Parasaurolophus (; meaning "beside crested lizard" in reference to Saurolophus) is a genus of hadrosaurid "duck-billed" dinosaur that lived in what is now western North America and possibly Asia during the Late Cretaceous period, about 76.5–73 million years ago. It was a large herbivore that could reach over long and weigh over , and were able to move as a biped and a quadruped. Three species are universally recognized: P. walkeri (the type species), P. tubicen, and the short-crested P. cyrtocristatus. Additionally, a fourth species, P. jiayinensis, has been proposed, although it is more commonly placed in the separate genus Charonosaurus. Remains are known from Alberta, New Mexico, and Utah, as well as possibly Heilongjiang if Charonosaurus is in fact part of the genus. The genus was first described in 1922 by William Parks from a skull and partial skeleton found in Alberta.
Parasaurolophus is a hadrosaurid, part of a diverse family of large Late Cretaceous ornithopods that are known for their range of bizarre head adornments, which were likely used for communication and increased hearing. This genus is known for its large, elaborate cranial crest, which forms a long curved tube projecting upwards and back from the skull in its largest form. Charonosaurus from China, which may have been its closest relative, had a similar skull and a potentially similar crest. Visual recognition of both species and sex, acoustic resonance, and thermoregulation have been proposed as functional explanations for the crest. It is one of the rarer hadrosaurids, known from only a handful of good specimens.
Discovery and naming
Meaning "near crested lizard", the name Parasaurolophus is derived from the Greek words para/παρα ("beside" or "near"), saurus/ ("lizard"), and lophos/λοφος ("crest"). It is based on ROM 768, a skull and partial skeleton missing most of the tail and the back legs below the knees, which was found by a field party from the University of Toronto in 1920 near Sand Creek along the Red Deer River in Alberta. These rocks are now known as the Campanian age Late Cretaceous period Dinosaur Park Formation. William Parks named the specimen P. walkeri in honor of Sir Byron Edmund Walker, the chairman of the Board of Trustees of the Royal Ontario Museum. Parasaurolophus remains are rare in Alberta, with only one other partial skull that is possibly from the Dinosaur Park Formation and three Dinosaur Park specimens lacking their skulls that possibly belong to the genus. In some faunal lists, there is a mention of possible P. walkeri material in the Hell Creek Formation of Montana, a rock unit of the late Maastrichtian age. This occurrence is not noted by Sullivan and Williamson in their 1999 review of the genus and has not been further elaborated upon elsewhere.
In 1921, Charles H. Sternberg recovered a partial skull (PMU.R1250) from what is now known as the slightly younger Kirtland Formation in San Juan County, New Mexico. This specimen was sent to Uppsala, where Carl Wiman described it as a second species, P. tubicen, in 1931. The specific epithet is derived from the Latin word tǔbǐcěn, meaning "trumpeter". A second, nearly complete P. tubicen skull (NMMNH P-25100) was found in New Mexico in 1995. Using computed tomography of this skull, Robert Sullivan and Thomas Williamson gave the genus a monographic treatment in 1999 that covered aspects of its anatomy and taxonomy, as well as the functions of its crest. Williamson later published an independent review of the remains that disagreed with the taxonomic conclusions.
John Ostrom described another good specimen (FMNH P27393) from New Mexico as P. cyrtocristatus in 1961. It includes a partial skull with a short, rounded crest and much of the postcranial skeleton except for the feet, neck, and parts of the tail. Its specific name is derived from the Latin words curtus, meaning "shortened" and cristatus, meaning "crested". The specimen was found in either the top of the Fruitland Formation or, more likely, the base of the overlying Kirtland Formation. The range of this species was described in 1979, when David B. Weishampel and James A. Jensen described a partial skull with a similar crest (BYU 2467) from the Campanian age Kaiparowits Formation of Garfield County, Utah. Since then, another skull has been found in Utah with the short, rounded P. cyrtocristatus crest morphology.
Species
Parasaurolophus is known from three certain species: P. walkeri, P. tubicen, and P. cyrtocristatus. All of them can be clearly distinguished from each other and have many differences. The first named species, therefore the type, is P. walkeri. One certain specimen from the Dinosaur Park Formation is referred to it, but many more are almost certainly referable. Like stated above, it is different from the other two species, with it having a simpler internal structure than P. tubicen, along with a straighter crest and different internal structuring than P. cyrtocristatus.
The next named species is P. tubicen, which is the largest of the Parasaurolophus species. It lived in New Mexico, where three specimens are known, and can be differentiated from its other species. It possesses a long and straight crest, with a very complex interior compared to the other species. All known specimens of P. tubicen come from the De-Na-Zin Member of the Kirtland Formation.
