Fourty meters in length, 17 meters height and 100 tons weight: the largest animals ever alive were the dinosaurs who reigned earth for more than 100 million years, twice as long as every other recent or fossil land vertebrate class. But why did no mammal ever reach that size of those giants of the primitive times? Palaeontologist Prof. Dr. Martin Sander and his collegue Dr. Marcus Clauss (University of Bonn, Germany) now give in a recent "Science"-paper a plausible explanation for the giantism of the sauropods: A bird like lung, laying eggs, a flexible metabolic rate and the lack of teeth allowed especially to the herbivores among the dinosaurs to get that big.

They just swallowed the food instead of chewing it with teeth. The teeth would have made the heads of such big animals disproportionately heavy. So there was no restriction for the length of a dinosaurs neck because the head remained relatively small compared to the body size. Digestion of the food was conducted by the enormous stomachs of the giants.


"Tradition and modern trend" - a secret of success 


But another interesting finding was revealed by the scientists: a combination of "old fashioned" and modern biological adaptations solved another problem which such large animals faced: their offspring hatched out of eggs, being very small compared to their parents. Egg laying is an evolutionary conservative way of reproduction which was retained by the dinosaurs. But having such small and numerous offspring would mean extremely quick growth to get that big one day. This was a special challenge to the dinosaurs metabolism. Due to latest research results obviously the sauropods had in their youth a metabolic rate comparable to that of mammals allowing a quick growth. But when getting taller and bigger this high metabolic rate would lead to overheating in the adult dinosaurs. So probably they reduced with growing body size their metabolic rate.


So modern evolutionary inventions like the high metabolic rate and the bird lung combined with laying eggs and swallowing the food instead of chewing it with highly specialized teeth was the secret of success for the giantism of the extinct mega animals.


Due to an asteroid we do not need to eat insects any more


Mammals (which  developed from small mouse-sized small insectivores to the recent speciose group including us - the modern man - after the extinction of the dinosaurs) compared to that have another combination: reproduction with live birth, grinding of the food with teeth and a high metabolic rate. This combination probably is the reason for the constraint of mammals body size.

If not an asteroid some 65 millions years ago had crashed onto earth and had annihilated the largest animals which ever existed on earth we  - the mammals  - still would run around and chew worms and insects like the first of our kind.


Source: Sander PM, Clauss M.: Sauropod gigantism. Science. 2008 Oct 10;322(5899):200-1

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