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Allosaurus

Skelettrekonstruktion von Allosaurus fragilis

Zeitliches Auftreten
Oberjura (Kimmeridgium bis Tithonium)
155,6 bis 145,5 Mio. Jahre
Fundorte
Systematik
Dinosaurier (Dinosauria)
Echsenbeckensaurier (Saurischia)
Theropoda
Carnosauria
Allosauridae
Allosaurus
Wissenschaftlicher Name
Allosaurus
Marsh, 1877
Arten
  • Allosaurus fragilis Marsh, 1877
  • ? Allosaurus maximus Chure, 1995
  • ? Allosaurus tendagurensis Janensch, 1925

Allosaurus war eine Gattung großer, theropoder Dinosaurier, welche vor 155 bis 145 Millionen Jahren im späten Jura lebte. Die ersten Allosaurus zugeordneten Funden wurden 1877 von Othniel Charles Marsh beschrieben. Er war ein großer, zweibeiniger (bipeder) Fleischfresser, mit einem großen Schädel und zahlreichen Reißzähnen. Allosaurus wurde im Schnitt 8,5 Meter lang, außergewöhnlich große Individuen könnten 12 Meter Länge erreicht haben. Im Vergleich zu seinen kräftig gebauten Beinen waren seine Arme mit drei Fingern klein, der Körper und Schädel wurden durch einen langen, schweren Schwanz ausbalanciert.

Er wird der Allosauridae zugeordnet, eine Gruppe von Carnosauriern innerhalb der Theropoda. Die interne Systematik von Allosaurus ist kompliziert, zahlreiche Arten sind womöglich nicht valide. Als gesichert gilt die Typusart Allosaurus fragilis.

Der Großteil der Allosaurus-Fossilien kommt aus der nordamerikanischen Morrison-Formation, eventuell ist auch Material aus Tansania und Portugal Allosaurus zuzordnen. Als größter Räuber der Morrison-Formation war er am Ende der Nahrungskette, und machte wohl Jagd auf große pflanzenfressende Dinosaurier, unter anderem Ornithopoden, Stegosaurier und Sauropoden. Einige Paläontologen vermuten, dass Allosaurus im Rudel jagte, während andere meinen, Allosaurus war innerartlich aggressiv.

Als einer der ersten, gut erforschten Raubdinosaurier ist Allosaurus einer der bekanntesten Dinosaurier, und kommt in etlichen Filmen und Dokumentationen vor.

Körperbau

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Verschieden große Allosaurus und das mögliche Synonym Epanterias im Vergleich mit einem Menschen

Allosaurus war ein typischer, großer Raubdinosaurier, mit kräftig gebautem Schädel, kurzem Hals, einem langen Schwanz und kurzen Armen. Allosaurus fragilis, die am besten bekannte Art, wurde im Schnitt 8,5 Meter lang [1], das nachweislich größte bekannte Exemplar maß 9,7 Meter[2] und wird auf ein Gewicht von 2,3 Tonnen geschätzt.[2]. Einzelne, ungewöhnlich große Knochen werdem manchmal als Überreste zwölf bis 13 Meter langer Tiere gedeutet.[3] Gewichtsangaben sind Gegenstand heftiger Diskussionen, seit 1980 wurden Angaben zwischen 1000 und 4000 Kilogramm gemacht.[4] John Foster, ein Spezialist über die Morrison-Formation meint, dass 1000 Kilogramm für ausgewachsene Exemplare wahrscheinlich sein, für die durchschnittlich großen Exemplare seien jedoch 700 Kilogramm realistischer.[5] Anhand von Computermodellen eines subadulten Allosaurus wurde jüngst ein Gewicht von 1500 Kilogramm angegeben, doch das Ergebniss variiert je nach Parametern von 1400 bis 2000 Kilogramm.[6]

Eine Reihe Fossilien großer Theropoden wurde Allosaurus zugeschrieben, sie gehören jedoch größtenteils zu anderen Gatungen. Der nahe verwandte Saurophaganax wurde womöglich 11 Meter lang[2], und gelegentlich wurde die einzige Art von Saurophaganax als Allosaurus maximus bezeichnet. Neuere Studien zeigen, dass er einer separaten Gattung angehört.[7] Ein anderes mögliches Allosaurus-Exemplar, welches oft Epanterias zugeordnet wurde, war 12,1 Meter lang.[2] Eine neuere Entdeckung stammt aus dem Peterson-Quarry der Morrison-Formation; der große Allosauride ist wahrscheinlich ebenfalls Saurophaganax zugehörig.[8]

Schädel

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Schädel im Oklahoma Museum of Natural History

