Terry D. Jones

The metabolic status of some Late Cretaceous dinosaurs

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 M0.72. 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.

Cursoriality in bipedal archosaurs

Modern birds have markedly foreshortened tails and their body mass is centred anteriorly, near the wings. 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 animals centre of mass; stride generation stems largely from parasagittal rotation of the lower leg about the knee joint. In contrast, bipedal dinosaurs had a centre of mass near the hip joint and rotated the entire hindlimb during stride generation. 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 dinosaur, 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.

Respiratory and Reproductive Paleophysiology of Dinosaurs and Early Birds

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.

Selective Factors Associated with the Origin of Fur and Feathers1

Abstract 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.

Juvenile Skeletal Structure and the Reproductive Habits of Dinosaurs

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.

Respiratory physiology of the dinosaurs

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.