Hypacrosaurus

*Hypacrosaurus altispinus*

***Hypacrosaurus*** (meaning "near the highest lizard" [Greek υπο-, hypo- = less + ακρος, akros, high], because it was almost but not quite as large as *Tyrannosaurus*) was a genus of duckbill dinosaur similar in appearance to *Corythosaurus*. Like *Corythosaurus*, it had a tall, hollow rounded crest, although not as large and straight. It is known from the remains of two species that spanned 75 to 67 million years ago, in the Late Cretaceous of Alberta, Canada, and Montana, USA, and is the latest hollow-crested duckbill known from good remains in North America. It was an obscure genus until the description of nests, eggs, and hatchlings belonging to *H. stebingeri*.

## Description

*Hypacrosaurus* is most easily distinguished from other hollow-crested duckbills (lambeosaurines) by its tall neural spines and the form of its crest. The neural spines, which project from the top of the vertebrae, are 5 to 7 times the height of the body of their respective vertebrae in the back, which would have given it a tall back in profile. The skull's hollow crest is like that of *Corythosaurus*, but is more pointed along its top, not as tall, wider side to side, and has a small bony point at the rear. Unlike other lambeosaurines, the passages for the airways do not form an S-curve in the crest (at least not in H. altispinus). The animal is estimated to have been around 9.1 meters long (30 feet), and to have weighed up to 4.0 tonnes (4.4 tons). As with most duckbills, its skeleton is otherwise not particularly remarkable, although some pelvic details are distinctive. Like other duckbills, it was a bipedal/quadrupedal herbivore. The two known species, *H. altispinus* and *H. stebingeri*, are not differentiated in the typical method, of unique characteristics, as *H. stebingeri* was described as transitional between the earlier *Lambeosaurus* and later *Hypacrosaurus*. Photographs of an adult *H. stebingeri* skull show an animal that looks very similar to *H. altispinus*.

## Discovery and history

The type remains of *Hypacrosaurus* remains were collected in 1910 by Barnum Brown for the American Museum of Natural History.

## Classification

*Hypacrosaurus* was a lambeosaurine hadrosaurid, and has been recognized as such since the description of its skull. Within the Lambeosaurinae, it is closest to *Lambeosaurus* and *Corythosaurus*, with Jack Horner and Phil Currie (1994) suggesting that *H. stebingeri* is transitional between *Lambeosaurus* and *H. altispinus*, and Michael K. Brett-Surman (1989) suggesting that *Hypacrosaurus* and *Corythosaurus* are the same genus. These genera, particularly *Corythosaurus* and *Hypacrosaurus*, are regarded as the "helmeted" or "hooded" branch of the lambeosaurines, and the clade they form is sometimes informally designated Lambeosaurini. Although Suzuki et al.'s 2004 redescription of *Nipponosaurus* found a close relationship between *Nipponosaurus* and *Hypacrosaurus stebingeri*, indicating that Hypacrosaurus may be paraphyletic, this was rejected in a later, more comprehensive reanalysis of lambeosaurines, which found the two species of *Hypacrosaurus* to form a clade without *Nipponosaurus*, with *Corythosaurus* and *Olorotitan* being the closest relatives.

The following cladogram illustrating the relationships of *Hypacrosaurus* and its close relatives was recovered in a 2012 phylogenetic analysis by Albert Prieto-Márquez, Luis M. Chiappe and Shantanu H. Joshi.

## Paleobiology

As a hadrosaurid, *Hypacrosaurus* 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 meters.

### Reproduction

*H. stebingeri* laid roughly spherical eggs of 20 by 18.5 centimeters, with embryos being 60 cm long. Hatchlings were around 1.7 meters long. Growth was faster than that of an alligator and comparable to ratite growth, for several years, based on the amount of bone growth seen between lines of arrested growth.

## JPInstitute.com Description

Hypacrosaurus was a plant-eating, duck-billed dinosaur that was as long as a school bus. It was discovered in 1910 by Barnum Brown, one of America's most famous dinosaur hunters. In 1993, nests were found that have greatly helped scientists understand how all dinosaurs cared for their babies.

There is still some debate as to whether or not this dinosaur should be reassigned to the genus Corythosaurus. Regardless of where it is classified, however, Hypacrosaurus has yielded a great deal of information about itself and other dinosaurs. In addition to the wealth of nest and nurturing information provided by this species, its skull has been used to study warm-blooded dinosaur theories.

Hypacrosaurus had unusually long neural spines and it had a fairly small head crest.

## Links

http://web.archive.org/web/20040214162549fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_hypac.html