Albertosaurus 1367 226138628 2008-07-17T00:08:21Z J. Spencer 568400 /* Description */ italics {{Taxobox | name = ''Albertosaurus'' | fossil_range = [[Late Cretaceous]] | image = Drumheller 150.jpg | image_caption = ''Albertosaurus'' skeleton at the [[Royal Tyrrell Museum]] in [[Alberta]] | image_width = 200px | regnum = [[Animal]]ia | phylum = [[Chordata]] | classis = [[Sauropsid]]a | superordo = [[Dinosauria]] | ordo = [[Saurischia]] | subordo = [[Theropoda]] | familia = [[Tyrannosauridae]] | genus = '''''Albertosaurus''''' | genus_authority = [[Henry Fairfield Osborn|Osborn]], 1905 | subdivision_ranks = [[Species]] | subdivision = *'''''A. sarcophagus''''' ([[Type (zoology)|type]]) <br/><small>Osborn, 1905</small> | synonyms = *?''[[Gorgosaurus]]'' <small>[[Lawrence Lambe|Lambe]], 1914</small> }} '''''Albertosaurus''''' ({{pronEng|ælˌbɝtoʊˈsɔrəs}}; meaning "Alberta lizard") is a [[genus]] of [[tyrannosaurid]] [[theropod]] [[dinosaur]] that lived in western [[North America]] during the Late [[Cretaceous]] [[Period (geology)|Period]], more than 70&nbsp;[[million years ago]]. The [[type species]], ''A. sarcophagus'', was restricted in [[range (biology)|range]] to the modern-day [[Canada|Canadian]] province of [[Alberta]], after which the genus is named. Scientists disagree on the content of the genus, with some recognizing ''[[Gorgosaurus libratus]]'' as a second species. As a tyrannosaurid, ''Albertosaurus'' was a [[biped]]al [[predator]] with tiny, two-fingered hands and a massive head with dozens of large, sharp teeth. It may have been at the top of the [[food chain]] in its local [[ecosystem]]. Although relatively large for a theropod, ''Albertosaurus'' was much smaller than its more famous relative ''[[Tyrannosaurus]]'', probably weighing less than 2&nbsp;[[metric ton]]s. [[Fossil]]s of more than thirty individuals have been recovered, providing scientists with a more detailed knowledge of ''Albertosaurus'' anatomy than is available for most other tyrannosaurids. The discovery of 22 individuals at one site provides evidence of [[pack hunter|pack]] behavior and allows studies of [[ontogeny]] and [[population biology]] which are impossible with lesser-known dinosaurs. ==Description== [[Image:Albertosaurus.png|thumb|left|''Albertosaurus sarcophagus'' with a human for scale]] ''Albertosaurus'' was smaller than the truly gigantic tyrannosaurids like ''[[Tarbosaurus]]'' and ''Tyrannosaurus''. Typical adults measured up to 9&nbsp;meters (30&nbsp;ft) long,<ref name=russell1970>{{cite_journal |last=Russell |first=Dale A. |authorlink=Dale Russell |year=1970 |title=Tyrannosaurs from the Late Cretaceous of western Canada |journal=National Museum of Natural Sciences Publications in Paleontology |volume=1 |pages=1–34}}</ref><ref name=holtz2004>{{cite_book |last=Holtz |first=Thomas R. |authorlink=Thomas R. Holtz, Jr. |year=2004 |chapter=Tyrannosauroidea |editor= [[David B. Weishampel|Weishampel, David B.]]; [[Peter Dodson|Dodson, Peter]]; & Osmólska, Halszka (eds.). |title=The Dinosauria |edition=Second Edition |publisher=University of California Press |location=Berkeley |pages=111-136 |isbn=0-520-24209-2}}</ref> while rare individuals of great age could grow to over 10&nbsp;meters (33&nbsp;ft) in length.<ref name=ericksonetal2006>{{cite_journal |last=Erickson |first=Gregory M. |coauthors= [[Phil Currie|Currie, Philip. J.]]; Inouye, Brian D.; & Wynn, Alice A. |year=2006 |title=Tyrannosaur life tables: an example of nonavian dinosaur population biology |journal=Science |volume=313 |pages=213–217 |doi=10.1126/science.1125721 |pmid=16840697}}</ref> Several independent mass estimates, obtained by different methods, suggest that an adult ''Albertosaurus'' weighed between 1.3&nbsp;[[tonne]]s (1.4&nbsp;[[short ton]]s)<ref name=ericksonetal2004>{{cite_journal |last=Erickson, Gregory M. |coauthors=Makovicky, Peter J.; [[Phil Currie|Currie, Philip J.]]; Norell, Mark A.; Yerby, Scott A.; & Brochu, Christopher A. |year=2004 |title=Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs |journal=Nature |volume=430 |pages=772–775 |doi=10.1038/nature02699}}</ref> and 1.7&nbsp;tonnes (1.9&nbsp;tons).<ref name=christiansenfarina2004>{{cite_journal |last=Christiansen |first=Per |coauthors=& Fariña, Richard A. |year=2004 |title=Mass prediction in theropod dinosaurs |journal=Historical Biology |volume=16 |issue=2-4 |pages=85–92 |doi=10.1080/08912960412331284313}}</ref> [[Image:AlbertosaurusSarcophagus-PartialJaws RoyalOntarioMuseum.png|thumb|Partial ''Albertosaurus'' jaws, Royal Ontario Museum.]] The massive [[skull]] of ''Albertosaurus'', perched on a short, ''S''-shaped neck, was approximately 1&nbsp;meter (3.3&nbsp;ft) long in the largest adults.