Australia's Arboreal Dinosaurs
A When most people imagine dinosaurs, they picture enormous animals walking across open ground. Yet dinosaurs occupied a much wider range of environments than this popular image suggests. Some were small, lightly built creatures, and palaeontologists have long debated whether certain species may have spent at least part of their lives in trees. Such animals are described as arboreal, meaning adapted to living or moving among branches. Fossils from several parts of the world have provided evidence of dinosaurs with features that may have helped them climb. In Australia, however, investigating this possibility is particularly difficult because the dinosaur fossil record is incomplete.
B During the Early Cretaceous, around 100 to 125 million years ago, southern Australia looked very different from the continent today. Australia was still connected to Antarctica and formed part of the enormous southern landmass known as Gondwana. Areas that are now in the state of Victoria lay much closer to the South Pole. Despite their high latitude, these regions supported forests, rivers and a surprisingly diverse collection of animals. The climate was cool by dinosaur standards, although it was considerably warmer than modern polar regions. Forests containing conifers, ferns and other plants covered much of the landscape. During certain parts of the year, daylight would have been limited for long periods. Animals living there therefore had to cope with unusual seasonal conditions. Fossil discoveries suggest that several types of small dinosaurs successfully inhabited this environment.
C One of the difficulties facing Australian palaeontologists is the limited nature of the fossil evidence. Complete dinosaur skeletons are rare in Australia, and many species are known from only a handful of bones. In southern Victoria, researchers have discovered numerous teeth and isolated skeletal remains belonging to different groups of meat-eating dinosaurs, or theropods. Small dinosaurs are particularly difficult to study because their delicate bones are less likely to survive the fossilisation process. After an animal died, its skeleton could be damaged by scavengers, flowing water or changes in the surrounding sediment. Small bones were easily scattered or destroyed. As a result, scientists may know that a small dinosaur existed without having enough of its skeleton to determine precisely how it moved or behaved.
D Determining whether a dinosaur climbed trees involves careful examination of its anatomy. Modern climbing animals often possess features that help them grip branches, maintain balance and move safely above the ground. These can include curved claws, flexible joints, long limbs and feet capable of grasping. However, individual features can be misleading. A strongly curved claw, for instance, might be useful for climbing, but it could also be used to catch prey or defend against predators. Similarly, long arms do not necessarily prove that an animal lived in trees. Palaeontologists must therefore examine several parts of the skeleton together rather than relying on a single characteristic. Comparisons with living animals can also be valuable. Birds are especially important because they are the surviving descendants of theropod dinosaurs. By studying the feet, claws and limb movements of modern birds that spend time in trees, researchers can identify characteristics that may have served similar purposes in extinct species.
E Discoveries outside Australia have demonstrated that at least some small dinosaurs possessed remarkable adaptations associated with life above the ground. Fossils from China, for example, include small feathered dinosaurs with long limbs and curved claws. Some even had extensive feathers on both their front and rear limbs. Researchers have proposed that certain species could climb trees and perhaps glide between them. Such discoveries have changed the traditional picture of dinosaurs as exclusively ground-dwelling animals. They have also contributed to an important debate concerning the origin of bird flight. According to one hypothesis, flight developed gradually among small dinosaurs that climbed trees and initially used feathered limbs to control their descent. Another theory proposes that the ancestors of birds were primarily ground runners and that wings developed for other purposes before true flight evolved.
F Could comparable animals have lived in prehistoric Australian forests? There are reasons to believe this is possible. Fossils from southern Australia show that several groups of relatively small theropods were present during the Cretaceous. Some may have been related to groups elsewhere in the world that included feathered species. Nevertheless, researchers must be cautious. No Australian fossil currently provides the same type of clear evidence for tree climbing as some exceptionally preserved specimens from Asia. Fossil feathers are particularly rare because soft structures usually decay before they can be preserved. Consequently, scientists often have to reconstruct Australian dinosaurs from bones alone. This does not mean that arboreal dinosaurs were absent. Forest environments would have provided numerous ecological opportunities. Trees offered insects, small vertebrates, seeds and other food sources, as well as possible protection from larger predators. For a lightweight dinosaur capable of climbing, the forest canopy may have represented an environment with relatively little competition.
G The possibility of tree-dwelling dinosaurs may also help scientists understand why small body size became increasingly important in some theropod groups. Moving through branches is easier for a lightweight animal than for a large one. Smaller animals place less pressure on branches and can move through narrow spaces that are inaccessible to larger predators. Some researchers have suggested that life in trees may have encouraged other evolutionary changes. Good depth perception would have been useful when judging distances between branches, while long arms could have helped an animal reach or balance. Feathers, which initially may have evolved for insulation or display, could later have assisted animals in controlling jumps and falls. Over many generations, a combination of these characteristics might have created animals increasingly suited to aerial movement. In this view, the transition from running dinosaur to flying bird was not a sudden event but a long process involving numerous intermediate stages.
H Finding stronger evidence in Australia will depend on future discoveries. New fossil sites are still being investigated, and improved technology allows scientists to obtain more information from specimens than was possible in the past. Three-dimensional scanning, microscopic examination and detailed comparisons with living animals can reveal features that were previously overlooked. The absence of complete fossils remains a major challenge, but even a single well-preserved skeleton could significantly alter current understanding. It might demonstrate that Australia's ancient polar forests contained dinosaurs capable of climbing, or it could show that the small species found there remained firmly on the ground. Either way, the search is important. Australia's fossil record provides a rare view of dinosaur life at high southern latitudes. Studying these animals can reveal not only how dinosaurs adapted to unusual climates but also how some of the smallest members of their group eventually developed lifestyles very different from those of the giants with which they shared the prehistoric world.