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Cardio-Pulmonary Anatomy in Theropod Dinosaurs: Implications from Extant Archosaurs

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Cardio-Pulmonary Anatomy in Theropod Dinosaurs: Implications from Extant Archosaurs JOURNAL OF MORPHOLOGY 270:1232–1246 (2009) Cardio-Pulmonary Anatomy in Theropod Dinosaurs: Implications From Extant Archosaurs Devon E. Quick* and John A. Ruben Department of Zoology, Oregon State University, Corvallis, Oregon 97331 ABSTRACT Although crocodilian lung and cardiovas- increased pulmonary surface area, more efficient cular organs are markedly less specialized than the gas diffusion at the lungs, greatly expanded oxy- avian heart and lung air-sac system, all living archo- gen carrying capacity of the blood, and increased saurs possess four-chambered hearts and heterogene- maximal cardiac output (Bennett, 1991; Ruben, ously vascularized, faveolar lungs. In birds, normal lung 1995). Furthermore, in birds and mammals there function requires extensive, dorsally situated nonvascu- larized abdominal air-sacs ventilated by an expansive is no mixing of deoxygenated systemic venous (pul- sternum and specially hinged costal ribs. The thin monary arterial) blood with oxygenated systemic walled and voluminous abdominal air-sacs are supported arterial (pulmonary venous) blood (see Fig. 1). laterally and caudally to prevent inward (paradoxical) Complete separation of pulmonary and systemic collapse during generation of negative (inhalatory) pres- circulations is achieved initially where interatrial sure: the synsacrum, posteriorly directed, laterally open and interventricular septa divide the heart into pubes and specialized femoral-thigh complex provide four chambers. This separation is maintained by requisite support and largely prevent inhalatory col- two distinct great vessels which emerge from the lapse. In comparison, theropod dinosaurs probably heart: the pulmonary artery (trunk) from the right lacked similarly enlarged abdominal air-sacs, and skel- ventricle and the single, systemic aortic arch from eto-muscular modifications consistent with their ventila- tion. In the absence of enlarged, functional abdominal the left ventricle. During fetal development there air-sacs, theropods were unlikely to have possessed a is communication between the two circuits via the specialized bird-like, air-sac lung. The likely absence of truncus arteriosus and the foramen ovale and both bird-like pulmonary function in theropods is inconsistent passages normally close at or before hatching or with suggestions of cardiovascular anatomy more sophis- birth. Should there be incomplete closure, systemic ticated than that of modern crocodilians. J. Morphol. oxygen delivery capacity and stamina can be 270:1232–1246, 2009. Ó 2009 Wiley-Liss, Inc. reduced significantly (Oelberg et al., 1998; Brick- ner et al., 2000; Hicks, 2002; Suchon et al., 2005). KEY WORDS: theropod heart; theropod lung; bird lung Importantly, oxygen delivery modifications associ- ated with endothermy are most critical not at rest but during bouts of routine or accelerated levels of INTRODUCTION activity (Bennett and Ruben, 1979). There exists a diversity of vertebrate cardiovas- In addition to the separation of oxygenated and cular morphologies ranging from the incompletely deoxygenated blood in endotherm cardiovascular divided heart and dual aortic arches of the lower systems, differential pressures are maintained in tetrapods to the fully partitioned heart and single the pulmonary and systemic vascular circuits. aortic arch of birds and mammals. In all cases, the Mean systolic pressures in the systemic circula- cardiovascular system delivers nutrients and oxy- tions of diverse endotherms vary between 87 and gen to tissues while simultaneously removing met- 216 cm H2O, while the resting pulmonary circula- abolic wastes including carbon dioxide. The range tions of men, ducks and chickens are considerably of vertebrate cardiovascular designs reflects, in lower, typically achieving less than 30 cm H2O part, varied metabolic demands in endotherms and ectotherms. Endotherms, whose field metabolic Additional Supporting Information may be found in the online version of this article. rates exceed those in similar-mass ectotherms by some 15–20 fold (Bennett, 1991; Bennett et al., *Correspondence to: Devon E. Quick, Department of Zoology, 2000), require substantially more oxygen and Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331. nutrients per unit time. Accordingly, in birds and E-mail: [email protected] mammals, the cardiovascular and respiratory sys- tems are likely to have evolved largely in response Received 8 September 2008; Revised 16 February 2009; to selection for enhanced exchange, transport and Accepted 21 February 2009 delivery of respiratory gasses. Published online 20 May 2009 in Specialized features which developed independ- Wiley