Schädel und Zähne von Allosaurus hatte eine für Theropoden seiner Größe normal großen Schädel, er wurde für ein 7,9 Meter langes Exemplars mit 84,5 Zentimetern Länge angegeben.[9] Jedes Prämaxillare (an der Schnauze gelegene Schädelknochen) hatte fünf Zähne mit mit D-förmigem Profil, jedes Maxillare (Knochen des Oberkiefers) und Mandibula (Knochen des Unterkiefers) trug 14 bis 17 Zähne. Die Zähne sind gesägt und wurden in Richtung Hals immer kürzer, schmaler und stärker gekrümmte. Sie fielen leicht aus und wurden kontinuierlich ersetzt, daher sind sie häufig zu finden.[3]

Über den Augen befand sich je ein auffälliger Knochenkamm. Sie wurden von den Tränenbeinen (Lacrimale) gebildet und variierten in Größe und Form.[3] Womöglich waren sie von Keratin bedeckt und erfüllten eine Reihe verschiedener Funktionen, eventuell als Sonnenblende für die Augen[3] oder für eine Form des Ausdrucksverhaltens.[9][10] Ein Knochenkamm, der sich von der Schnauze bis zur Stin zug, diente als Ansatzpunkt für Muskeln, wie bei Tyrannosauriden.[9]

In den Tränenbeinen fanden sich Vertiefungen, die womöglich Drüsen (etwa Salzdrüsen) enthielten[11] Im Maxillare fanden sich Kieferhöhlen, welche wohl mit dem Geruchssinn in Verbindung standen; vielleicht enthielten sie ein spezialisiertes Organ. Das Dach des Hirnschädels war dünn, womöglich eine Anpassung an die Thermoregulation des Hirns..[3] Schädel und Unterkiefer waren durch Gelenke verbunden; im Unterkiefer waren vordere und hintere Hälfte lose verbunden. Dies erlaubte dem Unterkiefer, sich nach außen zu verbiegen und das Maul weiter zu öffnen. [12]

Postcranial skeleton

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Allosaurus-Skelett

Allosaurus hatte neun Halswirbel, 14 Rückenwirbel, und fünf Sakralwirbel zur Unterstützung des Beckens.[13] Die genaue Zahl der Schnwanzwirbel ist nicht bekannt, da sie stark mit der Individualgröße variierte, meist werden 45 bis 50 Stück angegeben.[3][9]


There were hollow spaces in the neck and anterior back vertebrae.[3] Such spaces, which are also found in modern theropods (that is, the birds), are interpreted as having held air sacs used in respiration.[14] The rib cage was broad, giving it a barrel chest, especially in comparison to less derived theropods like Ceratosaurus.[15] Allosaurus had gastralia (belly ribs), but these are not common findings,[3] and they may have ossified poorly.[9] In one published case, the gastralia show evidence of injury during life.[16] A furcula (wishbone) was also present, but has only been recognized since 1996; in some cases furculae were confused with gastralia.[16][17] The ilium, the main hip bone, was massive, and the pubic bone had a prominent foot that may have been used for both muscle attachment and as a prop for resting the body on the ground. Madsen noted that in about half of the individuals from the Cleveland Lloyd Dinosaur Quarry, independent of size, the pubes had not fused to each other at their foot ends. He suggested that this was a sexual characteristic, with females lacking fused bones to make egg-laying easier.[3] This proposal has not attracted further attention, however.

Hand and claws of A. fragilis

The forelimbs of Allosaurus were short in comparison to the hindlimbs (only about 35% the length of the hindlimbs in adults)[18] and had three fingers per hand, tipped with large, strongly curved and pointed claws.[3] The arms were powerful,[9] and the forearm was somewhat shorter than the upper arm (1:1.2 Oberarmknochen/ulna ratio).[19] The wrist had a version of the semilunate carpal[20] also found in more derived theropods like maniraptorans. Of the three fingers, the innermost (or thumb) was the largest,[9] and diverged from the others.[19] The legs were not as long or suited for speed as those of tyrannosaurids, and the claws of the toes were less developed and more hoof-like than those of earlier theropods.[9] Each foot had three weight-bearing toes and an inner dewclaw, which Madsen suggested could have been used for grasping in juveniles.[3] There was also what is interpreted as the splint-like remnant of a fifth (outermost) metatarsal, perhaps used as a lever between the Achillessehne and foot.[21]

Classification

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Life restoration of Allosaurus fragilis.