<ref name=currie2003a>{{cite_journal|last=Currie |first=Philip J. |authorlink=Phil Currie |year=2003 |title=Allometric growth in tyrannosaurids (Dinosauria: Theropoda) from the Upper Cretaceous of North America and Asia |journal=Canadian Journal of Earth Sciences |volume=40 |issue=4 |pages=651–665 |url=http://www.ingentaconnect.com/content/nrc/cjes/2003/00000040/00000004/art00014 |doi=10.1139/e02-083}}</ref> Wide openings in the skull ([[fenestrae]]) reduced the weight of the head while also providing space for [[muscle]] attachment and [[sensory organ]]s. Its long jaws contained more than 60 [[banana]]-shaped teeth; larger tyrannosaurids possessed fewer teeth. Unlike most theropods, ''Albertosaurus'' and other tyrannosaurids were [[heterodont]], with teeth of different forms depending on their position in the mouth. The [[premaxilla]]ry teeth at the tip of the upper jaw were much smaller than the rest, more closely packed, and ''D''-shaped in [[cross section (geometry)|cross section]].<ref name=holtz2004/> Above the eyes were short bony crests that may have been brightly colored in life and used in courtship to attract a mate.<ref name="ageofdinosaursalbertosaurus" /> All tyrannosaurids, including ''Albertosaurus'', shared a similar body appearance. Typically for a theropod, ''Albertosaurus'' was bipedal and balanced the heavy head and [[torso]] with a long tail. However, tyrannosaurid forelimbs were extremely small for their body size and retained only two digits. The hind limbs were long and ended in a four-toed foot. The first digit, called the [[hallux]], was short and only the other three contacted the ground, with the third (middle) digit longer than the rest.<ref name=holtz2004/> ''Albertosaurus'' may have been able to reach speeds of 25-30 miles per hour.<ref name="ageofdinosaursalbertosaurus">"Albertosaurus." In: Dodson, Peter & Britt, Brooks & Carpenter, Kenneth & Forster, Catherine A. & Gillette, David D. & Norell, Mark A. & Olshevsky, George & Parrish, J. Michael & Weishampel, David B. ''The Age of Dinosaurs''. Publications International, LTD. p. 106-107. ISBN 0-7853-0443-6.</ref> ==Classification and systematics== {{userboxtop|align=right|bordercolor=lightgray|toptext=[[Cladistics#Cladograms|Cladogram]] of [[Tyrannosauridae]] highlighting the position of ''Albertosaurus''}} {{clade| style=font-size:80%;line-height:80% |label1=[[Tyrannosauridae]]&nbsp; |1={{clade |1=Tyrannosaurinae |label2=&nbsp;Albertosaurinae&nbsp; |2={{clade |1='''''Albertosaurus''''' |2=''[[Gorgosaurus]]'' }} }} }} {{userboxbottom}} ''Albertosaurus'' is a member of the theropod [[family (biology)|family]] Tyrannosauridae, in the subfamily Albertosaurinae. Its closest relative is the slightly older ''Gorgosaurus libratus'' (sometimes called ''Albertosaurus libratus''; see below).<ref name=currieetal2003>{{cite_journal |last=Currie |first=Philip J. |coauthors=Hurum, Jørn H; & Sabath, Karol. |authorlink=Phil Currie |year=2003 |title=Skull structure and evolution in tyrannosaurid phylogeny |journal=Acta Palaeontologica Polonica volume=48 |issue=2 |pages=227–234 |url=http://app.pan.pl/acta48/app48-227.pdf |format={{dead link|date=June 2008}} &ndash; <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3ACurrie+intitle%3ASkull+structure+and+evolution+in+tyrannosaurid+phylogeny&as_publication=Acta+Palaeontologica+Polonica+volume%3D48&as_ylo=2003&as_yhi=2003&btnG=Search Scholar search]</sup>}}</ref> These two species are the only described albertosaurines, although other undescribed species may exist.<ref name=currie2003b>{{cite_journal |last=Currie |first=Philip J. |authorlink=Phil Currie |year=2003 |title=Cranial anatomy of tyrannosaurids from the Late Cretaceous of Alberta |journal=Acta Palaeontologica Polonica |volume=48 |issue=2 |pages=191–226 |url=http://app.pan.pl/acta48/app48-191.pdf |format={{dead link|date=June 2008}} &ndash; <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3ACurrie+intitle%3ACranial+anatomy+of+tyrannosaurids+from+the+Late+Cretaceous+of+Alberta&as_publication=Acta+Palaeontologica+Polonica&as_ylo=2003&as_yhi=2003&btnG=Search Scholar search]</sup>}}</ref> [[Thomas R. Holtz, Jr.|Thomas Holtz]] found ''[[Appalachiosaurus]]'' to be an albertosaurine in 2004,<ref name=holtz2004/> but his more recent unpublished work locates it just outside Tyrannosauridae,<ref name=holtzdml2005>{{cite web |url=http://dml.cmnh.org/2005Sep/msg00345.html |title=RE: Burpee Conference (LONG) |accessdate=2007-06-18 |last=Holtz |first=Thomas R. |authorlink=Thomas R. Holtz, Jr. |date=2005-09-20}}</ref> in agreement with other authors.