InterScience (www.interscience.wiley.com) ently in both birds and mammals include DOI: 10.1002/jmor.10752 Ó 2009 WILEY-LISS, INC. CARDIO-PULMONARY ANATOMY 1233 Fig. 1. Diagrams of the heart and great vessels in crocodilians (left) and birds (right). Both possess fully subdivided, four-chambered hearts. Crocodilians, like many sauropsids, retain paired aortae which allow mixing of oxygenated and deoxygenated blood (modified from Goodrich, 1930). (Johansen, 1972; Seymour and Blaylock, 2000; divided into three functional chambers) acts as a Weidong et al., 2002). High systemic arterial pres- single pressure pump, synchronously ejecting sures must be maintained in endotherms to over- blood into two systemic aortic arches and the pul- come elevated total peripheral resistance in monary trunk; widely disparate pressures in sys- densely vascularized tissues. Similarly elevated temic and pulmonary circulations are not possible pulmonary pressures are untenable due to the nec- (Hicks, 1998). Thus, among several turtles and essarily thin respiratory membrane in the lung. Of nonvaranid lizards, systemic peak systolic pres- all the air breathing vertebrates, endotherms have sures vary between 30 and 80 cm H2O but pulmo- evolved the thinnest respiratory membranes which nary peak systolic pressures average only 30% facilitate faster blood oxygen loading (Maina, lower at 20–55 cm H2O (Hicks, 1998). The conse- 2002). However, the thinner the respiratory mem- quence of elevated pulmonary pressures, which brane, the more susceptible respiratory mem- can be 53 those of mammals, in combination branes are to pulmonary edema, effusion or paren- with particularly leaky capillary walls, is that chymal structural damage if perfused at elevated ectotherms produce 10–203 more plasma filtrate pressures. Significantly, similar adverse effects at the respiratory membrane (Burggren, 1982; have been recorded in diverse animals during Wang et al., 1998). This ‘‘wet lung’’ state increases maximal exercise (West and Mathieu-Costello, the barrier to gas exchange in the lung and may 1995; Wang et al., 2003; Ware and Matthay, 2005). contribute to the relatively lower oxygen extrac- Even in the most extreme example, the giraffe tion capacity of reptilian lungs when compared to maintains the highest recorded systemic arterial those of birds, although it does not explain similar pressure (>325 cm H2O) to allow adequate brain oxygen extraction values to those of mammals perfusion, but laboratory pulmonary arterial pres- with dry lungs (Bennett, 1973; Geist, 2000). sures peak at 65 cm H2O (Goetz et al., 1960), i.e., Fully partitioned four-chambered hearts and/or some 20% of systemic pressures. Thus, as endo- intermediate systemic vascular pressure differen- thermy developed along with elevated cardiac out- ces also occur in certain ectothermic animals. put to serve increased systemic tissue demands, so Crocodilians maintain systemic pressures some too did the fully partitioned four-chambered heart threefold those of the pulmonary circulatory pres- and physically separate arterial systems with mark- sures by utilizing a fully partitioned four-cham- edly different vascular resistances and pressures. bered heart (see Fig. 1), while Varanus and Python Most ectotherms have incompletely divided five- manage to do so with a five-chambered heart. chambered hearts and paired systemic aortic Under resting laboratory conditions, the left ven- arches. Upon contraction, the single ventricle (sub- tricle and right aorta of crocodilians develop mean Journal of Morphology 1234 D.E. QUICK AND J.A. RUBEN systolic pressures of 104 cm H2O; the right ventri- ence of central vascular shunts during the embry- cle develops only 63 cm H2O pressure and the pul- onic stages of all vertebrates, it is possible that monary trunk can be an even lower 21 cm H2O shunting is a plesiomorphic vertebrate character (Shelton and Jones, 1991). In the five-chambered lost in adult endotherms (Hicks, 2002). In endo- hearts of Varanus and Python, differential, crocodi- therms, as evolutionary pressures selected for lian-like pressures may also develop due to the increased oxygen delivery (increased cardiac out- presence of an almost complete muscular ridge put) to meet the elevated demands of the tissues, that effectively creates a temporary, four-cham- those characters reducing oxygen delivery bered condition during systole (Wang et al., 2003). (shunting) were lost, and others favoring increased As such, these seemingly endotherm-like cardio- oxygen extraction were acquired (thin respiratory vascular features (four-chambered hearts, differen- membrane, low pulmonary pressure), perhaps tial systemic/pulmonary pressures) are not exclu- resulting in the character states observed in mod- sive to birds and mammals. It has been previously ern endotherms and ectotherms. argued that these shared characters may
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