Most of the theories that I've heard involved higher oxygen content of the atmosphere, and plentiful food supply. Also most reptiles keep on growing all of their lives.
Googled the O2 exchange rate and came up with this- seems to be a subject of continuing study:
[LUNG VENTILATION AND GAS EXCHANGE IN THEROPOD DINOSAURS
John A. Ruben and his colleagues suggest (Reports, 14 Nov. 1997, p. 1267) that the lung
of theropod dinosaurs was most likely similar in form to that of several extant reptiles and was
therefore incapable of sustaining high oxygen (O2) exchange rates characteristic of endothermy.
We disagree for two reasons. First, we examined the comparative physiology literature and
determined that maximum oxygen exchange rates (VO2max) of some extant reptiles overlap the
oxygen consumption rates measured in some mammals during activity. Specifically,
exceptionally active reptiles with multicameral lungs (1) (for example, monitor lizards and sea
turtles) have values of VO2max that overlap or approach the oxygen exchange rates measured in
similar size mammals during activity (2). Therefore, the septate lung in those reptiles must be
capable of sustaining rates of gas flux characteristic of endotherms. However, mammals and
birds “typically” have a greater VO2max. Therefore, we addressed the question of what
modifications in the oxygen transport system of an extant reptile would be necessary to support
higher rates of oxygen consumption.
We used morphological and physiological measurements of extant reptiles and wellestablished
respiratory equations to model the gas exchange potential of the reptilian oxygen
delivery system and to examine the role of lung structure in constraining gas exchange. Each step
in the oxygen cascade is described by a set of respiratory equations and, consequently, it is
possible to describe mathematically the flux of oxygen through the entire cascade and to evaluate
the impact of modifications in any of its components (3). We used this approach to predict the
effects of modifying several parameters in the oxygen cascade on VO2max in a 1-kilogram
lizard, Varanus exanthematicus (4). Our analysis included four modifications: (i) a small increase
in the maximum cardiac output; (ii) an increased oxygen carrying capacity of the blood from
reptilian to mammalian values; (iii) an increase in maximum cardiac output combined with the
changes in blood oxygen-carrying capacity; and (iv) an increased respiratory gas exchange area
in the dorsal region of the lung through elaboration of the intercameral septa with a membranous
region in the ventral portion of the lung. Without modification of the lung structure, our analysis
predicts that changes in blood oxygen capacity and cardiac output support a VO2max that is 50%
of the value for a “typical” 1-kilogram (kg) mammal (5). However, if we combine these changes
with conservative modifications in lung morphology, we predict a VO2max that is nearly 70% of
the typical mammalian value. Our analysis indicates that modifications in several of the steps of
the oxygen cascade have a cumulative effect on VO2max (6). The resulting high oxygen flux rate
mandates an increase in lung ventilation that is 233% above the maximum level measured in
extant lizards.
Lizards have a mechanical constraint on simultaneous vigorous locomotion and costal
ventilation that arises from the design of the axial musculoskeletal system, and this mechanical
constraint was probably the primitive condition for all tetrapods (7). Consequently, the
fundamental change required to support sustainable high oxygen exchange rates was the
development of new mechanisms to increase ventilation (7). This constraint has been
circumvented to varying degrees in some extant lizards, for example, the use of the gular pump
to assist costal ventilation during activity (8) and in the lineages that gave rise to endotherms by
the evolution of ventilatory mechanics that are not limited by locomotor requirements (7).
Inadequate preservation of the soft-tissue components of the oxygen transport system
precludes accurate assessment of the aerobic potential of theropod dinosaurs. However, on the
basis of metabolic patterns in extant reptiles and our theoretical analysis, we find that the notion
that nonavian septate lungs constrain high oxygen flux rates is not supported. Our analysis
This was taken verbatim from Sciencemag.org.
Science Volume 281, Number 5373, Issue of 3 Jul 1998, p. 45.
suggests that modifications in lung structure were not a prerequisite for supporting higher
oxygen consumption rates. In the mammalian and archosaur lineages that evolved endothermy,
higher oxygen consumption rates could have been supported through changes in ventilatory
mechanics and increases in blood oxygen content and cardiac output.
