News & Media
BODY SIZE IN HERBIVOROUS DINOSAURS
Evolution is a fascinating field for a scientist. It unleashes the imagination, letting you retrodict what might have happened in the distant past to guide the development of life on earth, even though you don’t have direct proof of the validity of the conclusions. And so last year Science published a “Perspectives” piece (Sander and Clauss, 322:200-201) on sauropod gigantism which provides an answer to the question, how did these dinosaurs reach body sizes that remain unsurpassed in land-dwelling animals?
Sauropods made their first appearance in the late Triassic, and by about 150 million years ago were found world-wide except in what is now Antarctica. They were massive: some weighed in at 80 metric tons. The most well-known members of the sauropod family were Apatosaurus (the dinosaur formerly known as Brontosaurus), Brachiosaurus, and Diplodocus. They had body lengths of more than 40 meters, and could be over 17 meters in height. They were by far the most successful herbivores. Their huge bodies were supported by gigantic legs, and they had long necks and tails. The head was small, with no sophisticated masticatory apparatus; they could not grind or chew vegetation, which was swallowed whole. Sander and Clauss proposed that these organisms were able to achieve such tremendous body size through “a combination of phylogenetic heritage (lack of mastication, egg-laying) and a cascade of evolutionary innovations (high-growth rate, avian-style respiratory system, and a flexible metabolic rate)." They also suggested that the sauropod digestive system was specialized to store and slowly break down vegetation.
Now, I think the authors missed the boat. Although the arguments for each of these factors playing a role in the development of an enormous body size make some sense, it is curious that the authors missed what must have been the evolutionary prerequisite for all these other attributes to develop - the myelin sheath, which is the most important specialization of the vertebrate nervous system after the synapse. The sheath is produced by myelinating glial cells that synthesize a membrane (myelin) that wraps in a helical coil that provides insulation for the axon. The sheath is segmented, and there are openings where the axon is “bare” and where sodium channels cluster. The action potential “jumps” from node to node at great speed (saltatory conduction), much greater than if the nerve were nonmyelinated. A myelinated nerve can conduct up to 100 meters per second along a thin fiber (10 microns or so). This same 10 micron diameter axon in an unmyelinated state could only support an extremely slow rate of conduction, on the order of 1 meter per second.
Imagine a giant sauropod whose nerves are not myelinated. Bitten in the tail by a predator, it would take a full 40 seconds for action potentials to ascend the length of the tail, body and neck to reach its brain, and another 40 seconds for the return signal to the tail muscles. Is there any doubt that this reaction time would be incompatible with a successful defensive or escape response? However, myelinated nerves of the same diameter would require only 800 milliseconds for the round trip reaction time. Thus, we have to conclude that by myelinating the nervous system, massive body size became possible; it permitted survival by organisms that then became progressively larger, possibly because of secondary factors such as those proposed as Sander and Clauss.
For further reading see Zalc et al, 2008, The origin of the myelination program in vertebrates. Current Biology 18:R511-12.