Allosaurus was an allosaurid, a member of a family of large theropods within the larger group Carnosauria. The family name Allosauridae was created for this genus in 1878 by Othniel Charles Marsh,[22] but the term was largely unused until the 1970s in favor of Megalosauridae, another family of large theropods that eventually became a wastebasket taxon. This, along with the use of Antrodemus for Allosaurus during the same period, is a point that needs to be remembered when searching for information on Allosaurus in publications that predate James Madsen's 1976 monograph. Major publications using the name Megalosauridae instead of Allosauridae include Gilmore, 1920,[19] von Huene, 1926,[23] Romer, 1956 and 1966,[24][25] Steel, 1970,[26] and Walker, 1964.[27]

Following the publication of Madsen's influential monograph, Allosauridae became the preferred family assignment, but it too was not strongly defined. Semi-technical works used Allosauridae for a variety of large theropods, usually those that were larger and better-known than megalosaurids. Typical theropods that were thought to be related to Allosaurus included Indosaurus, Piatnitzkysaurus, Piveteausaurus, Yangchuanosaurus,[28] Acrocanthosaurus, Chilantaisaurus, Compsosuchus, Stokesosaurus, and Szechuanosaurus.[29] Given modern knowledge of theropod diversity and the advent of cladistic study of evolutionary relationships, none of these theropods is now recognized as an allosaurid, although several, like Acrocanthosaurus and Yangchuanosaurus, are members of closely related families.[14]

Allosauridae is one of four families in Carnosauria; the other three are Neovenatoridae,[30] Carcharodontosauridae and Sinraptoridae.[14] Allosauridae has at times been proposed as ancestral to the Tyrannosauridae (which would make it paraphyletic), one recent example being Gregory S. Paul's Predatory Dinosaurs of the World,[31] but this has been rejected, with tyrannosaurids identified as members of a separate branch of theropods, the Coelurosauria.[32] Allosauridae is the smallest of the carnosaur families, with only Saurophaganax and a currently unnamed French allosauroid accepted as possible valid genera besides Allosaurus in the most recent review.[14] Another genus, Epanterias, is a potential valid member, but it and Saurophaganax may turn out to be large examples of Allosaurus.[9] Recent reviews have kept the genus Saurophaganax and included Epanterias with Allosaurus.[4][14]

Discovery and history

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Early discoveries and research

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The discovery and early study of Allosaurus is complicated by the multiplicity of names coined during the Bone Wars of the late 19th century. The first described fossil in this history was a bone obtained secondhand by Ferdinand Vandiveer Hayden in 1869. It came from Middle Park, near Granby, Colorado, probably from Morrison Formation rocks. The locals had identified such bones as "petrified horse hoofs". Hayden sent his specimen to Joseph Leidy, who identified it as half of a tail vertebra, and tentatively assigned it to the European dinosaur genus Poekilopleuron as Poicilopleuron [sic] valens.[33] He later decided it deserved its own genus, Antrodemus.[34]

The skeletal mount of AMNH 5753, posed as scavenging an Apatosaurus.
AMNH 5753 in a Charles R. Knight life restoration.

Allosaurus itself is based on YPM 1930, a small collection of fragmentary bones including parts of three vertebrae, a rib fragment, a tooth, a toe bone, and, most useful for later discussions, the shaft of the right humerus (upper arm). Othniel Charles Marsh gave these remains the formal name Allosaurus fragilis in 1877. Allosaurus comes from the Greek allos/αλλος, meaning "strange" or "different" and sauros/σαυρος, meaning "lizard" or "reptile".[35] It was named 'different lizard' because its vertebrae were different from those of other dinosaurs known at the time of its discovery.[36][37] The species epithet fragilis is Latin for "fragile", referring to lightening features in the vertebrae. The bones were collected from the Morrison Formation of Garden Park, north of Cañon City.[36] Marsh and Edward Drinker Cope, who were in scientific competition, went on to coin several other genera based on similarly sparse material that would later figure in the taxonomy of Allosaurus. These include Marsh's Creosaurus[22] and Labrosaurus,[38] and Cope's Epanterias.[39]

In their haste, Cope and Marsh did not always follow up on their discoveries (or, more commonly, those made by their subordinates). For example, after the discovery by Benjamin Mudge of the type specimen of Allosaurus in Colorado, Marsh elected to concentrate work in Wyoming; when work resumed at Garden Park in 1883, M. P. Felch found an almost complete Allosaurus and several partial skeletons.[11] In addition, one of Cope's collectors, H. F. Hubbell, found a specimen in the Como Bluff area of Wyoming in 1879, but apparently did not mention its completeness, and Cope never unpacked it. Upon unpacking in 1903 (several years after Cope had died), it was found to be one of the most complete theropod specimens then known, and in 1908 the skeleton, now cataloged as AMNH 5753, was put on public view.[40] This is the well-known mount poised over a partial Apatosaurus skeleton as if scavenging it, illustrated as such by Charles R. Knight. Although notable as the first free-standing mount of a theropod dinosaur, and often illustrated and photographed, it has never been scientifically described.[41]

Mounted A. fragilis skeleton (USNM4734) at the National Museum of Natural History

