Agriotherium is the name of an extinct genus of bears, known as the giant short-faced bears of the family Ursidae. They occur first in Miocene Era deposits estimated to be about 13 million years old. Most appear about five million years ago, when there was an explosion of different kinds of bear species at a time of great environmental changes at the time of the Miocene-Pliocene boundary.
Four species of Agriotherium are currently recognised and their fossils have been found across Eurasia, Africa and North America until about 2.5million years ago. They have the widest distribution of any of the Ursidae and are the only bears known to have made it to subSaharan Africa. One specimen has been measured at 2.7 meters (9 feet) long and they are believed to have been anywhere from 100 to 600 kg in weight so they were indeed big bears. As their common name suggests, they are distinguished from modern bears by having a much shorter muzzle.
Like most living bears, these animals had large crushing teeth and probably lived on a mixed diet of vegetation and meat when it was available. It probably lived a similar lifestyle to modern bears, eating large quantities of vegetation but capable of killing anything it could catch, from rodents to unwary ungulates. However, because it was a large, obvious and probably fairly slow animal because of its bulk, it is more likely to taken advantage of scavenging opportunities and most of the meat in its diet probably came from scavenging rather than active hunting. Being such a large animal, it probably lived out in the open on grasslands rather than in heavily forested country.
Of course, Agriotherium is known only from fossils so speculations about its life style can at most be educated guesses based on its anatomy and what is known about the behaviour of modern bears. It lived at a time when large mammals dominated the landscapes. There were lots of different kinds of bears, wolves and other predators that took advantage of the huge herds of ungulates: deer and antelope, horses and camels. At http://www.science-art.com/image.asp?id=846 there is an artist's impression of what one of these bears would have looked like attacking a small ancient giraffe.
Why did they become extinct when they were so big and powerful? Did the climate change dramatically? They died out over two million years ago so we cannot blame humans for their demise. They were successful for some eleven million years but in the end they went the way of over 90% of all known species: they died out. More studies will have to be made before definite causes can be ascertained. As with most fossil species, the fossils raise more questions than answers. If only we had a time machine so we could have a look for ourselves. In the meantime we will just have to use our imaginations to try and understand the world and lifestyle of these extinct bears.