Cardio‐pulmonary anatomy in theropod dinosaurs: Implications from extant archosaurs

Although crocodilian lung and cardiovascular organs are markedly less specialized than the avian heart and lung air‐sac system, all living archosaurs possess four‐chambered hearts and heterogeneously vascularized, faveolar lungs. In birds, normal lung function requires extensive, dorsally situated nonvascularized abdominal air‐sacs ventilated by an expansive sternum and specially hinged costal ribs. The thin walled and voluminous abdominal air‐sacs are supported laterally and caudally to prevent inward (paradoxical) collapse during generation of negative (inhalatory) pressure: the synsacrum, posteriorly directed, laterally open pubes and specialized femoral‐thigh complex provide requisite support and largely prevent inhalatory collapse. In comparison, theropod dinosaurs probably lacked similarly enlarged abdominal air‐sacs, and skeleto‐muscular modifications consistent with their ventilation. In the absence of enlarged, functional abdominal air‐sacs, theropods were unlikely to have possessed a specialized bird‐like, air‐sac lung. The likely absence of bird‐like pulmonary function in theropods is inconsistent with suggestions of cardiovascular anatomy more sophisticated than that of modern crocodilians. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.