The multiplicity of early names complicated later research, with the situation compounded by the terse descriptions provided by Marsh and Cope. Even at the time, authors such as Samuel Wendell Williston suggested that too many names had been coined.[42] For example, Williston pointed out in 1901 that Marsh had never been able to adequately distinguish Allosaurus from Creosaurus.[43] The most influential early attempt to sort out the convoluted situation was produced by Charles W. Gilmore in 1920. He came to the conclusion that the tail vertebra dubbed Antrodemus by Leidy was indistinguishable from those of Allosaurus, and Antrodemus thus should be the preferred name because as the older name it had priority.[19] Antrodemus became the accepted name for this familiar genus for over fifty years, until James Madsen published on the Cleveland-Lloyd specimens and concluded that Allosaurus should be used because Antrodemus was based on material with poor, if any, diagnostic features and locality information (for example, the geological formation that the single bone of Antrodemus came from is unknown).[3] "Antrodemus" has been used informally for convenience when distinguishing between the skull Gilmore restored and the composite skull restored by Madsen.[44]

Cleveland-Lloyd discoveries

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Although sporadic work at what became known as the Cleveland-Lloyd Dinosaur Quarry in Emery County, Utah had taken place as early as 1927, and the fossil site itself described by William J. Stokes in 1945,[45] major operations did not begin there until 1960. Under a cooperative effort involving nearly 40 institutions, thousands of bones were recovered between 1960 and 1965.[3] The quarry is notable for the predominance of Allosaurus remains, the condition of the specimens, and the lack of scientific resolution on how it came to be. The majority of bones belong to the large theropod Allosaurus fragilis (it is estimated that the remains of at least 46 A. fragilis have been found there, out of at minimum 73 dinosaurs), and the fossils found there are disarticulated and well-mixed. Nearly a dozen scientific papers have been written on the taphonomy of the site, coming up with numerous contradictory explanations for how it formed. Suggestions have ranged from animals getting stuck in a bog, to becoming trapped in deep mud, to falling victim to drought-induced mortality around a waterhole, to getting trapped in a spring-fed pond or seep.[46] Regardless of the actual cause, the great quantity of well-preserved Allosaurus remains has allowed this genus to be known in detail, making it among the best-known theropods. Skeletal remains from the quarry pertain to individuals of almost all ages and sizes, from less than 1 meter (3.3 ft)[47] to 12 meters (39 ft) long, and the disarticulation is an advantage for describing bones usually found fused.[3]

Recent work: 1980s–present

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The period since Madsen's monograph has been marked by a great expansion in studies dealing with topics concerning Allosaurus in life (paleobiological and paleoecological topics). Such studies have covered topics including skeletal variation,[48] growth,[49][50] skull construction,[51] hunting methods,[52] the brain,[53] and the possibility of gregarious living and parental care.[54] Reanalysis of old material (particularly of large 'allosaur' specimens),[9][55] new discoveries in Portugal,[56] and several very complete new specimens[16][57][58] have also contributed to the growing knowledge base. Fossil footprints attributed to Allosaurus were discovered in Bałtów, Poland, by Polish paleontologist Gerard Gierliński in the early 21st century.[59]

"Big Al"

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Cast of "Big Al Two", HMNS

One of the more significant Allosaurus finds was the 1991 discovery of "Big Al" (MOR 693), a 95% complete, partially articulated specimen that measured about 8 meters (about 26 ft) in length. MOR 693 was excavated near Shell, Wyoming, by a joint Museum of the Rockies and University of Wyoming Geological Museum team.[60] This skeleton was discovered by a Swiss team, led by Kirby Siber. The same team later excavated a second Allosaurus, "Big Al Two", which is the best preserved skeleton of its kind to date.[58]

The completeness, preservation, and scientific importance of this skeleton gave "Big Al" its name; the individual itself was below the average size for Allosaurus fragilis,[60] and was a subadult estimated at only 87% grown.[61] The specimen was described by Breithaupt in 1996.[57] Nineteen of its bones were broken or showed signs of infection, which may have contributed to "Big Al's" death. Pathologic bones included five ribs, five vertebrae, and four bones of the feet; several damaged bones showed osteomyelitis, a bone infection. A particular problem for the living animal was infection and trauma to the right foot that probably affected movement and may have also predisposed the other foot to injury because of a change in gait.[61]

Species and taxonomy

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Vorlage:Main article

Allosaurus sp. skull (DINO 11541) from Dinosaur National Monument, still partially encased in matrix.

It is unclear how many species of Allosaurus there were. Seven species have been considered potentially valid since 1988 (A. amplexus,[9] A. atrox,[9] A. europaeus,[62] the type species A. fragilis,[14] the as-yet not formally described "A. jimmadseni",[7] A. maximus,[48] and A. tendagurensis[14]), although only a fraction are usually considered valid at any given time. Additionally, there are at least ten dubious or undescribed species that have been assigned to Allosaurus over the years, along with the species belonging to genera now sunk into Allosaurus. In the most recent review of basal tetanuran theropods, only A. fragilis (including A. amplexus and A. atrox as synonyms), "A. jimmadseni" (as an unnamed species), and A. tendagurensis were accepted as potentially valid species, with A. europaeus not yet proposed and A. maximus assigned to Saurophaganax.[14]

