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''Hypacrosaurus'' (meaning "near the highest lizard" reek υπο-, ''hypo-'' = less + ακρος, ''akros'', high because it was almost but not quite as large as ''
Tyrannosaurus ''Tyrannosaurus'' () is a genus of large theropod dinosaur. The type species ''Tyrannosaurus rex'' ( meaning 'king' in Latin), often shortened to ''T. rex'' or colloquially t-rex, is one of the best represented theropods. It lived througho ...
'') is an extinct
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of duckbill
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
similar in appearance to ''
Corythosaurus ''Corythosaurus'' (; ) is a genus of hadrosaurid "duck-billed" dinosaur from the Late Cretaceous Period (geology), period, about 77–75.7 million years ago, in what is now Laramidia, western North America. Its name is derived from the Anci ...
''. 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 The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...
of
Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...
, Canada, and
Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...
, United States, and is the latest hollow-crested duckbill known from good remains in
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
. It was an obscure genus until the discovery in the 1990s of
nest A nest is a structure built for certain animals to hold Egg (biology), eggs or young. Although nests are most closely associated with birds, members of all classes of vertebrates and some invertebrates construct nests. They may be composed of ...
s, eggs, and hatchlings belonging to ''H. stebingeri''.


Discovery and history

The type remains of ''Hypacrosaurus'' were collected in 1910 by
Barnum Brown Barnum Brown (February 12, 1873 – February 5, 1963), commonly referred to as Mr. Bones, was an American paleontologist. He discovered the first documented remains of ''Tyrannosaurus'' during a career that made him one of the most famous fossil ...
for the
American Museum of Natural History The American Museum of Natural History (AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. Located in Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 21 interconn ...
. The remains, a partial postcranial skeleton consisting of several vertebrae and a partial pelvis (AMNH 5204), came from along the
Red Deer River The Red Deer River is a river in Alberta and a small portion of Saskatchewan, Canada. It is a major tributary of the South Saskatchewan River and is part of the larger Saskatchewan / Nelson River, Nelson system that empties into Hudson Bay. T ...
near Tolman Ferry,
Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...
, Canada, from rocks of what is now known as the Horseshoe Canyon Formation ( early Maastrichtian,
Upper Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cret ...
). 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. Not long after, Richard Swann Lull and Nelda Wright identified an American Museum of Natural History skeleton (AMNH 5461) from the
Two Medicine Formation The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between 82.4 Ma and 74.4 Ma, during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky Mountai ...
of Montana as a specimen of ''
Procheneosaurus ''Lambeosaurus'' ( ) is a genus of hadrosaurid dinosaur that lived during the Late Cretaceous period of western North America. The first skull of ''Lambeosaurus'' found was used by palaeontologist Lawrence M. Lambe to justify the creation of ...
''. These and other
taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...
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, although not formally tested. The Two Medicine ''Procheneosaurus'', 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''.


Species

''H. altispinus'', the
type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
, is known from 5 to 10 articulated skulls with some associated skeletal remains, from juvenile to adult individuals found in the Horseshoe Canyon Formation. ''H. stebingeri'' is known from an unknown but substantial number of individuals, with an age range of embryos to adults. The
hypothesis A hypothesis (: hypotheses) is a proposed explanation for a phenomenon. A scientific hypothesis must be based on observations and make a testable and reproducible prediction about reality, in a process beginning with an educated guess o ...
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 In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
found that ''Nipponosaurus'' was more closely related to ''H. altispinus'' than ''H. stebingeri'' was to ''H. altispinus''. This was rejected by Evans and Reisz (2007), though. 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 The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cret ...
)
Two Medicine Formation The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between 82.4 Ma and 74.4 Ma, during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky Mountai ...
in
Glacier County, Montana Glacier County is located in the U.S. state of Montana. As of the 2020 census, the population was 13,778. The county is located in northwestern Montana between the Great Plains and the Rocky Mountains, known to the Blackfeet as the "Backbone ...
, and across the border in Alberta. These represent "the largest collection of baby skeletal material of any single species of hadrosaur known".


Description

''Hypacrosaurus'' is most easily distinguished from other hollow-crested duckbills ( lambeosaurines) by its tall
neural spines Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...
and the form of its crest. The neural spines, which project from the top of the
vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...
e, 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 long, and to have weighed up to 4.0 
tonnes The tonne ( or ; symbol: t) is a unit of mass equal to 1,000  kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton in the United States to distinguish it from the non-metric units of the s ...
(4.4  tons). As with most duckbills, its skeleton is otherwise not particularly remarkable, although some
pelvic The pelvis (: pelves or pelvises) is the lower part of an anatomical trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis or pelvic skeleton). ...
details are distinctive. Like other duckbills, it was a
bipedal Bipedalism is a form of terrestrial locomotion where an animal moves by means of its two rear (or lower) limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' ...
/
quadruped Quadrupedalism is a form of locomotion in which animals have four legs that are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four legs is said to be a quadruped (fr ...
al
herbivore A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat ...
. 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 ''Lambeosaurus'' ( ) is a genus of hadrosaurid dinosaur that lived during the Late Cretaceous period of western North America. The first skull of ''Lambeosaurus'' found was used by palaeontologist Lawrence M. Lambe to justify the creation of ...
'' and later ''Hypacrosaurus''.


