Stem-Bird Files: Marsh's Hatchetjaw

Carnosauria>Allosauroidea>Allosauridae>Allosaurinae>Allosaurus>A.fragilis
Othniel C.Marsh, 1877
Time: Kimmeridgian-Early Tithonian (155-150 million years ago)
Length: Up to 10.8 meters (35.4ft) when fully grown.
Height: Up to 2.9 meters (9.5ft) at the hips when fully grown.
Weight: Up to ~3.9 tonnes when fully grown.
Habitat: The Morrison Formation of multiple states in Western North America.
Ecology and hunting habits: Medium-large terrestrial predator, by far the most common large carnivorous theropod on its habitat. As proposed by Greg Paul based on comparisons of their jaw and neck muscles with those of sabertooth cats, Allosaurus most likely was capable of attacking prey larger than itself, unlike most theropods. This gave it access to the most abundant food source of the Morrison, juvenile sauropods, although it was also capable of hunting smaller and medium-sized prey.
Diet: Juveniles of sauropods such as Apatosaurus, Brachiosaurus, Barosaurus, Supersaurus and Camarasaurus suprems, with adults of smaller species like Diplodocus carnegii and C.lentus likely being in the menu as well; ornithopods such as Camptosaurus and tyreophorans like Stegosaurus and Hesperosaurus.
Social behavior: Due to their great abundance in some quarries (more than 75% of all theropod fauna in the Cleveland-Lloyd quarry is composed purely of Allosaurus) and the large or exceptionally well-defended prey items they had at choice, many have suggested that Allosaurus may have lived in packs to hunt down large sauropods and dangerous tyreophorans. However, currently there is no evidence of gregariousness in the species, and it is just as likely that the Allosaurus bonebeds represent communal feeding sites where juvenile and subadult individuals were killed in fights for food or dominance over the area; this is supported by evidence of several occurences of cannibalism between the species in these very locations, and is similar to the aggressive feeding behavior of modern komodo dragons or crocodiles in times of scarcity.
Locomotion: The speed of Allosaurus has been estimated by recent studies to max out at 21.4mph (33.7 kilometers an hour).
Competition: Hatchlings and juveniles would have competed with theropods like Coelurus, Koparion, Tanycolagreus and Ornitholestes, while older juveniles would face Ceratosaurus and adults had to contend with Torvosaurus and other species of allosaur, like A.maximus.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Allosaurus - perhaps not the cooperative pack hunter we're accustomed to, but a savage carnivore akin to the Deinonychus in Michael Crichton's Lost World or even ankle-biting Austroraptor players in The Isle...

Update 07/01/17: 
-Lineart completely remade.
-Replaced human scale figure (now it's the same as used in my charts)
-Updated common name (you may have noticed the, uh, disappearance of the ''Royal'' Hatchet.. see below for reason)
-More accurate size estimate, now along the centra as is the norm, instead of in a straight line. PS: yes, A.fragilis really did get that big. Or at least current phylogeny says so.
-Removed ''average adult'' estimates, they're dumb.
-Minor text corrections.

Based on skeletal reconstruction by Scott Hartman.