The superficial resemblance in size and shape between the Australasian crackbeak and the arbrosaurs of the northern continents has come about by convergent evolution. They both evolved from lightly built running ancestors and have become lightly built tree-dwelling animals, but whereas the arbrosaurs subsist on a diet of insects and smaller vertebrates, the crackbeak is strictly vegetarian.
The Crackbeak, Fortirostrum fructiphagum, is an arboreal basal ornithopod from the Queensland tropical rainforests, as well as the jungles of Southern Asia and Africa, in The New Dinosaurs: An Alternative Evolution.
High in the luxuriant branches of the tropical forest trees in the northeast of the Australian continent, a black and white creature throws itself from one branch to another and disappears among the foliage. At first glance it looks like an arbrosaur, one of the tree-living coelurosaurs that are found almost all over the world. Then it appears again, and its bright face, surmounted by a high crest, is its most obvious feature. This is a crackbeak - a tree-living variety of the successful basal ornithopods.
Crackbeak ancestors, Fulgurotherium, were lightly built running animals, evolved to run swiftly across open country. Now the legs have developed into jumping legs, and the lightweight feet have evolved for perching. The small first toe has turned round to face the rear, so the foot can now be used for grasping branches. The tail is still a long, stiff balancing organ but can now be used as a third leg; pressed against a tree trunk it gives the animal a firm anchor while feeding. As in the arbrosaurs, the crackbeak has developed a strong system of bones and muscles in the shoulder region to help it climb trees. The front feet have become dexterous hands. Crackbeaks are found in many other parts of the world, particularly in the tropical rainforests of the Afrotropical and Indomalayan ecozones (some Asian specimens evolving into the taddey), but it is only in Australia that they are so abundant and varied.
The crackbeak's bill is delicate enough to pick individual berries out of a bunch, yet powerful enough to crack open even the hardest nuts. The beak is used only for picking and cracking; the actual chewing is done by the back teeth. The horny crest on the head, along with the bright color of the face and dewlap, are used for signaling both to mates and to enemies.
The crackbeak hand, like those of almost all basal ornithopods, has a full complement of five fingers. All the fingers are long and mobile. Both the first and the fifth fingers are opposable, or able to cross over the palm of the hand in order to grasp things. These are very useful to the crackbeak while climbing in the branches and selecting food from between the twigs.