<ref name=carretal2005>{{cite_journal |last=Carr |first=Thomas D. |coauthors=Williamson, Thomas E.; & Schwimmer, David R. |year=2005 |title=A new genus and species of tyrannosauroid from the Late Cretaceous (middle Campanian) Demopolis Formation of Alabama |journal=Journal of Vertebrate Paleontology |volume=25 |issue=1 |pages=119–143 | doi = 10.1671/0272-4634(2005)025[0119:ANGASO]2.0.CO;2}}</ref> The other major subfamily of tyrannosaurids is the Tyrannosaurinae, including ''[[Daspletosaurus]]'', ''[[Tarbosaurus]]'' and ''Tyrannosaurus''. Compared with these robust tyrannosaurines, albertosaurines had slender builds, with proportionately smaller skulls and longer bones of the lower leg ([[tibia]]) and feet ([[metatarsal]]s and [[phalanges]]).<ref name=currie2003a/><ref name=currieetal2003/> ==Discovery and naming== ''Albertosaurus'' was named by [[Henry Fairfield Osborn]] in a very brief note at the end of his 1905 description of ''Tyrannosaurus rex''. The name honors Alberta, the Canadian province in which the first remains were found. The generic name also incorporates the [[Ancient Greek|Greek]] term ''σαυρος''/''sauros'' ("lizard"), the most common suffix in dinosaur names. The type species is ''A. sarcophagus'', which means "flesh-eater" and has the same [[etymology]] as the [[sarcophagus|funeral container]] with which it shares its name: a combination of the Ancient Greek words ''σαρξ''/''sarx'' ("flesh") and ''Φαγειν''/''phagein'' ("to eat").<ref name=osborn1905>{{cite_journal |last=Osborn |first=Henry F. |authorlink=Henry Fairfield Osborn |year=1905 |title=Tyrannosaurus and other Cretaceous carnivorous dinosaurs |journal=Bulletin of the American Museum of Natural History |volume=21 |pages=259–265 |url=http://hdl.handle.net/2246/1464}}</ref> More than thirty specimens of all ages are known to science.<ref name=ericksonetal2006/><ref name=currie2003b/> ===Early discoveries=== [[Image:red deer river.jpg|thumb|right|The [[Red Deer River]] near [[Drumheller]], [[Alberta]]. Almost three-quarters of all ''Albertosaurus'' remains have been discovered alongside the river, in outcrops like the ones on either side of this picture.]] The [[type specimen]] is a partial skull, collected in 1884 from an [[outcrop]] of the [[Horseshoe Canyon Formation]] alongside the [[Red Deer River]] in Alberta. This specimen and a smaller skull associated with some skeletal material were recovered by expeditions of the [[Geological Survey of Canada]], led by the famous [[geologist]] [[Joseph Tyrrell|Joseph B. Tyrrell]]. The two skulls were assigned to the preexisting species "Laelaps incrassatus" by [[Edward Drinker Cope]] in 1892,<ref name=cope1892>{{cite_journal |last=Cope |first=Edward D. |authorlink=Edward Drinker Cope |year=1892 |title=On the skull of the dinosaurian ''Laelaps incrassatus'' Cope |journal=American Philosophical Society, Proceedings |volume=30 |pages=240–245}}</ref> despite the fact that the name ''Laelaps'' was [[ICZN|preoccupied]] by a genus of [[mite]] and had been changed to ''[[Dryptosaurus]]'' in 1877 by [[Othniel Charles Marsh]]. Cope refused to recognize the new name created by his archrival Marsh, so it fell to [[Lawrence Lambe]] to change "Laelaps incrassatus" to ''Dryptosaurus incrassatus'' when he described the remains in detail in 1904.<ref name=lambe1904>{{cite_journal |last=Lambe |first=Lawrence M. |authorlink=Lawrence Lambe |year=1904 |title=On ''Dryptosaurus incrassatus'' (Cope) from the Edmonton Series of the Northwest Territory |journal=Contributions to Canadian Palaeontology |volume=3 |pages=1–27}}</ref> Shortly later, Osborn pointed out that ''D. incrassatus'' was based on generic tyrannosaurid teeth, so the two Alberta skulls could not be confidently referred to that species. The Alberta skulls also differed markedly from the remains of ''D. aquilunguis'', type species of ''Dryptosaurus'', so Osborn created the new name ''Albertosaurus sarcophagus'' for them in 1905. He did not describe the remains in any great detail, citing Lambe's complete description the year before.