Most meat-eating non-avian theropods have lost their collarbone. They do not need one since the forelimbs do not need to be particularly strong. One or two forms do possess this bone, however, and it is not clear whether their ancestors retained it from the time when they went about on four legs, or whether it 're-evolved' having been first lost a process known as secondary development. In Arbrosaurus and its relatives, the collarbone provides the anchorage for the very powerful arm muscles with which the animal can brachiate, or swing with an overarm action, through the branches of the trees.
The Tree Hopper (Arbrosaurus bernardi) is an agile, primate-like arbrosaur from The New Dinosaurs: An Alternative Evolution. It lives in African tropical rainforests.
One of the most widespread developments in the dinosaurs after the end of the Cretaceous period was the evolution of the arbrosaurs. These evolved from the small, primitive coelurosaur theropods. In Jurassic times the coelurosaurs gave rise to the birds, and the development of the arbrosaurs from the same stock was brought about by similar evolutionary processes. Perhaps the most typical of the modern arbrosaurs is the genus Arbrosaurus itself. Various species of this animal appear in almost all the zoogeographical realms.
The main feature that distinguishes the arbrosaurs from other coelurosaurs is the presence of a strong collarbone. In another coelurosaur offshoot, retention of the collarbone allowed for the development of strong flight muscles and paved the way for the evolution of the birds. In the case of the arbrosaurs the collar girdle provides support for the strong arms which are used for climbing and swinging about among the branches. Its endothermic (warm-blooded) physiology enables it to pursue an active hunting lifestyle. Its skull shows adaptations for this, with the big brain box, the eye sockets directed forwards giving stereoscopic vision, and the narrow, finely toothed jaws - ideal for winkling insects out of crannies in tree bark.
An arbrosaur, such as the tree hopper, is agile and perfectly at home in the trees, but is a very ungainly animal on the ground. When it is forced to, it can only move over the ground by a series of undignified leaps, keeping its long arms and heavy tail well up out of the way.
An arbrosaur's tail is a stiff, straight rod. It uses it for balance when leaping about among the trees. The long claws on the three main toes and the three fingers are useful both for finding purchase on branches and for ripping up bark for insects.