Parasaurolophus

Parasaurolophus 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. Its name means "beside crested lizard," in reference to Saurolophus. It should not be confused with Parasaurus.

It was a large herbivore that could reach over 9 metres (30 ft) long and weigh over 5 metric tons (5.5 short tons). It could move as a biped and as a quadruped.

Three species are universally recognized: P. walkeri, the type species, P. tubicen, and the short-crested P. cyrtocristatus. 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, and possibly Heilongjiang if Charonosaurus is part of the genus. The genus was first described in 1922 by William Parks from a skull and partial skeleton found in Alberta.

Parasaurolophus belongs to a diverse family of large Late Cretaceous ornithopods, the hadrosaurid group, which is known for its range of bizarre head adornments. These were likely used for communication and increased hearing. This genus is known for its large, elaborate crest, which forms a long curved tube that projects upwards and back from the skull in its largest form.

Scientists have proposed a few explanations for the crest, including visual recognition of both species and sex, acoustic resonance, and thermoregulation. Charonosaurus from China, which may have been its closest relative, had a similar skull and a potentially similar crest. Parasaurolophus is one of the rarer hadrosaurids, known from only a handful of good specimens.

Discovery and naming

The name Parasaurolophus means "near crested lizard" and comes from the Greek words for "beside," "lizard," and "crest." The first find, ROM 768, was a skull and partial skeleton discovered in 1920 near Sand Creek along the Red Deer River in Alberta by a field party from the University of Toronto. The rocks there are from the Campanian age of the Late Cretaceous period and are part of the Dinosaur Park Formation. William Parks named this specimen P. walkeri after Sir Byron Edmund Walker, chairman of the Board of Trustees of the Royal Ontario Museum. In 1921, Charles H. Sternberg found a partial skull (PMU.R1250) in the slightly younger Kirtland Formation of San Juan County, New Mexico. Carl Wiman of Uppsala described it in 1931 as a second species, P. tubicen, whose name comes from a Latin word meaning "trumpeter." In 1961, John Ostrom named a third species, P. cyrtocristatus, from a good specimen (FMNH P27393) that included a partial skull with a short, rounded crest. Its name comes from Latin words meaning "shortened" and "crested." Later, David B. Weishampel and James A. Jensen described a skull with a similar crest (BYU 2467) from the Kaiparowits Formation of Garfield County, Utah.

Parasaurolophus is known from three certain species: P. walkeri, which is the type species, P. tubicen, and P. cyrtocristatus. Each can be clearly told apart from the others. P. tubicen is the largest species and has a long, straight crest with a very complex inside structure. Its known specimens all come from the De-Na-Zin Member of the Kirtland Formation. P. cyrtocristatus is the smallest species and has the most curved crest. Some scientists have thought it might be a female of another species, but many other studies have found this is not so, because of differences in age, location, and crest structure. A 2014 study in PLoS ONE, led by Xing, found that Charonosaurus jiayensis was actually deep inside Parasaurolophus, creating a new species, P. jiayensis. If this is correct, the genus lasted until the K-Pg extinction and is known from two continents.

Description

Like most dinosaurs, the skeleton of Parasaurolophus is not fully known. The length of the type specimen of P. walkeri is estimated at 9.45 metres (31.0 ft). A 9 metres (30 ft) long individual would have weighed more than 5 metric tons (5.5 short tons). Gregory S. Paul estimated that an average adult of the type species would measure 7.5 metres (25 ft) long and weigh 2.6 metric tons (2.9 short tons). Its one known arm was short for a hadrosaurid, with a short but wide shoulder blade. The thighbone of P. walkeri measures 103 cm (41 in) long and is strong for its length compared to other hadrosaurids. The upper arm and pelvic bones were also heavily built. Like other hadrosaurids, it could walk on two legs or four. It probably looked for food on four legs but ran on two. The neural spines of the vertebrae were tall, which raised the height of the back over the hips. Skin impressions are known for P. walkeri, showing uniform, bump-like scales but no larger structures.

The most noticeable feature was the cranial crest that stuck out from the back of the head. It was made up of the premaxilla and nasal bones. The crest was hollow, with tubes leading from each nostril to the end of the crest before turning back down into the skull. The tubes were simplest in P. walkeri and more complex in P. tubicen. P. walkeri and P. tubicen had long crests with slight curvature, while P. cyrtocristatus had a short crest with a more circular shape.

Classification

As its name suggests, Parasaurolophus was first thought to be closely related to Saurolophus because of its similar crest. However, it was soon found to belong to the lambeosaurine group of hadrosaurids, while Saurolophus belongs to the hadrosaurine group. It is usually seen as a separate branch of the lambeosaurines, different 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 a clade called Parasaurolophini. P. cyrtocristatus, with its short, rounded crest, may be the most basal of the three known Parasaurolophus species, or it may instead be a subadult (not yet fully grown) or female P. tubicen.

The chart below shows how these animals are related. It is based on the 2007 re-study of Lambeosaurus magnicristatus.

