Re: disappearance of the big creatures

Hi sharc, Thanks for this interesting question. The fossil record is full of very large organisms, some unique, some larger relatives of today's species and they are intriguing. My favourite are the large dragonflies with wingspans of up to 75cm, which I'd love to see flying around today.
There is no one general reason why large organisms disappeared and a lot of debate about why certain groups became extinct. Undoubtedly some large species or their ancestors died out in the mass extinctions, some of which destroyed 95% of all creatures. Climate changes probably play a large role in many cases. There are fundamental biological limits to an animals size, based on physical factors such as heat distribution, circulation and the diffusion of nutrients to the body tissues and these are sensitive to the environment in which the creature lives. If it's warm and food is abundant, you can thrive as a large, slothful creature that soaks up heat from sunshine and needs one large meal every day or so. If it becomes cooler that may affect both your metabolism and your food source and you may do better as a smaller creature with a high metabolic rate, that can forage over a wide area. This is why much of the debate over dinosaur extinction centres around whether they were cold-blooded, sluggish animals or more nimble and warm blooded. It's worth noting too that large animals exist today in climatically quite stable regions (e.g. blue whales in the ocean depths, elephants in the interiors of large continents).
Here in Australia there is great debate over the disappearance of Australian megafauna, which is linked to climatic change around 50 000 years ago when the interior of the continent suddenly dried up. As for the Pleistocene animals, debate rages over the influence of early human hunters-some say they played a large role, others that climatic change was far more significant.
There are some great web sites discussing these things:

htt p://cas.bellarmine.edu/tietjen/images/debating_extinction.htm
Discusses extinctions in Australia.
http://www.bagheer a.com/inthewild/ext_woollym.htm
Discusses the North American megafauna.
http://www.mov.vic .gov.au/dinosaurs/mammintro.stm
Another site discussing the ice-age mammals.
htt p://www.museum.state.il.us/exhibits/larson/LP_extinction.html
Illinois State Museum site on late Pleistocene extinctions.
htt p://www.museum.state.il.us/exhibits/larson/LP_extinction.html
http ://www.austmus.gov.au/biodiversity/factsheets/fs_mega2.html
These last 2 illustrate the extremes of the humans versus climate change debate!

Neil Saunders