'''Hypacrosaurus''' (meaning "near the highest lizard", because it was almost but not quite as large as ''
Tyrannosaurus'')
[ A new trachodont dinosaur, ''Hypacrosaurus'', from the Edmonton Cretaceous of Alberta, , Barnum, Brown, Bulletin of the American Museum of Natural History, 1913 ][ Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs, , Benjamin S., Creisler, Indiana University Press, 2006, ] 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 lived about 76 to 68 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,
[ Hadrosaurian Dinosaurs of North America, , Richard Swann, Lull, Geological Society of America, 1942, ] 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 nasal cavity of lambeosaurine hadrosaurids (Reptilia:Ornithischia): comparative anatomy and homologies, , David B., Weishampel, Journal of Paleontology, 1981 ] 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.][ Dinosaurs: The Encyclopedia, , Donald F., Glut, McFarland & Co, 1997, ] 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''.[ Dinosaur Eggs and Babies, , John R., Horner, Cambridge University Press, 1994, ] 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.[ On the genus ''Stephanosaurus'', with a description of the type specimen of ''Lambeosaurus lambei'', Parks, , Charles Whitney, Gilmore, Canada Department of Mines Geological Survey Bulletin (Geological Series), 1924 ] Within the Lambeosaurinae, it is closest to ''Lambeosaurus'' and ''Corythosaurus'',[ The Dinosauria, , John R., Horner, University of California Press, 2004, ] 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.][ A revision of the Hadrosauridae (Reptilia:Ornithischia) and their evolution during the Campanian and Maastrichtian. Ph.D. dissertation, , Michael K., Brett-Surman, Graduate School of Arts and Sciences of The George Washington University, 1989, ] 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,[ ''Nipponosaurus sachaliensis'' (Dinosauria; Ornithopoda): anatomy and systematic position within Hadrosauridae, , Daisuke, Suzuki, Journal of Vertebrate Paleontology, 2004 ] 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.[ Anatomy and relationships of ''Lambeosaurus magnicristatus'', a crested hadrosaurid dinosaur (Ornithischia) from the Dinosaur Park Formation, Alberta, , David C., Evans, Journal of Vertebrate Paleontology, 2007 ]
Discovery and history
The type remains of ''Hypacrosaurus'' remains were collected in 1910 by Barnum Brown for the American Museum of Natural History.[ The remains, a partial postcranial skeleton consisting of several vertebrae and a partial pelvis (AMNH 5204), came from along the Red Deer River near Tolman Ferry, Alberta, Canada, from rocks of what is now known as the Horseshoe Canyon Formation (early Maastrichtian, Upper Cretaceous). Brown described these remains, in combination with other postcranial bones, in 1913 as a new genus that he considered to be like ''Saurolophus''.][ No skull was known at this time, but two skulls were soon discovered and described.]
During this period, the remains of small hollow-crested duckbills were described as their own genera and species. The first of these that figure into the history of ''Hypacrosaurus'' was ''Cheneosaurus tolmanensis'', based on a skull and assorted limb bones, vertebrae, and pelvic bones from the Horseshoe Canyon Formation.[ On ''Cheneosaurus tolmanensis'', a new genus and species of trachodont dinosaur from the Edmonton Cretaceous of Alberta, , Lawrence M., Lambe, The Ottawa Naturalist, 1917 ] Shortly thereafter, William Diller Matthew identified an American Museum of Natural History skeleton (AMNH 5340) as ''Procheneosaurus'', from the Two Medicine Formation of Montana.[ Canadian dinosaurs, , William Diller, Matthew, Natural History, 1920 ] 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 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''.[ Taxonomic implications of relative growth in lambeosaurine dinosaurs, , Peter, Dodson, Systematic Zoology, 1975 ] This idea has become accepted,[ although not formally tested. Matthew's ''Prochenosaurus'', meanwhile, was not quite like the other ''Procheneosaurus'' specimens studied by Dodson, and for good reason: it was much more like a species that would not be named until 1994, ''H. stebingeri''.][ ]
''H. stebingeri''
The new species ''Hypacrosaurus stebingeri'' was named for a variety of remains, including hatchlings with associated eggs and nests, found near the top of the late Campanian (Upper Cretaceous) Two Medicine Formation in Glacier Creek, Montana, and across the border in Alberta. These represent "the largest collection of baby skeletal material of any single species of hadrosaur known".