Paleobiology

Parasaurolophus was a large bipedal and quadrupedal plant-eater. Its skull allowed a grinding motion similar to chewing, and it ate plants such as leaves, twigs, and pine needles. Its teeth were constantly replaced, and it held food in its jaws with a cheek-like organ. Robert Bakker noted that its narrower beak suggests it could feed more carefully than related dinosaurs with broad, crestless beaks. It could have reached plants up to about 4 m (13 ft) above the ground. Many adult fossils are known, along with a juvenile described in 2013. Numbered RAM 14000 and nicknamed Joe after a volunteer at the Raymond M. Alf Museum of Paleontology, this young animal was found in the Kaiparowits Formation in 2009 by a joint expedition from the museum and The Webb Schools. It was about one year old when it died and is the most complete and youngest Parasaurolophus found so far. Its nearly complete skull was only missing part of the maxilla on the left side. Erosion split the skull down the middle, possibly while it rested on the bottom of a river bed, and after repair it looked roughly like a trapezoid from the side. Its crest was low and rounded, unlike the long, tube-shaped crests of adults.

The crest's purpose has been studied for a long time. Many early ideas, such as use as a snorkel suggested by Alfred Sherwood Romer, have been discredited, and it is now thought to have had several roles. These include visual display to help identify species and whether an animal was male or female, sound for communication, and temperature control. James Hopson suggested that the small-crested P. cyrtocristatus was the female form, while Thomas Williamson suggested it was the juvenile form, but neither idea became widely accepted. A juvenile braincase shows a small upward flare of the frontal bones, which supports the idea that a small tube-shaped crest was present in young animals. Many researchers have proposed that the crest acted as a resonating chamber that produced low frequencies between 55 and 720 Hz to alert others. Hopson also suggested that hadrosaurids could hear high sounds, based on their inner ears being similar to those of crocodiles. The large crest may also have been a visual signal, and the large eye sockets and scleral rings suggest that sight was important. Artists once pictured a skin frill, like that of the basilisk lizard, on the crest, as shown by Charles R. Knight and in the Walt Disney film Fantasia, but this idea has since fallen out of favor. One specimen of P. walkeri may show a pathology, a notch in the vertebrae at the base of the neck. A 2020 analysis suggested that injuries to the shoulder and ribs may have come from a falling tree, and that the animal survived for months to perhaps years afterward.

Paleoecology

Parasaurolophus walkeri, from the Dinosaur Park Formation, lived alongside many other prehistoric animals. These included famous dinosaurs such as the horned Centrosaurus, Chasmosaurus, and Styracosaurus; the ornithomimids Struthiomimus; the duckbills Gryposaurus and Corythosaurus; the tyrannosaurids Gorgosaurus and Daspletosaurus; and the armored Edmontonia, Euoplocephalus, and Dyoplosaurus. It was a rare member of this group. The area was made of low, flat land with rivers and floodplains that became more swampy and were affected by marine conditions as the Western Interior Seaway moved westward. The climate was warmer than present-day Alberta, with no frost, but it had wetter and drier seasons. Conifers appear to have been the main canopy plants, with an understory of ferns, tree ferns, and angiosperms. Some of the less common hadrosaurs here, such as Parasaurolophus, may be the remains of animals that died while passing through the region, and they might have lived in upland areas to nest or feed. Their presence at northern fossil sites, along with Kritosaurus, may show animals moving between otherwise separate northern and southern environments.

In New Mexico, the Fruitland Formation was a swampy lowland near the shore of the Cretaceous Interior Seaway when P. cyrtocristatus lived there. Its neighbors included the ceratopsian Pentaceratops sternbergii, the pachycephalosaur Stegoceras novomexicanum, and fossils that may belong to Tyrannosauridae, ?Saurornitholestes langstoni, ?Struthiomimus, Ornithopoda, ?Chasmosaurus, ?Corythosaurus, Hadrosaurinae, Hadrosauridae, and Ceratopsidae. Slightly later, P. tubicen is found in the Kirtland Formation, which holds fossils of fishes such as Melvius chauliodous, crurotarsans such as Brachychampsa montana and Denazinosuchus kirtlandicus, ornithischians such as the hadrosaurids Anasazisaurus horneri, Naashoibitosaurus ostromi, and Kritosaurus navajovius, saurischians such as the troodontid "Saurornitholestes" robustus, pterosaurs such as Navajodactylus boerei, and turtles such as Denazinemys nodosa. The Kirtland Formation begins at 74.55 ± 0.22 million years ago and ends at around 73.05 ± 0.25 million years ago. In Utah, the Kaiparowits Formation was deposited between 76.6 and 74.5 million years ago, according to Argon-argon radiometric dating. Then, the Western Interior Seaway split North America into two landmasses, Laramidia to the west and Appalachia to the east. The plateau was an ancient floodplain with large channels, peat swamps, ponds, and lakes, and the climate was wet and humid. Parasaurolophus shared its paleoenvironment with dromaeosaurid theropods, the troodontid Talos sampsoni, ornithomimids like Ornithomimus velox, tyrannosaurids like Teratophoneus, armored ankylosaurids, the duckbilled hadrosaur Gryposaurus monumentensis, the ceratopsians Utahceratops gettyi, Nasutoceratops titusi, and Kosmoceratops richardsoni, and the oviraptorosaurian Hagryphus giganteus. Other animals included chondrichthyans (sharks and rays), frogs, salamanders, turtles, lizards, and crocodilians like the apex predator Deinosuchus, as well as early mammals such as multituberculates, marsupials, and insectivorans.