<ref name=osborn1905/> Both specimens (CMN 5600 and 5601) are stored in the [[Canadian Museum of Nature]] in [[Ottawa]]. ===Dry Island bonebed=== In 1910, American paleontologist [[Barnum Brown]] uncovered the remains of a large group of ''Albertosaurus'' at another [[quarry]] alongside the Red Deer River. Because of the large number of bones and the limited time available, Brown's party did not collect every specimen, but made sure to collect remains from all of the individuals they could identify in the [[bonebed]]. Among the bones deposited in the [[American Museum of Natural History]] collections in [[New York City]] are seven sets of right [[metatarsal]]s, along with two isolated toe bones that did not match any of the metatarsals in size. This indicated the presence of at least nine individuals in the quarry. The [[Royal Tyrrell Museum of Palaeontology]] rediscovered the bonebed in 1997 and resumed fieldwork at the site, which is now located inside [[Dry Island Buffalo Jump Provincial Park]].<ref name=currie1998>{{cite_journal |last=Currie |first=Philip J. |authorlink=Phil Currie |year=1998 |title=Possible evidence of gregarious behavior in tyrannosaurids |journal=Gaia |volume=15 |pages=271–277 |url=http://www.mnhn.ul.pt/geologia/gaia/21.pdf}}(not printed until 2000)</ref> Further excavation from 1997 to 2005 turned up the remains of 13 more individuals of various ages, including a diminutive two-year-old and a very old individual estimated at over 10&nbsp;meters (33&nbsp;ft) in length. None of these individuals are known from complete skeletons, and most are represented by remains in both museums.<ref name=ericksonetal2006/><ref name=ericksonetal2004/> ===''Gorgosaurus libratus''=== In 1913, [[paleontologist]] [[Charles Hazelius Sternberg]] recovered another tyrannosaurid skeleton from the slightly older [[Dinosaur Park Formation]] in Alberta. Lawrence Lambe named this dinosaur ''[[Gorgosaurus]] libratus'' in 1914.<ref name=lambe1914>{{cite_journal |last=Lambe |first=Lawrence M. |authorlink=Lawrence Lambe |year=1914 |title=On a new genus and species of carnivorous dinosaur from the Belly River Formation of Alberta, with a description of the skull of ''Stephanosaurus marginatus'' from the same horizon |journal=Ottawa Naturalist |volume=28 |pages=13–20}}</ref> Other specimens were later found in Alberta and [[Montana]]. Finding few differences to separate the two genera, [[Dale Russell]] declared the name ''Gorgosaurus'' a [[junior synonym]] of ''Albertosaurus'', which had been named first, and ''G. libratus'' was renamed ''Albertosaurus libratus'' in 1970. This addition extended the temporal range of the genus ''Albertosaurus'' backwards by several million years and its geographic range southwards by hundreds of kilometers.<ref name=russell1970/> In 2003, [[Phil Currie]] compared several tyrannosaurid skulls and came to the conclusion that the two species are more distinct than previously thought. The decision to use one or two genera is rather arbitrary, as the two species are [[sister taxa]], more closely related to each other than to any other species. Recognizing this, Currie nevertheless recommended that ''Albertosaurus'' and ''Gorgosaurus'' be retained as separate genera, as they are no more similar than ''Daspletosaurus'' and ''Tyrannosaurus'', which are almost always separated. In addition, several albertosaurine specimens have been recovered from [[Alaska]] and [[New Mexico]], and Currie suggested that the ''Albertosaurus''-''Gorgosaurus'' situation may be clarified once these are described fully.<ref name=currie2003b/> Most authors have followed Currie's recommendation,<ref name=holtz2004/><ref name=ericksonetal2004/><ref name=ricklefs2007>{{cite_journal |last=Ricklefs |first=Robert E. |year=2007 |title=Tyrannosaur ageing |journal=Biology Letters |volume=3 |issue=2 |pages=214–217 |doi=10.1098/rsbl.2006.0597}}</ref> but some have not.<ref name=carretal2005/> ===Other discoveries=== [[William Parks]] described a new species, ''Albertosaurus arctunguis'', based on a partial skeleton excavated near the Red Deer River in 1928,<ref name=parks1928>{{cite_journal |last=Parks |first=William A. |authorlink=William Parks |year=1928 |title=''Albertosaurus arctunguis'', a new species of therapodous dinosaur from the Edmonton Formation of Alberta |journal=University of Toronto Studies, Geological Series |volume=25 |pages=1–42}}</ref> but this species has been considered identical to ''A. sarcophagus'' since 1970.<ref name=russell1970/> Parks' specimen (ROM 807) is housed in the [[Royal Ontario Museum]] in [[Toronto]]. Six more skulls and skeletons have since been discovered in Alberta and are housed in various Canadian museums. Fossils have also been reported from the American states of Montana, New Mexico, and [[Wyoming]], but these probably do not represent ''A. sarcophagus'' and may not even belong to the genus ''Albertosaurus''.<ref name=holtz2004/><ref name=currie2003b/> ''Albertosaurus megagracilis'' was based on a small tyrannosaurid skeleton from the [[Hell Creek Formation]] of Montana.