Parasaurolophus

Parasaurolophus (/ˌpærəsɔːˈrɒləfəs/ PARR-ə-saw-ROL-ə-fəs or /ˌpærəˌsɔrəˈloʊfəs/ PARR-ə- SAWR-ə-LOH-fəs; meaning "near crested lizard" in reference to Saurolophus) is a genus of ornithopod dinosaur that lived in what is now North America during the Late Cretaceous Period, about 76.5–73 million years ago.[1] It was a herbivore that walked both as a biped and a quadruped. Three species are recognized: P. walkeri (the type species), P. tubicen, and the short-crested P. cyrtocristatus. Remains are known from Alberta (Canada), and New Mexico and Utah (USA). The genus was first described in 1922 by William Parks from a skull and partial skeleton found in Alberta.

Parasaurolophus was a hadrosaurid, part of a diverse family of Cretaceous dinosaurs known for their range of bizarre head adornments. This genus is known for its large, elaborate cranial crest, which at its largest forms a long curved tube projecting upwards and back from the skull. Charonosaurus from China, which may have been its closest relative, had a similar skull and potentially a 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.

Description
Like most dinosaurs, the skeleton of Parasaurolophus is incompletely known. The length of the type specimen of P. walkeri is estimated at 9.5 m (31 ft), and its weight is estimated at 2.5 tonnes (2.8 short tons).[2] Its skull is about 1.6 m (5 ft 3 in) long, including the crest, whereas the type skull of P. tubicen is over 2 m (6 ft 7 in) long, indicating a larger animal.[3] Its single known forelimb was relatively short for a hadrosaurid, with a short but wide shoulder blade. The thighbone measures 103 cm (41 in) long in P. walkeri and is robust for its length when compared to other hadrosaurids.[4] 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.[6] The neural spines of the vertebrae were tall, as was common in lambeosaurines;[4] 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.

The most noticeable feature was the cranial crest, which protruded from the rear of the head and was made up of the premaxillae and nasal bones. The P. walkeri type specimen has a notch in the neural spines near where the crest would hit the back, but this may be a pathology peculiar to this individual.[4] William Parks, who named the genus, hypothesized that a ligament ran from the crest to the notch to support the head.[7] Although this idea seems unlikely,[2] Parasaurolophus is sometimes restored with a skin flap from the crest to the neck. 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.[8] While P. walkeri and P. tubicen had long crests with only 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.[7] However, it was soon reassessed as a member of the lambeosaurine subfamily of hadrosaurids—Saurolophus is an hadrosaurine.[10] It is usually interpreted as a separate offshoot of the lambeosaurines, distinct from the helmet-crested Corythosaurus, Hypacrosaurus, and Lambeosaurus.[6][11][12] 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,[13] and the two may form a clade Parasaurolophini.[12] P. cyrtocristatus, with its short, rounder crest, may be the most basal of the three known Parasaurolophus species,[12] or it may represent subadult or female specimens of P. tubicen.[14]

The following cladogram is after the 2007 redescription of Lambeosaurus magnicristatus (Evans and Reisz, 2007):