Mitt. Mus. Nat.kd. Berl ., Geowiss. Reihe 2 (1999) 75-79 19.10.1999 Hypothetical Lung Structure of Brachiosaurus (Dinosauria: Sauropoda) Based on Functional Constraints Steven F Perry' & Christian Reuters With 2 figures Abstract Comparison of avian and crocodylian lung structure suggests a basic archosaurian structural type, consisting of four rows of chambers that radiate independently from an intrapulmonary bronchus. The later structure is cartilage-reinforced cranially. Caudally, the cartilage reinforcement diminishes and the radiating pattern of tubular chambers becomes irregular. The pre- sence of extensive pneumatic spaces in the dorsal vertebrae combined with the extremely large size of Brachiosaurus suggests that the dorsal margin of the lungs was strongly attached to the body wall, thus restricting lung inflation in this region . By analogy with similarly structured testudine lungs, it is highly probable that the ventral part of the very large and cavernous lungs consisted of sac-like chambers, which acted as air reservoirs and also ventilated the gas exchange tissue, which was concentrated dorsally. Key words: Brachiosaurus, crocodile, Aves, turtle, Archosauria, lung, parenchyma. Zusammenfassung Die Strukturen der Vogel- und der Krokodillunge deuten auf einen basalen Bauplan der Archosaurierlunge hin, der aus vier Reihen von Kammern besteht, die von einem intrapulmonalen Bronchus entspringen . Dieser ist cranial knorpelig verstärkt . Caudal vermindert sich die knorpelige Verstärkung und das Verteilungsmuster der Kammern wird ungleichmäßig. Die An- wesenheit von pneumatischen Räumen in den Wirbelkörpern und die extreme Größe von Brachiosaurus deuten daraufhin, daß der dorsale Bereich der Lunge mit der Körperwand verbunden war. Dies schränkte die Belüftung der Lunge in dieser Region ein. In Analogie zu den ähnlich strukturierten Schildkrötenlungen ist es höchst wahrscheinlich, daß der ventrale Teil der sehr großen Lungen aus sackartigen Kammern bestand, die als Luftreservoir dienten und das dorsal liegende Gas- austauschgewebe ventilierten. Schlüsselwörter: Brachiosaurus, Krokodil, Vogel, Schildkröte, Archosaurier, Lunge, Gasaustauschgewebe . Introduction les autres." (Cuvier 1812). Recently this principle has returned to vogue (Thomason 1995), and has Since lungs of the sauropod Brachiosaurus (Ja- been applied to the reconstruction of theropod nensch 1937) and of other non-avian dinosaurs lungs (Perry 1992). are not fossilized, we must rely on indirect evi- In addition, exceptionally well-preserved ther- dence to reconstruct the structure of the respira- opods, in which the hindgut and the liver can be tory system. The principle of using recent analo- located, have served to intensify the debate re- gues to help reconstruct the soft tissue of fossil garding the structure and function of lungs in organisms was developed and exploited by Cu- dinosaurs (Ruben et al. 1997, 1999; Sasso & vier in the early 1800'' and is most explicitly sta- Signore 1998). To date, however, fossilized lung ted in his law of the correlation of parts: "Toute tissue has not been found. etre organise forme un ensemble . .. dont tout les Fortunately the phylogenetic position of birds parties se correspondent mutuellment.. Aucune relative to other dinosaurs as well as of croco- de ses parties ne peut changer sans que les au- dylians within the Archosauria is well established tres changent aussi, et par consequent chacune (Weishampel et al. 1990) . Comparison of croco- d'elles, prise separement, indique et donne toute dylian and avian lung structure, both adult and t Institut für Zoologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany. E-mail: [email protected]; [email protected] Received March 1999, accepted June 1999 76 Perry, E S. & C. Reuter, Hypothetical Lung Structure of Brachiosaurus embryonic, reveals a basic archosaurian structur- intrapulmonary bronchus appear to constitute al type (Fig. 1), which, when compared with that the basic structural type. The medial migration of of testudines and of lepidosaurs can be shown to the intrapulmonary bronchus together with the exhibit apomorphies. Using this basic structural spiral rotation of the chamber orifices about the type as a starting point, we can consider the intrapulmonary bronchus (proceeding caudad, a functional constraints likely to have been present right-hand spiral in the left lung, a left-hand spir- in Brachiosaurus as a means of refining this al in the right lung) modifies the basic plan simi- model. The results of this hypothetical construct larly in both crocodylian and in avian lungs. No- are presented here together with some physiolo- tably in the dorsal part of the lung, the chambers gical implications for the respiratory, cardiovas- are