A molecular model for the evolution of endothermy in the Theropod-Bird lineage

Ectothermy is a primitive state; therefore, a shared common ancestor of cro codiles, dinosaurs, and birds was at some point ectothermic. Birds, the ext ant descendants of the dinosaurs, are endothermic. Neither the metabolic tr ansition within this lineage nor the place the dinosaurs held along the ect othermic-endothermic continuum is defined. This paper presents a conceptual model for the evolution of endothermy in the theropod-bird lineage. It is recognized that other animals (some fish, insects, etc.) are functionally e ndothermic. However, endothermy in other clades is beyond the scope of this paper, and we address the onset of endothermy in only the theropod/bird cl ade. The model begins with simple changes in a single gene of a common ance stor, and it includes a series of concomitant physiological and morphologic al changes, beginning perhaps as early as the first archosaurian common anc estor of dinosaurs and crocodiles. These changes continued to accumulate wi thin the theropod-avian lineage, were maintained and refined through select ive forces, and culminated in extant birds. Metabolic convergence or homopl asy is evident in the inherent differences between the endothermy of mammal s and the endothermy of extant birds. The strength and usefulness of this m odel lie in the phylogenetic, genetic, evolutionary, and adaptive plausibil ity of each of the suggested developmental steps toward endothermy. The mod el, although conceptual in nature, relies on an extensive knowledge base de veloped by numerous workers in each of these areas. In addition, the model integrates known genetic, metabolic, and developmental aspects of extant ta xa that phylogenetically bracket theropod dinosaurs for comparison with inf ormation derived from the fossil record of related extinct taxa. (C) 2001 W iley-Liss, Inc.