Classification

''Hypacrosaurus'' was a lambeosaurine
hadrosaurid Hadrosaurids (), also hadrosaurs or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod fami ...
, 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 In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
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 Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
, this was rejected in a later, more comprehensive reanalysis of lambeosaurines, which found the two species of ''Hypacrosaurus'' to form a
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
without ''Nipponosaurus'', with ''Corythosaurus'' and '' Olorotitan'' being the closest relatives. The following
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
illustrating the relationships of ''Lambeosaurus'' and its close relatives was recovered in a 2022 phylogenetic analysis by Xing Hai and colleagues. Unlike other modern analyses, they found the genus '' Magnapaulia'' to be within ''Hypacrosaurus'', indicating it could be a potential third species.


Paleobiology

As a
hadrosaurid Hadrosaurids (), also hadrosaurs or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod fami ...
, ''Hypacrosaurus'' would have been a
bipedal Bipedalism is a form of terrestrial locomotion where an animal moves by means of its two rear (or lower) limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' ...
/
quadruped Quadrupedalism is a form of locomotion in which animals have four legs that are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four legs is said to be a quadruped (fr ...
al
herbivore A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat ...
, eating a variety of
plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s. Its skull permitted a grinding motion analogous to
chewing Chewing or mastication is the process by which food is crushed and ground by the teeth. It is the first step in the process of digestion, allowing a greater surface area for digestive enzymes to break down the foods. During the mastication proc ...
, and its
teeth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...
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 The cheeks () constitute the area of the face below the eyes and between the nose and the left or right ear. ''Buccal'' means relating to the cheek. In humans, the region is innervated by the buccal nerve. The area between the inside of th ...
-like organ. Its feeding range would have extended from the ground to about above.


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
cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...
s that come in two types, with two functions. Nasal
olfactory The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, it ...
turbinates are found in all living
tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...
s and function in smell. Respiratory turbinates function to prevent water loss through evaporation and are found only in
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s and
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s, modern
endotherms An endotherm (from Ancient Greek, Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily ...
(warm-blooded animals) who could lose a great deal of water while breathing because they breathe more often than comparably sized
ectotherm An ectotherm (), more commonly referred to as a "cold-blooded animal", is an animal in which internal physiological sources of heat, such as blood, are of relatively small or of quite negligible importance in controlling body temperature.Dav ...
s (cold-blooded animals) to support their higher
metabolism Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...
.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.


Thermoregulation

Examining the oxygen-isotope ratio from the bones from different parts of an extinct animal's body should indicate which thermoregulation mode an animal used during its lifetime. An endothermic (warm-blooded) animal should maintain a very similar body temperature throughout its entire body (which is called homeothermy) and therefore there should be little variation in the oxygen-isotope ratio when measured in different bones. Alternatively, the oxygen-isotope ratio differs considerably when measured throughout the body of an organism with an ectothermic (cold-blooded) physiology.Martin, A.J. (2006). Introduction to the Study of Dinosaurs. Second Edition. Oxford, Blackwell Publishing. 560 pp. . Oxygen-isotope ratios calculated for ''Hypacrosaurus'' suggesting that the ratios varied little, indicating that ''Hypacrosaurus'' was a homeotherm, and likely was endothermic. This is in contrast to the Ruben et al. (1996) finding that ''Hypacrosaurus'' was not warm-blooded, which was based on the absence of nasal turbinates (see Crest functions subsection, above).


Nests and growth

''Hypacrosaurus stebingeri'' laid roughly spherical eggs of , with embryos long. Hatchlings were around long. Studies of lines of growth (i.e. lines of von Ebner) in the teeth of embryonic ''H. stebingeri'' suggests plesiomorphically long incubation times, with a minimum incubation time of 171.4 days for ''H. stebingeri''. 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 An alligator, or colloquially gator, is a large reptile in the genus ''Alligator'' of the Family (biology), family Alligatoridae in the Order (biology), order Crocodilia. The two Extant taxon, extant species are the American alligator (''A. mis ...
and comparable to
ratite Ratites () are a polyphyletic group consisting of all birds within the infraclass Palaeognathae that lack keels and cannot fly. They are mostly large, long-necked, and long-legged, the exception being the kiwi, which is also the only nocturnal ...
growth, for several years, based on the amount of bone growth seen between lines of arrested growth (analogous to
growth ring Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed in a tree. As well as dating them, this can give data for dendroclimatology, the study of climate ...
s in trees). 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. Secondary cartilage has been found in the skull of a hatchling specimen of ''H. stebingeri''.