Saurophaganax or A. maximus and Apatosaurus

A. amplexus, A. atrox, A. fragilis, "A. jimmadseni", and A. maximus are all known from remains discovered in the Kimmeridgian–Tithonian Upper Jurassic-age Morrison Formation of the United States, spread across the states of Colorado, Montana, New Mexico, Oklahoma, South Dakota, Utah, and Wyoming. A. fragilis is regarded as the most common, known from the remains of at least sixty individuals.[14] Debate has gone on since the 1980s regarding the possibility that there are two common Morrison Formation species of Allosaurus, with the second known as A. atrox;[9][63] recent work has followed a "one species" interpretation,[14] with the differences seen in the Morrison Formation material attributed to individual variation.[64][65] A. europaeus was found in the Kimmeridgian-age Porto Novo Member of the Lourinhã Formation,[62] but may be the same as A. fragilis.[66] A. tendagurensis was found in Kimmeridgian-age rocks of Tendaguru, in Mtwara, Tanzania.[67] Although the most recent review tentatively accepted it as a valid species of Allosaurus, it may be a more basal tetanuran,[68] or simply a dubious theropod.[1] Although obscure, it was a large theropod, possibly around 10 meters long (33 ft) and 2.5 metric tons (2.8 short tons) in weight.[2]

A. tendagurensis tibia, Naturkunde Museum Berlin

Allosaurus is regarded as a probable synonym of the genera Antrodemus, Creosaurus, Epanterias, and Labrosaurus.[14] Most of the species that are regarded as synonyms of A. fragilis, or that were misassigned to the genus, are obscure and were based on scrappy remains. One exception is Labrosaurus ferox, named in 1884 by Marsh for an oddly formed partial lower jaw, with a prominent gap in the tooth row at the tip of the jaw, and a rear section greatly expanded and turned down.[69] Later researchers suggested that the bone was pathologic, showing an injury to the living animal,[19] and that part of the unusual form of the rear of the bone was due to plaster reconstruction.[70] It is now regarded as an example of A. fragilis.[14] Other remains thought to pertain to Allosaurus have come from across the world, including Australia,[71] Siberia,[72] and Switzerland,[1] but these fossils have been reassessed as belonging to other dinosaurs.

The issue of synonyms is complicated by the type specimen of Allosaurus fragillis (catalogue number YPM 1930) being extremely fragmentary, consisting of a few incomplete vertebrae, limb bone fragments, rib fragments, and a tooth. Because of this, several scientists have noted that the type specimen, and thus the genus Allosaurus itself or at least the species A. fragillis, is technically a nomen dubium ("dubious name", based on a specimen too incomplete to compare to other specimens or to classify). In an attempt to fix this situation, Gregory S. Paul and Kenneth Carpenter (2010) submitted a petition to the ICZN to have the name A. fragillis officially transferred to a more complete specimen (as a neotype). This request is currently pending review.[73]

Paläoökologie

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Locations in the Morrison Formation (yellow) where Allosaurus remains have been found.

Allosaurus war der häufigste, große Raubdinosaurier der Morrison-Formation, 75 % aller Funde von Theropoden sind ihm zuzuordnen[5]. Er war wohl der Spitzenprädator seiner Zeit, und stand am oberen Ende der Nahrungskette.[74] Die Morrison-Formation war im späten Jura eine semi-aride Landschaft mit Flussauen, und ausgeprägten Wechsel zwischen Regen- und Trockenzeit.[75], die Vegetation bestand aus Wäldern von Koniferen und Baumfarnen and Flussufern und Farnsavannen mit wenigen Bäumen.[76]

Die Morrison-Formation ist reich an Fossilien. Die Flora bestandt aus Schachtelhalmen, Farnen, Palmfarnen, Ginkgogewächsen und diversen Koniferen. An kleineren Landwirbeltieren fanden Paläontologen Frösche, Schwanzlurche, Schildkröten, Brückenechsen, Echsen, Krokodile, verschiedene Flugsaurier, und frühe Vorfahren der Säugetiere wie Docodonten, Multituberculaten, Symmetrodonten und Triconodonten. Theropoden aus der Morrison-Formation waren unter anderem Ceratosaurus, Ornitholestes und Torvosaurus. Die Sauropoden der Morrison-Formation werden Apatosaurus, Brachiosaurus, Camarasaurus und Diplodocus zugeordnet, weitere Pflanzenfresser (aus der Gruppe der Vogelbeckensaurier) waren Camptosaurus, Dryosaurus und Stegosaurus.[77]

Die portugiesischen Formationen ähnelten der Morrison-Formation stark. Zahlreiche Dinosaurier waren in Portugal ebenfalls präsent (etwa Allosaurus, Ceratosaurus, Torvosaurus und Apatpsaurus), oder durch ähnliche Gegenstücke vertreten (Brachiosaurus und Lusotitan, Camptosaurus und Draconyx. [78]

Outcrop of the Brushy Basin Member of the Morrison Formation, west of Green River, Utah, on the Colorado Plateau.