<ref name=paul1988>{{cite_book |last=Paul |first=Gregory S. |authorlink=Gregory S. Paul |year=1988 |title=Predatory Dinosaurs of the World. |publisher=Simon & Schuster |location=New York |pages=464pp |isbn=978-0671619466}}</ref> It was renamed ''[[Dinotyrannus]]'' in 1995,<ref name=olshevsky1995>{{cite_journal |last=Olshevsky |first=George. |authorlink=George Olshevsky |year=1995 |title=(The origin and evolution of the tyrannosaurids.) |journal=Kyoryugaku Saizensen (Dino Frontline) |volume=9 |pages=92–119 |language=Japanese}}</ref> but is now thought to represent a juvenile ''Tyrannosaurus rex''.<ref name=currie2003a/> ==Paleobiology== ===Growth pattern=== [[Image:Tyrantgraph.png|thumb|400px|right|A graph showing the hypothesized growth curves (body mass versus age) of four tyrannosaurids, with ''Albertosaurus'' drawn in <span style="color:red">red</span><ref name=ericksonetal2004/>]] Most age categories of ''Albertosaurus'' are represented in the [[fossil record]]. Using bone [[histology]], the age of an individual animal at the time of death can often be determined, allowing growth rates to be estimated and compared with other species. The youngest known ''Albertosaurus'' is a two-year-old discovered in the Dry Island bonebed, which would have weighed about 50&nbsp;kilograms (110&nbsp;[[pound (mass)|lb]]) and measured slightly more than 2&nbsp;meters (7&nbsp;ft) in length. The 10&nbsp;meter (33&nbsp;ft) specimen from the same quarry is the oldest and largest known, at 28 years of age. When specimens of intermediate age and size are plotted on a graph, an ''S''-shaped growth curve results, with the most rapid growth occurring in a four-year period ending around the sixteenth year of life, a pattern also seen in other tyrannosaurids. The growth rate during this phase was 122&nbsp;kilograms (268&nbsp;lb) per year, based on an adult 1.3&nbsp;[[tonne]]s (1.4&nbsp;[[short ton]]s). Other studies have suggested higher adult weights; this would affect the magnitude of the growth rate but not the overall pattern. Tyrannosaurids similar in size to ''Albertosaurus'' had similar growth rates, although the much larger ''Tyrannosaurus rex'' grew almost five times faster (601&nbsp;kilograms [[1325&nbsp;lb]] per year) at its peak.<ref name=ericksonetal2006/> The end of the rapid growth phase suggests the onset of [[sexual maturity]] in ''Albertosaurus'', although growth continued at a slower rate throughout the animals' lives.<ref name=ericksonetal2006/><ref name=ericksonetal2004/> Sexual maturation while still actively growing appears to be a shared trait among small<ref name=ericksonetal2007>{{cite_journal |last=Erickson |first=Gregory M. |coauthors= Curry Rogers, Kristi; Varricchio, David J.; Norell, Mark.; & Xu, Xing |year=2007 |title=Growth patterns in brooding dinosaurs reveals the timing of sexual maturity in non-avian dinosaurs and genesis of the avian condition |journal=Biology Letters |volume=3 |pages=558–561 |doi=10.1098/rsbl.2007.0254}}</ref> and large<ref name=lee_werning2008>{{cite_journal |last=Lee |first=Andrew H. |coauthors= Werning, Sarah |year=2008 |title=Sexual maturity in growing dinosaurs does not fit reptilian growth models |journal=PNAS |volume=105 |pages=582–587 |doi=10.1073/pnas.0708903105 |pmid=18195356}}</ref> dinosaurs as well as in large mammals such as humans and [[elephant]]s.<ref name=lee_werning2008/> This pattern of relatively early sexual maturation differs strikingly from the pattern in birds, which delay their sexual maturity until after they have finished growing.<ref name=ricklefs2007/><ref name=lee_werning2008/> ===Life history=== [[Image:Mcgill redpath m albertosaurus.jpg|thumb|''Albertosaurus'' skeleton at the [[Redpath Museum]], [[Montreal]]]] Most known ''Albertosaurus'' individuals were aged 14 years or more at the time of death. Juvenile animals are rarely found as fossils for several reasons, mainly [[Biased sample|preservation bias]], where the smaller bones of younger animals were less likely to preserved by fossilization than the larger bones of adults, and collection bias, where smaller fossils are less likely to be noticed by collectors in the field.