Why have land mammals never evolved any species as gigantic as the sauropod dinosars?
We believe that the large dinosaurs were thermo-regulators. This means that they overcame the inefficiencies of a large body in order to have a sort of pseudo warm blooded system. There is evidence in the bones of some creatures for both warm and cold blooded characteristic vasculation. Basically what happens is that with such a large girth it would take the beast a very long time to cool. Compared with smaller reptiles that constantly need sun in order to have energy. Also moving a body around like that would generate alot of heat. There is no need for mammals to grow this large for heat because they are warm blooded. A large body size for a warm blooded animal would be a liability. For the sauropods it may have been a failed trial at warm blood. This is something that appears to be common in evolution.
