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News: THE SEARCH FOR ATLANTIS IN CUBA
A Report by Andrew Collins
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Allosaurus

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Author Topic: Allosaurus  (Read 2340 times)
Moonfire
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« Reply #30 on: December 13, 2009, 02:25:27 am »



Cast of "Big Al Two"
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Moonfire
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« Reply #31 on: December 13, 2009, 02:26:28 am »

Species and taxonomy

It is unclear how many species of Allosaurus there were. Seven species have been considered potentially valid since 1988 (A. amplexus,[8] A. atrox,[8] A. europaeus,[60] the type species A. fragilis,[13] the as-yet not formally described "A. jimmadseni",[6] A. maximus,[46] and A. tendagurensis[13]), 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.[13]
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Moonfire
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« Reply #32 on: December 13, 2009, 02:28:52 am »

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.[13] 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;[8][61] recent work has followed a "one species" interpretation,[13] with the differences seen in the Morrison Formation material attributed to individual variation.[62][63] A. europaeus was found in the Kimmeridgian-age Porto Novo Member of the Lourinhã Formation,[60] but may be the same as A. fragilis.[64] A. tendagurensis was found in Kimmeridgian-age rocks of Tendaguru, in Mtwara, Tanzania.[65] Although the most recent review tentatively accepted it as a valid species of Allosaurus, it may be a more basal tetanuran,[66] 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]
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Moonfire
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« Reply #33 on: December 13, 2009, 02:29:02 am »

Allosaurus is regarded as a probable synonym of the genera Antrodemus, Creosaurus, Epanterias, and Labrosaurus.[13] 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.[67] Later researchers suggested that the bone was pathologic, showing an injury to the living animal,[18] and that part of the unusual form of the rear of the bone was due to plaster reconstruction.[68] It is now regarded as an example of A. fragilis.[13] Other remains thought to pertain to Allosaurus have come from across the world, including Australia,[69] Siberia,[70] and Switzerland,[1] but these fossils have been reassessed as belonging to other dinosaurs.
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« Reply #34 on: December 13, 2009, 02:29:50 am »



Allosaurus sp. skull (DINO 11541) from Dinosaur National Monument, still partially encased in matrix.
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« Reply #35 on: December 13, 2009, 02:30:19 am »

Paleoecology

Allosaurus was the most common large theropod in the vast tract of Western American fossil-bearing rock known as the Morrison Formation, accounting for 70 to 75% of theropod specimens,[5] and as such was at the top trophic level of the Morrison food web.[71] The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons, and flat floodplains.[72] Vegetation varied from river-lining forests of conifers, tree ferns, and ferns, to fern savannas with rare trees.[73]
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« Reply #36 on: December 13, 2009, 02:30:37 am »

The Morrison Formation has been a rich fossil hunting ground, holding fossils of green algae, fungi, mosses, horsetails, ferns, cycads, ginkgoes, and several families of conifers. Other fossils discovered include bivalves, snails, ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, terrestrial and aquatic crocodylomorphans, several species of pterosaur, numerous dinosaur species, and early mammals such as docodonts, multituberculates, symmetrodonts, and triconodonts. Such dinosaurs as the theropods Ceratosaurus, Ornitholestes, and Torvosaurus, the sauropods Apatosaurus, Brachiosaurus, Camarasaurus, and Diplodocus, and the ornithischians Camptosaurus, Dryosaurus, and Stegosaurus are known from the Morrison.[74] The Late Jurassic formations of Portugal where Allosaurus is present are interpreted as having been similar to the Morrison but with a stronger marine influence. Many of the dinosaurs of the Morrison Formation are the same genera as those seen in Portuguese rocks (mainly Allosaurus, Ceratosaurus, Torvosaurus, and Apatosaurus), or have a close counterpart (Brachiosaurus and Lusotitan, Camptosaurus and Draconyx).[75]
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« Reply #37 on: December 13, 2009, 02:31:00 am »

Allosaurus coexisted with fellow large theropods Ceratosaurus and Torvosaurus in both the United States and Portugal,[75] The three appear to have had different ecological niches, based on anatomy and the location of fossils. Ceratosaurs and torvosaurs may have preferred to be active around waterways, and had lower, thinner bodies that would have given them an advantage in forest and underbrush terrains, whereas allosaurs were more compact, with longer legs, faster but less maneuverable, and seem to have preferred dry floodplains.[76] Ceratosaurus, better known than Torvosaurus, differed noticeably from Allosaurus in functional anatomy by having a taller, narrower skull with large, broad teeth.[42] 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.[77]
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« Reply #38 on: December 13, 2009, 02:31:42 am »



Locations in the Morrison Formation (yellow) where Allosaurus remains have been found.
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« Reply #39 on: December 13, 2009, 02:32:29 am »



Outcrop of the Brushy Basin Member of the Morrison Formation, west of Green River, Utah, on the Colorado Plateau.
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« Reply #40 on: December 13, 2009, 02:33:17 am »

Paleobiology

Life history


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.[47]
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« Reply #41 on: December 13, 2009, 02:33:55 am »

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.[78]
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« Reply #42 on: December 13, 2009, 02:34:16 am »

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.[48] 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.[79]
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« Reply #43 on: December 13, 2009, 02:35:00 am »



Mounted cast of a juvenile specimen, A. "jimmadseni", North American Museum of Ancient Life
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« Reply #44 on: December 13, 2009, 02:35:29 am »

Feeding

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.[80] 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.[81] 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.[8] Another possibility is that it preferred to hunt juveniles instead of fully grown adults.[5][61] Research in the 1990s and 2000s 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.[50]
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