<ref name=roachbrinkman2007>{{cite_journal |last=Roach |first= Brian T. |coauthors=& Brinkman, Daniel T. |year=2007 |title=A reevaluation of cooperative pack hunting and gregariousness in ''Deinonychus antirrhopus'' and other nonavian theropod dinosaurs |journal=Bulletin of the Peabody Museum of Natural History |volume=48 |issue=1 |pages=103–138 |url=http://www.peabody.yale.edu/scipubs/abstracts/abs_b48-1b.html |doi=10.3374/0079-032X(2007)48[103:AROCPH]2.0.CO;2 |format={{dead link|date=June 2008}} &ndash; <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3A+intitle%3AA+reevaluation+of+cooperative+pack+hunting+and+gregariousness+in+%27%27Deinonychus+antirrhopus%27%27+and+other+nonavian+theropod+dinosaurs&as_publication=Bulletin+of+the+Peabody+Museum+of+Natural+History&as_ylo=2007&as_yhi=2007&btnG=Search Scholar search]</sup>}}</ref> Young ''Albertosaurus'' are relatively large for juvenile animals, but their remains are still rare in the fossil record compared with adults. It has been suggested that this phenomenon is a consequence of [[life history]], rather than bias, and that fossils of juvenile ''Albertosaurus'' are rare because they simply did not die as often as adults did.<ref name=ericksonetal2006/> A hypothesis of ''Albertosaurus'' life history postulates that [[hatchling]]s died in large numbers, but have not been preserved in the fossil record due to their small size and fragile construction. After just two years, juveniles were larger than any other predator in the region aside from adult ''Albertosaurus'', and more fleet of foot than most of their prey animals. This resulted in a dramatic decrease in their mortality rate and a corresponding rarity of fossil remains. Mortality rates doubled at age twelve, perhaps the result of the physiological demands of the rapid growth phase, and then doubled again with the onset of sexual maturity between the ages of fourteen and sixteen. This elevated mortality rate continued throughout adulthood, perhaps due to high physiological demands, stress and injuries received during intraspecific competition for mates and resources, and eventually, the ever-increasing effects of [[senescence]]. The higher mortality rate in adults may explain their more common preservation. Very large animals were rare because few individuals survived long enough to attain such sizes. High infant mortality rates, followed by reduced mortality among juveniles and a sudden increase in mortality after sexual maturity, with very few animals reaching maximum size, is a pattern observed in many modern large mammals, including elephants, [[African buffalo]], and [[rhinoceros]]. The same pattern is also seen in other tyrannosaurids. The comparison with modern animals and other tyrannosaurids lends support to this life history hypothesis, but bias in the fossil record may still play a large role, especially since more than two-thirds of all ''Albertosaurus'' specimens are known from one locality.<ref name=ericksonetal2006/><ref name=ricklefs2007/> ===Pack behavior=== [[Image:Albertosaurus.jpg|thumb|left|''Albertosaurus'' models, [[Royal Tyrell Museum]].]] The Dry Island bonebed discovered by Barnum Brown and his crew contains the remains of 22 ''Albertosaurus'', the most individuals found in one locality of any Cretaceous theropod, and the second-most of any large theropod dinosaur behind the ''[[Allosaurus]]'' assemblage at the [[Cleveland Lloyd Dinosaur Quarry]] in [[Utah]]. The group seems to be composed of one very old adult; eight adults between 17 and 23 years old; seven sub-adults undergoing their rapid growth phases at between 12 and 16 years old; and six juveniles between the ages of 2 and 11 years, who had not yet reached the growth phase.<ref name=ericksonetal2006/> The near-absence of [[herbivore]] remains and the similar state of preservation between the many individuals at the ''Albertosaurus'' bonebed quarry led Phil Currie to conclude that the locality was not a predator trap like the [[La Brea Tar Pits]] in [[California]], and that all of the preserved animals died at the same time. Currie claims this as evidence of pack behavior.<ref name="currie1998"/> Other scientists are skeptical, observing that the animals may have been driven together by drought, flood or for other reasons.<ref name=ericksonetal2006/><ref name=roachbrinkman2007/><ref name=eberthmccrea2001>{{cite_journal |last=Eberth |first=David A. |coauthors=& McCrea, Richard T. |year=2001 |title=Were large theropods gregarious? |journal=Journal of Vertebrate Paleontology |volume=21 |issue=(Supplement to 3 - Abstracts of Papers, 61st Annual Meeting of the Society of Vertebrate Paleontology) |pages=46A}}(published abstract only)</ref> There is abundant evidence for gregarious behavior among herbivorous dinosaurs, including [[ceratopsia]]ns and [[hadrosaur]]s.