Species
''H. altispinus'', the type species, is known from 5 to 10 articulated skulls with some associated skeletal remains, from juvenile to adult individuals. ''H. stebingeri'' is known from an unknown but substantial number of individuals, with an age range of embryos to adults.[ The hypothesis that ''H. altispinus'' and ''H. stebingeri'' form a natural group excluding other known hadrosaur species may be incorrect, as noted in Suzuki ''et al.'s 2004 redescription of ''Nipponosaurus''; their phylogenetic analysis found that ''Nipposaurus'' was more closely related to ''H. altispinus'' than ''H. stebingeri'' was to ''H. altispinus''.][ This was rejected by Evans and Reisz (2007), though.]
Paleoecology
''H. altispinus'' shared the Horseshoe Canyon Formation with fellow hadrosaurids ''Edmontosaurus'' and ''Saurolophus'', hypsilophodont ''Parksosaurus'', ankylosaurid ''Euoplocephalus'', nodosaurid ''Edmontonia'', horned dinosaurs ''Montanoceratops'', ''Anchiceratops'', ''Arrhinoceratops'', and ''Pachyrhinosaurus'', pachycephalosaurid ''Stegoceras'', ostrich-mimics ''Ornithomimus'' and ''Struthiomimus'', a variety of poorly-known small theropods including troodontids and dromaeosaurids, and the tyrannosaurs ''Albertosaurus'' and ''Daspletosaurus''.[ The Dinosauria, , David B., Weishampel, University of California Press, 2004, ] 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 Horned Dinosaurs: A Natural History, , Peter, Dodson, Princeton University Press, 1996, ] 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.[ ''H. altispinus'' may have preferred to stay more landward.]
The slightly older Two Medicine Formation, home to ''H. stebingeri'', was also populated by another well-known nesting hadrosaur, ''Maiasaura'', as well as the troodontid ''Troodon'', which is also known from nesting traces. The tyrannosaurid ''Daspletosaurus'', caenagnathid ''Chirostenotes'', dromaeosaurid ''Bambiraptor'', armored dinosaurs ''Edmontonia'' and ''Euoplocephalus'', hypsilophodont ''Orodromeus'', hadrosaur ''Prosaurolophus'', and horned dinosaurs ''Achelousaurus'', ''Brachyceratops'', ''Einiosaurus'', and ''Styracosaurus ovatus'' were also present.[ This formation was more distant from the Western Interior Seaway, higher and drier, with a more terrestrial influence.][ Taphonomy of three dinosaur bone beds in the Upper Cretaceous Two Medicine Formation of northwestern Montana: evidence for drought-related mortality, , Raymond R., Rogers, Palaios, 1990 ]
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.
Nests and growth
''H. stebingeri'' laid roughly spherical eggs of 20 by 18.5 cm (7.9 by 7.3 inches), with embryos 60 cm long (23.6 in). Hatchlings were around 1.7 m long (5.6 ft). Young and embryonic individuals had deep skulls with only slight expansion in the bones that would one day form the crest.[ 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 (analogous to growth rings in trees).][ Growth rate of ''Hypacrosaurus stebingeri'' as hypothesized from lines of arrested growth and whole femur circumference, , Lisa N., Cooper, Journal of Vertebrate Paleontology, 1999 ]
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.[ The crest and its associated nasal passages have also figured in the debate about dinosaur endothermy, specifically in discussions about nasal turbinates. ]
Turbinates are thin bones or cartilages that come in two types, with two functions. Nasal olfactory turbinates are found in all living tetrapods and function in smell. Respiratory turbinates function to prevent water loss through evaporation and are found only in birds and mammals, modern endotherms (warm-blooded animals) who could lose a great deal of water while breathing because they breathe more often than comparably-sized ectotherms (cold-blooded animals) to support their higher metabolism.[Chinsamy, Anusuya; and Hillenius, Willem J. (2004). "Physiology of nonavian dinosaurs". ''The Dinosauria'', 2nd. 643-659.] 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.[ The metabolic status of some Late Cretaceous dinosaurs, , J. A., Ruben, Science, 1996 ]
References
External links
★ ''Hypacrosaurus'' in the Dinosaur Encyclopedia at DinoRuss' Lair
★ ''Hypacrosaurus'', from the Canadian Museum of Nature
★ ''Hypacrosaurus'' in The Natural History Museum's Dino Directory
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