tubular, whereas apically, ventrally and caud- cular and locomotor systems. ally they are saccular. This tendency reaches an extreme in birds, where the tubular chambers form secondary bronchi and the saclike dilata- Functional anatomy of reptilian lungs tions, the airsacs (Fig. 1). Although testudines possess multicham- Comparison of crocodylian and avian lungs and platynotan lizards also bered lungs, as also must have been the case The lungs of crocodylians and of birds are sepa- with ancestral synapsids, the extremely asym- rated ventrally from the remaining body cavity metric (monopodial) branching pattern appears by a postpulmonary septum (Duncker 1978). The unique to archosaurs. lungs themselves are multichambered (Fig. 1) . In the cranial part of the lung, four rows of Lung parenchyma chambers (dorsal, ventral, medial and lateral), each consisting of four or five chambers, fol- The gas exchange tissue, or parenchyma, of rep- lowed caudally by an inconstant number of tilian lungs can vary in its depth, in the diameter chambers, all emanating from an unbranched of the smallest terminal units, and in its distribu- CL T P l J CL FaveCLr Single-chambered V T~ <ä~ a i= Transitional \ I EdicuCL A 11 101, N, Trabecular Mult hambered ~V' RzeGY~9'1'py G4J`'v~ .~EiJ Homogeneous~ g 101t -0,17 l .. ~ ,~cr &~-"'°`"W'" eteroge erT ous Fig . 2. Schematic representation of three variables in the Fig. 1 . Simplified representation of possible phylogenetic re- macroscopic structure of reptilian lungs. Illustrating the lationships among major groups of archosaurs. 1, Archo- structural type of lung are lung schemata of a scincid lizard sauria (Euparkeria as beginning of line); (1), presumed (single-chambered), an iguanid lizard (transitional) and a sea euparkerian lung type ; 2, Pterosauria ; (2), presumed avian- turtle (multi-chambered); illustrating the parenchymal distri- like lung with cranial airsacs ; 3, Crocodilia ; (3), Crocodilian bution are lung schemata of a teju lizard (homogeneous, lung type ; 4, Aves; (4), Avian lung-airsac system; 5, Coeluro- upper) and a caiman (homogeneous, lower), as well as a scin- sauria (Saurischia : Theropoda) ; (5), presumed carnosaurian/ cid lizard (heterogeneous, upper) and a varanid lizard (het- early coelurosaurian lung type ; 6, Carnosauria (Saurischia : erogeneous, lower) . The various parenchymal types can be Theropoda), for presumed lung see (5) ; 7, Sauropoda ; (7), found within a single heterogeneously partitioned lung, or presumed multisaccular sauropodian lung type ; 8, Stego- they can be characteristic of an entire homogeneously parti- sauria (Ornitischia) ; 9, Ankylosauria; 10, Ceratopsia; 11, tioned lung . In general, even within a heterogeneous lung, a Hardrosauridae (Ornitischia : Ornithopoda) ; 12, Iguanodonti- particular parenchymal type will predominate . (CL, central dae (Ornithischia : Ornithopoda) ; (8-12), presumed broncho- lumen of lung or chamber; E, edicula ; F, faveolus ; IB, intra- alveolar-like lung type of Ornitischia (modified after Perry pulmonary bronchus; P, parenchymal layer; T, smooth-muscu- 1989) lar core of trabeculae) Mitt . Mus. Nat.kd. Berl ., Geowiss. Reihe 2 (1999) 77 tion within the lung. Lungs with homogeneously Reptiles tend to have large lungs, which, like distributed parenchyma (Fig. 2) are intrinsically those of mammals, encase the heart and extend stable and are often freely suspended in the from the dorsal arch of the ribs to the sternum. body cavity. Those with a highly heterogeneous A muscular diaphragm does not develop in the parenchymal distribution, however, are typically postpulmonary septum of reptiles and the lungs firmly attached to the concavity of the body often extend caudally into the pelvic region. Cro- wall. This construction prevents collapse of the codylians represent an exception . In this group, densely partitioned dorsal regions. Thus, dorsal the lungs abut the liver caudally and thus are attachment is indicative of heterogeneous par- limited in their caudal extension . They are small enchymal distribution (Fig. 2). relative to the body mass, in keeping with the The surface-to-volume ratio in the parenchy- buoyancy requirements of a diving animal. In- ma of reptilian lungs is extremely variable, ran- spiration occurs when the liver is pulled caudally ging from 18 cm2/cm3 in the testudine Trachemys by the diaphragmaticus muscle, thus stretching scripta to nearly 200 cm2/cm3 within the single- the lungs in the conically broadening thoracic chambered lungs of scincomorph lizards (Perry cavity (Gans & Clark 1978) . This liver retraction 1983). These values compare with 230 cm2/cm3 mechanism - commonly called the "hepatic pis- for the short-nosed echidna, 750 cm2/cm3 for the ton" - does not exist