Arbrosaurs are a family of primarily tree-dwelling maniraptoran coelurosaurs in the alternate timeline of the Cenozoic era in The New Dinosaurs: An Alternative Evolution.
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 small, primitive coelurosaurs. 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 from the tropical rainforests of Africa. 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's tail is usually 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.
Most arbrosaurs are able to leap great distances from branch to branch and from tree to tree, in order to look for more prey.
In the deciduous forests of North America live a vast variety of different arbrosaurs, each one specifically adapted to a particular way of life, and are also so different from other arbrosaur species of other areas in the world. As on almost all the other continents, the arbrosaurs fill the trees of North America. Some eat insects, others consume fruit and berries, and some prey upon other arbrosaurs. The trees are alive with myriads of tiny insectivorous arbrosaurs, each one differing from the next by the different colors of the feathery pelt, by different display tufts on the head and tail, and by slight differences in the shape and size of the head. The arrangement of the skull and jaws depends upon the diet. Feeding both in the trees and on the ground, the tiny kinds of arbrosaurs have short, thick jaws to crunch up beetles, or long thin jaws to dig for buried larvae and worms. All the arbrosaurs have the same light build, long, springing legs and thin toes. The body is small and balanced for running and jumping by the long, stiff tail.
The airy canopy of Central and South American tropical forests is alive with little arbrosaurs (and close relatives) adapted to feed on the thousands upon thousands of different types of insects that make their homes there. Some of the arbrosaurs, however, have abandoned their insectivorous way of life and have evolved into totally new herbivorous forms.
As in all other tropical forest areas the arboreal animals of the Indomalayan realm have evolved into a vast variety of strange forms. Many of these have developed from the arbrosaurs of the other realms.