The small Late Jurassic compsognathids and coelurids are the most primitive definite avetheropods, although the first is barely in the order. Beyond these there are two main avetheropod clades. One is the classic allosaur-tyrannosaur group, which also includes the small ornitholestians and aublysodonts. These are advanced relative to other theropods because they have robust suspensoriums (walking dinosaur costume). Another distinctive character is a central cannon bone that is L-shaped at the upper end. A subtle character, it is important because it is consistent regardless of how reduced this bone becomes, and because it distinguishes the allosaur-tyrannosaur group from other avetheropods in which this element is always straight at its upper end (Realistic dinosaur costume).
Although tyrannosaurs and their aublysodont relatives share some qualities with the sickle-clawed dromaeosaurs discussed below,r2 they are really united with allosaurs above the ornitholestians. Allosaurs and tyrannosaurs share unusual plate-like processes that project forward from the braincase and run up to the skull roof (Animatronic dinosaur), and a more flexible extra joint in the lower jaw. Aublysodonts and tyrannosaurs share even more similarities with each other: D-cross-sectioned front teeth followed by bigger blades behind, small premaxillas in the tip of the snout, deep maxillas behind them, extremely slender shoulder blades, big pubic boots-and most notable of all-atrophied arms and hands. Allosaurs have some minor specializations that tyrannosaurs lack, and this made me think at first that they were sister groups with a corrunon Jurassic ancestor. But Cretaceous allo saurs became increasingly tyrannosaur-like, so it really looks like advanced Cretaceous allosaurs directly spawned the aublysodont- tyrannosaurs. Sankar Chatterj ee disagrees, contending that differing peg-and-socket articulations in the allosaurs and tyrannosaurs show they directly and independently evolved from different thecodonts.t3 But the theropods’ “pegs and sockets” are just interlocking bumps, not at all like the thecodonts’systems. Otherwise, the dinosaurs’mesotarsal ankles are extremely alike, and completely different from the thecodonts’ crurotarsal ones (Animatronic dinosaur for sale). The same is true of the rest of these animals. The allosaur-tyrannosaur clade is a sister group to the other major avetheropod clade, the “true” protobirds and birds. Protobirds are less advanced than the allosaur-tyrannosaurs in their retention of slender cheek bones, strong intermandibular joints, and straight-ended central cannon bones. But protobirds have many avian features not found in most other dinosaurs, although as mentioned earlier some may have been mimicked by earlier “Protoavis” and its relatives. Perhaps the first protobird is famous Archaeopteryx, often considered the original bird. This little animal’s very large arms bore fully aerodynamic feathered wings.