How the dino soared

Birds are among the most familiar of all living creatures. Today, there are over 9,000 species, and the study of avian biology (especially behaviour and ecology) has attracted the attention of battalions of naturalists. Birds in flight have impressed artists and poets from all cultures with their seemingly effortless mastery of the skies, and birdsong has inspired musicians through the centuries. On a more pragmatic note, we enjoy our Christmas turkey and scrambled eggs. Television documentaries inform us of the most intimate moments in the lives of blue tits, flamingos, and ostriches, and we all like to feed the ducks in the park. This familiarity is deceptive, however, and many aspects of bird biology and evolution remain mysterious.

The origin and early evolutionary history of birds has provoked considerable controversy among palaeontologists and ornithologists. Where do birds come from? How did flight evolve? How are the various groups of living and fossil birds related to one another? In The Rise of Birds, Sankar Chatterjee has attempted to answer some of these questions. The resulting book is likely to provoke controversy of its own, as it contains many ideas which fly in the face of current orthodoxy.

The book is aimed at a wide audience, including palaeontologists, ornithologists, and amateur naturalists. The style, however, is rather dry, and it is unlikely that a novice would be able to cope with the dense anatomical terminology.

For many, the most contentious issue raised in this volume will be the treatment of an enigmatic little animal named Protoavis. This first hit the headlines in the late 1980s when Chatterjee (who discovered and described its incomplete remains) claimed that Protoavis was the earliest bird. Prior to this, the earliest-known bird was Archaeopteryx, the feathered fossil from the Late Jurassic (about 150 million years ago) of Germany, much beloved by evolutionary biologists. Protoavis was recovered from the Late Triassic rocks of western Texas, and as such predates Archaeopteryx by around 75 million years. Not surprisingly, Chatterjee's announcement prompted a furore of criticism from the biological community, and many distinguished palaeontologists denounced Protoavis as a jumbled mixture of reptile bones, irrelevant to the study of avian evolution.

Despite this tirade, Chatterjee remains convinced that Protoavis is the first bird, and he provides a detailed description of the fossil in the first part of the book. The author highlights a large number of anatomical features which, if correctly interpreted, do look suspiciously avian. Paradoxically, many of these features also suggest that Protoavis was more advanced than Archaeopteryx, even though it occurs so much earlier. If Chatterjee is correct, then palaeontologists will have to seek the origin of birds in the Triassic period - much earlier than conventional wisdom suggests. Furthermore, the discovery of a Triassic bird would indicate that there were huge gaps in the pre-Archaeopteryx bird fossil record, waiting to be filled. Could Chatterjee be right? Many palaeontologists remain unconvinced. It is notable, however, that very few people (apart from Chatterjee) have given the remains of Protoavis the careful scrutiny they deserve. As a result, Protoavis remains in limbo, desperately in need of further study.

The following chapters move into much less controversial territory, and provide a comprehensive and up-to-date review of the fossil record of birds from the Jurassic and Cretaceous periods. Most attention is given to those species which are known from skeletal remains, though the author also surveys the occurrences of isolated feathers, eggs, and footprints. The reader is supplied with brief summaries of the anatomy, classification, and distribution of these birds, along with additional information on their purported lifestyles. Background information on the habitats of the birds helps to flesh out their biology and behaviour more vividly. These chapters are both interesting and useful, though their breadth of scope has prevented Chatterjee from providing more than anecdotal accounts of some of the early bird species. This is not a substantial criticism, given the limitations of time and space, but the lack of information on some bird groups was slightly disappointing. This brevity reaches extremes in Chatterjee's treatment of more recent bird groups, with only a single chapter devoted to the last 65 million years of bird evolution. Still, as most of the "interesting" evolutionary questions are concerned with events that occurred in much earlier times, this seems to be a justifiable course of action.

It has been known for well over a century that birds are the highly modified descendants of reptiles. Following a protracted debate, in which a number of different reptile groups (including crocodiles) were proposed as the ancestors of birds, a consensus has emerged, with most palaeontologists supporting the idea that birds are the direct descendants of small carnivorous dinosaurs. From the outset of the book, Chatterjee strongly supports the consensus view, and goes to some lengths to highlight the many similarities in the skeletons of birds and dinosaurs which indicate their kinship. The author takes this comparison further, demonstrating how many of the characteristic features of birds are modified versions of structures that can be found in their dinosaur progenitors. This exercise allows Chatterjee to detail the functional changes that were associated with the transformation of a bipedal ground-dwelling dinosaur into a winged and feathered flying machine. These sections of the book have much to commend them and they contain a mass of information. Unfortunately, it all comes across as rather dogmatic. Although most palaeontologists support the dinosaur theory, there is a small, but vociferous, minority who do not. Chatterjee pays lip-service to several competing hypotheses of bird origins, but the treatment is all too brief and presents little in the way of discussion. This same criticism can also be levelled at the rest of the book. Competing viewpoints are seldom discussed at length, and the reader is not really presented with enough evidence to make up his or her own mind.

One of the most original sections of the book is the chapter which deals with the origin of flight. The majority of biologists who work on this problem fall into one of two camps. One school of thought advocates the "ground up" theory, while the other school promotes the "trees down" hypothesis. Briefly, proponents of the "ground up" theory have suggested that the ancestor of birds was a swift bipedal runner which made occasional leaps into the air in pursuit of prey. In contrast, the "trees down" lobby suggests that birds are the descendants of tree-climbing animals that originally jumped from tree to tree. Chatterjee swiftly demolishes the "ground up" theory on biomechanical and ecological grounds, and goes on to provide his own detailed version of the "trees down" theory. I found Chatterjee's scenario both intriguing and plausible, and it is certainly worthy of further consideration. His seven-stage model includes a number of surprises, including the reconstruction of the "proto-bird" as a small tree-dwelling dinosaur akin to Velociraptor, which used its long tail as a prop, and its sharp claws as hooks while clambering up tree trunks.

The Rise of Birds is no unbiased textbook account, but represents Chatterjee's own highly personal view of avian evolution. As such, this is an interesting and provocative document, and one that should be read with an open, but critical, mind. Reading it was by turns instructive, irritating, frustrating, and fascinating. I hope that it provokes much thought, and much more work in an area which new discoveries have recently revitalised.

Paul M. Barrett is research fellow in palaeontology, Trinity College, Cambridge.