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Pterosaurs were the first vertebrates to achieve active flight, with some derived forms reaching enormous size. Accumulating fossil evidence confirms earlier indications that selection for large size in these flying forms resulted in a light, yet strong skeleton characterized by fusion of many bones of the trunk. However, this process also added mechanical constraints on the mobility of the thorax of large pterosaurs that likely limited the options available for lung ventilation. We present an alternative hypothesis to recent suggestions of an avian-like mechanism of costosternal pumping as the primary means of aspiration. An analysis of the joints among the vertebrae, ribs, sternum, and pectoral girdle of large pterosaurs indicates limited mobility of the ribcage and sternum. Comparisons with modes of lung ventilation in extant amniotes suggests that the stiffened thorax, coupled with mobile gastralia and prepubic bones, may be most consistent with an extracostal mechanism for lung ventilation in large pterodactyloids, perhaps similar to a crocodile-like visceral displacement system.

Conventional wisdom notwithstanding, fur and feathers are unlikely to have arisen in direct association with elevated metabolic rates in early mammals, birds, or their ancestors. A complete insulative fur coat probably appeared first in the earliest mammals long after mammalian ancestors (therapsids) had attained mammalian, or near-mammalian, metabolic rates. The evolution of feathers was unlinked to the evolution of modern avian metabolic rates since early, fully flighted birds (i.e., Archaeopteryx) retained an ectothermic metabolic status. Recent claims of “feathered dinosaurs” should be regarded with caution.

Longisquama insignis was an unusual archosaur from the Late Triassic of central Asia. Along its dorsal axis Longisquama bore a series of paired integumentary appendages that resembled avian feathers in many details, especially in the anatomy of the basal region. The latter is sufficiently similar to the calamus of modern feathers that each probably represents the culmination of virtually identical morphogenetic processes. The exact relationship of Longisquama to birds is uncertain. Nevertheless, we interpret Longisquama 's elongate integumentary appendages as nonavian feathers and suggest that they are probably homologous with avian feathers. If so, they antedate the feathers of Archaeopteryx , the first known bird from the Late Jurassic.

Summary Dinosaurs were among the most distinctive and successful of all land vertebrates. Attempts at reconstructing their biology have become commonplace. However, given the absence of closely comparable living models, deciphering their physiology necessarily remains speculative and determination of their metabolic status has been particularly problematical. Nevertheless, many paleontologists have advocated the notion that they were probably ‘‘warm-blooded’’ (endothermic), thus providing a model supposedly essential to the interpretation of these animals as having led particularly active, interesting lives. Those suppositions notwithstanding, the apparent absence of respiratory turbinates in dinosaurs, as well as likely ectothermic patterns of thermoregulation in very early birds, argues strongly that these animals were unlikely to have achieved the metabolic status of modern terrestrial endotherms. These data are not necessarily inconsistent with current models of active lifestyles of dinosaurs. BioEssays 20:852‐859, 1998. r 1998 John Wiley & Sons, Inc.

In our report “Lung structure and function in theropod dinosaurs and early birds” (14 Nov., p. [1267][1]), we inadvertently substituted an incorrect photo in place of an illustration that should have appeared as figure 1D. Here we present the correct figure (below, 1X), which illustrates the

Reptiles and birds possess septate lungs rather than the alveolar-style lungs of mammals. The morphology of the unmodified, bellowslike septate lung restricts the maximum rates of respiratory gas exchange. Among taxa possessing septate lungs, only the modified avian flow-through lung is capable of the oxygen–carbon dioxide exchange rates that are typical of active endotherms. Paleontological and neontological evidence indicates that theropod dinosaurs possessed unmodified, bellowslike septate lungs that were ventilated with a crocodilelike hepatic-piston diaphragm. The earliest birds ( Archaeopteryx and enantiornithines) also possessed unmodified septate lungs but lacked a hepatic-piston diaphragm mechanism. These data are consistent with an ectothermic status for theropod dinosaurs and early birds.

Pterosaurs were the first vertebrates to achieve active flight, with some derived forms reaching enormous size. Accumulating fossil evidence confirms earlier indications that selection for large size in these flying forms resulted in a light, yet strong skeleton characterized by fusion of many bones of the trunk. However, this process also added mechanical constraints on the mobility of the thorax of large pterosaurs that likely limited the options available for lung ventilation. We present an alternative hypothesis to recent suggestions of an avian-like mechanism of costosternal pumping as the primary means of aspiration. An analysis of the joints among the vertebrae, ribs, sternum, and pectoral girdle of large pterosaurs indicates limited mobility of the ribcage and sternum. Comparisons with modes of lung ventilation in extant amniotes suggests that the stiffened thorax, coupled with mobile gastralia and prepubic bones, may be most consistent with an extracostal mechanism for lung ventilation in large pterodactyloids, perhaps similar to a crocodile-like visceral displacement system.

