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 in the 1990s.
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. The animal is estimated to have been around 9.1 meters long (30 feet), 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.
Classification
Hypacrosaurus was a lambeosaurinae
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, 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.
Discovery and history
]]
The
type remains of
Hypacrosaurus remains were collected in 1910 by
Barnum Brown for the
American Museum of Natural History. Not long after,
Richard Swann Lull and Nelda Wright identified an American Museum of Natural History skeleton (AMNH 5461) from the
Two Medicine Formation of Montana as a specimen of
Procheneosaurus. These and other
taxa were accepted as valid genera until the 1970s, when
Peter Dodson showed that it was more likely that the "cheneosaurs" were the
juveniles of other established lambeosaurines. Although he was mostly concerned with the earlier,
Dinosaur Park Formation genera
Corythosaurus and
Lambeosaurus, he suggested that
Cheneosaurus would turn out to be composed of juvenile individuals of the contemporaneous
Hypacrosaurus altispinus. This idea has become accepted, The dinosaurs from this formation are sometimes known as Edmontonian, after a
land mammal age, and are distinct from those in the formations above and below. The Horseshoe Canyon 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.
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 ~4 m (13 ft) above. Research by Lisa Cooper and colleagues on
H. stebingeri indicates that this animal may have reached reproductive maturity at the age of 2 to 3 years, and reached full size at about 10 to 12 years old. The circumference of the
thigh bone at postulated reproductive maturity was about 40% that of its circumference at full size. The postulated growth rate of
H. stebingeri outpaces those of tyrannosaurids (predators of hypacrosaurs) such as
Albertosaurus and
Tyrannosaurus; rapidly-growing hypacrosaurs would have had a better chance to reach a size large enough to be of defensive value, and beginning reproduction at an early age would also have been advantageous to a prey animal.
Crest functions
]]
The hollow crest of
Hypacrosaurus most likely had social functions, such as a visual signal allowing individuals to identify sex or species, and providing a resonating chamber for making noises. Ruben and others in 1996 concluded that respiratory turbinates were probably not present in
Nanotyrannus,
Ornithomimus or
Hypacrosaurus based on
CT scanning, thus there was no evidence that those animals were warm-blooded.
References
External links
Hypacrosaurus, from the Canadian Museum of Nature
Hypacrosaurus in The Natural History Museum's Dino Directory
Category:Dinosaurs of North America
Category:Cretaceous dinosaurs
Category:Hadrosaurs