Cells

In 2020, Alida M. Bailleul and colleagues reported cartilage traces on a hatchling specimen of ''H. stebingeri''. The team performed
histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...
analyses on skull and limb bones of nestling individuals of the specimen MOR 548, a large nesting ground in the
Two Medicine Formation The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between 82.4 Ma and 74.4 Ma, during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky Mountai ...
attributed to ''H. stebingeri'', and the results showed calcified cartilage within a supraoccipital bone, and upon microscopic magnification,
chondrocyte Chondrocytes (, ) are the only cells found in healthy cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans. Although the word '' chondroblast'' is commonly used to describe an immatu ...
-like structures were found. Several of these structures were preserved in the final stages of
mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
, with some preserving putative traces of celular nuclei. Bailleul and colleagues isolated some of these cells in order to be tested with
DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
staining Staining is a technique used to enhance contrast in samples, generally at the Microscope, microscopic level. Stains and dyes are frequently used in histology (microscopic study of biological tissue (biology), tissues), in cytology (microscopic ...
: stains DAPI and PI. They also exposed
emu The emu (; ''Dromaius novaehollandiae'') is a species of flightless bird endemism, endemic to Australia, where it is the Tallest extant birds, tallest native bird. It is the only extant taxon, extant member of the genus ''Dromaius'' and the ...
chondrocytes, and these tied up to DNA fragments. ''H. stebingeri'' cells tested positive to possible chemical markers of DNA, in a similar way to the emu cells, suggesting the potential preservation of this molecule. The team concluded that the find was not a product of fossil contamination, and DNA may last much longer than previously assumed.


Paleopathology

The discovery of tooth marks in the fibula of a ''Hypacrosaurus'' specimen inflicted by a bite from the teeth of a tyrannosaurid indicated that this, and other hadrosaurids were either preyed upon or scavenged by large theropod dinosaurs during the Late Cretaceous period.


Paleoecology


Taphonomy

The large, monospecific assemblage of ''Hypacrosaurus stebingeri'' in Montana was interpreted as a group of dinosaurs that was killed by a volcanic ashfall. This assemblage is considered autochthonous, meaning that the remains are thought to have been buried on or near the same spot where the individuals died. The variety of ages in this group supports that this was a biocoenosis—an actual life assemblage of animals.Behrensmeyer, A. K. 1991. Terrestrial vertebrate accumulations. In Allison P. A. and Briggs D. E. G. (Eds), Taphonomy: Releasing the Data Locked in the Fossil Record. New York: Plenum Press. The cause of death in a volcanic ashfall is suffocation by the ash and by the gases released from volcanic eruptions. The preservation of this diverse group of dinosaurs provides researchers with a growth series, which is a sequence of growth stages from juvenile to adult.


Environment

''H. altispinus'' shared the Horseshoe Canyon Formation with fellow hadrosaurids '' Edmontosaurus'' and '' Saurolophus'', hypsilophodont ''
Parksosaurus ''Parksosaurus'' (meaning "William Parks (paleontologist), William Parks's lizard") is a genus of neornithischian dinosaur from the Maastrichtian, early Maastrichtian-age Upper Cretaceous Horseshoe Canyon Formation of Alberta, Canada. It is based ...
'', ankylosaurid '' Anodontosaurus'', nodosaurid '' Edmontonia'', horned dinosaurs '' Montanoceratops'', '' Anchiceratops'', '' Arrhinoceratops'', and '' Pachyrhinosaurus'', pachycephalosaurid '' Stegoceras'', ostrich-mimics '' Ornithomimus'' and '' Struthiomimus'', a variety of poorly known small
theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...
s including troodontids and dromaeosaurids, and the tyrannosaurid '' Albertosaurus''. 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 Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...
, the shallow sea that covered the midsection of North America through much of the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
. ''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'', dromaeosaurids '' Bambiraptor'' and '' Saurornitholestes'', armored dinosaurs ''Edmontonia'', '' Oohkotokia'', and '' Scolosaurus'', hypsilophodont '' Orodromeus'', hadrosaur '' Prosaurolophus'', and horned dinosaurs '' Achelousaurus'', '' Brachyceratops'', '' Einiosaurus'', and '' Rubeosaurus'' were also present. This formation was more distant from the Western Interior Seaway, higher and drier, with a more terrestrial influence.


See also

* Timeline of hadrosaur research


References


External links


''Hypacrosaurus'', from the Canadian Museum of Nature

''Hypacrosaurus'' in The Natural History Museum's Dino Directory
{{Taxonbar, from=Q311563 Lambeosaurinae Dinosaur genera Maastrichtian dinosaurs Horseshoe Canyon Formation Two Medicine Formation Taxa named by Barnum Brown Taxa named by Jack Horner Taxa named by Philip J. Currie Fossil taxa described in 1913 Fossil taxa described in 1994 Dinosaurs of Canada Dinosaurs of the United States no:Hypacrosaurus