James W. Hicks
Colleen G. Farmer
Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697,
USA]
And then we have this from Encarta-I guess I’ll never know:
[Earth’s environment during the dinosaurian era was far different than it is today. The days were several minutes shorter than they are today because the gravitational pull of the sun and the moon have over time had a braking influence on Earth’s rotation. Radiation from the Sun was not as strong as it is today because the Sun has been slowly brightening over time.
Other changes in the environment may be linked to the atmosphere. Carbon dioxide, a gas that traps heat from the Sun in Earth’s atmosphere—the so-called greenhouse effect—was several times more abundant in the air during the dinosaurian age. As a result, surface temperatures were warmer and no polar ice caps could form.
The pattern of continents and oceans was also very different during the age of dinosaurs. At the beginning of the dinosaurian era, the continents were united into a gigantic supercontinent called Pangaea (all lands), and the oceans formed a vast world ocean called Panthalassa (all seas). About 200 million years ago, movements of Earth’s crust caused the supercontinent to begin slowly separating into northern and southern continental blocks, which broke apart further into the modern continents by the end of the dinosaurian era.
As a result of these movements of Earth’s crust (see Plate Tectonics), there was less land in equatorial regions than there is at present. Deserts, possibly produced by the warm, greenhouse atmosphere, were widespread across equatorial land, and the tropics were not as rich an environment for life forms as they are today. Plants and animals may have flourished instead in the temperate zones north and south of the equator.
The most obvious differences between dinosaurian and modern environments are the types of life forms present. There were fewer than half as many species of plants and animals on land during the Mesozoic Era than there are today. Bushes and trees appear to have provided the most abundant sources of food for dinosaurs, rather than the rich grasslands that feed most animals today. Although flowering plants appeared during the dinosaurian era, few of them bore nuts or fruit.
The animals of the period had slower metabolisms and smaller brains, suggesting that the pace of life was relatively languid and the behavior patterns were simple. The more active animals—such as ants, wasps, birds, and mammals—first made their appearance during the dinosaurian era but were not as abundant as they are now.
IV Behavior and Physiology
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The behavior of dinosaurs was governed by their metabolism and by their central nervous system. The dinosaurs’ metabolism—the internal activities that supply the body’s energy needs—affected their activity level. It is unclear whether dinosaurs were purely endothermic (warm-blooded), like modern mammals, or ectothermic (cold-blooded), like modern reptiles. Endotherms regulate their body temperature internally by means of their metabolism, rather than by using the temperature of their surroundings. As a result, they have higher activity levels and higher energy needs than ectotherms. Ectotherms have a slower metabolism and regulate their body temperature by means of their behavior, taking advantage of external temperature variations by sunning themselves to stay warm and resting in the shade to cool down. By determining whether dinosaurs were warm- or cold-blooded, paleontologists could discover whether dinosaurs behaved more like modern mammals or more like modern reptiles.]
And Wiki gives me this:
“The climate of the Cretaceous is less certain and more widely disputed. Higher levels of carbon dioxide in the atmosphere are thought to have caused the world temperature gradient from north to south to become almost flat: temperatures were about the same across the planet. Average temperatures were also higher than today by about 10°C. In fact, by the middle Cretaceous, equatorial ocean waters (perhaps as warm as 20 °C in the deep ocean) may have been too warm for sea life, and land areas near the equator may have been deserts despite their proximity to water. The circulation of oxygen to the deep ocean may also have been disrupted. For this reason, large volumes of organic matter that was unable to decompose accumulated, eventually being deposited as “black shale”.
Not all of the data support these hypotheses, however. Even with the overall warmth, temperature fluctuations should have been sufficient for the presence of polar ice caps and glaciers, but there is no evidence of either. Quantitative models have also been unable to recreate the flatness of the Cretaceous temperature gradient.[citation needed]
Oxygen levels in the Mesozoic atmosphere were probably lower (12 to 15 %) than today’s level (20 to 21 %). Some researchers have postulated levels of 12 % because that was assumed to be the lowest concentration at which natural combustion could occur. However, a 2008 study concludes that at least 15 % is necessary.”