<ref name=horner1997>{{cite_book |last=Horner |first=John R. |authorlink=Jack Horner (paleontologist) |year=1997 |chapter=Behavior |editor=[[Phil Currie|Currie, Philip J.]]; & Padian, Kevin. (eds.). |title=Encyclopedia of Dinosaurs |publisher=Academic Press |location=San Diego |pages=45–50 |isbn=0-12-226810-5}}</ref> However, only rarely are so many dinosaurian predators found at the same site. Small theropods like ''[[Deinonychus]]'',<ref name=maxwellostrom1995>{{cite journal |last=Maxwell |first=W. Desmond |coauthors=& [[John Ostrom|Ostrom, John H.]] |year=1995 |title=Taphonomy and paleobiological implications of ''Tenontosaurus''-''Deinonychus'' associations |journal=Journal of Vertebrate Paleontology |volume=15 |issue=4 |pages=707–712}}</ref> ''[[Coelophysis]]'' and ''[[Megapnosaurus|Megapnosaurus (Syntarsus) rhodesiensis]]''<ref name=raath1990>{{cite_book |last=Raath |first=Michael A. |year=1990 |chapter=Morphological variation in small theropods and its meaning in systematics: evidence from ''Syntarsus rhodesiensis'' |editor=[[Kenneth Carpenter|Carpenter, Kenneth]] & [[Phil Currie|Currie, Philip J.]] |title=Dinosaur Systematics: Approaches and Perspectives |publisher=Cambridge University Press |location=Cambridge |pages=91–105 |isbn=0-521-43810-1}}</ref> have been found in aggregations, as have larger predators like ''Allosaurus'' and ''[[Mapusaurus]]''.<ref name=coriacurrie2006>{{cite_journal |last=Coria |first=Rodolfo A. |authorlink=Rodolfo Coria |coauthors=& [[Phil Currie|Currie, Philip J.]] |year=2006 |title=A new carcharodontosaurid (Dinosauria, Theropoda) from the Upper Cretaceous of Argentina |journal=Geodiversitas |volume=28 |issue=1 |pages=71–118 |url=http://www.mnhn.fr/museum/front/medias/publication/7653_g06n1a4.pdf}}</ref> There is some evidence of gregarious behavior in other tyrannosaurids as well. Fragmentary remains of smaller individuals were found alongside "Sue," the ''Tyrannosaurus'' mounted in the [[Field Museum of Natural History]] in [[Chicago]], and a bonebed in the [[Two Medicine Formation]] of Montana contains at least three specimens of ''Daspletosaurus'', preserved alongside several hadrosaurs.<ref name=currieetal2005>{{cite_book |last=Currie |first=Philip J. |authorlink=Phil Currie |coauthors=Trexler, David; [[Eva Koppelhus|Koppelhus, Eva B.]]; Wicks, Kelly; & Murphy, Nate. |year=2005 |chapter=An unusual multi-individual tyrannosaurid bonebed in the Two Medicine Formation (Late Cretaceous, Campanian) of Montana (USA) |editor=[[Kenneth Carpenter|Carpenter, Kenneth]] (ed.). |title=The Carnivorous Dinosaurs |publisher=Indiana University Press |location=Bloomington |pages=313–324 |isbn=978-0253345394}}</ref> These findings may corroborate the evidence for social behavior in ''Albertosaurus'', although some or all of the above localities may represent temporary or unnatural aggregations.<ref name=currie1998/> Others have speculated that instead of social groups, at least some of these finds represent [[Komodo dragon]]-like mobbing of carcasses, where aggressive competition leads to some of the predators being killed and [[cannibal]]ized.<ref name=roachbrinkman2007/> Currie also offers speculation on the pack-hunting habits of ''Albertosaurus''. The leg proportions of the smaller individuals were comparable to those of [[Ornithomimid]]s, which were probably among the fastest dinosaurs. Younger ''Albertosaurus'' were probably equally fleet-footed, or at least faster than their prey. Currie hypothesized that the younger members of the pack may have been responsible for driving their prey towards the adults, who were larger and more powerful, but also slower.<ref name=currie1998/> Juveniles may also have had different lifestyles than adults, filling predator [[Ecological niche|niche]]s between the enormous adults and the smaller contemporaneous theropods, the largest of which were two [[orders of magnitude]] smaller than adult ''Albertosaurus'' in mass.<ref name=holtz2004/> A similar situation is observed in modern Komodo dragons, with hatchlings beginning life as small [[insectivore]]s before growing to become the dominant predators on their islands.