Allosaurus lebte zusammen mit einigen anderen, großen Theropoden wie Ceratosaurus und Torvosaurus, sowohl in den USA als auch in Portugal.[78] Sie besetzten anscheinend verschiedene ökologische Nischen, dies ist aus Körperbau und Fundort ersichtlich. Ceratosaurus und Torvosaurus bewegten sich wohl öfter um die bewachsenen Flussufer; ihr kleinerer, dünnerer Körper wäre in Wäldern und Unterholz vorteilhaft gewesen. Allosaurus hingegen war kompakter gebaut, mit längeren Beinen, also schneller, aber weniger wendig. Er bevorzugte die trockenen Farnsavannen.[79] Ceratosaurus unterschied sich merklich von Allosaurus durch seinen längeren, schmaleren Schädel mit großen, breiten Zähnen.[44] 


Allosaurus was itself a potential food item to other carnivores, as illustrated by an Allosaurus pubic foot marked by the teeth of another theropod, probably Ceratosaurus or Torvosaurus. The location of the bone in the body (along the bottom margin of the torso and partially shielded by the legs), and the fact that it was among the most massive in the skeleton, indicates that the Allosaurus was being scavenged.[80]

Paleobiology

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Life history

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The wealth of Allosaurus fossils, from nearly all ages of individuals, allows scientists to study how the animal grew and how long its lifespan may have been. Remains may reach as far back in the lifespan as eggs—crushed eggs from Colorado have been suggested as those of Allosaurus.[1] Based on histological analysis of limb bones, the upper age limit for Allosaurus is estimated at 22 to 28 years, which is comparable to that of other large theropods like Tyrannosaurus. From the same analysis, its maximum growth appears to have been at age 15, with an estimated growth rate of about 150 kilograms (330 lb) per year.[49]

Medullary bone tissue, also found in dinosaurs as diverse as Tyrannosaurus and Tenontosaurus, has been found in at least one Allosaurus specimen, a shin bone from the Cleveland-Lloyd Quarry. Today, this bone tissue is only formed in female birds that are laying eggs, as it is used to supply calcium to shells. Its presence in the Allosaurus individual establishes sex and shows she had reached reproductive age. By counting growth lines, it was shown that she was 10 years old at death, so sexual maturity in Allosaurus was attained well before maximum growth and size.[81]

Mounted cast of a juvenile specimen, A. "jimmadseni", North American Museum of Ancient Life

The discovery of a juvenile specimen with a nearly complete hindlimb shows that the legs were relatively longer in juveniles, and the lower segments of the leg (shin and foot) were relatively longer than the thigh. These differences suggest that younger Allosaurus were faster and had different hunting strategies than adults, perhaps chasing small prey as juveniles, then becoming ambush hunters of large prey upon adulthood.[50] The thigh bone became thicker and wider during growth, and the cross-section less circular, as muscle attachments shifted, muscles became shorter, and the growth of the leg slowed. These changes imply that juvenile legs has less predictable stresses compared with adults, which would have moved with more regular forward progression.[82]

Feeding

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Allosaurus and Stegosaurus skeletons, Denver Museum of Nature and Science

Paleontologists accept Allosaurus as an active predator of large animals. Sauropods seem to be likely candidates as both live prey and as objects of scavenging, based on the presence of scrapings on sauropod bones fitting allosaur teeth well and the presence of shed allosaur teeth with sauropod bones.[83] There is dramatic evidence for allosaur attacks on Stegosaurus, including an Allosaurus tail vertebra with a partially healed puncture wound that fits a Stegosaurus tail spike, and a Stegosaurus neck plate with a U-shaped wound that correlates well with an Allosaurus snout.[84] However, as Gregory Paul noted in 1988, Allosaurus was probably not a predator of fully grown sauropods, unless it hunted in packs, as it had a modestly sized skull and relatively small teeth, and was greatly outweighed by contemporaneous sauropods.[9] Another possibility is that it preferred to hunt juveniles instead of fully grown adults.[5][63] Research in the 1990s and first decade of the 21st century may have found other solutions to this question. Robert T. Bakker, comparing Allosaurus to Cenozoic sabre-toothed carnivorous mammals, found similar adaptations, such as a reduction of jaw muscles and increase in neck muscles, and the ability to open the jaws extremely wide. Although Allosaurus did not have sabre teeth, Bakker suggested another mode of attack that would have used such neck and jaw adaptations: the short teeth in effect became small serrations on a saw-like cutting edge running the length of the upper jaw, which would have been driven into prey. This type of jaw would permit slashing attacks against much larger prey, with the goal of weakening the victim.[52]

Allosaurus attacking, based on the theories of Bakker (1998) and Rayfield et al. (2001).

