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The Mesozoic Aviary

Paul M. Barrett

Glorified Dinosaurs: The Origin and Early Evolution of Birds. Luis M. Chiappe. x + 263 pp. John Wiley and Sons, 2007. $69.95.

Charles Darwin famously declared that the origin of flowering plants was an "abominable mystery": The highly modified structure and reproductive habits of the angiosperms set them apart from all other plants and, until recently, confounded attempts to identify the common ancestor of roses, grasses and oak trees. However, Darwin's adage might equally well have been applied to many other plant and animal groups with specialized and idiosyncratic anatomy or an incomplete fossil record.


150-million-year-old%20featherClick to Enlarge ImageIn 1860, just one year after publication of On the Origin of Species, a 150-million-year-old fossil—a finely detailed impression of a single feather (shown at left)—was found in Solnhofen, Germany. The following year the same strata yielded a nearly complete skeleton of a feathered animal that would become an icon of evolution—the earliest known bird, Archaeopteryx lithographica. The origins of birds were particularly mysterious: Warm blood and the ability to fly could potentially unite them with mammals, whereas certain skeletal features and the possession of scales linked them with reptiles. The discovery of Archaeopteryx provided a definitive "intermediate form" that indicated the strong evolutionary relationship between birds and reptiles, offering 19th-century biologists much-needed support for Darwin's arguments. But even though Archaeopteryx demonstrated the link between birds and reptiles, there was little consensus about which particular group of reptiles was closest to birds. This uncertainty, in turn, was a major obstruction to meaningful discussions on the origin of key avian features, including flight and feathers.

Over the past 150 years, numerous candidates have been proposed in the search for the closest reptilian relatives of birds, including crocodiles, pterosaurs, various primitive Triassic archosaurs ("thecodonts") and the enigmatic Triassic reptiles Longisquama (a tiny creature with very long, superficially featherlike scales fanning out from its back) and Protoavis (proposed as the oldest known bird, a claim that has been vigorously disputed by most paleontologists). However, an overwhelming amount of evidence now shows that, beyond reasonable doubt, the
ancestors of birds are to be found among the meat-eating theropod dinosaurs, of which the gargantuan Tyrannosaurus rex is the most famous example.

Thomas Henry Huxley (Darwin's pugilistic "bulldog") first proposed this idea in the 1860s, and John Ostrom of Yale University expanded and elaborated on it in a seminal 1976 paper demonstrating the presence of many detailed similarities between the skeletons of birds and those of small predatory dinosaurs, such as Deinonychus. These features were not present in other reptile groups and provided strong support for the dinosaurian ancestry of birds, a hypothesis that has now emerged as the consensus view among the majority of paleontologists. Put simply, birds are feathered dinosaurs that can fly.

Birds have a relatively poor fossil record in comparison with that of other vertebrate groups: Weathering, abrasion and the activities of other organisms easily destroy their lightweight bones. For many years, the record of Mesozoic birds consisted only of Archaeopteryx and several Cretaceous forms, notably Hesperornis and Ichthyornis, which together were responsible for shaping our entire knowledge of early avian history. However, the past 20 years have witnessed the discovery of a staggering number of new Cretaceous bird localities, many characterized by conditions that have allowed the preservation of these fragile skeletons.


Archaeopteryx%20skeletonClick to Enlarge ImageLuis Chiappe, a central figure in this enterprise, has been involved in describing many new bird species from the Cretaceous rocks of Argentina, Spain, Mongolia and China and in deducing their evolutionary relationships and paleobiology. As one of the world's leading paleornithologists, he is ideally qualified to guide us through the burgeoning Mesozoic aviary and to reveal its numerous surprises.

Chiappe's new book, Glorified Dinosaurs, opens with a wide-ranging introduction to the anatomy of birds, their fossil record and the methods used to reconstruct evolutionary relationships, along with short excursions into plate tectonics and fossilization. The text also provides information on the phylogenetic position of birds among vertebrates in general, concluding (unsurprisingly) that they should be considered as part of the dinosaur radiation.

This theme is explored further in the second chapter, which summarizes the various lines of evidence supporting the close relationship of birds and theropods. The evidence ranges from the many detailed (and sometimes esoteric) similarities in skeletal anatomy to developmental biology and the presence of birdlike nesting behaviors in dinosaurs. Perhaps the most convincing evidence, however, has been the discovery of spectacularly preserved dinosaur specimens from the Lower Cretaceous Yixian Formation of Liaoning Province, China, which not only share numerous skeletal characters with birds but also possess feathers, a feature previously thought to be uniquely avian.

These dinosaurs, including Sinornithosaurus (a dromaeosaurid with a downy coat and birdlike shoulder and pelvic girdles) and Caudipteryx (a turkey-sized oviraptorosaur with a short, feathered tail and beaklike snout) show that feathers did not evolve in conjunction with flight (as had been postulated), but appeared well before flight and must have had some other function (maybe they were for display or insulation) before being co-opted to form part of the bird wing. Moreover, work on theropod and bird evolutionary history now shows that many of the other features previously associated with birds and with flight actually appeared first in nonavian theropod dinosaurs.

Chiappe dismisses other theories relating to bird origins relatively quickly, which might represent a missed opportunity. Although none of these alternative hypotheses are supported by the same volume of evidence as the dinosaur-bird connection, a small but vociferous band of ornithologists remains bitterly opposed to the dinosaurian origin of birds. This schism is partly due to disagreements over the methodology used to construct evolutionary trees and to differences of opinion regarding the anatomy of various key animals. This book would have been the ideal vehicle for pointing out the flaws and weaknesses of these competing hypotheses, so as to lay them to rest once and for all.

To some extent the discussion of bird origins is merely a prelude to the bulk of the book, which deals with the early diversification of birds. Again, discoveries from Liaoning are key to many of the recent advances that have been made in this field, but specimens from other corners of the world have added important elements to the story. This new material is revealing previously inaccessible information on the sequence of evolutionary changes among early birds, including improvements to the flight apparatus, and has also yielded clues on dietary and ecological specializations. Cretaceous birds were surprisingly diverse and included a variety of groups, such as the enantiornithines, which did not survive the great mass extinction that wiped out the nonavian dinosaurs. Many of these ancient birds have not been given the coverage they deserve in other popular treatments, and the comprehensive account in this volume represents one of its major strengths.

Chiappe is to be congratulated on this excellent book. The text provides sufficient detail to satisfy the specialist but is written in a pleasant style that will be easily accessible to the lay reader. Glorified Dinosaurs represents the most up-to-date review of early birds currently available and benefits from being the first to incorporate detailed information on the exciting recent discoveries of birds (and other feathered dinosaurs) into a wider context. The volume is well illustrated with numerous photographs of specimens and many color reconstructions. It will appeal to a wide audience of bird lovers, dinosaur-philes and people interested in evolutionary biology, serving not only as an entertaining popular introduction to the subject but also as a thorough primer for college-level courses. Finally, it should leave you convinced: That bluebird in your backyard really is a living dinosaur.

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