Allosaurus

Allosaurus is a theropod dinosaur genus from the Late Jurassic, and lived between 155 and 145 million years ago. Its name means „different lizard”, referring to its concave vertebrae which were quite unusual at the time of its discovery. Allosaurus was scientifically described based on its fossils by Othniel Charles Marsh in 1877[1]. The first fossils of Allosaurus were found in the Morrison Formation, in the United States, and recently fossils of the species have been reported form Portugal, Europe, too[2]. However, the discovery and early history of this dinosaur is complicated.

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Known occurrences of Allosaurus fossils.

Description and anatomy

Allosaurus was a massive bipedal carnivorous dinosaur with a light but robust skull which was 1–1.5 m long, 14–17 sharp and serrated teeth in each maxilla, becoming shorter, narrower, and more curved posteriorly. The teeth were easily shed, and then they were replaced wit new ones, resulting Allosaurus teeth to be common fossils[3]. The skull had a pair of horns above the eyes, somewhat anteriorly to them; these were extensions of the lacrimal bones. These horns were fragile and probably covered by keratin sheath, and might served several functions, e.g. they could be useful sunshades for the eyes. The length of Allosaurus was about 10 meters (with a maximum length of 13m)[1] and weighed approximately 1000 kilograms, had large and strong hindlimbs with curved and pointed claws, and small, powerful, three-fingered arms, and a long and muscular tail. Of the three fingers of the forelimb, the innermost was the greatest[4].

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Allosaurus fragilis. Source: By Fred Wierum - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=47577505

Valid species

The taxonomy of the genus is complicated, and there is three valid species in it: A. fragilis, A. europaeus, and A. jimmadseni[5]. A. fragilis is the type species of the genus, which was described by Marsh in 1877[6]. The fossils of at least 60 specimens are known, all form the Morrison Formation in the United States. A. jimmadseni was described based on two nearly complete skeletons, and differs from the type species in several characters, e.g. in having a cheekbone with a straight lower margin. It was found also in the Morrison Formation[5]. A. europaeus from the Iberian peninsula differs from the latter two species in details of the skull[7].

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Allosaurus jaws. Source: By I, Steveoc 86, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=2869688

Paleobiology

Allosaurus was probably the apex predator of the ecosystem it lived within, probably hunting mostly for large herbivorous dinosaurs, like stegosaurids, ornithopods and sauropods. As there was found Allosaurus fossils from almost all ages of specimens, it is possible for scientists to study the life history and estimate the lifespan of the individuals. Based on scientific research it seems that specimens reached their maximum length at the age of 15, with a growth rate of approximately 150 kilograms per years[8]. A study based on the discovery of a juvenile Allosaurus suggests that younger specimens were faster than the adults, and also might have different hunting strategies, probably chasing smaller preys, and upon adulthood they gradually become ambush predators hunting for for large dinosaurs, like Stegosaurus and sauropods[9]. It is possible that Allosaurus could prey on fully grown sauropods only if it hunted in packs[4]. Robert T. Bakker have suggested that the species also showed parental care, and probably adults brought food to youngs until they become strong enough for hunting[10]. Territorial fights between packs were also likely[11]. However, a different interpretation of the fossil evidences were also suggested, which concluded that Allosaurus and other theropod dinosaurs had largely aggressive interactions with conspecifics, and even cannibalism were likely[12][13].

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Allosaurus skeleton. Source: By Nekarius - File:Allosaurus AMNH.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=39833686

Scientific references

[1] Liddell & Scott (1980): Greek–English Lexicon, Abridged Edition. Oxford: Oxford University Press. ISBN 978-0-19-910207-5.

[2] Pérez-Moreno, B.P.; Chure, D. J.; Pires, C.; Marques Da Silva, C.; Dos Santos, V.; Dantas, P.; Povoas, L.; Cachao, M.; Sanz, J. L. (1999): On the presence of Allosaurus fragilis (Theropoda: Carnosauria) in the Upper Jurassic of Portugal: First evidence of an intercontinental dinosaur species. Journal of the Geological Society. 156 (3): 449–452. doi:10.1144/gsjgs.156.3.0449.

[3] Madsen, James H., Jr. (1993) [1976]: Allosaurus fragilis: A Revised Osteology. Utah Geological Survey Bulletin 109 (2nd ed.). Salt Lake City: Utah Geological Survey.

[4] Paul, Gregory S. (1988): Genus Allosaurus. Predatory Dinosaurs of the World. New York: Simon & Schuster. pp. 307–313. ISBN 978-0-671-61946-6.

[5] Glut, Donald F. (1997): Allosaurus. Dinosaurs: The Encyclopedia. Jefferson, North Carolina: McFarland & Co. pp. 105–117. ISBN 978-0-89950-917-4.

[6] Marsh, O.C. (1877): Notice of new dinosaurian reptiles from the Jurassic formation. American Journal of Science and Arts. 14 (84): 514–516. doi:10.2475/ajs.s3-14.84.514.

[7] Mateus, O.; Walen, A.; Antunes, M.T. (2006): The large theropod fauna of the Lourinha Formation (Portugal) and its similarity to that of the Morrison Formation, with a description of a new species of Allosaurus. In Foster, John R.; Lucas, Spencer G. (eds.). Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin, 36. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. pp. 123–129.

[8] Bybee, P.J.; Lee, A.H.; Lamm, E.T. (2006): Sizing the Jurassic theropod dinosaur Allosaurus: Assessing growth strategy and evolution of ontogenetic scaling of limbs. Journal of Morphology. 267 (3): 347–359. doi:10.1002/jmor.10406.

[9] Foster, John R.; Chure, Daniel J. (2006): Hindlimb allometry in the Late Jurassic theropod dinosaur Allosaurus, with comments on its abundance and distribution. In Foster, John R.; Lucas, Spencer G. (eds.). Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin, 36. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. pp. 119–122.

[10] Bakker, R.T. (1997): Raptor Family values: Allosaur parents brought giant carcasses into their lair to feed their young. In Wolberg, Donald L.; Sump, Edmund; Rosenberg, Gary D. (eds.). Dinofest International, Proceedings of a Symposium Held at Arizona State University. Philadelphia: Academy of Natural Sciences. pp. 51–63.

[11] Tanke, D.H. (1998). Head-biting behavior in theropod dinosaurs: Paleopathological evidence. Gaia (15): 167–184.

[12] Roach, B.T.; Brinkman, D.L. (2007): A reevaluation of cooperative pack hunting and gregariousness in Deinonychus antirrhopus and other nonavian theropod dinosaurs. Bulletin of the Peabody Museum of Natural History. 48 (1): 103–138. doi:10.3374/0079-032X(2007)48[103:AROCPH]2.0.CO;2.

[13] Bakker, R.T.; Bir, G. (2004): Dinosaur crime scene investigations: theropod behavior at Como Bluff, Wyoming, and the evolution of birdness. In Currie, Philip J.; Koppelhus, Eva B.; Shugar, Martin A.; Wright, Joanna L. (eds.). Feathered Dragons: Studies on the Transition from Dinosaurs to Birds. Bloomington and Indianapolis: Indiana University Press. pp. 301–342. ISBN 978-0-253-34373-4.