaur works, such as David B. Norman, who discussed hadrosaurid display at length and included a life restoration of such an adaptation in action.[22]

Highlited juvenile remains from block MPC-D 100/764, representing possibly four individuals

Cranial ontogeny of S. angustirostris

In 2015 Leonard Dewaele and colleagues described a small and partial nest containing several juveniles of S. angustirostris. The specimen (MPC-D 100/764) was recovered from the notorious Dragon's Tomb assambleage of the Nemegt Formation. The team noted that among remains, three or even four juveniles can be recognized, and two fragmentary eggshells were found in association. Juveniles within this block were identified as perinates, as they had skull lengths less than five percent of the length of the skulls of the adults, indicating they were in the earliest developmental stage at the time of their deaths. Based on these juveniles, Dewaele and team indicated that during the ontogeny of S. angustirostris the distinct crest found in adults was poorly developed in infancy, the snout grew proportionally longer, the orbit became more oval-shaped, the doming of the frontal became less prominent, and the coronoid process became higher.[23]

Bell and team in 2018 described the famous Dragon's Tomb assambleage of the Altan Uul II locality, Nemegt Formation, which contains a large-sized bonebed of S. angustirostris. This bonebed is largely monodominant (one dominant species), with at least three size-classes (juveniles, subadults, and adults) of S. angustirostris. Examinations made to Dragon's Tomb suggest that at least 21 Saurolophus individuals can be currently found. The team indicated that this bonebed has a minimum size of about 2000 m2, which suggest that over 100 Saurolophus carcasses may have contributed to the event. However, they discussed that even though evidence clearly reflects a catastrophic mass-mortality of a social group of S. angustirostris and provide the first evidence of gregariousness in this taxon, the exact conditions and cause surrounding the group death can not be determined. Bell and team also noted that while Dragon's Tomb provides direct evidence for social behaviour in S. angustirostris, there is yet no evidence for it in S. osborni. Nevertheless, gregariousness is apparently widespread in hadrosaurines.[24]

Left humerus of S. angustirostris MPC-D 100/764, showing multiple bite marks

David W.E. Hone and Mahito Watabe in 2011 reported the left humerus of a nearly complete S. angustirostris skeleton (MPC-D 100/764) from the Bügiin Tsav locality of the Nemegt Formation, which was heavily damaged from bite marks attributed to the sympatric Tarbosaurus. As suggested by the lack of damage to the rest of the skeleton (such as large wounds in skeletal remains indicative of predation), this tyrannosaurid was likely scavenging an already dead S. angustirostris. It is unlikely that a large-bodied predator such as Tarbosaurus would have left sparse feeding traces on a single humerus having an entire carcass to feed on. The humerus shows three distinctive feeding methods, interpreted as punctures, drag marks, and bite−and−drag marks. Hone and Watabe noted that bite marks were mostly located at the deltopectoral crest, suggesting that this Tarbosaurus was actively selecting which biting style employ to scavenge the bone.[25]

Comparisons between the scleral rings of Saurolophus and modern birds and reptiles suggest that it may have been cathemeral, active throughout the day at short intervals.[26]

S. osborni is known only from the upper part (unit 4) of the Horseshoe Canyon Formation. The formation is interpreted as having a significant marine influence, due to an encroaching Western Interior Seaway, the shallow sea that covered the midsection of North America through much of the Cretaceous.[27] S. osborni may have preferred to stay inland.[7] It lived alongside other dinosaur species including the ornithopods Hypacrosaurus altispinus and Parksosaurus warreni, ankylosaurid Anodontosaurus lambei, pachycephalosaurid Sphaerotholus edmontonense, ornithomimids Ornithomimus brevitertius and an unnamed species of Struthiomimus, small theropods including Atrociraptor marshalli and Albertonykus borealis, and the tyrannosaurid Albertosaurus sarcophagus.[28] The dinosaurs from this formation form part of the Edmontonian land vertebrate age.[27] A 2001 study suggested that Saurolophus osborni was part of a distinct inland fauna characterized by an association between Anchiceratops ornatus and it, while the contemporary coastal fauna was characterized by the association of Pachyrhinosaurus canadensis and Edmontosaurus regalis.[29] However, the association between S. osborni and Anchiceratops was later noted to be in error, Anchiceratops only occurs lower in the Horseshoe Canyon Formation, before the major transgression of the Western Interior Seaway represented by the Drumheller Marine Tongue.[30]

Restoration of a S. angustirostris herd in their native Nemegt Formation alongside a Homalocephale pair

S. angustirostris was one of the largest herbivores of the Nemegt Formation, which lacked large ceratopsians, but had sauropods and a more diverse theropod fauna. Unlike other Mongolian formations like the well-known Djadochta Formation that includes Velociraptor and Protoceratops, the Nemegt is interpreted as being well-watered region, like the Dinosaur Park Formation in Alberta.[27] When examined, the rock facies of the Nemegt formation suggest the presence of stream and river channels, mudflats, and shallow lakes. Sediments also indicate that a rich habitat existed, offering diverse food in abundant amounts that could sustain Cretaceous dinosaurs.[31] It coexisted with the rare hadrosaurid Barsboldia, flat-headed pachycephalosaurian Homalocephale and domed Prenocephale, the large ankylosaurid Saichania, rare titanosaurs sauropods Nemegtosaurus and Opisthocoelicaudia, the alvarezsaurid Mononykus, three types of troodontids including Zanabazar, several oviraptorosaurians including Rinchenia and Nemegtomaia, the ornithomimosaurs Anserimimus and Gallimimus, and the giant theropods Deinocheirus and Therizinosaurus, including the tyrannosaurid Tarbosaurus.[28]

S. angustirostris was common, and would have been an important large herbivore in the Nemegt Formation. By comparison, S. osborni was rare in the Horseshoe Canyon Formation, and faced competition from other duckbills (genus Hypacrosaurus).