And while most dinosaur palaeontologists also believe long-necked dinosaurs - collectively known as sauropods - behaved this way, a leading Adelaide University researcher has just published new data which suggests otherwise.
Dr Roger Seymour, from Adelaide's Department of Environmental Biology, has co-authored a paper in the prestigious Proceedings of the Royal Society in London, which argues it was physically impossible for sauropods to behave in this way. The paper argues that due to the sauropods' possible heart size and metabolic rates, the only way they could have functioned on land was with a horizontal neck.
Dr Seymour based his findings on his research of the factors which determine heart size in animals. He has spent the past 24 years collecting data on heart morphology and arterial blood pressure in reptiles, birds and mammals to determine how blood pressure influences the thickness of the heart wall. His research is directed at understanding the evolution of vertebrate cardiovascular systems.
His research shows that heart size in all animals depends on two factors: the vertical distance of the head above the heart, and whether the animal was cold- or warm-blooded. For example, the giraffe has exceptionally high blood pressure and an enlarged heart because it has to pump blood up its long neck. Birds and mammals also have relatively large hearts because they are warm-blooded, while cold-blooded reptiles have low metabolic rates, low blood pressures and smaller hearts.
"We have determined that the left ventricle in a warm-blooded Barosaurus, for instance, would have needed to weigh about 2000kg to pump the blood its brain needed," he says. "This is impossible for at least three reasons. First, it would be difficult to fit such a heart in the available space; second, the heart would use more energy than the entire remainder of the body, and third; the thick walls would be mechanically so inefficient that they would expend more energy deforming themselves than in actually pumping the blood."
There only two viable solutions to the dinosaurs' blood pressure problem, Dr Seymour says: either they were restricted to holding their necks horizontally, that they were cold-blooded animals with low blood flow rates.
"We admit that they could have had a vertical neck, with blood supplied from a smaller heart, but only if they had a low metabolic rate typical of a cold-blooded reptile. Even in this case, however, the heart wall still would have been relatively thick and inefficient for pumping.
"The question whether dinosaurs were warm or cold-blooded has been debated over the last 30 years, but the metabolic rates of sauropods will probably never be known with certainty. In either case, it appears unlikely that these animals lifted their heads high as commonly depicted."
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