Analysis of the nasal region in fossils of three theropod dinosaurs ( Nanotyrannus, Ornithomimus, and Dromaeosaurus ) and one ornithischian dinosaur ( Hypacrosaurus ) showed that their metabolic rates were significantly lower than metabolic rates in modern birds and mammals. In extant endotherms and ectotherms, the cross-sectional area of the nasal passage scales approximately with increasing body mass M at M 0.72 . However, the cross-sectional area of nasal passages in endotherms is approximately four times that of ectotherms. The dinosaurs studied here have narrow nasal passages that are consistent with low lung ventilation rates and the absence of respiratory turbinates.

Modern birds have markedly foreshortened tails and their body mass is centred anteriorly, near the wings1,2,3,4,5. To provide stability during powered flight, the avian centre of mass is far from the pelvis, which poses potential balance problems for cursorial birds. To compensate, avians adapted to running maintain the femur subhorizontally, with its distal end situated anteriorly, close to the animal's centre of mass; stride generation stems largely from parasagittal rotation of the lower leg about the knee joint6,7,8,9,10,11,12. In contrast, bipedal dinosaurs had a centre of mass near the hip joint and rotated the entire hindlimb during stride generation4,5,6,7,8,11,12,13. Here we show that these contrasting styles of cursoriality are tightly linked to longer relative total hindlimb length in cursorial birds than in bipedal dinosaurs. Surprisingly, Caudipteryx , described as a theropod dinosaur14,15, possessed an anterior centre of mass and hindlimb proportions resembling those of cursorial birds. Accordingly, Caudipteryx probably used a running mechanism more similar to that of modern cursorial birds than to that of all other bipedal dinosaurs. These observations provide valuable clues about cursoriality in Caudipteryx , but may also have implications for interpreting the locomotory status of its ancestors.

Ultraviolet light analysis of a fossil of the theropod dinosaurScipionyx samniticusrevealed that the liver subdivided the visceral cavity into distinct anterior pleuropericardial and posterior abdominal regions. In addition,Scipionyxapparently had diaphragmatic musculature and a dorsally attached posterior colon. These features provide evidence that diaphragm-assisted lung ventilation was present in theropods and that these dinosaurs may have used a pattern of exercise physiology unlike that in any group of living tetrapods.

Skeletal ontogeny in extant archosaurians (crocodilians and birds) indicates that the morphology of the perinatal pelvic girdle is an indicator of overall developmental maturity [that is, altriciality (nestbound) versus precociality (mobile and relatively independent)]. Comparison of the skeletal anatomy of perinatal extant archosaurians and perinatal dinosaurs suggests that known dinosaur hatchlings were precocial. These data are consistent with the overall similarity in nesting behavior of dinosaurs and modern crocodilians.

In terms of their diversity and longevity, dinosaurs and birds were/are surely among the most successful of terrestrial vertebrates. Unfortunately, interpreting many aspects of the biology of dinosaurs and the earliest of the birds presents formidable challenges because they are known only from fossils. Nevertheless, a variety of attributes of these taxa can be inferred by identification of shared anatomical structures whose presence is causally linked to specialized functions in living reptiles, birds, and mammals. Studies such as these demonstrate that although dinosaurs and early birds were likely to have been homeothermic, the absence of nasal respiratory turbinates in these animals indicates that they were likely to have maintained reptile-like (ectothermic) metabolic rates during periods of rest or routine activity. Nevertheless, given the metabolic capacities of some extant reptiles during periods of elevated activity, early birds were probably capable of powered flight. Similarly, had, for example, theropod dinosaurs possessed aerobic metabolic capacities and habits equivalent to those of some large, modern tropical latitude lizards (e.g., Varanus), they may well have maintained significant home ranges and actively pursued and killed large prey. Additionally, this scenario of active, although ectothermic, theropod dinosaurs seems reinforced by the likely utilization of crocodilian-like, diaphragm breathing in this group. Finally, persistent in vivo burial of their nests and apparent lack of egg turning suggests that clutch incubation by dinosaurs was more reptile- than birdlike. Contrary to previous suggestions, there is little if any reliable evidence that some dinosaur young may have been helpless and nestbound (altricial) at hatching.

Attenuation is the anelastic process which dissipates seismic energy by conversion to heat, thus decreasing the amplitude and modifying the frequency and phase content of a propagating wavelet. Laboratory measurements show seismic phase velocity and attenuation are dependent upon the fluid saturation and the product of frequency and pore‐fluid viscosity, with a peak in attenuation between the seismic and sonic bands. The dominant mechanism by which seismic energy is dissipated in the upper crust is local viscous fluid flow in pores of small aspect ratio. Phenomenologically, this behavior is modeled as a series of linear viscoelastic elements with a narrow distribution of relaxation times, where velocity and attenuation are related through the Hilbert transform. This model may be generalized to include constant-Q behavior, as observed in dry rocks. Solutions to the wave equation may be generated for an arbitrary frequency dependence of phase velocity and Q. When Q is nearly independent of frequency, the impulse response is asymmetric and the power spectrum is a straight line with slope proportional to Q. When Q is roughly proportional or inversely proportional to frequency, the impulse response is nearly symmetrical and the power spectrum is nonlinear. These results indicate that the frequency content, phase spectrum, and velocity may be strong indicators of the type of pore fluid in a formation. Alternatively, if the type of pore fluid is known, these attributes could be used to monitor temperature changes of a formation.