Allosaurus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Carnosauria > Allosauroidea > Allosauridae

Overview: Aside from the infamous Tyrannosaurus, Allosaurus is probably one of the best known theropods to the general public. It’s been featured in various documentaries, encyclopedias, and a number of popular works of fiction. Fossils attributed to the genus are primarily known from what is now the western United States, but also along the coast of Portugal, within rock layers dated to the Late Jurassic. Within those regions, Allosaurus would’ve been among the largest predators. It’s often speculated to have been a specialized sauropod killer, probably hunting the young or sick, but we can assume it would’ve hunted other prey as well. It was lightly built in comparison to some other theropods, but still quite powerful, sporting fairly strong arms equipped with three claws on each hand. The skull wasn’t as strongly built as those of tyrannosaurs, so its jaws were mainly suited to stripping flesh, not crushing bone. However, the jaws could flex quite a bit, allowing it to open its mouth remarkably wide to inflict massive wounds. One other notable trait were the prominent crests it sported in front of each eye, running along its snout, likely used for visual display.

Fossils that probably belong to Allosaurus were first reported in the late 1860’s, found in Colorado, consisting of partial tail vertebrae. In 1870, paleontologist Joseph M. Leidy would refer this bone to a new genus called Antrodemus. Othniel C. Marsh would later describe somewhat more complete fossils in 1877, establishing Allosaurus as a genus. Part of its name was taken from the Greek word for “other” or “different”, inspired by features on its vertebrae, setting it apart from other dinosaur genera Marsh had previously described. Studies in the 1920’s found the fossils of both genera to be from the same animal. Antrodemus was named first, so for a number of decades, paleontologists would use said name to refer to this animal. However, in the 1970’s, an influential study noted that Antrodemus was based on very scant material that may lack diagnostic traits, leading to Allosaurus coming back into use. Many more Allosaurus specimens have been found over the past century, in many cases being relatively complete, making it a very well understood theropod. Currently, there are up to four potential species – A. fragilis, A. europaeus, A. jimmadseni, and possibly A. anax.

Allosaurus fragilis is the type species of Allosaurus, found in the Morrison Formation of the United States, as is the case with Allosaurus jimmadseni. Allosaurus europaeus, from Portugal’s Lourinhã Formation, may be invalid or the same as A. fragilis, according to one recent study. Allosaurus anax is a bit more controversial. Its fossils were, for many years, attributed to a related and now dubious genus called Saurophaganax. Recent studies have found said genus to be based on chimeric fossils from a sauropod, mixed with those of a very large Allosaurus species, hence A. anax. Estimates for the size of Allosaurus vary. Adults averaged around eight to nine meters long, but isolated fossils of A. anax suggest it could grow to be much larger, up to eleven meters or more. All species belong to the family Allosauridae, a part of the larger superfamily Allosauroidea, making them earlier or basal kin of the carcharodontosaurids. In the Morrison Formation, it may’ve coexisted with Stegosaurus, Camptosaurus, Diplodocus, Apatosaurus, Camarasaurus, Ceratosaurus, and Torvosaurus. The latter two were also large-bodied theropods, possibly occupying different hunting niches.