<ref name=auffenberg1981>{{cite_book |last=Auffenberg |first=Walter |year=2000 |title=The Behavioral Ecology of the Komodo Monitor |publisher=University Press of Florida |location=Gainesville |pages=460pp. |isbn=978-0813006215}}</ref> However, as the preservation of behavior in the fossil record is exceedingly rare, these ideas cannot readily be tested. ==Paleoecology== [[Image:Horseshoe Canyon.jpg|thumb|right|The [[Horseshoe Canyon Formation]] is exposed in its [[Type locality (geology)|type section]] at [[Horseshoe Canyon (Alberta)|Horseshoe Canyon]], [[Alberta]].]] All identifiable fossils of ''Albertosaurus sarcophagus'' are known from the Horseshoe Canyon Formation in Alberta. This [[geologic formation]] dates to the early [[Maastrichtian]] stage of the [[Late Cretaceous]] Period, 73 to 70 [[Mega-annum|Ma]] (million years ago). Immediately below this formation is the [[Bearpaw Shale]], a [[Marine (ocean)|marine]] formation representing a section of the [[Western Interior Seaway]]. The seaway was receding as the climate cooled and sea levels subsided towards the end of the Cretaceous, exposing land that had previously been underwater. It was not a smooth process, however, and the seaway would periodically rise to cover parts of the region throughout Horseshoe Canyon times before finally receding altogether in the years after. Due to the changing sea levels, many different environments are represented in the Horseshoe Canyon Formation, including offshore and near-shore marine habitats and coastal habitats like [[lagoon]]s, [[estuary|estuaries]] and [[tidal flat]]s. Numerous [[coal]] seams represent ancient [[peat]] [[swamp]]s. Like most of the other [[vertebrate]] fossils from the formation, ''Albertosaurus'' remains are found in deposits laid down in the [[river delta|deltas]] and [[floodplain]]s of large rivers during the later half of Horseshoe Canyon times.<ref name=eberth1997>{{cite_book |last=Eberth |first=David A. |year=1997 |chapter=Edmonton Group |editor=[[Phil Currie|Currie, Philip J.]] & Padian, Kevin (eds.). |title=Encyclopedia of Dinosaurs |publisher= Academic Press |location=San Diego |pages=199-204 |isbn=0-12-226810-5}}</ref> The [[fauna]] of the Horseshoe Canyon Formation is well-known, as vertebrate fossils, including those of dinosaurs, are quite common. [[Shark]]s, [[Batoidea|rays]], [[sturgeon]]s, [[bowfin]]s, [[gar]]s and the gar-like ''[[Aspidorhynchus]]'' made up the fish fauna. Mammals included [[multituberculate]]s and the [[marsupial]] ''[[Didelphodon]]''. The saltwater [[plesiosaur]] ''[[Leurospondylus]]'' has been found in marine sediments in the Horseshoe Canyon, while freshwater environments were populated by [[turtle]]s, ''[[Champsosaurus]]'', and [[crocodilia]]ns like ''[[Leidyosuchus]]'' and ''[[Stangerochampsa]]''. Dinosaurs dominate the fauna, especially hadrosaurs, which make up half of all dinosaurs known, including the genera ''[[Edmontosaurus]]'', ''[[Saurolophus]]'' and ''[[Hypacrosaurus]]''. Ceratopsians and [[ornithomimid]]s were also very common, together making up another third of the known fauna. Along with much rarer [[Ankylosauria]]ns and [[pachycephalosaur]]s, all of these animals would have been prey for a diverse array of carnivorous theropods, including [[troodontid]]s, [[dromaeosaurid]]s, and [[caenagnathid]]s. Adult ''Albertosaurus'' were the [[apex predator]]s in this environment, with intermediate niches possibly filled by juvenile albertosaurs.<ref name=eberth1997/> ==References== {{reflist|2}} ==External links== {{commons|Albertosaurus}} {{portalpar|Dinosaurs}} *Some entries in [http://paleo.amnh.org/fossil/seek.html Online Collections Database] at the American Museum of Natural History: **[http://paleo.amnh.org/fossil/show.html?cat_num=FR%205218 AMNH 5218] (numerous pictures of bones included) **[http://paleo.amnh.org/fossil/show.html?cat_num=FR%205226 AMNH 5226] (pictures of a series of tail vertebrae included) *[http://web.archive.org/web/20070125140546/staff.washington.edu/eoraptor/Tyrannosauroidea.html#Albertosaurussarcophagus List of specimens] from The Theropod Database. *[http://web.archive.org/web/20070108133913/http://www.paleograveyard.com/albertosaurus.html Skeletal images] at The Graveyard. *[http://gsc.nrcan.gc.ca/paleochron/31_e.php A brief history] of Joseph B. Tyrrell and his discovery of the [[holotype]] skull of ''Albertosaurus sarcophagus''. *[http://www.nm.blm.gov/features/dino_lift/dino_lift.html Operation Dino-Lift], detailing the discovery, excavation, and preparation of a possible ''Albertosaurus'' from [[Bureau of Land Management]] land in New Mexico. {{featured article}} [[Category:Cretaceous dinosaurs]] [[Category:Dinosaurs of North America]] [[Category:Tyrannosaurs]] {{Link FA|pt}} {{Link FA|sv}} [[cs:Albertosaurus]] [[de:Albertosaurus]] [[eo:Albertosaŭro]] [[es:Albertosaurus]] [[fi:Albertosaurus]] [[fr:Albertosaurus]] [[he:אלברטוזאור סרקופגוס]] [[hu:Albertosaurus]] [[it:Albertosaurus sarcophagus]] [[ja:アルバートサウルス]] [[lv:Albertozaurs]] [[lt:Albertozauras]] [[nl:Albertosaurus]] [[pl:Albertozaur]] [[pt:Albertossauro]] [[ru:Альбертозавр]] [[simple:Albertosaurus]] [[sk:Albertosaurus]] [[sv:Albertosaurus sarcophagus]] [[zh:艾伯塔龙]]