The Evolution Of The Air Sac System

The complex air-sac-lung system found in birds had to have evolved from a reptilian, sac-like lung, and all evidence indicates that this happened in the middle Triassic. The most primitive thero-pods from this time (the first dinosaurs) do not show bone pneumatization, but their ribs became double headed, showing that the rib cage itself was capable of a great ventilation capacity. Perhaps as a consequence of going bipedal, these dinosaurs may have switched from the more primitive abdominal...

Conclusions

The fact that the air sac system survived the Triassic mass extinction, which killed off many of the more common lineages of Triassic animals, such as the phytosaurs, most cynodonts, and many primitive reptilian groups, is, to us, persuasive. The filter that was the Triassic mass extinction was one where method of respiration seemingly mattered. One of us (Ward, 2006) showed that of the various lung types present at the beginning of this extinction, the clades with air sac respiratory system...

References

Alonso PD, Milner AC, Ketcham RA, Cookson MJ, Rowe TB. 2004. The avian nature of the brain and inner ear of Archaeopteryx. Nature 430 666-669. Balanoff AM, Xu X, Kobayashi Y, Matsufune Y, Norell MA. 2009. Cranial osteology of the theropod dinosaur Incisivosaurus gauthieri (Theropoda Oviraptoro-sauria). American Museum Novitates 3651 1-35. Barsbold R. 1983. Carnivorous dinosaurs from the Cretaceous of Mongolia. Joint Soviet-Mongolian Paleontological Expedition Transactions 19 5-120. (In Russian)...

Oxygen Through Time

The first step in uncovering past atmospheric gas compositions comes from understanding the carbon cycle, which comes with a long-term component, the movement of carbon into, and out of rocks, and a shorter term cycle, the movement of carbon in and out of air, water, and organisms. It is the long-term cycle that most importantly affected the levels of oxygen and carbon dioxide over the past 500 million years. The most important processes affecting levels of oxygen have been the rates of organic...

Institute of Vertebrate Paleontology and Paleoanthropology Beijing China 2Los Angeles County Museum Los Angeles USA

Birds are the most diverse group of living land vertebrates on the planet, yet the origin of the clade is one of the most heated and longest debates in scientific history (Witmer, 2001, 2002). Recent years have witnessed incredibly rapid growth in the fossil record of Cretaceous birds and theropod dinosaurs and today, the idea that birds are part of the diverse evolutionary radiation of maniraptoran theropods is nearly universally accepted and supported by similarities in skeletal morphology,...

Respiration And Bipedalism

The fossil record shows that the earliest true dinosaurs were bipedal, and came from more primitive bipedal thecodonts slightly earlier in the Triassic. Thecodonts (diapsids) were the ancestors of the lineage giving rise to the crocodiles as well, and may have been either warm blooded or heading in that way. We see bipedalism as a recurring body plan in this group, and there were even bipedal crocodiles early on. Why bipedalism, and how could it have been an adaptation to low oxygen Our view is...

Roadmap To The Dinosaurian Heritage Of Birds

Despite their radically different body plans, birds inherited a mosaic of anatomical traits from various stages of vertebrate history. A host of discoveries over the past five decades provide a detailed road map to how the highly specialized avian anatomy was assembled over the evolutionary lineage leading to birds and demonstrates that many of the traits that are considered uniquely avian among extant amniotes actually arose before the origin of birds themselves. While most of our...

Preface

The scope of this book ranges widely, from bio-molecular aspects of avian biology to details of the anatomy of dinosaurs. However, it is not just a simple compilation of current material. Its purpose is to help bridge a gap that has developed between those who study birds as fossils and those who study the living animals. The size of that gap has much to do with two controversies related to the evolution of birds that remained unresolved for much of the 20th century. One involved the origin of...

List of Contributors

HERCULANO ALVARENGA Museu de Historia Natural de Taubate, Taubate, Brazil TATSURO ANDO Ashoro Museum of Paleontology, Hokkaido, Japan F. KEITH BARKER Department of Ecology, Evolution and Behavior and, Bell Museum of Natural History, University of Minnesota, St. Paul, USA ALYSSA BELL University of Southern California, Los Angeles, California, USA ROBERT BERNER Yale University, New Haven, USA SARA BERTELLI Museum f r Naturkunde, Berlin, Germany ESTELLE BOURDON American Museum of Natural History,...