Similar conclusions were drawn by another study using finite element analysis on an Allosaurus skull. According to their biomechanical analysis, the skull was very strong but had a relatively small bite force. By using jaw muscles only, it could produce a bite force of 805 to 2,148 N, less than the values for alligators (13,000 N), lions (4,167 N), and leopards (2,268 N), but the skull could withstand nearly 55,500 N of vertical force against the tooth row. The authors suggested that Allosaurus used its skull like a hatchet against prey, attacking open-mouthed, slashing flesh with its teeth, and tearing it away without splintering bones, unlike Tyrannosaurus, which is thought to have been capable of damaging bones. They also suggested that the architecture of the skull could have permitted the use of different strategies against different prey; the skull was light enough to allow attacks on smaller and more agile ornithopods, but strong enough for high-impact ambush attacks against larger prey like stegosaurids and sauropods.[51] Their interpretations were challenged by other researchers, who found no modern analogues to a hatchet attack and considered it more likely that the skull was strong to compensate for its open construction when absorbing the stresses from struggling prey.[85] The original authors noted that Allosaurus itself has no modern equivalent, that the tooth row is well-suited to such an attack, and that articulations in the skull cited by their detractors as problematic actually helped protect the palate and lessen stress.[86] Another possibility for handling large prey is that theropods like Allosaurus were "flesh grazers" which could take bites of flesh out of living sauropods that were sufficient to sustain the predator so it would not have needed to expend the effort to kill the prey outright. This strategy would also potentially have allowed the prey to recover and be fed upon in a similar way later.[14] An additional suggestion notes that ornithopods were the most common available dinosaurian prey, and that allosaurs may have subdued them by using an attack similar to that of modern big cats: grasping the prey with their forelimbs, and then making multiple bites on the throat to crush the trachea.[5] This is compatible with other evidence that the forelimbs were strong and capable of restraining prey.[20]

Allosaurus skull cast in front view at the Museum für Naturkunde, Berlin

Other aspects of feeding include the eyes, arms, and legs. The shape of the skull of Allosaurus limited potential binocular vision to 20° of width, slightly less than that of modern crocodilians. As with crocodilians, this may have been enough to judge prey distance and time attacks.[87] The similar wide field of view suggests that allosaurs, like modern crocodilians, were ambush hunters.[88] The arms, compared with those of other theropods, were suited for both grasping prey at a distance or clutching it close,[20] and the articulation of the claws suggests that they could have been used to hook things.[19] Finally, the top speed of Allosaurus has been estimated at 30 to 55 kilometers per hour (19 to 34 miles per hour).[89]

Social behavior

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Allosaurus has long been regarded in the semitechnical and popular literature as an animal that preyed on sauropods and other large dinosaurs by hunting in groups.[11][28][63] Robert T. Bakker has extended social behavior to parental care, and has interpreted shed allosaur teeth and chewed bones of large prey animals as evidence that adult allosaurs brought food to lairs for their young to eat until they were grown, and prevented other carnivores from scavenging on the food.[54] However, there is actually little evidence of gregarious behavior in theropods,[14] and social interactions with members of the same species would have included antagonistic encounters, as shown by injuries to gastralia[16] and bite wounds to skulls (the pathologic lower jaw named Labrosaurus ferox is one such possible example). Such head-biting may have been a way to establish dominance in a pack or to settle territorial disputes.[90]

Although Allosaurus may have hunted in packs,[91] it has recently been argued that Allosaurus and other theropods had largely aggressive instead of cooperative interactions with other members of their own species. The study in question noted that cooperative hunting of prey much larger than an individual predator, as is commonly inferred for theropod dinosaurs, is rare among vertebrates in general, and modern diapsid carnivores (including lizards, crocodiles, and birds) very rarely cooperate to hunt in such a way. Instead, they are typically territorial and will kill and cannibalize intruders of the same species, and will also do the same to smaller individuals that attempt to eat before they do when aggregated at feeding sites. According to this interpretation, the accumulation of remains of multiple Allosaurus individuals at the same site, e.g. in the Cleveland-Lloyd quarry, are not due to pack hunting, but to the fact that Allosaurus individuals were drawn together to feed on other disabled or dead allosaurs, and were sometimes killed in the process. This could explain the high proportion of juvenile and subadult allosaurs present, as juveniles and subadults are disproportionally killed at modern group feeding sites of animals like crocodiles and Komodo dragons. The same interpretation applies to Bakker's lair sites.[92] There is some evidence for cannibalism in Allosaurus, including Allosaurus shed teeth found among rib fragments, possible tooth marks on a shoulder blade,[93] and cannibalized allosaur skeletons among the bones at Bakker's lair sites.[79]

Brain and senses

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The brain of Allosaurus, as interpreted from spiral CT scanning of an endocast, was more consistent with crocodilian brains than those of the other living archosaurs, birds. The structure of the vestibular apparatus indicates that the skull was held nearly horizontal, as opposed to strongly tipped up or down. The structure of the inner ear was like that of a crocodilian, and so Allosaurus probably could have heard lower frequencies best, and would have had trouble with subtle sounds. The olfactory bulbs were large and seem to have been well suited for detecting odors, although the area for evaluating smells was relatively small.[53]

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Allosaurus model in Bałtów, Poland.

