Just Big Lizards
Contents Updated: Thursday, August 05, 1999
"No non-dinosaur larger than a turkey walked the land in the age of the dinosaurs."
Astonishing Creatures
Many would say it is absurd to imagine intelligence in dinosaurs, cold-blooded, lumbering beasts with brains the size of walnuts. But is it so absurd? Were dinosaurs so primitive?
They were considered so for many years but in the 70s, two American professors, John Ostrom and Robert Bakker, drove a coach and four through previously held dogmas about these astonishing beasts. They argued convincingly that dinosaurs were not just big lizards, cold-blooded and sluggish—dinosaurs were hot-blooded. Like mammals and, especially, like birds they had a high metabolic rate and an active lifestyle. These new ideas provide a base for the reassessment of intelligence in dinosaurs. Hot-bloodedness changes the picture completely. The evidence for it is crucially important to the thesis presented here and justifies some detailed attention in this chapter.
The earliest large fossils that had been found were recognized by the authorities of the time like Baron Cuvier, Dr Mantell and the Reverend Buckland as being those of reptiles, similar to the monitor lizard and the iguana. These modern lizards are all cold-blooded. Strictly their blood is not cold but, unlike birds and mammals, they have no internal thermostat to keep their temperature constant. In cold weather their blood temperature drops and they become sluggish and inactive, but when the sun shines they bask in it until their temperature rises, then they become active and seek some sustenance. If the sun gets too hot they shelter in the shade or try to expose the minimum surface area to the sun’s rays to absorb less heat to prevent overheating.
When dinosaur fossils were first recognized, being lizard-like, they were classified as being of cold-blooded animals—though Richard Owen, who first categorized the dinosaurs, realized that, in many respects, they were more advanced than lizards and crocodiles. But it was the picture of a giant lizard that prevailed rather than that of a more advanced creature—until Bakker and Ostrom came along.
Owen’s definitive paper was written in 1841, yet almost 130 years later Robert Bakker could write that paleontologists still regarded dinosaurs as overgrown lizards in respect of their everyday behavior. Because crocodiles and lizards spent much of their lives in inactivity, sunning themselves on a convenient rock, the brontosaurus was pictured as a rotund, long necked crocodile moving about slowly and infrequently, and basking whenever it could in the sun.
The Physiology of Dinosaurs?
Was this picture of such amazingly successful animals correct? Were they really just big lizards?
If they were, their physiology, the physical construction of their bodies, would have meant that they were severely handicapped creatures. Let us take a look at these handicaps.
Lungs
Lungs are spongy air sacs with millions of tiny, thin walled lobes containing circulating blood to absorb oxygen from the air. The smaller the tiny air lobes, the greater the surface area exposed to the inhaled air and the more efficient the absorption of oxygen. In a lizard these lobes are not miniscule as they are in mammals, they are larger and correspondingly inefficient.
In the human mammal destruction of the walls between the tiny air cells , by smoking cigarettes, for example, reduces the surface available for the exchange of oxygen causing a chronic and totally debilitating disease, emphysema. People with emphysema suffer from oxygen deficiency and have to move slowly. Pure oxygen inhalers help a little. Lizards effectively have to live their whole lives with emphysema: they are unable to take in much oxygen, their blood does not get much and their activity is correspondingly curtailed.
In contrast, birds, which have some lizard-like features (witness the scales on their legs, for example), have highly efficient lungs to support their active lifestyle. Yet many dinosaurs had hollow bones containing additional air sacs connected to their lungs just like birds. Did they have bird-like lungs with subsidiary air sacs pushing air steadily through in one direction, much more efficient than mammals’ lungs in which the air travels first one way then the other. This alone could have ensured the dinosaurs’ superiority for millions of years. But did it also make them more prone to gaseous or particulate pollution contributing to their eventual extinction?
Heart
A lizard’s heart is particularly inefficient because it has only one main pumping chamber or ventricle whereas mammals have two. Two ventricles are more efficient. The oxygenated blood from the lungs can be kept separated from the oxygen depleted blood returning from the body.
In humans blue babies are born with a hole in their heart, not a hole to the outside of the heart—blood does not escape—a hole between two of the chambers of the heart allowing oxygenated and spent blood to mix. Consequently, the capacity of the blood for oxygen is not fully utilized. The blue baby’s arteries always carry some spent blood, which is dark in color unlike oxygenated blood which is bright red, giving it a blue look instead of the normal pink.
In a lizard the two bloodstreams have to enter the single ventricle where they mix just as they do in the heart of a blue baby. Lizards are like mammals with a hole in the heart.
A zoologist, Roger Seymour of Adelaide University, researched into the connection between the difference in height between the heart and the brain of sauropod dinosaurs and the blood pressure needed to push the blood up to that height. If the animals were browsing from high branches then high blood pressures would obviously be required (a giraffe’s blood pressure is twice that of a man) and to generate such high pressures the heart would have to be sophisticated. It would certainly need four chambers not two like lizards.
High browsing dinosaurs, if no others, must have had four-chambered hearts like this and, if they had them, it is probable that the others had too.
Bone
Bone is not simply a dead inorganic material which only provides the internal framework of a vertebrate’s body, it is also an important living organ equivalent to the heart and the lungs. The bone marrow is where blood cells are made and the bones act as a reserve of calcium, a mineral that is essential in the functioning of muscle.
Naturally the efficient working of bone in these tasks depends upon a liberal supply of blood to transport the organ’s products to the sites in the body where they are needed. Mammals and birds have a complex system of blood supply through the bones but in reptiles the blood supply is much more feeble. Growth is slow in the warm days of summer and stops altogether in the winter giving annual growth rings like those in trees or on tortoise shells.
The exchange of calcium between the bones and the blood occurs in structures called the Haversian canals which are relatively lacking in reptile bones. In 1988 Robin Reid of Belfast University published an extensive survey of bone structure. He found that reptiles could have a mammalian type of bone structure, particularly if they were large or grew in hot temperatures.
What of dinosaurs? Sometimes they had reptilian and sometimes mammalian types of bone but most dinosaur bone is of the warm-blooded variety, whereas it is unusual for reptiles to have it. Reptilian physiology obliges reptiles to be slow and to conserve their energy. Compared with mammals they suffer from almost crippling defects of lungs, heart and bone. Unlike mammals their bodies simply cannot sustain continuous effort. That is why they move jerkily and spend long periods totally motionless.
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