In 1961, the third species, P. cyrtocristatus was named by John Ostrom. Its three known specimens have been found in the Fruitland and Kaiparowits formations of Utah and New Mexico. The second specimen, the first known from the Kaiparowits Formation, was originally unassigned to a specific taxon. Of the Parasaurolophus species, P. cyrtocristatus is the smallest and has the most curved crest. Because of its possession of the two above features, it has often been speculated that it was a female of P. walkeri or P. tubicen, which were all thought to be males, although P. tubicen lived approximately a million years later. As noted by Thomas Williamson, the type material of P. cyrtocristatus is about 72% the size of P. tubicen, close to the size at which other lambeosaurines are interpreted to begin showing definitive sexual dimorphism in their crests (~70% of adult size). Even though many scientists have supported the possible fact of P. cyrtocristatus being a female, many other studies have found that it is not because of the differences in age, distribution, and the large differences in the crest and its internal structure.
A study published in PLoS ONE in 2014 found that one more species could be referred to Parasaurolophus. This study, led by Xing, found Charonosaurus jiayensis was actually nested deeply inside Parasaurolophus, which created the new species P. jiayensis. If this species is indeed inside Parasaurolophus, then the genus therefore lasted until the K-Pg extinction and is known from two continents.
Description
Like most dinosaurs, the skeleton of Parasaurolophus is incompletely known. The length of the type specimen of P. walkeri is estimated at , and allometry-based body mass estimates indicate that a long individual would have weighed more than . Gregory S. Paul estimated that an average adult individual of the type species would measure long and weigh . The skull of the P. walkeri type specimen is about long, including the crest. Its single known arm was relatively short for a hadrosaurid, with a short but wide shoulder blade. The thighbone measures long in P. walkeri and is robust for its length when compared to other hadrosaurids. The upper arm and pelvic bones were also heavily built.
Like other hadrosaurids, it was able to walk on either two legs or four. It probably preferred to forage for food on four legs, but ran on two. The neural spines of the vertebrae were tall, as was common in lambeosaurines. At their tallest over the hips, they increased the height of the back. Skin impressions are known for P. walkeri, showing uniform tubercle-like scales, but no larger structures.
Skull
The most noticeable feature was the cranial crest that protruded from the rear of the head and was made up of the premaxilla and nasal bones. The crest was hollow, with distinct tubes leading from each nostril to the end of the crest before reversing direction and heading back down the crest and into the skull. The tubes were simplest in P. walkeri, and more complex in P. tubicen, where some tubes were blind and others met and separated. While P. walkeri and P. tubicen had long crests with slight curvature, P. cyrtocristatus had a short crest with a more circular profile.
Classification As its name implies, Parasaurolophus was initially thought to be closely related to Saurolophus because of its superficially similar crest. However, it was soon reassessed as a member of the lambeosaurine subfamily of hadrosaurids—Saurolophus is a hadrosaurine. It is usually interpreted as a separate offshoot of the lambeosaurines, distinct from the helmet-crested Corythosaurus, Hypacrosaurus, and Lambeosaurus. Its closest known relative appears to be Charonosaurus, a lambeosaurine with a similar skull (but no complete crest yet) from the Amur region of northeastern China. The two may form the clade called Parasaurolophini. P. cyrtocristatus, with its short, rounded crest, may be the most basal of the three known Parasaurolophus species or it may represent subadult or female specimens of P. tubicen.
The following cladogram is after the 2007 redescription of Lambeosaurus magnicristatus (Evans and Reisz, 2007):
Paleobiology
Diet and feeding As a hadrosaurid, Parasaurolophus was a large bipedal and quadrupedal herbivore, eating plants with a sophisticated skull that permitted a grinding motion analogous to chewing. Its teeth were continually being replaced and were packed into dental batteries containing hundreds of teeth, but only a relative handful of which were in use at any time. It used its beak to crop plant material, which was held in the jaws by a cheek-like organ. Vegetation could have been taken from the ground up to a height of around . As noted by Robert Bakker, lambeosaurines have narrower beaks than hadrosaurines, implying that Parasaurolophus and its relatives could feed more selectively than their broad-beaked, crestless counterparts. Parasaurolophus had a diet consisting of leaves, twigs, and pine needles which would imply that it was a browser.
Growth
Parasaurolophus is known from many adult specimens, and a juvenile described in 2013, numbered RAM 14000 and nicknamed Joe, after a volunteer at the Raymond M. Alf Museum of Paleontology (RAM). The juvenile was discovered in the Kaiparowits Formation in 2009. Excavated by the joint expedition by museum and The Webb Schools, the juvenile has been identified as around only one year old when it died. Referred to Parasaurolophus sp., the juvenile is the most complete, as well as youngest Parasaurolophus ever found, and measures . This individual fits neatly into the currently known Parasaurolophus growth stages, and lived approximately 75 million years ago. Even though no complete skull of the intermediate age between RAM 14000 and adult Parasaurolophus has been found yet, a partial braincase of about the right size is known. At 25% of the total adult size, the juvenile show that crest growth of Parasaurolophus began sooner than in related genera, such as Corythosaurus. It has been suggested that Parasaurolophus adults bore such large crests, especially when compared to the related Corythosaurus, because of this difference in age between when their crests started developing. The crest of the juvenile is not long and tubular like the adults, but low and hemispherical.