Along with Tyrannosaurus, Allosaurus has come to represent the quintessential large, carnivorous dinosaur in popular culture. It is a common dinosaur in museums, due in particular to the excavations at the Cleveland Lloyd Dinosaur Quarry; by 1976, as a result of cooperative operations, 38 museums in eight countries on three continents had Cleveland-Lloyd allosaur material or casts.[3] Allosaurus is the official state fossil of Utah.[94]

Allosaurus has been depicted in popular culture since the early years of the 20th century. It is top predator in both Arthur Conan Doyle's 1912 novel, The Lost World, and its 1925 film adaptation, the first full-length motion picture to feature dinosaurs.[95] Allosaurus was used as the starring dinosaur of the 1956 film The Beast of Hollow Mountain,[29] and the 1969 film The Valley of Gwangi, two genre combinations of living dinosaurs with Westerns. In The Valley of Gwangi, Gwangi is billed as an Allosaurus, although Ray Harryhausen based his model for the creature on Charles R. Knight's depiction of a Tyrannosaurus. Harryhausen sometimes confuses the two, stating in a DVD interview "They're both meat eaters, they're both tyrants... one was just a bit larger than the other."[96] Allosaurus appeared in the second episode of the 1999 BBC television series Walking with Dinosaurs and the follow-up special The Ballad of Big Al, which speculated on the life of the "Big Al" specimen, based on scientific evidence from the numerous injuries and pathologies in its skeleton.[97]

References

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  2. ↑ a b c d e Mickey Mortimer: And the largest Theropod is... The Dinosaur Mailing List, 21. Juli 2003, abgerufen am 8. September 2007.
  3. ↑ a b c d e f g h i j k l m n o James H., Jr. Madsen: Allosaurus fragilis: A Revised Osteology (= Utah Geological Survey Bulletin '''109'''). 2nd Auflage. Utah Geological Survey, Salt Lake City 1993 ( [1976]).
  4. ↑ a b John R. Foster: Paleoecological Analysis of the Vertebrate Fauna of the Morrison Formation (Upper Jurassic), Rocky Mountain Region, U.S.A. (= New Mexico Museum of Natural History and Science Bulletin '''23'''). New Mexico Museum of Natural History and Science, Albuquerque 2003, S. 37.
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  6. ↑ Karl T. Bates, Falkingham, Peter L.; Breithaupt, Brent H.; Hodgetts, David; Sellers, William I.; and Manning, Phillip L.: How big was 'Big Al'? Quantifying the effect of soft tissue and osteological unknowns on mass predictions for Allosaurus (Dinosauria:Theropoda). In: Palaeontologia Electronica. 12. Jahrgang, Nr. 3, 2009, S. unpaginated (palaeo-electronica.org [abgerufen am 13. Dezember 2009]).
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  77. ↑ Daniel J. Chure, Litwin, Ron; Hasiotis, Stephen T.; Evanoff, Emmett; and Carpenter, Kenneth: Paleontology and Geology of the Upper Jurassic Morrison Formation. Hrsg.: Foster, John R.; and Lucas, Spencer G. (= New Mexico Museum of Natural History and Science Bulletin, '''36'''). New Mexico Museum of Natural History and Science, Albuquerque, New Mexico 2006, The fauna and flora of the Morrison Formation: 2006, S. 233–248.
  78. ↑ a b Octávio Mateus: Paleontology and Geology of the Upper Jurassic Morrison Formation. Hrsg.: Foster, John R.; and Lucas, Spencer G. (= New Mexico Museum of Natural History and Science Bulletin, '''36'''). New Mexico Museum of Natural History and Science, Albuquerque, New Mexico 2006, Jurassic dinosaurs from the Morrison Formation (USA), the Lourinhã and Alcobaça Formations (Portugal), and the Tendaguru Beds (Tanzania): A comparison, S. 223–231.
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  80. ↑ Daniel J. Chure: Prey bone utilization by predatory dinosaurs in the Late Jurassic of North America, with comments on prey bone use by dinosaurs throughout the Mesozoic. In: Gaia. 15. Jahrgang, 2000, ISSN 0871-5424, S. 227–232 (ul.pt [PDF]).
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  96. ↑ Return to the Valley, a documentary on the Region 1 Valley of Gwangi DVD.
  97. ↑ Benton, M.J. (2001). "The science of 'Walking with Dinosaurs'". Teaching Earth Sciences, 24: 371-400.
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